conjugated polymer-inorganic semiconductor hybrid solar cells

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Conjugated polymer–inorganic semiconductor hybrid solar cells

Tingting Xu and Qiquan Qiao*

Received 6th November 2010, Accepted 22nd December 2010

DOI: 10.1039/c0ee00632g

Polymer–inorganic semiconductor hybrid solar cells have attracted extensive research and attention as

a promising approach to achieve cost effective solar energy. Power conversion efficiencies exceeding 3%

have been achieved for polymer–inorganic hybrid solar cells. However, these efficiencies are still lower

than those of polymer-fullerene solar cells, which have recently reached up to 8.13%. In this article, we

review the recent developments including device operation mechanism, cell structures, polymer and

inorganic materials, and various approaches to improve cell performance. In addition, the dependence

of power conversion efficiency on the polymer bandgap and the lowest unoccupied molecular orbital

(LUMO) using several typical inorganic acceptors including TiO2, ZnO and CdSe are presented andmay provide guidance for the engineering of donor polymers.

1. Introduction

Sunlight is an abundant and renewable energy resource, and

converting sunlight into electricity has been regarded as one of

the most promising approaches to provide clean energy.

Conventional silicon solar cells usually require high purity Si

materials and complicated fabrication processing; therefore they

are not cost effective for wide range applications.1 Organic or

organic-inorganic hybrid solar cells become a promising candi-

date for offering low cost solar energy, since they can be fabri-

cated by solution-based processing, such as dip coating, screenprinting, ink jet printing, painting and roll-to-roll processing.2,3

They also have the advantages of being lightweight and

mechanically flexible.4,5 Polymer-fullerene solar cells have

recently achieved energy conversion efficiency exceeding 8%.6

In recent years, conjugated polymer–inorganic semiconductor

hybrid systems haveattractedextensive attentionand research.7–18

These devices combine the advantages from both organic and

inorganic materials.19 Conjugated polymers (e.g., P3HT), when

self-organized into crystal structure, can own a high hole

mobility,20 and can also be easily processed onto the surfaces of

both rigid and flexible substrates. Nanoscale inorganic materials

exhibit different optical absorption and photocurrent generation

properties from bulk materials due to their quantum size

confinement. They have advantages including relatively high

electron mobility, high electron affinity and good thermal

stability.21 Solution-processible nanostructured inorganic semi-

conductors also provide the possibility to have a large interfacial

area for efficient exciton dissociation when blending with soluble

polymers.21 One-dimensional (1-D) ordered nanostructure inor-

ganic semiconductors aligned on a substrate can provide an

ideally straight pathway for carrier transport. Generