the study of organic dyes for p-type dye- sensitized solar ...349533/fulltext01.pdf · synthesis...

Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan i Stockholm

framlägges till offentlig granskning för avläggande av doktorsexamen i kemi med

inriktning mot organisk kemi torsdagen den 23 september kl 10.00 i sal F3, KTH,

Lindstedtsvägen 26, Stockholm. Avhandlingen försvaras på engelska. Opponent är

Professor Michael Grätzel, Ecole Polytechnique Fédérale de Lausanne, Switzerland.

The Study of Organic Dyes for p-Type Dye-

Sensitized Solar Cells

Peng Qin

Doctoral Thesis

Stockholm 2010

ISBN 978-91-7415-695-9

ISSN 1654-1081

TRITA-CHE-Report 2010:28

© Peng Qin, 2010

Universitetsservice US AB, Stockholm

Peng Qin, 2010: “The Study of Organic Dyes for p-Type Dye-Sensitized Solar

Cells” Organic Chemistry, KTH Chemical Science and Engineering, Royal

Institute of Technology, SE-100 44 Stockholm, Sweden.

Abstract

This thesis concerns the study of D–π–A type dyes as sensitizers for NiO-

based p-type dye-sensitized solar cells. The focus has been on the design and

synthesis of efficient dyes and the identification of parameters limiting the

solar cell performance.

We have developed a new design strategy for the dyes: upon photoexcitation

of the dye, the electron density is moving from the part that is attached to the

semiconductor towards the part which is pointing away. This intramolecular

charge transfer provides an efficient pathway for the following charge transfer

processes. The first organic dye, composed of a triphenylamine (TPA) moiety

as the electron-donor, dicyanovinyl groups as the electron-acceptors and linked

by thiophene units, showed much better photovoltaic performance than other

dyes reported at the same time, turning it into a model for future dye design.

A series of dyes with different energy levels were synthesized and

characterized on NiO-based devices using iodide/triiodide as redox couple.

Lower photovoltaic performance was obtained for the dye with less negative

reduction potential due to the insufficient driving force for dye regeneration.

We have investigated the symmetric and unsymmetric structures of the dyes.

The breaking of molecular symmetry did not significantly broaden the

absorption spectrum, or improve the efficiency. In addition, we have tuned the

molecular structure to prevent charge recombination. Increasing the distance

between the anchoring group and the electron-acceptor was an effective way to

improve the device efficiency. Besides TPA-based compounds, a zinc

porphyrin dye was also synthesized and tested in p-type solar cells. However,

the solar cell performed less well due to its narrow absorption band and the

tendency for aggregation. Co-sensitization of the TPA-based dye with the

porphyrin dye did not result in higher photovoltaic performance.

After optimization of the dye structure, the highest overall conversion

efficiency was achieved for the P5-sensitized solar cell, based on 1.5 μm NiO

film prepared from NiCl2 and the F108 template precursor, and an acetonitrile-

based electrolyte.

Keywords: organic dye, sensitizer, TPA, donor, linker, acceptor, energy level,

p-type, solar cell, nickel oxide, photovoltaic, IPCE, efficiency, charge transfer

Abbreviations

DSSC (DSC) dye-sensitized solar cell

NiO nickel oxide

WE working electrode

CE counter electrode

FTO fluorine-doped tin oxide

CB conduction band

VB valence band

AM air mass

IPCE incident photon-to-current conversion efficiency

APCE absorbed photon-to-current conversion efficiency

Voc open-circuit voltage

Jsc short-circuit photocurrent density

ff fill factor

η overall solar energy to electricity conversion efficiency

LHE light harvesting efficiency

Φinj the quantum yield of charge injection

ηcoll charge collection efficiency

λ wavelength of the light

I light intensity

ɛ molar extinction coefficient

A absorbance

q elementary charge

NA Avogadro’s constant

c concentration

Jlim limiting current density

τh hole lifetime

τtr hole transport time

HOMO highest occupied molecular orbital

LUMO lowest unoccupied molecular orbital

ICT intramolecular charge transfer

E(D/D+) ground state oxidation potential of the dye

E(D/D–) ground state reduction potential of the dye

E0–0 0–0 transition energy

NHE normal hydrogen electrode

CV cyclic voltammetry

NMR nuclear magnetic resonance

TBA(PF6) tetrabutylammonium hexafluorophosphate

Fc/Fc+ ferrocene/ferrocenium

TPA triphenylamine

dppf diphenylphosphinoferrocene

dba dibenzylideneacetone

N3 cis-di(thiocyanato)bis(2,2′-bipyridine-4,4′-dicarboxylate)

ruthenium(II)

N719 bis(tetrabutylammonium)cis-di(thiocyanato)bis(2,2′-bipyridine

-4-COOH,4′-COO–)ruthenium(II)

F108 polyethyleneoxide133-polypropyleneoxide50-

polyethyleneoxide133

NBS N-bromosuccinimide

DDQ 2,3-dichloro-5,6-dicyano-p-benzoquinone

TCNE tetracyanoethylene

TFA trifluoroacetic acid

DCM dichloromethane

THF tetrahydrofuran

DMF N,N-dimethylformamide

List of Publications

This thesis is based on the following papers, referred to in the text by their

Roman numerals I–V:

I. Design of an Organic Chromophore for p-Type Dye-Sensitized

Solar Cells

Peng Qin, Hongjun Zhu, Tomas Edvinsson,

Gerrit Boschloo,

Anders

Hagfeldt, and Licheng Sun

J. Am. Chem. Soc. 2008, 130, 8570–8571.

II. High Incident Photon-to-Current Conversion Efficiency of p-Type

Dye-Sensitized Solar Cells Based on NiO and Organic

Chromophores

Peng Qin, Mats Linder, Tore Brinck, Gerrit Boschloo, Anders


Adv. Mater. 2009, 21, 2993–2996.

III. Double-Layered NiO Photocathodes for p-Type DSSCs with Record

IPCE

Lin Li, Elizabeth A. Gibson, Peng Qin, Gerrit Boschloo, Mikhail

Gorlov, Anders Hagfeldt, and Licheng Sun

Adv. Mater. 2010, 22, 1759–1762.

IV. Synthesis and Mechanistic Studies of Organic Chromophores with

Different Energy Levels for p-Type Dye-Sensitized Solar Cells

Peng Qin, Joanna Wiberg, Elizabeth A. Gibson, Mats Linder, Lin Li,

Tore Brinck, Anders Hagfeldt,

Bo Albinsson, and Licheng Sun

J. Phys.Chem.C 2010, 114, 4738–4748.

V. Structural Modifications to Triphenylamine Based Chromophores

for Control of Charge Recombination in p-Type Dye-Sensitized

Solar Cells

Peng Qin, Elizabeth A. Gibson,

Mats Linder, Tore Brinck, Anders


Manuscript

Paper not included in this thesis:

VI. Tuning the HOMO and LUMO Energy Levels of Organic

Chromophores for Dye Sensitized Solar Cells

Daniel P. Hagberg, Tannia Marinado, Karl Martin Karlsson, Kazuteru

Nonomura, Peng Qin, Gerrit Boschloo, Tore Brinck, Anders Hagfeldt,

and Licheng Sun

J. Org. Chem. 2007, 72, 9550–9556.

Table of Contents Abstract

Abbreviations

List of publications

1. Introduction 1.1. p-Type dye-sensitized solar cells (p-type DSSCs) ........................................ 2 1.2. Dyes for NiO-based p-type DSSCs............................................................. 4 1.3. The aim of this thesis ................................................................................ 7

2. General characterization methods 2.1. Structural characterization ......................................................................... 8 2.2. Optical characterization ............................................................................ 8 2.3. Electrochemical characterization ................................................................ 9 2.4. Photovoltaic parameters characterizing the performance of p-type DSSCs ..... 9

2.4.1. Incident photon-to-current conversion efficiency .................................... 9 2.4.2. Current–voltage characteristics .............................................................. 10

3. Design of D–π–A dyes for p-type DSSCs 3.1. Donor–π-conjugated linker–acceptor (D–π–A) dyes. .................................. 11 3.2. Aim of the study .................................................................................... 14 3.3. TPA-based dye (P1) for p-type DSSCs ..................................................... 14

3.3.1. Synthesis ................................................................................................ 14 3.3.2. Optical and electrochemical characterization ......................................... 16 3.3.3. Solar cell performance of P1 .................................................................. 18

3.4. Conclusions ........................................................................................... 19 4. The study of organic dyes with symmetric and unsymmetric

structures: modification of the conjugated linker 4.1. Aim of the study .................................................................................... 20 4.2. Modification of dye structure for p-type DSSCs ........................................ 20

4.2.1. Synthesis ................................................................................................ 21 4.2.2. Optical and electrochemical characterization ......................................... 23

4.3. Optimization of the electrolyte for p-type DSSCs ...................................... 25 4.4. Modification of the NiO film for p-type DSSCs ......................................... 28

4.4.1. 1.2 μm NiO film with Ni(OH)2 precursor .............................................. 28 4.4.2. 1.2 μm NiO film with NiCl2 and the F108 template precursor ............... 29

4.5. Conclusions ........................................................................................... 32 5. The study of organic dyes with different thermodynamic driving

force: modification of the electron-withdrawing group 5.1. Aim of the study .................................................................................... 33 5.2. Synthesis ............................................................................................... 34 5.3. Optical and electrochemical characterization ............................................. 35 5.4. Solar cell performance ............................................................................ 37

5.5. Conclusions ........................................................................................... 39 6. The study of organic dyes for control of charge recombination:

modification of the conjugated linker 6.1. Aim of the study .................................................................................... 40 6.2. Synthesis ............................................................................................... 41 6.3. Optical and electrochemical characterization ............................................. 44 6.4. Solar cell performance ............................................................................ 45 6.5. Conclusions ........................................................................................... 47

7. The study of porphyrin dye and its application for co-sensitization

in p-type DSSCs 7.1. Aim of the study .................................................................................... 48 7.2. Synthesis ............................................................................................... 49 7.3. Optical and electrochemical characterization ............................................. 51 7.4. Solar cell performance ............................................................................ 53 7.5. Conclusions ........................................................................................... 58

8. Concluding remarks

Acknowledgements

References

Appendix A

Appendix B

Till min familj

1

1. Introduction

Energy is essential to just about every aspect of human life. From the

beginning of the last century, the energy demand has gone up quickly due to an

increasing global population, leading to the depletion of fossil fuels, the key

energy sources of today, at an alarming rate.1 The use of fossil fuels also has

negative effects on the environment.2 The increased concentration of

greenhouse gases due to the combustion of fossil fuels plays an important role

in climate variations.3 The worldwide power consumption is expected to rise in

the next decades. This increasing energy demand in the near future forces us to

seek environmentally clean alternative energy resources.

Solar energy is expected to be a good candidate for a future renewable energy

source. The energy provided by the sun in one hour is larger than the energy

consumption globally in an entire year.4 However, capturing solar energy and

converting it to chemical or electrical energy efficiently with a low cost is still

a big challenge. Photovoltaic cells are one of the devices being used to capture

solar energy. The crystalline silicon based photovoltaic devices are the most

widely used at present with solar to electricity conversion efficiencies of

approximately 25%.5,6

They are now at a mature state of technical

development, but the high cost of manufacturing restricts their competition

with traditional energy sources. So the cost-effective photovoltaic devices

receive enormous attention. Different inorganic materials, such as amorphous

silicon, cadmium telluride (CdTe), copper indium gallium diselenide (CIGS),

gallium arsenide (GaAs), and both the single junction and multijunction cell

technologies have been investigated, giving efficiencies of 10–32% (under AM

1.5 standard sunlight).6 But the toxicity or scarcity of the materials become

drawbacks of these kind of devices for large scale use. In the 1990s, organic-

based photovoltaic cells, which employ organic constituents for light

harvesting or charge carrier transport, were extensively studied. Since the

organic materials are abundantly available, this may lead to a relatively low

cost for device fabrication. However, their efficiencies at present are lower

than those based on inorganic materials.7,8

The dye-sensitized solar cell (DSSC) belongs to organic-based photovoltaic

cells. It uses molecules to absorb photons and separates the two functions of

light harvesting and charge-carrier transport. The advantage of this kind of

solar cell is that it is compatible with various supporting materials and can be

produced under mild conditions, making it significantly less expensive than the

earlier cell design. The breakthrough for DSSCs occurred in 1991, when

2

O’Regan and Grätzel reported the devices with efficiencies of 7–8%, based on

a ruthenium complex and nanoporous TiO2 film.9 Until now most research in

this field has focused on the sensitization of n-type semiconductors, such as

TiO2 and ZnO (n-type DSSCs), and conversion efficiencies of more than 11%

have been achieved.10–12

In a n-type DSSC, which contains a photoactive anode and a passive cathode,

an electron is injected from the excited dye to the conduction band of the n-

type semiconductor. Inversing the working principle of this, p-type

semiconductors can also be sensitized and an active photocathode can be

assembled (p-type DSSCs). Until now there have been very few studies on the

sensitization of p-type semiconductors and the efficiencies are relatively low.

However, the investigation of p-type DSSCs leads to the development of a new

kind of photon-to-electric device and opens the possibility of increasing the

efficiencies of existing n-type DSSCs by making tandem cells in the future, in

which both electrodes are photoactive.

1.1. p-Type dye-sensitized solar cells (p-type DSSCs)

The p-type DSSC is a sandwich structured device composed of a photoactive

working electrode (cathode), a passive counter electrode (anode), and redox

electrolyte (Figure 1). The main components are: 1) a fluorine-doped SnO2

(FTO) glass substrate, 2) a nanostructured p-type semiconductor layer, 3) a

light-absorbing layer, normally the dye, 4) a platinized (Pt) conducting glass

counter electrode, and 5) the electrolyte. In our case, the p-type semiconductor

is nickel oxide (NiO), which has a band gap of ~3.6 eV and has the advantage

of good stability and transparency. The electrolyte is a liquid system containing

the iodide/triiodide redox couple.

3

2) nanostructured semiconductor film

3) dye molecule

5) electrolyte

4) platinized conducting substrate

e-

CE

(counter electrode)(working electrode)

WE

LOAD

Ox

Red

1) fluorine-doped SnO2 (FTO) glass substrate

Figure 1. Schematic illustration of p-type DSSC.

In p-type DSSCs, visible light absorption by the dye is followed by electron

transfer from the valence band of the semiconductor to the dye (or, in other

words, hole injection from the dye to the valence band of the semiconductor).

The dye is then regenerated by electron transfer from the reduced dye to the

oxidized species (I3–) in the electrolyte (Figure 2). If the reduced dye can not

react with the electrolyte within the charge-separated lifetime, it may

recombine with the hole in the semiconductor (geminate recombination). The

holes in the semiconductor move to the back collector of the working electrode

and the reduced species (I–) in the electrolyte diffuses to the Pt electrode. This

charge collection gives rise to a cathodic photocurrent in the external circuit.

Dye|NiO + hv → Dye*|NiO (excitation)

Dye*|NiO → Dye

–|NiO + h

+(NiO) (hole injection)

Dye–|NiO

+ h

+(NiO) → Dye|NiO (geminate recombination)

Dye–|NiO + 1/2 I3

– → Dye|NiO + 3/2 I

– (dye regeneration)

3/2 I– → 1/2 I3

– + e

– (Pt)

(regeneration of I3

– at the counter electrode)

4

Figure 2. Schematic diagram illustrating the key processes in a p-type DSSC under

illumination.

1.2. Dyes for NiO-based p-type DSSCs

The molecular structure of a dye plays an important role in DSSCs. After

absorption of light, charge separation is initiated at the dye|NiO surface. The

performance of a solar cell strongly depends on the light harvesting ability and

the relative energy levels of the dye, as well as the kinetics of the charge

transfer processes at different interfaces. In order to increase the efficiency of a

DSSC, the relationship between the chemical structure of the dye and the

photovoltaic performance of the cell has to be examined. The next section

illustrates the organic dyes so far developed for NiO-based p-type DSSCs.

In 1999 Lindquist and co-workers described the preparation and

characterization of a photovoltaic cell based on the sensitization of a

nanostructured NiO electrode with tetrakis(4-carboxyphenyl)porphyrin or

erythrosin B (Figure 3).13

The cathodic photocurrent they observed was

explained by hole injection from the dye to the valence band of NiO.

Unfortunately, the photovoltaic properties were quite poor, giving the highest

incident photon-to-current conversion efficiency (IPCE) of 0.24% and 3.44%,

and the overall conversion efficiency (η) of 0.0033% and 0.0076% for the

porphyrin and erythrosin B, respectively. The same group later published a

tandem DSSC consisting of a N3|TiO2(N3:cis-di(thiocyanato)bis(2,2′-

bipyridine-4,4′-dicarboxylate)ruthenium (II), Ru(dcbpyH2)2(NCS)2)

photoanode and an erythrosin B|NiO photocathode. Although the photovoltage

was higher than those of the individual cells, a significant drop of photocurrent

was observed compared with the N3|TiO2 based cell alone.14

With a similar

5

idea, Suzuki and co-workers used a merocyanine (MC) dye instead of

erythrosin B in the cathode, which resulted in a Jsc value of 3.62 mA cm–2

and

a Voc value of 0.918 V, with Jsc of 1.00 and 5.83 mA cm–2

, Voc of 0.093 and

0.762 V for MC|NiO and N3|TiO2 based DSSCs, respectively, under the same

conditions.15

All the results indicated that the photocurrent from the NiO-based

cathode was quite low and could not match well with the TiO2-based anode,

leading to a limitation of both the photocurrent and the overall conversion

efficiency of the tandem cell.

N

NH N

HN

COOH

COOH

COOH

HOOC

O

II

NaO

I

O

I

COONa

erythrosin B

tetrakis(4-carboxyphenyl)porphyrin

N

SCH CH

N

S

C18H37

CH2COOH

S

O

MC

Figure 3. The molecular structures of tetrakis(4-carboxyphenyl)porphyrin, erythrosin B

and MC.

In order to find out the reasons limiting the photocathode performance, the

dynamics of photoinduced charge transfer between the p-type semiconductor

and the excited dye, together with the subsequent recombination, were

investigated by Hammarström, Hagfeldt, Odobel and their co-workers.16,17

Two different dyes, coumarin 343 and a phosphorus porphyrin, were used as

sensitizers (Figure 4). According to their results, the low efficiency of NiO-

based solar cells could be rationalized by the remarkably fast recombination

between the reduced dye and the hole in the semiconductor, which prevented

the efficient hole collection and dye regeneration. Besides the charge transfer

process, the effect of the I–/I3

– redox couple was also investigated at the same

time, based on both coumarin dyes and cyanine dyes, showing more

information about the mechanism of photocurrent generation in p-type

system.18–20

6

N

N N

N

Ar

Ar

Ar

ArP

COOH

O

O

COOH

Ar =

t-Bu

t-Bu

phosphorus porphyrincoumarin 343(C343)

N O

OH

O

O

Figure 4. The molecular structures of C343 and phosphorus porphyrin.

We initially broke the record of IPCE for p-type DSSCs with a donor-acceptor

dye, which was shown in chapter 3. After that, research aiming at more

efficient dyes has been performed. A peryleneimide dye (PI) and the

corresponding covalently linked peryleneimide-naphthalenediimide dyad

(NDI-PI) were published in 2008 (Figure 5).21

For the latter dyad, the

existence of a naphthalenediimide unit led to a long-lived charge-separated

state, and retarded charge recombination between the hole and the reduced

dye, leading to a higher IPCE of 4.0% compared with that of 1.3% for PI. This

NDI-PI dyad has recently reached the efficiency of 0.2% and IPCE of 31%

with a cobalt-based redox mediator.22

N

O

O

OO

O

t-Bu

t-Bu

COOH

t-Bu

t-Bu

N

O

O

OO

O

t-Bu

t-Bu

COOH

NN

O

O

O

O

C8H17

PI NDI-PI

Figure 5. The molecular structures of PI and NDI-PI.

Another series of donor-acceptor dyes were published by Bach and co-workers

in 2010, using triphenylamine as the donor, perylenemonoimide as the

acceptor, and oligothiophene as the conjugated chain (Figure 6).23

Based on

7

1.25 μm NiO film prepared by commercially available NiO nanopowder, a Voc

of 0.218 V and a Jsc of 5.35 mA cm–2

were obtained for dye 3, giving η of

0.41%. This is the highest efficiency for p-type DSSCs until now. The tandem

cell combined with N719|TiO2 (N719: (Bu4N)2[Ru(dcbpyH)2(NCS)2]) anode

exhibited the highest efficiency of 2.42%, still less efficient than the equivalent

N719|TiO2 based solar cell (η = 5.9%).

N

O

OS

SN

OH

OH

O

O

C6H13

n

1: n = 12: n = 23: n = 3

C6H13

Figure 6. Molecular structures of the dyes developed by Bach and co-workers.23

1.3. The aim of this thesis

The aim of this thesis was to design and synthesize organic dyes as sensitizers

for NiO-based p-type DSSCs, and to investigate the effect of molecular

structures on the photophysical, electrochemical properties, as well as the

photovoltaic performance.

A model of the dye structure was initially designed. Using this model as the

standard, different kinds of structural modifications were investigated,

showing their influence on the light harvesting ability, charge-separation

property and energy levels. With this strategy, we hoped that we could identify

the parameters limiting the performance of p-type DSSCs and determined the

guidelines of designing more efficient p-type dyes in the future.

8

2. General characterization methods

2.1. Structural characterization

All the compounds synthesized in this thesis were characterized by 1H NMR,

13C NMR and mass spectrometry. NMR spectra were measured on a Bruker

Avance 400 spectrometer at 400 (100) MHz or a Bruker Avance DMX 500 at

500 (125) MHz. Mass spectra were performed by using a Q-Tof Micro mass

spectrometer equipped with Z-spray ionization source. The detailed

information will be shown in later chapters.

2.2. Optical characterization

The absorption spectroscopy is used to identify the optical properties of the

dye both in solution and adsorbed on the film. The wavelength of the light (λ),

together with the absorbance (A) at this wavelength, is recorded. The spectra in

our work were recorded on a Lambda 750 UV–vis spectrometer.

The molar extinction coefficient (ε) is particularly useful when comparing

spectra of different compounds and determining the relative strength of light

absorbing functions. ε is defined as equation 1:

c l=

A (1)

Where A is the absorbance, c is the sample concentration (mol L–1

), and l is the

length of light path through the sample (cm). For a given compound, a standard

graph of ε versus λ can be drawn, which is effectively “concentration-

corrected”.

Fluorescence is the emission of light from the singlet excited state by a

substance that has absorbed light or other electromagnetic radiation. In most

cases, absorption of light with a certain wavelength induces an emission of

light with a longer wavelength (lower energy) due to thermal losses. The

fluorescence spectra in this thesis were measured on a Cary Eclipse

fluorescence spectrophotometer. From the intersection of the normalized

absorption and emission spectra, the 0–0 transition energy (E0–0) of the dye can

be estimated.

9

2.3. Electrochemical characterization

Cyclic voltammetry (CV) is a type of potentiodynamic electrochemical

measurement and has been widely used to study the electrochemical properties

of a compound. The spectra in this thesis were performed with a CH

Instruments 660 potentiostat using 0.1 M tetrabutylammonium

hexafluorophosphate (TBA(PF6)) in different solvents as the supporting

electrolyte. A glassy carbon working electrode, a platinum counter electrode

and an Ag/Ag+ reference electrode were used. The voltage was measured

between the working electrode and the reference electrode and the current was

measured between the working electrode and the counter electrode. The data

were then plotted as current versus voltage. The system was initially calibrated

with ferrocene/ferrocenium (Fc/Fc+) and the potential measured versus Fc/Fc

+

was added +0.63 V compared to the normal hydrogen electrode (NHE).

2.4. Photovoltaic parameters characterizing the performance of p-type

DSSCs

The photovoltaic performance of DSSCs is mainly characterized by the

following parameters: IPCE, open-circuit voltage (Voc), short-circuit

photocurrent density (Jsc), fill factor (ff), and the overall conversion efficiency

(η). The IPCE was recorded using a monochromatic light from a system

consisting of a Xenon lamp, a monochromator, and appropriate filters. I–V

characteristics were measured using a Keithley source/meter under simulated

sunlight from a Newport 300 W solar simulator, giving light with an intensity

of 100 mW cm–2

. Both systems were calibrated against a certified reference

solar cell (IR-filtered silicon solar cell, Fraunhofer ISE, Freiburg, Germany).

2.4.1. Incident photon-to-current conversion efficiency

“The IPCE value corresponds to the photocurrent density produced in the

external circuit under monochromatic illumination of the cell divided by the

photon flux that strikes the cell”.7 It is obtained by equation 2:

IPCE (λ) =1240 Jph

λ I(2)

Jph is the short-circuit photocurrent density generated by the monochromatic

light, λ and I are the wavelength and intensity of the monochromatic light.

The IPCE can also be expressed as the product of the light harvesting

efficiency (LHE), the quantum yield of hole injection (Φinj), and the efficiency

of collecting the holes at the FTO substrate (ηcoll).

10

IPCE (λ) LHE (λ)Φinjηcoll= (3)

The absorbed photon-to-current conversion efficiency (APCE) can be

estimated from the ratio of IPCE and LHE. It is a better measure of the

intrinsic light conversion efficiency.

APCE (λ) = IPCE (λ) LHE (λ)/ (4)

2.4.2. Current–voltage characteristics

The measurement of a J–V curve is an effective way to evaluate the

photovoltaic performance of a DSSC. A standard test condition of air mass 1.5

(AM 1.5) with a light intensity of 100 mW cm–2

was used for solar cell

characterization. From the J–V curve, the following parameters can be

obtained: Voc, Jsc, ff and η (Figure 7).

Voc

Jsc

Jmax

Vmax V / V

J / mA cm-2

Pmax

Figure 7. Current–voltage characteristics of DSSCs.

The Voc of a p-type DSSC corresponds to the difference between the quasi-

Fermi level of the semiconductor and the redox potential of the electrolyte. The

position of the quasi-Fermi level is in the band gap, close to the valence band

of p-type semiconductor. Jsc is the photocurrent per unit area under short-

circuit condition. It is related to the optical properties of the dye, as well as to

different dynamic processes in the cell. The ff of a solar cell is defined as the

maximum power output (Jmax × Vmax) divided by the product of Jsc and Voc.

From the parameters discussed above, the η of a solar cell can be calculated by

the ratio of the maximum output electrical power to the energy of incident

sunlight (Is).

η Jsc Voc ff Is

ff = Jmax Vmax VocJsc/

= /

(5)

(6)

11

3. Design of D–π–A dyes for p-type DSSCs

(Paper I)

The dye in the solar cell is responsible for capturing sunlight. Its structure

influences the light harvesting ability, as well as the kinetics of charge transfer

processes in the whole system. To be a good sensitizer: 1) a dye should have

broad absorption band and a high molar extinction coefficient for efficient light

harvesting; 2) there must be at least one anchoring group for adsorption of the

dye on the semiconductor surface; 3) the energy levels of a dye should match

well with those of the semiconductor and electrolyte, providing sufficient

driving force for hole injection and dye regeneration; 4) to minimize charge

recombination between the hole in the semiconductor and the reduced dye, a

long-lived charge-separated state should exist; 5) dye aggregation on the

semiconductor surface, leading to nonradiative decay of the excited dye

(through intermolecular energy transfer), should be avoided. These

requirements can be fulfilled by the modification of dye structures.

3.1. Donor–π-conjugated linker–acceptor (D–π–A) dyes.

The D–π–A system has been widely used in the design of organic dyes for n-

type DSSCs. The general design of n-type dyes consists of both electron-donor

(D) and electron-acceptor (A), to which the anchoring group is attached on the

electron-acceptor part. In the case of p-type DSSCs, the dye injects a hole

instead of an electron after light excitation. After that, the reduced dye reacts

with the oxidized species in the electrolyte to be regenerated to its original

state. Therefore, the electron-acceptor should locate away from the

semiconductor surface, giving low electron density close to the surface after

photoexcitation and facilitating the following charge transfer. On the basis of

this strategy, the design of organic dyes used in this thesis follows this three-

unit approach but with the anchoring group on the electron-donor part (Figure

8).

The photoexcitation of a D–π–A dye is followed by intramolecular charge

transfer (ICT) from the donor to the acceptor moiety of the dye. This charge

separation facilitates rapid hole injection and restricts the charge recombination

between the hole in the semiconductor and the reduced dye. The ICT property

of a D–π–A dye is strongly dependent on the electron-donating ability of D,

the electron-withdrawing ability of A, as well as the electronic characteristic of

the π-conjugated bridge. It can be tuned through chemical modification of each

component.

12

Figure 8. The designing model for p-type dyes.

During the last decade, different arylamines, such as triphenylamine

(TPA),24,25

coumarin,26,27

indoline,28,29

and carbazole,30,31

have been

investigated as the electron-donating group. Most of them exhibited

satisfactory electron-donating ability. Chromophores with the TPA moiety as

the electron-donor have shown long-lived charge-separated state and good

hole-transporting ability.32,33

In addition, its nonplanar configuration with steric

hindrance can be used to prevent the unfavorable aggregation on the surface. In

the past few years, the dyes with TPA moiety have been widely investigated as

sensitizers in n-type DSSCs (Figure 9).34–39

N

O

O N

O

O

SCOOH

CN

D11

N

n-C6H13O

n-C6H13O

S

S

S

OO

COOHNC

C217

N

N

O COOH

CN

C12H25

TH305

N

O

O

S

Si

S

O O

S

NC

COOH

C219

Figure 9. Examples of TPA-based dyes used in n-type DSSCs.

13

The conjugated linker in a D–π–A system acts as both a component for light

harvesting and a channel for charge transport. A good conjugated linker should

promote the absorption of light over a wide region, and at the same time,

facilitate charge transfer. The thiophene unit is an appealing class of

conjugated linker used in photovoltaic cells. It provides effective conjugation

without affecting the stability of the dyes (Figure 10).40–43

N

S

S S

COOH

NC

MK-3

N O O

S

SCN

COOH

NKX-2677

N

S

CN

COOH

D5

NS S

S

N

S

NC

COOH

2

Figure 10. Dyes with different thiophene-based conjugated linkers.

Besides the electron-donor and π-conjugated linker, a push-pull system is

completed by introducing an electron-acceptor. There are a number of different

electron-acceptors reported so far and their electron-withdrawing abilities have

a big influence on the optical properties and energy levels of the compounds

(Figure 11).44–46

The dicyanovinyl group was chosen in our first test.

N

O

O

S CN

NC

N

Br

Br

O

NCCN

N S S

N

N

O

O

S

Figure 11. Dyes with different electron-acceptors.

14

The dye binds to the semiconductor surface through an anchoring group.

Strong binding is desired to avoid dye desorption, and is beneficial for efficient

charge transfer. Dyes with different anchoring groups, such as carboxylic acid,

phosphonic acid, thiol derivatives, have been published.47

Among them,

carboxylic acid has already been used on the NiO surface in early publications.

Possible binding modes of carboxylic acid include monodentate (ester-like),

bidentate chelating, and bidentate bridging (Figure 12).

M

O

R

O O O

R

O O

R

M M M

monodentate bidentate chelating bidentate bridging

Figure 12. Different binding modes of carboxylic acid on a metal oxide surface.

3.2. Aim of the study

We designed a D–π–A dye using a triphenylamine moiety as the electron-

donor, dicyanovinyl groups as the electron-acceptors, and linked the units by

thiophene rings (Figure 13). The difference between this dye and the normal n-

type dyes is that the anchoring group is now on the electron-donor part. We

hoped that this kind of new design could promote efficient hole injection and

the following dye regeneration in p-type system, and improved the solar cell

performance.

HOOC N

S

S

CN

CN

CN

CN

P1

Figure 13. The molecular structure of P1.

3.3. TPA-based dye (P1) for p-type DSSCs

3.3.1. Synthesis

P1 was synthesized following the route depicted in Scheme 1. The

commercially available compound 4-(diphenylamino)benzaldehyde (1) was

brominated with bromine in dichloromethane (DCM), affording dibromo-

15

substituted product 2.45

Oxidation of compound 2 with silver oxide under basic

conditions yielded compound 3, containing the carboxylic acid instead of the

original formyl group.48

The anchoring group was now connected with the

TPA moiety. The thiophene-based conjugated linkers were introduced by

Suzuki coupling.49

Different reaction conditions (catalyst, temperature and

time) were investigated. The best choice for the catalyst was

Pd(dppf)Cl2∙CH2Cl2 due to its good stability. Under microwave irradiation, this

reaction showed moderate yield within very short reaction time. The final step

was the condensation of the intermediate 4 with malononitrile in the presence

of triethylamine according to the Knoevenagel reaction.24

Scheme 1. Synthetic route for P1

NOHC NOHC

Br

Br

NHOOC

Br

Br

Br2

DCM

77%

Ag2O

NaOH

ethanol / toluene

78%

HOOC N

S

S

4

P1

CHO

CHO

NC CN

Et3N

ethanol

60%

Pd(dppf)Cl2·CH2Cl2

methanol / toluene

K2CO3

61%

SOHC

B(OH)2

1 2 3

Fe

P

P

Pd(II)Cl

Cl

Pd(dppf)Cl2

80 °C, 20 min

rt, 24 h

90 °C, 2 h

The 1H NMR spectrum of P1 is shown in Figure 14. All the protons from the

aromatic rings can be seen clearly. The proton from the carboxylic acid

exhibited a very weak signal which appeared between 11 to 12 ppm. The

structure was further proved by 13

C NMR and HR-MS.

16

ppm (t1)0.05.010.0

ppm (t1)7.508.008.50

1.9

3

2.0

3

2.0

0

2.0

6

2.0

1

4.0

3

4.0

7

2 3

45

67

8

Figure 14. 1H NMR spectrum of P1 in (CD3)2CO.

3.3.2. Optical and electrochemical characterization

The normalized absorption and emission spectra of P1 in acetonitrile solution

are shown in Figure 15. The absorption spectrum exhibits a minor absorption

band and a prominent one, with maxima at 345 and 468 nm, respectively. The

molar extinction coefficient is 5.8 × 104 M

–1 cm

–1 at the maximum absorption

in the visible region. From the intersection of the normalized absorption and

emission spectra, E0–0 of 2.25 eV is extracted. After adsorbing P1 on a 600 nm

NiO film, the absorption spectrum is broader than that in solution,

accompanied by a red shift, due to the interaction of the dye with the

semiconductor surface.

0

0.2

0.4

0.6

0.8

1

300 400 500 600 700 800

Norm

aliz

ed

ab

so

rptio

n

Wavelength (nm)

550 nm

(2.25 eV)

Norm

aliz

ed

em

issio

n

Figure 15. Normalized absorption (dot line) and emission (solid line) spectra of P1 in

acetonitrile solution, and the normalized absorption spectrum of P1 adsorbed on a 600

nm NiO film (dash line).

17

The ground-state oxidation and reduction potentials of P1 were measured by

cyclic voltammetry in dry acetonitrile containing 0.1 M TBA(PF6). Two

oxidation peaks and one reduction peak were obtained. The respective

oxidation and reduction potentials, E(D/D+) and E(D/D

–), were 0.68, 1.24, and

–1.46 V vs. Fc/Fc+

(1.31, 1.87, and –0.83 V vs. NHE). The corresponding

excited state energies, E(D*/D

+) and E(D

*/D

–), were estimated to be –0.94 and

1.42 V vs. NHE.

In p-type DSSCs, there are two possible ways for charge transfer after light

excitation. The hole injection from the excited dye (D*) to the valence band of

NiO could happen first, followed by electron transfer from the reduced dye to

the oxidized species (I3–) in the electrolyte. Alternatively, electron transfer

firstly from D* to the oxidized species in the electrolyte is also possible, the

dye is then regenerated by hole injection from the oxidized dye to the

semiconductor. Ultrafast spectroscopy has been performed on other similar

systems, and the results suggest that the former mechanism is dominant.16,17

For P1, it is noted that E(D*/D

–) is more positive than the valence band of NiO

(~0.5 V vs. NHE), giving substantial driving force for hole injection. The

potential of the reduced dye, E(D/D–), is more negative than the redox

potential of I–/I3

– and far below the bottom of the conduction band of NiO,

providing sufficient driving force for dye regeneration. This indicates that the

energy levels of P1 are appropriate for the existing NiO based p-type system.

The electron distribution of P1 at different energy levels was calculated. As

shown in Figure 16, the electron density at the highest occupied molecular

orbital (HOMO) is fully delocalized over all the conjugated system, and the

electron density at the lowest unoccupied molecular orbital (LUMO) moves

away from the anchoring group, towards the electron-acceptor part. This

intramolecular charge transfer is beneficial for p-type dyes.

Figure 16. The optimized structure (left) of P1, the frontier molecular orbitals of the

HOMO (middle) and LUMO (right) calculated with density functional theory (DFT) on

a B3LYP/6-31 + G(d) level (The calculation was performed by Tomas Edvinsson,

Uppsala University).

18

3.3.3. Solar cell performance of P1

The NiO film used here was made from a Ni(OH)2 precursor, which was

prepared as described in the literature50

with a weight concentration of ca. 20%

in ethanol. Doctor-blading of this Ni(OH)2 precursor onto a fluorine-doped

SnO2 glass (Pilkington TEC8), followed by sintering at 300 C for 30 min,

gave a transparent film with a thickness of 600 nm. The prepared NiO film

(0.32 cm2) was immersed in a 0.3 mM dye solution (in ethanol) for 16 h at

room temperature, then assembled with a platinized counter electrode using a

50 μm thick thermoplastic frame (Surlyn 1702). The electrolyte, composed of

0.5 M LiI and 0.1 M I2 in propylene carbonate, was introduced through a hole

drilled in the counter electrode which was sealed afterword.

Figure 17 illustrates the IPCE spectrum and J–V curve of P1. DSSC based on

P1 shows a maximum IPCE of 18%. It is the first dye to give such a high

photon-to-current conversion efficiency in a p-type DSSC. Under standard

sunlight irradiation (AM 1.5, 100 mW cm–2

), the P1-sensitized solar cell

exhibited a Jsc of 1.52 mA cm–2

, a Voc of 110 mV, and a ff of 0.31, giving an

overall conversion efficiency of 0.05% in the preliminary tests.

Under the same conditions, C343 and N3 were also tested for comparison

(Table 1). C343, the most popular dye used in p-type DSSCs, gave a

significantly lower photovoltaic performance than P1. N3, the most efficient

Ru dye used in n-type DSSCs, showed almost no photocurrent at all.

0

5

10

15

20

400 450 500 550 600 650 700 750 800

NiO-P1NiO-N3NiO-C343

IPC

E (

%)

Wavelength (nm)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.02 0.04 0.06 0.08 0.1 0.12

NiO-P1NiO-N3NiO-C343

Ph

oto

cu

rre

nt

(mA

cm

-2)

Photovoltage (V)

Figure 17. Comparison of the IPCE spectra and J–V characteristics of P1-, C343-, and

N3-sensitized NiO based solar cells (This work was done together with Dr. Hongjun

Zhu).

19

Table 1. Photovoltaic performance of NiO based dye-sensitized solar cells sensitized

with P1, C343, and N3.

Jsc / mA cm–2 Voc / mV ff η / %

P1 1.52 110 0.31 0.052

C343 0.78 70 0.32 0.017

N3 0.02 35 0.31 0.000

3.4. Conclusions

We have designed and synthesized an organic dye, P1, and investigated its

optical and electrochemical properties, as well as the photovoltaic performance

used in NiO based p-type DSSCs. The device based on P1 showed maximum

18% of IPCE and 0.05% of overall conversion efficiency, which was much

higher than other dyes reported by that time. This work broke the record of

IPCE for p-type DSSCs, and was highlighted by JACS Beta for its potential

contribution to the tandem solar cells. Therefore, this D–π–A system became a

model for the future design of p-type dyes. The next task was to further

optimize the dye, modify the NiO film and the electrolyte in order to improve

the efficiency of p-type DSSCs.

20

4. The study of organic dyes with symmetric and

unsymmetric structures:

modification of the conjugated linker

(Paper II and III)


The light harvesting ability is one of the most important properties of the dyes

used for DSSCs. Broad absorption spectrum can be obtained either by

structural modification or by co-sensitization of different dyes with

complementary absorption bands. In this chapter, we will pay more attention to

the first approach. For the p-type dye with two electron-acceptors (P1), the

prominent absorption band in the visible region is mainly due to the

HOMO→LUMO and a weak transition of the HOMO→LUMO+1 character.

The first transition is dominant and the second one gives a contribution to

broaden the first band. Two near-degenerate energy profiles of LUMO and

LUMO+1 are observed for a symmetric molecule. If this symmetry is broken,

the rearrangement of the molecular orbital could possibly lead to widely

separated excitations from HOMO→LUMO and HOMO→LUMO+1, and

broaden the absorption spectrum. On the basis of this hypothesis, we have

designed a series of dyes with symmetric and unsymmetric structures, and

have investigated their optical properties and photovoltaic performance. At the

same time, modifications of the NiO film and the electrolyte were also studied.

4.2. Modification of dye structure for p-type DSSCs

Based on the first dye, P1, with thiophene units as the conjugated linkers, other

systems were considered for comparison. From earlier studies, a phenylene

unit is another possibility.39,51

Therefore, a new symmetric dye, with phenylene

units as the conjugated linkers (P4), and an unsymmetric dye, with the

combination of thiophene and phenylene moiety (P6), were synthesized for

this study (Figure 18).

21

HOOC N

S

S

CN

CN

CN

CN

P1

HOOC N

S

P6

CN

CN

CN

CN

HOOC N

P4

CN

CN

CN

CN

Figure 18. The molecular structures of P1, P4, and P6.

4.2.1. Synthesis

The synthetic routes for P4 and P6 are shown in Scheme 2. Compound 3 was

prepared as described in the previous chapter. The Suzuki coupling reaction

was performed with compound 3 and 4-formylphenyl boronic acid under

microwave irradiation, giving the di-substituted aldehyde 5 in moderate yield.

Since the polarities of the mono-substituted and di-substituted products were

quite similar, the separation of these two compounds by column

chromatography was performed with care, with a slow increase in the polarity

of the eluent. Finally, P4 was obtained by the condensation of aldehyde 5 with

malononitrile in anhydrous N,N-dimethylformamide (DMF).52

For P6, a

similar method to that used for compound 5 was applied. Compound 3 was

reacted with 5-formyl-2-thiophene boronic acid and 4-formylphenyl boronic

acid stepwise, giving the unsymmetric intermediate 7. Condensation of

aldehyde 7 with malononitrile in dry DMF gave the target dye P6.

From the

1H NMR spectrum, when phenylene units are used instead of

thiophene, the protons (Ha) from the dicyanovinyl group shift to the upfield

part of the spectrum (Figure 19). These two protons (Hb and Hc) in the

unsymmetric dye P6 are located at almost the same place, compared to those

in the symmetric dyes (Figure 20).

22

Scheme 2. Synthetic routes for P4 and P6

NHOOC

Br

Br

HOOC N

5

P4

NC CN

50%

Pd(dppf)Cl2·CH2Cl2

methanol / toluene

70 °C, 1 h

K2CO3

59%3

OHC B(OH)2

CHO

CHO

DMF

SOHC

B(OH)2

Pd(dppf)Cl2·CH2Cl2

K2CO3

methanol / toluene

70 °C, 25 min

33%

HOOC N

Br

S

CHO

6 7

B(OH)2

Pd(dppf)Cl2·CH2Cl2

K2CO3

methanol / toluene

70 °C, 25 min

81%

OHC

HOOC N

S

CHO

CHO

NC CN

DMF

42%

P6

85 °C, 5 h

85 °C, 8 h

ppm (t1)0.05.010.0

Ha

ppm (t1)7.508.008.50

6.0

4

4.0

0

4.0

0

3.9

7

1.9

3

1.9

5


23

ppm (t1)0.05.010.0

Hb

ppm (t1)7.508.008.50

1.0

6

2.0

0

2.0

0

2.1

2

4.0

0

5.1

2

1.9

6

1.0

4

0.9

4

Hc


4.2.2. Optical and electrochemical characterization

The absorption spectra of P1, P4, and P6 in THF solution are shown in Figure

21 and the physical parameters are listed in Table 2. P1 was measured in the

same solvent for comparison. The spectra of all the compounds show two

absorption bands, one in the UV region and another one in the visible region.

When phenylene unit is used instead of thiophene in the linker part, a blue shift

occurs (P4). The molar extinction coefficient of P4 is lower than P1 in the

visible region, but it has stronger absorption in the UV region. The absorption

spectrum of the unsymmetric dye P6, where these two different linkers are

combined, is located between P1 and P4 instead of overlapping the region of

the two symmetric dyes. From these results, it seems that the breaking of

molecular symmetry does not broaden the absorption spectrum as we expected

and the molar extinction coefficient of P6 is even lower than P1 and P4. When

excited within the π–π* band, a strong luminescence was observed with

maxima at 624 and 617 nm for P4 and P6, respectively.

24

300 400 500 600

0

20000

40000

60000

80000

Mola

r E

xtinction C

oeffic

ien

t / M

-1cm

-1

Wavelength / nm

P1

P4

P6

Figure 21. Absorption spectra of P1, P4, and P6 in THF solution.

The redox properties of P4 and P6 were measured by cyclic voltammetry in

dry acetonitrile. The first oxidation and reduction potentials were 0.69 and –

1.36 V, and 0.69 and –1.48 V vs. Fc/Fc+ for P4 and P6, respectively. The

corresponding potentials of P1 were 1.32 and –0.83 V vs. NHE for E(D/D+)

and E(D/D–) with our new instrument, almost the same as the earlier data. It is

noted that the different conjugated linkers have an effect on the reduction

potentials of the dyes, but little influence on the oxidation potentials. From the

energy diagram shown before for the p-type system (Figure 2), all the dyes

show sufficient driving force for hole injection and dye regeneration.

Table 2. Optical and electrochemical properties of the dyes.

Dye λabs (ε /M–1 cm–1) a

/nm

λema

/nm E0–0

b

/eV

E(D/D+)c

/V vs. NHE

E(D/D–)c

/V vs. NHE

E(D*/D–) d

/V vs. NHE

P1 348(34720);481(57900) 618 2.25 1.32 –0.83 1.42

P4 332(68720);430(45200) 624 2.38 1.32 –0.73 1.65

P6 331(31350);466(38240) 617 2.28 1.32 –0.85 1.43

aThe absorption and emission spectra were measured in THF solution. bE0–0 was estimated from the

intersection of the normalized absorption and emission curves. cThe ground-state oxidation and

reduction potentials of the dyes, E(D/D+) and E(D/D–), were obtained in dry acetonitrile with 0.1

M TBA(PF6) and 0.1 V s–1 scan rate dE(D*/D–) was estimated from E(D/D–) and E0–0 (E(D*/D–) =

E(D/D–) + E0–0).

From DFT calculations (Figure 22), the phenylene unit in P4 shows less

coplanarity, which leads to a higher degree of frontier orbital separation. It is

25

also noted that the unsymmetric dye P6 has the widest separation between

LUMO and LUMO+1, but unfortunately this is not reflected in a broadening of

the absorption spectrum.

Figure 22. Frontier orbital energies (vertical lines) and isodensity plots of P1 (top), P4

(middle), and P6 (bottom). The two near-degenerate unoccupied orbitals are shown.

(The calculation was performed by Mats Linder).

4.3. Optimization of the electrolyte for p-type DSSCs

Optimization of the electrolyte was investigated in order to improve the

efficiency of existing p-type system. In DSSCs, ions can be adsorbed on the

semiconductor surface, thereby affecting the surface charge and energy. The

position of the valence band edge moves down in energy in the presence of

positive adsorbed ions,13

which is an effective way to increase the Voc in a p-

type DSSC. The electrolyte used in our earlier work was 0.5 M LiI and 0.1 M

I2 in propylene carbonate. In order to check the influence of positive ion on the

Voc, different concentrations of LiI were tested in propylene carbonate based

electrolyte. Figures 23 and 24 show the solar cell performance based on P1

and P4, with 0.5 M and 1.0 M LiI (1.2 μm NiO film, Ni(OH)2 precursor). The

photovoltage of both P1 and P4 increase about 20 mV with higher LiI

concentration, which leads to an increased efficiency. Further increased LiI

concentration did not give significantly higher voltage (Table 3).

26

400 500 600 700 800

0

4

8

12

16

IPC

E / %

Wavelength / nm

P1

P4

0.00 0.02 0.04 0.06 0.08 0.10

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Photo

curr

ent density / m

A c

m-2

Photovoltage / V

P1

P4

Figure 23. Comparison of the IPCE spectra and J–V characteristics of P4- and P1-

sensitized solar cells with 0.5 M LiI and 0.1 M I2 in propylene carbonate as the

electrolyte.

400 500 600 700 800

0

4

8

12

16

20

IPC

E / %

Wavelength / nm

P1

P4

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Photo

curr

ent density / m

A c

m-2

Photovoltage / V

P1

P4


sensitized solar cells with 1.0 M LiI and 0.1 M I2 in propylene carbonate as the

electrolyte.

Table 3. Photovoltaic performance of P4- and P1-sensitized solar cells based on

different electrolytes.

Voc / mV Jsc / mA cm–2 ff η / %

P1 (electrolyte 1)a 102 1.27 0.32 0.04

P4 (electrolyte 1) 94 0.90 0.33 0.03

P1 (electrolyte 2)b 123 1.26 0.34 0.05

P4 (electrolyte 2) 115 0.91 0.36 0.04

P1 (electrolyte 3)c 110 2.51 0.29 0.08

P4 (electrolyte 3) 101 2.48 0.36 0.09

a Electrolyte 1: 0.5 M LiI and 0.1 M I2 in propylene carbonate. bElectrolyte 2: 1.0 M LiI and 0.1 M

I2 in propylene carbonate. cElectrolyte 3: 1.0 M LiI and 0.1 M I2 in acetonitrile.

27

Besides the influence of LiI concentration, the solvent effect was also

investigated. Acetonitrile, the most typical solvent in the electrolyte for n-type

DSSCs, was used in our study. The mass transport of the species in different

electrolytes can be determined by the diffusion coefficient. Fast mass transport

is beneficial for efficient dye regeneration, thus preventing charge

recombination. Due to the large excess of I–

in our electrolyte, the current is

limited mainly by the diffusion of I3–.

The diffusion coefficient D(I3–) was determined by measuring the limiting

current density Jlim, according to the following equation,53,54

Jlim = 2nqD(I3–)c(I3

–)NA/d (7)

where n is the number of electrons, q is the elementary charge (1.60 × 10–19

C),

c(I3–) is the concentration of I3

–, and NA is the Avogadro’s constant (6.02 × 10

23

mol–1

). Cyclic voltammetry was performed to measure Jlim using a cell

consisting of two platinized conducting glass electrodes, which were separated

at a fixed distance d (43 μm) and connected to a potentiostat, giving Jlim of

0.106 and 0.014 A cm–2

for acetonitrile and propylene carbonate based

electrolytes, respectively. D(I3–) was calculated according to equation 7,

yielding values of 1.18 × 10–5

(acetonitrile) and 1.56 × 10–6

cm2 s

–1 (propylene

carbonate) for these two electrolytes. This result was in agreement with the

lower viscosity of acetonitrile (0.344 mPa∙s) compared to that of propylene

carbonate (2.512 mPa∙s),55

which led to improved ion transport in the

electrolyte.

Sandwich-type solar cells were assembled using sensitized nanocrystalline

NiO film (1.2 μm, Ni(OH)2 precursor) as the working electrode, platinized

conducting glass as the counter electrode, 1.0 M LiI and 0.1 M I2 in

acetonitrile as the electrolyte. The IPCE spectra are illustrated in Figure 25.

The highest IPCE values for P1- and P4-sensititzed solar cells are 35% and

44%, respectively, which is a significant improvement from the earlier data

and the IPCE of P4 is higher than P1 in this case. The APCEs of these two

dyes with both acetonitrile and propylene carbonate based electrolyte were

calculated in order to learn more about the intrinsic light conversion efficiency.

The corresponding APCE values were 47% and 25% for P1, 65% and 29% for

P4. Since the solvent of the electrolyte was the only difference between these

devices, this result implied that the fast ion transport in acetonitrile based

electrolyte had a positive effect on efficient hole collection and/or dye

regeneration, leading to higher APCEs for both P1 and P4. A possible

explanation for the higher APCE for P4, especially in the acetonitrile based

electrolyte, is due to the slightly more twisted phenylene linkers which localize

the unpaired electron more strongly in the electron-acceptor part, resulting in a

28

long-lived charge-separated state. Alternatively, dyes with different conjugated

linkers may associate with triiodide (or iodine) differently,56

and could be

related to different regeneration kinetics.

Under standard sunlight irradiation, the P4-sensitized solar cell exhibited a Jsc

of 2.48 mA cm–2

, a Voc of 0.10 V, and a ff of 0.36, giving an overall conversion

efficiency of 0.09%. Under the same conditions, the corresponding values for

the P1-sensitized solar cell were 2.51 mA cm–2

, 0.11 V, 0.29, and 0.08%,

respectively. The significant increase of Jsc and η was mainly due to the fast

mass transport in the acetonitrile based electrolyte.

400 500 600 700 800

0

10

20

30

40

50

IPC

E / %

Wavelength / nm

P1

P4

NiO film without dye

0.00 0.02 0.04 0.06 0.08 0.10 0.12

0.0

0.5

1.0

1.5

2.0

2.5

Ph

oto

cu

rre

nt

de

nsity /

mA

cm

-2

Photovoltage / V

P1

P4


sensitized solar cells with 1.0 M LiI and 0.1 M I2 in acetonitrile as the electrolyte.

4.4. Modification of the NiO film for p-type DSSCs

4.4.1. 1.2 μm NiO film with Ni(OH)2 precursor

The thickness of the NiO film was increased from an initial 600 nm to 1.2 μm

by increasing the aging time of Ni(OH)2 precursor, and careful control of the

sintering time, producing a higher dye loading. Thus, a higher internal surface

area was obtained.

With the propylene carbonate based electrolyte, increasing the film thickness

did not result in a large increase in cell efficiency. However, with the

optimized acetonitrile based electrolyte and a higher concentration of LiI, both

the IPCE and η reached almost twice the value obtained earlier. This could be

explained by the higher dye loading and the faster ion transport described

previously.

29

4.4.2. 1.2 μm NiO film with NiCl2 and the F108 template precursor

The NiCl2 and the triblock co-polymer F108 (polyethyleneoxide133-

polypropyleneoxide50-polyethyleneoxide133, PEO133PPO50PEO133) template

precursor was firstly published by Suzuki and co-workers for preparation of a

nanoporous NiO electrode for p-type DSSCs.57

With this precursor, a film

formed by uniformly distributed NiO nanoparticles with small interparticle

voids was obtained, resulting in a large surface area and a high photocurrent.

Based on their work, a substantial increase in photocurrent by using a stepwise

procedure for NiO preparation was described. This work was mainly done by

my co-workers, Lin Li and Elizabeth A. Gibson. The details can be found in

Paper III.

The NiO film was prepared by two steps of doctor-blading the precursor

solution onto conducting glass substrate, followed by sintering at 450 °C for

30 min after each deposition. The films (1.2 μm) prepared in this way were

more porous and compact than the previous Ni(OH)2 based method, allowing

an increase in dye loading, and resulting in a significant improvement of the

LHE. In addition, hole transport kinetic studies demonstrated better charge

collection efficiency for this film.

The solar cell performance based on this new film with an active area of 0.32

cm2 is shown in Figure 26. All the three dyes were used for this test. The

highest IPCE values for P1-, P4-, and P6-sensititzed solar cells are 63%, 56%,

and 46%, respectively. P4 and P6 show a blue-shift, consistent with their

absorption spectra, and the maximum IPCE of the unsymmetric dye, P6, is

lower than both P1 and P4.

Under standard sunlight irradiation, the P1-sensitized solar cell exhibited a Jsc

of 5.48 mA cm–2

, a Voc of 0.084 V, and a ff of 0.33, giving η of 0.15%. Under

the same conditions, the P4- and P6-sensitized solar cells gave Jsc of 3.77 and

4.15 mA cm–2

, Voc of 0.087 and 0.096 V, ff of 0.34 and 0.35, and η of 0.11%

and 0.14%, respectively. The low efficiencies of the P4- and P6-sensitized

solar cells are mainly due to the lower photocurrent, although they have

slightly higher photovoltage. Therefore, with the film based on the template

precursor, both the IPCE and efficiency of the device are significantly

improved compared to those prepared from the Ni(OH)2 precursor. The

improvement for P1-sensitized device is larger than P4.

30

400 500 600 700 800

0

10

20

30

40

50

60

70

IPC

E / %

Wavelength / nm

P1

P4

P6

NiO film without dye

0.00 0.02 0.04 0.06 0.08 0.10

0

1

2

3

4

5

6

Photo

curr

ent

density / m

A c

m-2

Photovoltage / V

P1

P4

P6

Figure 26. Comparison of the IPCE spectra and J–V characteristics of P1-, P4-, and

P6-sensitized solar cells with 1.0 M LiI and 0.1 M I2 in acetonitrile as the electrolyte

(This work was done together with Lin Li).

The absorption and LHE spectra of the sensitized films are shown in Figure

27. The absorbance of the film with P1 is higher than those with P4 and P6,

either due to more dye loading or the higher molar extinction coefficient of the

dye. The LHE is in proportion to the absorbance of the sensitized film. As

expected, P1 shows the highest LHE, indicating that it can absorb more

photons compared to other dyes under the same conditions. Although the

molar extinction coefficient of P4 is higher than P6, the sensitized films show

almost the same LHEs. P4 is, however, more blue-shifted.

31

400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

1.2

(A)

Ab

so

rba

nce

Wavelength / nm

P1|NiO

P4|NiO

P6|NiO

400 500 600 700 800

0

20

40

60

80

100

(B)

LH

E / %

Wavelength / nm

P1|NiO

P4|NiO

P6|NiO

Figure 27. (A) The absorption and (B) the LHE spectra of P1, P4, and P6 adsorbed on

1.2 μm NiO films.

The hole lifetime (τh) and transport time (τtr) for the solar cells based on P1,

P4, and P6 were investigated (Figure 28). Small-modulation photovoltage

measurement58

under open-circuit condition was used to measure the lifetime

of the holes in NiO. Lifetime measurements are performed in order to compare

the charge recombination kinetics between the holes in NiO and the reduced

species in the electrolyte. At higher voltage, the concentration of holes in the

valence band of NiO is increased. Therefore, there is more chance for charge

recombination, leading to shorter hole lifetimes. This is the reason why a

decrease of τh with Voc is observed for all the dyes. With the same Voc, there is

not a big difference for the hole lifetime based on different dyes.

The hole transport time was obtained from small-modulation transient

photocurrent measurement under short-circuit condition,58

and was recorded at

different light intensity. It provides information about how fast the hole is

transported through the NiO film. Under short-circuit condition the majority of

holes will be extracted at the back contact, so τtr can be obtained by monitoring

the photocurrent changes. As shown in Figure 28(B), the P1-sensitized solar

cell gives the shortest τtr, followed by P6 and P4. Since the fast hole transport

is beneficial for hole collection and current generation, this trend is consistent

with the Jsc we got before.

32

0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12

0.01

0.1

1

(A)

Ho

le life

tim

e /

s

Voc

/ V

P1

P4

P6

0.01 0.1 1

0.01

0.1

(B)

Ho

le tra

nspo

rt tim

e / s

Light intensity / sun

P1

P4

P6

Figure 28. (A) Hole lifetime as function of Voc, and (B) hole transport time as function

of the light intensity.

4.5. Conclusions

Two organic dyes with different conjugated linkers were designed and

synthesized in order to investigate the influence of symmetric and

unsymmetric structures of the dyes on the optical properties and the

photovoltaic performance. From the absorption spectra, the breaking of

molecular symmetry did not broaden the absorption band, possibly due to the

absorption regions of the two symmetric dyes were too close to each other. An

unsymmetric dye with the combination of two dramatically different units

could be considered in future work. After optimization of the electrolyte and

the NiO film, the solar cell using the symmetric thiophene based dye showed

the highest efficiency, followed by the unsymmetric dye and the symmetric

phenylene based dye.

33

5. The study of organic dyes with different

thermodynamic driving force:

modification of the electron-withdrawing group

(Paper IV)


Interfacial charge transfer processes, such as hole injection, dye regeneration,

and charge recombination, are crucial to determine the efficiency of DSSCs.

According to the working principle shown before, the dye plays an important

role in most of these processes, thereby becoming a significant parameter to be

considered for an efficient solar cell. In this chapter, we will pay more

attention to the influence of the energy levels of a dye on the thermodynamic

driving force for different charge transfer processes and the solar cell

performance. It is well known that for a suitable dye, the energy levels should

match well with those of the semiconductor and the electrolyte for sufficient

driving force for hole injection and dye regeneration. How is the photovoltaic

performance be influenced by the energy levels of the dye? Are there any

limiting energy requirements for the charge transfer to occur? In order to solve

these problems, we designed a series of dyes, P2, P3, and P7, with different

energy levels, by altering the electron-acceptors (Figure 29). We anticipate that

the study of their effects on individual charge transfer reactions and the

corresponding solar cell performance will enable us to direct our approach to

the design of optimized dyes, in terms of the optical properties, energy levels

and the cell performance, in the future.

34

HOOC N

S

S

CN

CN

CN

CN

HOOC N

S

S

HOOC N

S

S

CN

CN

CN

CN

HOOC N

S

S

N

N

N

N

O

O

S

O

O

S

NC

NC

NCCN

NC CN

P1 P2

P3 P7

Figure 29. The molecular structures of P1, P2, P3, and P7.

5.2. Synthesis

The synthetic routes to prepare P2, P3, and P7 are shown in Scheme 3.

Compound 2 and 4 were prepared as described in chapter one. P2 and P7 were

synthesized by the condensation of compound 4 with 1,3-diethyl-2-

thiobarbituric acid and 2-(3,5,5-trimethylcyclohex-2-enylidene)malononitrile,

respectively. A catalytic amount of piperidine was used as the catalyst for the

synthesis of P7. However, using a similar method, the yield of P2 was quite

low. Finally, P2 was obtained by mixing the two reagents in acetic anhydride

and stirring at 120 °C for 4 h.46

The yield of this reaction was still low, but no

further effort was made to improve it. The synthesis of P3 was based on the

Suzuki coupling reaction of compound 2 with 2-thiophene boronic acid under

the similar conditions described before. The following oxidation reaction

yielded compound 9. Finally, compound 9, upon treatment with

tetracyanoethylene (TCNE) in DMF, gave the target dye P3.44

35

Scheme 3. Synthetic routes for P2, P3, and P7

HOOC N

S

S

CHO

CHO

N

N

O

O S

P2

19%

P7

CN

CN

40%

8

OHC N

S

S

SB(OH)2

83%

NC

NC CN

CN

HOOC N

S

S

P3

73% 24%

9

OHC N

Br

Br

4

2

acetic anhydride

120 °C, 4 h

piperidine

acetonitrile

82 °C, 15 h

Pd(dppf)Cl2·CH2Cl2

K2CO3

methanol / toluene

70 °C, 40 min

Ag2O

NaOH

toluene / ethanol

rt, 36 h

DMF

100 °C, 23 h

5.3. Optical and electrochemical characterization

The absorption spectra of the dyes in THF solution and adsorbed on NiO films

are shown in Figure 30. The corresponding physical parameters are listed in

Table 4. In THF solution, all the dyes exhibit a prominent band in the visible

region due to the π–π* transition. The spectra show a clear red-shift with

stronger electron-acceptors, with the maximum absorption changing from 481

nm for P1 to 574 nm for P3. By adsorbing the dyes on NiO films, although the

molar extinction coefficients of P2 and P7 are higher than P1, the absorbance

of these two dyes is lower, especially for P2. This could possibly be explained

by a lower dye loading, originating from the increased molecular size. For P3,

with an extra cyano group on each electron-acceptor (compared to P1), the

absorbance is the lowest on the film, indicating that the light harvesting ability

of this dye is poor.

36

300 400 500 600 700 800

0

20000

40000

60000

80000

(A)

M

ola

r E

xtin

ctio

n C

oe

ffic

ien

t / M

-1cm

-1

Wavelength / nm

P1

P2

P3

P7

400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

1.2

(B)

Ab

so

rba

nce

Wavelength / nm

P1|NiO

P2|NiO

P3|NiO

P7|NiO

Figure 30. Absorption spectra of P1, P2, P3, and P7 in THF solution (A) and adsorbed

on 1.2 μm NiO films (B).


aThe absorption and emission spectra were measured in THF solution. b E0–0 was estimated from

the intersection of the normalized absorption and emission curves. cThe ground-state oxidation and

reduction potentials of the dyes, E(D/D+) and E(D/D–), were measured in dry THF (P2 and P3)

and acetonitrile (P1 and P7) with 0.1 M TBA(PF6). dE(D*/D–) was estimated from E(D/D–) and E0–

0 (E(D*/D–) = E(D/D–) + E0–0).

The ground-state redox potentials of the dyes were measured by CV in dry

acetonitrile or THF containing 0.1 M TBA(PF6) with 0.1 V s–1

scan rate. P2

and P3 were slightly soluble in acetonitrile. Therefore, they were measured in

THF. As shown in Table 4, the energy levels of E(D*/D

–) range from 1.34 to

1.50 V vs. NHE, which are more positive than the valence band of NiO.

Therefore, all the dyes have sufficient driving force for hole injection, and the

driving force of P3 is larger than others. Different electron-acceptors have a

larger influence on E(D/D–)

than E(D

*/D

–). Stronger electron-withdrawing

ability corresponds to a less negative E(D/D–) potential. This less negative

potential leads to a small difference with the redox potential of the electrolyte,


/nm

λema

/nm

E0–0b

/eV

E(D/D+)c

/V vs. NHE

E(D/D–)c

/V vs. NHE

E(D*/D–)d

/V vs. NHE

P1 348(34720);481(57900) 618 2.25 1.32 –0.83 1.42

P2 337(28292);404(25048);

531(64005)

668 2.04 1.30 –0.70 1.34

P3 349(27140);401(22530);

574(40990)

783 1.86 1.40 –0.36 1.50

P7 302(32356);357(42642);

490(75005)

711 2.07 1.28 –0.66 1.41

37

giving a lower thermodynamic driving force for dye regeneration. From Figure

31, it is seen that the driving force to regenerate P3 is 0.47 V smaller than P1.

In the next section, its effect on dye regeneration process and the solar cell

performance will be described.

NiO

VB

CB

h+

e-

I-/I3-

P2*/P2-

P7*/P7-

P1*/P1-

P3*/P3-

P2/P2-P1/P1-

P3/P3-

P7/P7-

Figure 31. Schematic energy level diagram with different dyes.

5.4. Solar cell performance

Sandwich-type solar cells were assembled using P1-, P2-, P3-, and P7-

sensitized NiO films (NiCl2 and the F108 template precursor, 1.2 μm, 0.32 cm2)

as the working electrode, platinized conducting glass as the counter electrode,

1.0 M LiI and 0.1 M I2 in acetonitrile as the electrolyte. As shown in Figure 32,

DSSCs based on P1 produce a maximum IPCE of 63%, which is the highest in

this series of dyes. P2, P3, and P7 show red-shifted IPCE spectra. The devices

based on them give lower IPCEs than P1, with the maximum values of 32%,

6%, and 26%, respectively, under the same conditions.

Under standard sunlight irradiation, the P1-sensitized solar cell gave an overall

conversion efficiency of 0.15%, followed by P7 (0.09%) and P2 (0.07%). P3,

with the strongest electron-acceptor, exhibited the lowest IPCE and efficiency.

As shown in Table 5, both the photovoltage and photocurrent decrease with the

increase of electron-withdrawing ability in this D–π–A system.

38

400 500 600 700 800

0

10

20

30

40

50

60

70

(A)

IPC

E / %

Wavelength / nm

P1

P2

P3

P7

NiO film

0.00 0.02 0.04 0.06 0.08 0.10

0

1

2

3

4

5

6

(B)Ph

oto

cu

rre

nt d

en

sity / m

A c

m-2

Photovoltage / V

P1

P2

P3

P7

Figure 32. Comparison of the IPCE spectra and J–V characteristics of P1-, P2-, P3-,

and P7-sensitized solar cells with 1.0 M LiI and 0.1 M I2 in acetonitrile as the

electrolyte (This work was done together with Lin Li).

Table 5. Photovoltaic performance of DSSCs based on P1, P2, P3, and P7.


P1 84 5.48 0.33 0.15

P2 63 3.37 0.31 0.07

P3 55 1.36 0.34 0.03

P7 80 3.37 0.35 0.09

In order to know whether the solar cell performance is related to the energy

levels of the dye, the kinetics of the hole injection and dye regeneration

processes were studied for the two dyes with the highest and lowest efficiency,

39

P1 and P3. The hole injection process was investigated by femtosecond

transient absorption spectroscopy. The injection efficiency was higher than 90

% for both dyes, and the injection of P3 was even faster than P1. This can be

explained by the larger driving force for hole injection induced by the stronger

electron-acceptor. The dye regeneration process was studied by photoinduced

absorption spectroscopy, which indicated that P1 could be regenerated

efficiently by the I–/I3

– based electrolyte, whereas P3 could not, leading to a

consumption of the holes by fast charge recombination with the reduced dye.

This work was mainly done by my co-workers, Joanna Wiberg and Elizabeth

A. Gibson.

According to the literature, the upper limit of Eº(I3–/I2

–) is about –0.35 V vs.

NHE.59

The driving force for dye regeneration is actually the energy difference

between E(D/D–) and the one-electron reduction potential of the I3

–, Eº(I3

–/I2

–).

As discussed before, E(D/D–) of P3 is less negative and it lies quite close to

Eº(I3–/I2

–), leading to an insufficient driving force for dye regeneration. This is

possibly the main reason to explain its slow regeneration process, as well as

the poor solar cell performance.

5.5. Conclusions

A series of organic dyes with different electron-withdrawing groups were

designed and synthesized in order to investigate the effect of energy levels of a

dye on different charge transfer processes, as well as the photovoltaic

performance of p-type DSSCs. Stronger electron-withdrawing groups caused a

red-shift of the absorption spectrum and a less negative E(D/D–) potential,

which led to both a low IPCE and efficiency. The kinetic studies of the two

dyes with the highest and lowest efficiency showed injection efficiencies of

higher than 90% for both dyes. However, the dye with the strongest electron-

withdrawing group could not be regenerated efficiently due to the insufficient

driving force for oxidizing the reduced dye by I3–. This is proposed to be the

main reason for the poor solar cell performance. This work indicated that the

dye with similar or more negative reduction potential than P1 might be worth

to try. Meanwhile, the optical properties, especially the light harvesting ability,

should be considered when tuning the energy levels.

40

6. The study of organic dyes for control of charge

recombination: modification of the

conjugated linker

(Paper V)


The charge recombination between the holes in the NiO film and other

electron-donors is a primary reason for the efficiency loss in p-type DSSCs.

The two main processes responsible for the hole consumption are the

combination with the reduced dye and the reduced species in the electrolyte. In

the first case, process depends on the electronic coupling between the dye and

the NiO film and this recombination is competitive with dye regeneration. To

slow the second recombination, protection of the semiconductor surface is

necessary. The recombination can be inhibited either by the modification of

dye structure or by additives in the electrolyte which prevent the approach of

the reduced species to the surface. In this chapter we will pay more attention to

the modification of the dye structure.

Two ways of derivatizing P1, which showed the best solar cell

performance so far in our study, were considered: either one phenylene

unit was introduced between the anchoring group and the electron-donor in

order to increase the distance between the hole in the NiO and the unpaired

electron in the reduced dye after photoexcitation (P5), or alkyl chains were

introduced on the conjugated linkers to prevent the apporach of I– ions to

the surface by steric hindrance (P10) (Figure 33). Devices sensitized with

P1, P5, and P10 were compared to see which way was more effective to

prevent the charge recombination and improve the solar cell performance.

41

HOOC N

S

S

CN

CN

CN

CN

P1

N

S

S

CN

CN

CN

CN

P5

HOOC HOOC N

S

S

CN

CN

CN

CN

P10


6.2. Synthesis

The synthetic routes for P5 and P10 are shown in Schemes 4 and 5. The

first step in the synthesis of P5 was a Suzuki coupling of 4,4 -́

dibromotriphenylamine (10) with 5-formyl-2-thiophene boronic acid.60

The

palladium complex, tris(dibenzylidene acetone)dipalladium [Pd2(dba)3],

was used as the catalyst and K3PO4 as the base. After refluxing at 100 °C

for 18 h under N2 atmosphere, the main product was the mono-substituted

compound. The yield of the target di-substituted product was lower than

that obtained by the earlier method using Pd(dppf)Cl2·CH2Cl2 as the

calalyst. After bromination with N-bromosuccinimide (NBS), compound

12, with a free bromo group, was obtained. This was then reacted with 4-

carboxyphenyl boronic acid, introducing one more phenylene unit together

with the anchoring group. The final step was, again, the condensation of

the aldehyde with malononitrile in the presence of triethylamine.

The preparation of P10 started with the coupling of compound 2 with 3-

hexylthiophene-2-boronic acid pinacol ester under microwave irradiation,

followed by the oxidation reaction. Formylation of the thiophene moiety

using phosphorus oxychloride and DMF was employed for preparation of

the aldehyde, which was necessary for the further condensation to yield the

target dye.

42


N

Br

Br

N

S

S

CHO

CHO

Br N

S

S

CHO

CHO

N

S

S

CHO

CHO

HOOC P5

[Pd2(dba)3]

PCy3

K3PO4

dioxane / H2O

33%

NBS

THF

90%

B(OH)2

Pd(dppf)Cl2·CH2Cl2

HOOC

methanol / toluene

70 °C, 30 min

K2CO3

68%

NC CN

Et3N

acetonitrile

67%

10 11 12

13

SOHC

B(OH)2

100 °C, 18 h

83 °C, 2.5 h


OHC N

Br

Br2

OHC N

S

S

Pd(dppf)Cl2·CH2Cl2

methanol / toluene

70 °C, 25 min

K2CO3

85%14

SB

O

O

Ag2O

rt, 24 h

75%

NaOH

ethanol / toluene

HOOC N

S

S16

P10

CHO

CHO

NC CN

Et3N

ethanol

41%

HOOC N

S

S15

POCl3

61%

DMF

83 °C, 2 h

43

From the 1H NMR spectra, four more protons at positions 9 and 10 are

observed for P5 than for P1, indicating that one more phenylene unit is

introduced (Figure 34). For P10, the protons in the alkyl chains can be seen

clearly in the upfield part of the spectrum. The small signal around 12.8 ppm is

due to the proton from the carboxylic acid (Figure 35).

ppm (t1)0.05.010.0

8

9 7

52

10

63

4

ppm (t1)7.508.00

1.9

4

2.0

6

2.0

8

2.0

0

2.0

0

4.0

4

8.1

6


ppm (t1)0.05.010.015.0

4.0

0

12.0

8

6.0

7

3.9

6

0.9

1

7

2

65 4

3

1

8

9

10-1213

ppm (t1)6.507.007.508.008.509.009.50

2.0

0

4.0

6

3.9

9

4.0

5

1.9

8

Figure 35. 1H NMR spectrum of P10 in (CD3)2SO.

44


The optical and electrochemical properties of the dyes are listed in Table 6. As

presented in Figure 36(A), the absorption spectrum of P5 has one intense band

centered at 499 nm, which is 18 nm red-shifted compared to P1, due to the

extension of the π-conjugation. P5 also shows the highest molar extinction

coefficient in this series of dyes. This property is favorable for light harvesting

and photocurrent generation in thin-film photovoltaic devices. With the

introduction of hexyl chains on thiophene moiety, a blue-shift of the spectrum

and a lower molar extinction coefficient are observed for P10. A similar trend

has previously been published for the dyes used in n-type DSSCs.61

It can be

explained by the distortion of the molecule, which lowers the overlap between

the interacting orbitals. After adsorbing the dyes on NiO films, the absorbance

of P10 is dramatically lower than both P5 and P1, owing to either a lower

molar extinction coefficient or a less dye loading.

300 400 500 600 700

0

10000

20000

30000

40000

50000

60000

70000

(A)

Mo

lar

Extin

ctio

n C

oe

ffic

ien

t / M

-1cm

-1

Wavelength / nm

P1

P5

P10

400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

(B)

Ab

so

rba

nce

Wavelength / nm

P1

P5

P10

Figure 36. Absorption spectra of P1, P5, and P10 in THF solution (A) and adsorbed on

1.5 μm NiO films (B).

The oxidation and reduction potentials of the dyes were determined using CV,

in dry acetonitrile. As shown in Table 6, the E(D*/D

–) of P10 is more positive

than P1 and P5, indicating that the driving force for hole injection is larger for

P10 than for the others, although all of these energy levels are sufficiently

lower than the valence band of NiO. After hole injection, the dye is in its

reduced form. P5 exhibits the most negative reduction potential, followed by

P1 and P10. Therefore, the driving force for dye regeneration is the greatest

for P5. The fast dye regeneration is also beneficial for preventing the

competing reaction, charge recombination between the holes and the reduced

dye molecules.

45



/nm

λema

/nm

E0–0b

/eV

E(D/D+)c

/V vs. NHE

E(D/D–) c

/V vs. NHE

E(D*/D–) d

/V vs. NHE

P1 348(34720);481(57900) 618 2.25 1.32 –0.83 1.42

P5

P10

353(38992);499(59141)

316(37595);448(46462)

638

611

2.19

2.31

1.23

1.34

–0.88

–0.73

1.31

1.58

aThe absorption and emission spectra were measured in THF solution. bE0–0 was estimated from the

intersection of the normalized absorption and emission curves. cThe ground-state oxidation and

reduction potentials of the dyes, E(D/D+) and E(D/D–), were measured in dry acetonitrile with 0.1

M TBA(PF6). dE(D*/D–) was estimated from E(D/D–) and E0–0 (E(D*/D–) = E(D/D–) + E0–0).


The IPCE spectra of DSSCs sensitized with the three dyes are shown in Figure

37. 1.5 μm NiO films (0.32 cm2) prepared from NiCl2 and the F108 template

precursor were used in the measurement. DSSCs based on P1, P5, and P10

produced maximum IPCEs of 56%, 60%, and 44%, respectively. The IPCE

spectra gradually became broader toward the red region, from P10 to P1, then

P5, consistent with the trend observed from the absorption spectra. As

expected, P5 showed the highest LHE, followed by P1. The LHE of P10 was

the lowest, indicating the significantly lower light harvesting ability of this

dye.

The photovoltaic performance of the solar cells based on these dyes is

summarized in Table 7. Alkyl chains have been shown to have a blocking

effect in TiO2-based n-type DSSCs, which results in a longer electron lifetime

and a higher Voc.61

However, in our case, the dye with alkyl chains displayed a

disappointing photovoltaic performance. Both the Voc and Jsc were low for

P10, thus leading to a lower overall conversion efficiency than P1. The lower

Jsc could possibly be due to the undesired molecular distortion with the

presence of alkyl chains, which has a negative effect on both the optical

properties and the dye loading. Although the alkyl chains are introduced to

prevent the approach of I– to the NiO surface and slow down the charge

recombination with the holes, the hole lifetime of the device based on P10 is

found to be similar in magnitude compared to those with P1 and P5. As a

result, the Voc is not improved.

A promising result was found for P5. With one more phenylene unit

introduced between the electron-donor and the anchoring group, P5 exhibited

the best solar cell performance in this series of dyes. Besides the influence of

46

the increased light harvesting ability due to the extended conjugation, the large

spatial separation between the anchoring group and the electron-acceptor part

is helpful to retard the charge recombination. this leads to a long charge-

separated state, which is beneficial for efficient hole collection and dye

regeneration, and can be used to explain the higher Jsc and η.

400 500 600 700 800

0

10

20

30

40

50

60

70

(A)

IP

CE

/ %

Wavelength / nm

P1

P5

P10

0.00 0.02 0.04 0.06 0.08 0.10

0

1

2

3

4

5

6

(B)

Ph

oto

cu

rre

nt

de

nsity /

mA

cm

-2

Photovoltage / V

P1

P5

P10

Figure 37. Comparison of the IPCE spectra and J–V characteristics of P1-, P5-, and

P10-sensitized solar cells with 1.0 M LiI and 0.1 M I2 in acetonitrile as the electrolyte.

Table 7. Photovoltaic performance of DSSCs based on P1, P5, and P10.


P1 91 4.50 0.31 0.13

P5 97 5.23 0.31 0.16

P10 86 3.30 0.33 0.09

47

6.5. Conclusions

Two different ways of tuning the molecular structures of the dyes to prevent

charge recombination in p-type DSSCs were presented in this chapter. With the

increase of the distance between the anchoring group and the electron-

acceptor, a better photovoltaic performance was obtained, possibly due to the

slower charge recombination between the hole in NiO and the unpaired

electron in the reduced dye. This modification seemed to be an effective way

to improve the performance of p-type dyes. However, in the other case,

attempts to block the semiconductor surface with alkyl chains did not succeed.

The performance of the dye with alkyl chains was worse than that without

them. In future work, shorter alkyl chains could be introduced closely to the

anchoring group instead of on the linker part for comparison.

48

7. The study of porphyrin dye and its application for

co-sensitization in p-type DSSCs


Porphyrin based chromophores are well documented to be used for conversion

of solar energy into storable chemical energy.62

Most porphyrins possess an

intense Soret band at 400–450 nm and moderate Q bands at 500–650 nm,

which make them suitable to be used as the light harvester in DSSCs.63

However, the narrow Soret band and relatively weak Q bands limit their light-

harvesting abilities to some extent. Previous work showed that elongation of π-

conjugation and loss of symmetry were effective ways to broaden the

absorption band and increase the intensity of the Q bands, leading to an

improvement in light absorption.64

Accordingly, an unsymmetric zinc(II)

porphyrin dye (P9) was designed as the sensitizer for p-type DSSCs. The

5,10,15,20-tetraphenylporphyrinatozinc motif was functionalized with one

carboxyl group on the meso-phenyl ring, to ensure the single attachment of the

dye on the NiO surface. Trifluoromethyl groups were introduced on the other

phenyl rings in order to pull the electron density away from the NiO surface.

The porphyrin dye is also a suitable choice for co-sensitization. The high molar

extinction coefficient around 420 nm is complementary to our earlier dye (P1).

To counteract the decreased dye loading for the individual dyes after co-

sensitization, a new dye (P8), with similar structure but smaller molecular size

compared with P1, was designed and used together with P9 for comparison.

49

HOOC N

S CN

CN

P8

HOOC N

S

S

CN

CN

CN

CN

P1

N

N N

N

CF3

F3C

CF3

COOHZn

P9

+


7.2. Synthesis

The synthesis of the meso-tetrasubstituted porphyrin was accomplished by the

Lindsey method.65,66

Mix-condensation of one equivalent of 4-formyl benzoic

acid and three equivalents of 4-trifluoromethyl benzaldehyde with pyrrole, in

the presence of BF3·Et2O in dry DCM, followed by oxidation with 2,3-

dichloro-5,6-dicyano-p-benzoquinone (DDQ), afforded mixtures of several

porphyrins.67

Compound 17 was separated by column chromatography over

silica gel. Zinc porphyrin P9 was obtained by treatment of compound 17 with

zinc acetate in chloroform for 2 h (Scheme 6).

From the 1H NMR spectra (Figure 39), the disappearance of the signal for the

NH protons (–2.97 ppm) is observed for P9 compared to that of compound 17.

This, together with the result from the MS, indicates that the porphyrin ring is

coordinated with the metal.

The synthetic route for P8 is depicted in Scheme 7. Starting from the

commercially available compound, 4-bromotriphenylamine (18), formylation

of one phenyl moiety occurred via the Vilsmeier reaction, yielding the mono-

substituted compound 19 as the main product. Attempts were made to obtain

compound 19 by bromination of 4-formyltriphenylamine, but proved to be

problematic since the mono- and dibromo-substituted products were not easily

to be separated by column chromatography. From the formylated product 19,

the synthesis followed a similar procedure to that used for P1: oxidation with

silver oxide, followed by Suzuki coupling with 5-formyl-2-thiophene boronic

50

acid. Finally, P8 was obtained by the condensation of compound 21 with

malononitrile.


NH

CF3

CHO

COOH

CHO

+ +

N

NH N

HN

CF3

F3C

CF3

COOH

1) TFA

BF3·Et2O

DCM

2) DDQ

Et3N

N

N N

N

CF3

F3C

CF3

COOHZn

Zn(OAc)2

chloroform / methanol

P9

17

59%

6%

62 °C, 2 h

N

NH N

HN

CF3

F3C

CF3

COOH

N

N N

N

CF3

F3C

CF3

COOHZn

Figure 39. 1H NMR spectra of P9 and compound 17 in (CD3)2SO.

51


N

Br

N

Br

N

Br

OHC HOOC

NHOOC

S

CHO

NHOOC

S

CN

CN

POCl3

DMF

72%

Ag2O

NaOH

ethanol / toluene

85%

SOHC

B(OH)2

Pd(dppf)Cl2·CH2Cl2

K2CO3

methanol / toluene

70 °C, 20 min

60%

Et3N

ethanol

NC CN

72%

18 19 20

21 P8

rt, 24 h

83 °C, 2 h


The absorption and emission spectra of P9 in DCM solution are displayed in

Figure 40. The absorption spectrum shows a strong Soret band with maximum

at 422 nm and moderate Q bands (λabs = 551, 585 nm). After excitation, a

strong luminescence was measured with maxima at 599 and 644 nm, arising

from the porphyrin core. From the intersection of the normalized absorption

and emission spectra, E0–0 was determined to be 2.20 eV for P9.

400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

0.0

0.2

0.4

0.6

0.8

No

rma

lize

d E

mis

sio

n

No

rma

lize

d A

bso

rba

nce

Wavelength / nm

P9 abs

P9 Em

Figure 40. Normalized absorption (solid line) and emission (dash line) spectra of P9 in

DCM solution.

In DCM solution, the absorption band of P8 is a little blue-shifted compared to

that of P1, and the molar extinction coefficient is much lower due to less

52

conjugation. As shown in Figure 41, the molar extinction coefficient of the

Soret band of P9 is much higher than P1 and P8.

Figure 41. Absorption spectra of P1, P8, and P9 in DCM solution.

The oxidation and reduction potentials of P9 were measured by CV in dry

DCM containing 0.1 M TBA(PF6) (Figure 42). The data are listed in Table 8.

From the optical and electrochemical measurements, the driving force for hole

injection and dye regeneration is thermodynamically feasible.

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-15

-10

-5

0

5

10

15

Curr

ent / A

Voltage / V (vs Fc/Fc+)

P9

Figure 42. Cyclic voltammogram of P9 in dry DCM with Fc/Fc+ couple as the internal

standard.

53



/nm

λema

/nm E0-0

b

/eV

E(D/D+)c

/V vs. NHE

E(D/D–)c

/V vs. NHE

E(D*/D–)d

/V vs. NHE

P1 351(29987);485(53781) 620 2.24 1.32 –0.83 1.41

P8 345(19300);479(27517) 623 2.24 1.31 –0.70 1.54

P9 422(571950);551(27017);

585(4133)

599;

644

2.20 1.14 –1.14 1.06

aThe absorption and emission spectra were measured in DCM solution. bE0–0 was estimated from

the intersection of the normalized absorption and emission curves. cThe ground-state oxidation and

reduction potentials of the dyes were measured in dry acetonitrile (P1 and P8) and DCM (P9) with

0.1 M TBA(PF6). E(D/D+) and E(D/D–) indicate the first oxidation and reduction potential. dE(D*/D–) was estimated from E(D/D–) and E0–0 (E(D*/D–) = E(D/D–) + E0–0).


NiO films (0.32 cm2) were prepared from NiCl2 and the F108 template

precursor. The film was dipped into a 0.3 mM dye solution in methanol for P1,

P8, and P9, respectively. For the co-sensitization measurements, the dye

solutions were prepared by mixing P1 or P8 with P9 in a volume ratio of 1:1.

Figure 43 shows the absorption spectra of the dyes on 1.5 μm NiO films. For

P9, upon adsorption on a NiO electrode, a strong shoulder peak appears at 400

nm, due to dye aggregation on the surface. After co-adsorption with P1 or P8,

the shoulder peak decreases with respect to the main peak originating from the

monomer, indicating that the presence of P1 or P8 restricts the aggregation of

the porphyrin dye on the film. In both cases, the co-sensitized electrodes

(P1+P9 and P8+P9) show lower dye loading of P1 or P8 compared to those

sensitized with individual dyes.

400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

(A)

Ab

so

rban

ce

Wavelength / nm

P1

P1+P9

P9

400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

(B)

Ab

so

rba

nce

Wavelength / nm

P8

P8+P9

P9

Figure 43. Absorption spectra of the dyes on 1.5 μm NiO films (sensitizing time: P9 1h;

other samples 15h).

54

The IPCE spectra of the solar cells based on the individual dyes as well as the

co-sensitized electrodes are shown in Figure 44. 1.0 M LiI and 0.1 M I2 in

acetonitrile was used as the electrolyte in all the cases. The IPCE spectrum of

P9 matches well with the corresponding absorption spectrum, but the values

are much lower than P1 and P8, probably due to dye aggregation on the film.

As mentioned before, the use of porphyrin dye allows us to complement the

absorption between 400–450 nm on the basis of the earlier TPA-based dyes. In

both cases, there is a clear peak around 420 nm after co-sensitization,

corresponding to the Soret band of P9. Unfortunately, after co-sensitization the

IPCEs are lower than those with single TPA-based dyes, P1 or P8, especially

in the second case.

400 500 600 700 800

0

10

20

30

40

50

60

(A)

IPC

E / %

Wavelength / nm

P1

P1+P9

P9

400 500 600 700 800

0

10

20

30

40

50

(B)

IPC

E / %

Wavelength / nm

P8

P8+P9

P9

Figure 44. Comparison of the IPCE spectra with different dyes on 1.5 μm NiO films

(sensitizing time: P9 1h; other samples 15h).

Under standard sunlight irradiation, the P9-sensitized solar cells gave an

efficiency of 0.02%, with both lower photovoltage and photocurrent compared

to the TPA-based dyes (Table 9). But this dye still worked better than other

porphyrin dyes used in p-type DSSCs before, which showed the photocurrent

density of less than 0.1 mA cm–2

, and the highest IPCE value of 2.5% at the

most intense absorption in the Soret band.13,17 The smaller TPA-based dye, P8,

was less efficient than P1 under the same conditions. That could be mainly due

to its lower molar extinction coefficient. After co-sensitization, an overall

conversion efficiency of 0.12% was obtained for the P1+P9 system, which was

a little lower than the devices based on P1 (0.13%). This could be explained by

the decreased dye loading of P1, as well as the small contribution from P9.

The same trend was observed for the P8+P9 system, and the efficiency after

co-sensitization decreased even more (Figure 45).

55

0.00 0.02 0.04 0.06 0.08 0.10

0

1

2

3

4

5

(A)

Photo

curr

ent den

sity / m

A c

m-2

Photovoltage / V

P1

P1+P9

P9

0.00 0.02 0.04 0.06 0.08

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

(B)

Ph

oto

cu

rren

t de

nsity / m

A c

m-2

Photovoltage / V

P8

P8+P9

P9

Figure 45. Comparison of J–V characteristics with different dyes on 1.5 μm NiO films

(sensitizing time: P9 1h; other samples 15h).

The influence of the NiO film thickness on co-sensitization was investigated.

As shown in Figure 46, more dyes can be adsorbed when the film thickness is

increased from 0.8 to 1.5 μm. For the first system, the dye loading of both P1

and P9 increases more than two-fold with the thicker film, while, in the second

case the dye loading of P8 does not increase as much. One reason for this

could be a different arrangement of the dyes on the film, or different

intermolecular coordination between the dyes. Further increase of film

thickness to 2.0 μm does not change the absorbance too much.

400 500 600 700

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

(A)

Absorb

ance

Wavelength / nm

P1+P9 0.8 m NiO

P1+P9 1.5 m NiO

P1+P9 2.0 m NiO

400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

(B)

Ab

so

rba

nce

Wavelength / nm

P8+P9 0.8 m NiO

P8+P9 1.5 m NiO

P8+P9 2.0 m NiO

Figure 46. Absorption spectra of the dyes with different thickness of NiO films

(sensitizing time: 15h).

The IPCE spectra and J–V characteristics of the solar cells with different film

thickness are shown in Figure 47 and 48. The data are listed in Table 9. For the

P1+P9 system, the co-sensitized electrode, based on a 1.5 μm NiO film,

exhibited the highest efficiency, showing much higher Voc, Jsc, as well as IPCE

than the thin film (0.8 μm) device owing to more dye loading. With even

thicker film (2.0 μm), the IPCE did not increase anymore. Although the Jsc

rised to 4.54 mA cm–2

, the Voc decreased to a lower value than that based on a

56

0.8 μm film. The decrease of Voc with thicker NiO film was also observed for

the P8+P9 system. In both cases, the best solar cell performance was obtained

with a 1.5 μm NiO film.

400 500 600 700 800

0

10

20

30

40

50

IPC

E / %

Wavelength / nm

P1+P9 0.8 m NiO

P1+P9 1.5 m NiO

P1+P9 2.0 m NiO

400 500 600 700 800

0

5

10

15

20

25

30

35

(B)

IPC

E / %

Wavelength / nm

P8+P9 0.8 m NiO

P8+P9 1.5 m NiO

P8+P9 2.0 m NiO

Figure 47. Comparison of the IPCE spectra with different thickness of NiO films


0.00 0.02 0.04 0.06 0.08 0.10

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

(A)

Pho

tocurr

en

t de

nsity / m

A c

m-2

Photovoltage / V

P1+P9 0.8 m NiO

P1+P9 1.5 m NiO

P1+P9 2.0 m NiO

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

0.0

0.5

1.0

1.5

2.0

2.5

(B)

Ph

oto

cu

rre

nt d

en

sity / m

A c

m-2

Photovoltage / V

P8+P9 0.8 m NiO

P8+P9 1.5 m NiO

P8+P9 2.0 m NiO

Figure 48. Comparison of J–V characteristics with different thickness of NiO films


The effect of the sensitizing time was also investigated. For the P1+P9 system,

both the IPCE and the overall conversion efficiency increased with longer

sensitizing time. The enhanced IPCE (Figure 50 A) corresponded well with the

amount of dye loading (Figure 49 A). The optimum sensitizing time for

obtaining the highest efficiency was 15 h, yielding a Voc of 91 mV, a Jsc of 3.95

mA cm–2

, and an overall conversion efficiency of 0.12%. Further increase of

the sensitizing time did not improve the solar cell performance, probably since

most of the binding site had already been occupied. For the P8+P9 system, the

longer sensitizing time did not influence the dye loading significantly (Figure

49 B). In fact, the IPCEs even decreased with longer sensitizing time, from 1 h

to 15 h (Figure 50 B). The highest efficiency was obtained with the sensitizing

time of either 4.5 h or 15 h (Figure 51 B).

57

Table 9. Photovoltaic performance of DSSCs based on P1, P8, and P9 with different

thickness of the NiO film.

Thickness / μm Voc / mV Jsc / mA cm–2 ff η / %

P1 0.8 88 3.93 0.31 0.11

1.5 91 4.50 0.31 0.13

2.0 76 5.03 0.33 0.13

P1+P9 0.8 80 2.73 0.32 0.07

1.5 91 3.95 0.33 0.12

2.0 63 4.54 0.31 0.09

P8 0.8 72 2.39 0.33 0.06

1.5 76 3.12 0.33 0.08

2.0 64 3.62 0.31 0.07

P8+P9 0.8 71 1.60 0.34 0.04

1.5 67 2.23 0.33 0.05

2.0 53 2.31 0.33 0.04

P9 0.8 68 0.69 0.35 0.02

1.5 61 0.99 0.35 0.02

2.0 45 1.40 0.33 0.02

400 500 600 700

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

Ab

sorb

ance

Wavelength / nm

P1+P9 1 h

P1+P9 4.5 h

P1+P9 15 h

400 500 600 700

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

(B)

Ab

so

rba

nce

Wavelength / nm

P8+P9 1 h

P8+P9 4.5 h

P8+P9 15 h

Figure 49. Absorption spectra of the dyes on 1.5 μm NiO films with different

sensitizing time.

58

400 500 600 700 800

0

5

10

15

20

25

30

35

40

45

50

IPC

E /

%

Wavelength / nm

P1+P9 1h

P1+P9 4.5 h

P1+P9 15 h

400 500 600 700 800

0

10

20

30

40

IPC

E / %

Wavelength / nm

P8+P9 1h

P8+P9 4.5 h

P8+P9 15 h

Figure 50. Comparison of the IPCE spectra of the dyes on 1.5 μm NiO films with

different sensitizing time.

0.00 0.02 0.04 0.06 0.08 0.10

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Ph

oto

cu

rre

nt d

en

sity / m

A c

m-2

Photovoltage / V

P1+P9 1 h

P1+P9 4.5 h

P1+P9 15 h

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

0.0

0.5

1.0

1.5

2.0

2.5

Pho

tocurr

en

t de

nsity / m

A c

m-2

Photovoltage / V

P8+P9 1 h

P8+P9 4.5 h

P8+P9 15 h

Figure 51. Comparison of J–V characteristics of the dyes on 1.5 μm NiO films with


Table 10. Photovoltaic performance of DSSCs based on P1+P9 and P8+P9 with


Time / h Voc / mV Jsc / mA cm–2 ff η / %

P1+P9 1.0 84 3.03 0.32 0.08

4.5 84 3.74 0.32 0.10

15.0 91 3.95 0.33 0.12

P8+P9 1.0 65 1.90 0.34 0.04

4.5 73 1.95 0.34 0.05

15.0 67 2.23 0.33 0.05

7.5. Conclusions

A zinc(II) porphyrin dye, P9, was designed and synthesized as the sensitizer

for p-type DSSCs. It performed better than other porphyrin dyes used in p-type

59

DSSCs before, but gave much lower efficiency than the TPA-based

compounds, possibly due to the narrow absorption bands and dye aggregation

on the semiconductor surface. Structure modifications of the broadening of the

absorption bands and the increase of charge separation within the molecule by

introducing additional electron-donating and/or electron-withdrawing groups,

and the introduction of bulky groups to prevent dye aggregation should be

considered for further improvement of cell performance. This porphyrin dye

was also used for co-sensitization. In order to investigate the influence of

molecular size on co-sensitization, a smaller TPA-based dye, P8, was made for

comparison. P8 gave a worse solar cell performance than P1. The co-

sensitization of P9 with P1 or P8 performed less well than those sensitized

with individual TPA-based dyes. Advantageously, this work illustrated the

possibility of modifying the molecular structure of the porphyrin dye, and

combining two dyes with complementary absorption spectra to increase the

efficiency of p-type DSSCs. Hopefully it would motivate future research in

this field.

60

8. Concluding remarks

The aim of this thesis was to study organic dyes for p-type DSSCs: design and

synthesis of efficient dyes, and the identification of parameters limiting the

solar cell performance. By modifying the dye structures, both the optical

properties and the energy levels were changed, which had a big effect on the

light harvesting ability, as well as on different charge transfer processes.

In order to increase the light harvesting efficiency, broadening of the

absorption band was attempted, either by the breaking of molecular symmetry

or by the co-sensitization of different dyes with complementary absorption

spectra. However, none of these methods gave the desired result. The breaking

of molecular symmetry by using different conjugated linkers did not

substantially broaden the absorption spectrum, or improve the solar cell

performance compared to the symmetric dyes. In the second case, although the

co-sensitization of TPA-based dye with a zinc porphyrin dye broadened the

absorption spectrum compared to individual dyes, the poor photovoltaic

performance of the porphyrin dye, together with the lower dye loading, led to a

similar or even worse efficiency.

The influence of the energy levels of a dye on the solar cell performance was

investigated by making a series of dyes with different electron-withdrawing

groups. The dye with the strongest electron-withdrawing group exhibited a less

negative E(D/D–) potential, and a poor solar cell performance. From kinetic

studies, it was determined that the dye could not be regenerated efficiently due

to the insufficient driving force for oxidizing the reduced dye by I3–, indicating

that this E(D/D–) potential might reach the limit for efficient dye regeneration.

Attempts were made to block the charge recombination between the holes in

the NiO film and the reduced dye and/or the reduced species in the electrolyte,

by either increasing the distance between the anchoring group and the electron-

acceptor part, or introducing alkyl chains on the conjugated linkers. A

promising result was obtained in the first case. In the second case, the

efficiency was even worse compared with the dye without alkyl chains, mainly

due to the improper location of the alkyl chains.

Besides the modification of dye structure, the optimization of NiO film and the

electrolyte were also presented. Using NiCl2 and the F108 template precursor

instead of the original Ni(OH)2, both the IPCE and efficiency could be

significantly improved. In addition, the electrolytes based on different

concentrations of LiI and different solvents were studied. Finally, the highest

61

overall conversion efficiency was achieved by the P5-sensitized solar cells

based on a 1.5 μm NiO film and acetonitrile-based electrolyte, which was a

significant increase compared to the commercially available dyes used before,

and made this design to be a model for the p-type dyes.

Considering future dye design, it is of interest to further increase the lifetime

of the charge-separated state by introducing the conjugated linkers either

between the anchoring group and the electron-donor, or between the electron-

donor and electron-acceptor, and introduce shorter alkyl chains closely to the

anchoring group to protect the surface. In addition, dyes with broad absorption

spectrum, high molar extinction coefficient, good stability and strong binding

ability should be considered.

62

Acknowledgements

I would like to express my sincere gratitude to all the people who have helped

and worked with me.

First and foremost, my supervisor, Professor Licheng Sun, for giving me the

opportunity to join the group, guiding and encouraging me during the last four

years. It was a great and fantastic experience to work with you.

Professor Anders Hagfeldt, Dr. Gerrit Boschloo, Professor Bo Albinsson,

Professor Lars Kloo and Professor Tore Brinck for encouraging discussions

and nice collaborations.

Licheng Sun, Anders Hagfeldt, Christina Moberg, Gerrit Boschloo, Elizabeth

A. Gibson, Karl Martin Karlsson, and Ze Yu for valuable comments on this

thesis.

My co-authors: Elizabeth A. Gibson (Uppsala University), Lin Li and Hongjun

Zhu for share of knowledge and nice collaboration. Tomas Edvinsson (Uppsala

University) and Mats Linder for calculations and discussions. Joanna Wiberg

(Chalmers University of Technology) for ultra laser spectroscopy

collaborations.

Tannia Marinado, Yunhua Xu, Haining Tian, and Wei Zhang for helpful and

excellent discussion. Yan Gao and Samir Andersson for MS measurements. Ze

Yu for the help of the diffusion coefficient measurement.

All present and former members of the Sun group, Yunhua, Misha, Samir,

Lele, Lianpeng, Lin, Haining, Ilkay, Sven, Fuyu, Dapeng, Viktor, Fujun,

Thitinum, Jingnan, and Xien for being intelligent and spreading a nice

atmosphere throughout the group, in particular my excellent collaborators

Daniel, Martin, and Erik for pleasant time in the lab.

The Swedish Research Council, the Swedish Energy Agency, the Knut and

Alice Wallenberg Foundation and the China Scholarship Council (CSC) for

financial supports.

The Aulin-Erdtman foundation and the Knut and Alice Wallenberg foundation

for travel grants.

All the former and present members of the Hagfeldt group for valuable

discussions.

63

Ulla Jacobsson, Lena Skowron, and Henry Challis for all kinds of help with

general things in the Department.

All former and present co-workers at the Department of organic chemistry for

great working environment.

I would like to thank my parents for endless love and encouragement. Thank

you for believing and supporting me all the time.

64

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68

Appendix A

The following is a description of my contribution to the publications I to V,

Paper I: Major contribution to the initiation of the project. Responsible for the

dye synthesis and characterization, wrote the main part of the manuscript.

Paper II: Major contribution to the initiation of the project. Responsible for the

dye synthesis, characterization, wrote the main part of the manuscript.

Paper III: Synthesized the dye used for studying, helped with the discussion

and formulation of the manuscript.

Paper IV: Major contribution to the initiation of the project. Responsible for

the dye synthesis and characterization, wrote the main part of the manuscript.

Paper V: Major contribution to the initiation of the project. Responsible for the

dye synthesis and characterization, wrote the main part of the manuscript.

69

Appendix B

This appendix contains experimental procedures and analytical data of the

compounds mentioned in the thesis but not included in the publications.

Chapter 4:

4-{(4-Bromophenyl)-[4-(5-formylthiophen-2-yl)phenyl]amino}benzoic acid

(6)

To a solution of compound 3 (98.4 mg, 0.22 mmol) and Pd(dppf)Cl2·CH2Cl2

(16.3 mg, 0.02 mmol) in toluene (1.5 mL) was added a solution of 5-formyl-2-

thiophene boronic acid (78.0 mg, 0.50 mmol) and K2CO3 (110.4 mg, 0.80

mmol) in methanol (2 mL). The mixture was heated by microwave oven

irradiation at 70 °C for 25 min and then poured into a saturated solution of

ammonium chloride (60 mL). Extracted with ethyl acetate (3 × 100 mL), the

combined organic phase was washed with brine and dried over anhydrous

sodium sulfate. After the solvents were evaporated, the residue was purified by

column chromatography over silica gel using petroleum ether/acetone mixture

as the eluent to give the product (35 mg, 33%) as yellow solid. 1H NMR (500

MHz, (CD3)2CO, 25°C): δ = 7.13–7.17 (m, 4H), 7.23 (d, J = 8.4 Hz, 2H), 7.57

(d, J = 8.4 Hz, 2H), 7.64 (d, J = 4.5 Hz, 1H), 7.80 (d, J = 8.4 Hz, 2H), 7.95–

7.97 (m, 3H), 9.94 ppm (s, 1H). 13

C NMR (125 MHz, (CD3)2CO, 25°C): δ =

118.0, 122.7, 125.1, 125.3, 125.9, 128.5, 128.5, 129.5, 132.1, 133.7, 139.2,

143.2, 146.7, 148.5, 151.8, 153.5, 167.1, 183.7 ppm. ESI–MS: m/z: 476.0 [M–

H]–.

4-{(4'-Formylbiphenyl-4-yl)-[4-(5-formylthiophen-2-yl)phenyl]amino}

benzoic acid (7)


(8.2 mg, 0.01 mmol) in toluene (1.5 mL) was added a solution of 4-

formylphenyl boronic acid (45.0 mg, 0.30 mmol) and K2CO3 (69.0 mg, 0.50

mmol) in methanol (2 mL). The mixture was heated by microwave oven







MHz, (CD3)2CO, 25°C): δ = 7.20 (d, J = 8.1 Hz, 2H), 7.28 (d, J = 7.7 Hz, 2H),

7.33 (d, J = 7.8 Hz, 2H), 7.64–7.65 (m, 1H), 7.81–7.84 (m, 4H), 7.93–8.03(m,

7H), 9.94 (s, 1H), 10.09 ppm (s, 1H). 13

C NMR (125 MHz, DMSO, 25°C): δ =

121.9, 124.6, 124.9, 125.0, 125.7, 127.0, 127.8, 128.0, 128.6, 130.3, 131.1,

70

134.6, 134.9, 139.5, 141.5, 145.0, 146.3, 147.2, 150.3, 152.3, 166.8, 183.9,

192.7 ppm. ESI–MS: m/z: 502.1 [M–H]–.

4-([4'-(2,2-Dicyanovinyl)biphenyl-4-yl]-{4-[5-(2,2-dicyanovinyl)thiophen-

2-yl]phenyl}amino)benzoic acid (P6)

A mixture of compound 7 (60.4 mg, 0.12 mmol) and malononitrile (33.0 mg,

0.50 mmol) in anhydrous DMF (8 mL) was stirred at 85 °C for 8 h. After the

solvents were evaporated, the residue was purified by column chromatography

over silica gel using pentane/ethyl acetate mixture as the eluent to give the

product (30 mg, 42%) as red solid. 1H NMR (500 MHz, (CD3)2CO, 25°C): δ =

7.23 (d, J = 8.6 Hz, 2H), 7.29 (d, J = 8.5 Hz, 2H), 7.34 (d, J = 8.4 Hz, 2H),

7.74 (d, J = 4.0 Hz, 1H), 7.85–7.89 (m, 4H), 7.98–8.01 (m, 5H), 8.15 (d, J =

8.3 Hz, 2H), 8.35 (s, 1H), 8.42 ppm (s, 1H). 13

C NMR (125 MHz, (CD3)2CO,

25°C): δ = 76.2, 82.1, 114.2, 114.6, 115.1, 115.3, 123.6, 125.6, 125.8, 126.0,

126.8, 128.2, 128.6, 128.9, 129.5, 131.3, 132.2, 132.5, 135.0, 135.8, 142.7,

146.4, 148.0, 149.3, 151.6, 152.6, 156.1, 160.9, 167.2 ppm. HR–MS (TOF MS

ESI) m/z calcd for C37H20N5O2S [M–H]–: 598.1343, found: 598.1304.

Chapter 7:

4-[10,15,20-Tris(4-trifluoromethylphenyl)porphyrin-5-yl]benzoic acid (17)

Pyrrole (0.35 mL, 5.0 mmol), 4-formyl benzoic acid (188.0 mg, 1.25 mmol)

and 4-trifluoromethyl benzaldehyde (0.5 mL, 3.75 mmol) were dissolved in

dry DCM (500 mL) and bubbled with N2 for 20 min. Trifluoroacetic acid

(TFA) (0.58 mL, 7.5 mmol) and boron trifluoride diethyl etherate (BF3·Et2O)

(9 μL, 75.0 μmol) were added and the reaction mixture was stirred at room

temperature for 1 h. DDQ (0.85 g, 3.75 mmol) in toluene (5 mL) was added

into the mixture, then stirring for an additional hour. After addition of Et3N

(1.04 mL, 7.5 mmol) the mixture was concentrated to 1/5 of the initial volume

and filtered through silica column (DCM/methanol). The second fluorescent

band was collected by column chromatography over silica gel using

DCM/methanol mixture as the eluent to give the product (69 mg, 6%) as

purple solid. 1

H NMR (500 MHz, (CD3)2SO, 25°C): δ = –2.97 (s, 2H), 8.19–

8.20 (m, 6H), 8.32–8.33 (m, 2H), 8.37–8.38 (m, 2H), 8.44–8.45 (m, 6H), 8.84

(s, 8H), 13.33 ppm (s, 1H). ESI–MS: m/z 861.2 [M–H]–.

Zn(ΙΙ)4-[10,15,20-tris(4-trifluoromethylphenyl)porphyrin-5-yl]benzoic

acid (P9)

To a solution of compound 17 (35.0 mg, 0.04 mmol) in chloroform (40 mL)

was added a solution of Zn(OAc)2 (147.0 mg, 0.80 mmol) in methanol (6 mL).

The mixture was refluxed for 2 h and cooled to room temperature. The reaction

mixture was washed with water (3 × 30 mL) and dried over anhydrous sodium

sulfate. After solvent removal, reprecipitation from DCM/pentane gave P9 (22

71

mg, 59%) as purple red solid. 1H NMR (500 MHz, (CD3)2SO, 25°C): δ = 8.16–

8.18 (m, 6H), 8.25–8.27 (m, 2H), 8.41–8.43 (m, 8H), 8.79 (s, 6H), 8.86–8.88

ppm (m, 2H). HR–MS (TOF MS ESI) m/z calcd for C48H24F9N4O2Zn [M–H]–:

923.1053, found: 923.1064.

4-[(4-Bromophenyl)phenylamino]benzaldehyde (19)

Phosphorus oxychloride (1.3 mL, 13.89 mmol) was added dropwise to dry

DMF (4.0 mL) at 0 °C under N2, to which 4-bromotriphenylamine (1.5 g, 4.63

mmol) in DMF (8 mL) was added. The mixture was stirred at 100 °C for 3 h.

After cooling to room temperature, the mixture was poured into ice-water (100

mL) and basified with 1 M NaOH to neutral. Extracted with DCM (3 × 100

mL), the organic phase was washed with brine and dried with anhydrous

sodium sulfate. The residue was purified by column chromatography on silica

gel using pentane/DCM mixture as the eluent to give the product (1.17 g, 72%)

as yellow oil-like compound. 1H NMR (500 MHz, DMSO, 25°C): δ = 6.94 (d,

J = 8.7 Hz, 2H), 7.11 (d, J = 8.7 Hz, 2H), 7.18 (d, J = 7.5 Hz, 2H), 7.24 (t, J =

7.5 Hz, 1H), 7.42 (t, J = 8.0 Hz, 2H), 7.56 (d, J = 8.7 Hz, 2H), 7.74 (d, J = 8.7

Hz, 2H), 9.79 ppm (s, 1H). 13

C NMR (125 MHz, (CD3)2CO, 25°C): δ = 118.0,

120.6, 126.4, 127.3, 128.5, 130.8, 130.9, 131.9, 133.6, 146.5, 146.8, 153.5,

190.7 ppm. ESI–MS: m/z: 352.3 [M+H]+.

4-[(4-Bromophenyl)phenylamino]benzoic acid (20)

Silver oxide (1.45 g, 6.24 mmol) was suspended in ethanol (50 mL) containing

sodium hydroxide (3.12 g, 78.0 mmol). To the solution was added compound

19 (0.55 g, 1.56 mmol) dissolved in toluene (4 mL). The mixture was stirred at

room temperature for 24 h. The solution was then decanted and precooled HCl

(10%, 60 mL) was added slowly. Extracted with ethyl acetate (3 × 100 mL),

the organic phase was washed with water and dried with anhydrous sodium

sulfate. Filtered and the solvent was evaporated. The residue was purified by

column chromatography on silica gel using DCM/ethyl acetate mixture as the

eluent to give the product (0.49 g, 85%) as white solid. 1

H NMR (500 MHz,

DMSO, 25°C): δ = 6.92 (d, J = 7.3 Hz, 2H), 7.04 (d, J = 7.0 Hz, 2H), 7.13 (d, J

= 7.1 Hz, 2H), 7.19 (t, J = 6.3 Hz, 1H), 7.39 (t, J = 6.7 Hz, 2H), 7.52 (d, J = 7.0

Hz, 2H), 7.80 (d, J = 7.3 Hz, 2H), 12.60 ppm (s, 1H). 13

C NMR (125 MHz,

DMSO, 25°C): δ = 116.1, 120.0, 123.7, 125.0, 125.8, 127.0, 130.0, 130.9,

132.6, 145.7, 145.8, 150.7, 166.9 ppm. ESI–MS: m/z: 366.2 [M–H]–.

4-{[4-(5-Formylthiophen-2-yl)phenyl]phenylamino}benzoic acid (21)


(40.8 mg, 0.05 mmol) in toluene (3.0 mL) was added a solution of 5-formyl-2-

thiophene boronic acid (212.2 mg, 1.36 mmol) and K2CO3 (298.1 mg, 2.16

mmol) in methanol (4.0 mL). The mixture was heated by microwave oven


72






MHz, (CD3)2CO, 25°C): δ = 7.10 (d, J = 8.6 Hz, 2H), 7.19–7.26 (m, 5H), 7.43

(t, J = 7.5 Hz, 2H), 7.62 (t, J = 3.7 Hz, 1H), 7.78 (d, J = 8.6 Hz, 2H), 7.93–7.96

(m, 3H), 9.93 (s, 1H), 11.00 ppm (s, 1H). 13

C NMR (125 MHz, (CD3)2CO,

25°C): δ = 122.1, 124.6, 125.0, 125.6, 126.1, 127.3, 128.4, 129.1, 130.8, 132.0,

139.2, 143.1, 147.2, 148.9, 152.3, 153.7, 167.1, 183.7 ppm. ESI–MS: m/z

398.1 [M–H]–.

4-({4-[5-(2,2-Dicyanovinyl)thiophen-2-yl]phenyl}phenylamino)benzoic

acid (P8)

To a solution of compound 21 (100.0 mg, 0.25 mmol) in ethanol (40 mL) were

added malononitrile (52.0 mg, 0.79 mmol) and triethylamine (10 μL). The

mixture was refluxed at 83 °C for 2 h. After cooling to room temperature water

(10 mL) was added, the solution was extracted with ethyl acetate and dried

with anhydrous sodium sulfate. After solvent removal, the residue was purified

by column chromatography over silica gel using acetone/petroleum ether

mixture as the eluent to give the product (80 mg, 72%) as red solid. 1

H NMR

(500 MHz, (CD3)2CO, 25°C): δ = 7.13 (d, J = 8.5 Hz, 2H), 7.21–7.28 (m, 5H),

7.44 (t, J = 7.0 Hz, 2H), 7.72 (d, J = 4.0 Hz, 1H), 7.82 (d, J = 8.5 Hz, 2H),

7.95–7.97 (m, 3H), 8.40 (s, 1H), 11.05 ppm (s, 1H). 13

C NMR (125 MHz,

(CD3)2CO, 25°C): δ = 76.1, 114.6, 115.3, 122.7, 125.0, 125.1, 125.4, 126.3,

127.4, 128.0, 128.7, 130.9, 132.0, 134.9, 142.7, 147.1, 149.6, 152.0, 152.6,

156.3, 167.1 ppm. HR–MS (TOF MS ESI) m/z calcd for C27H16N3O2S [M–H]–:

446.0969, found: 446.0956.

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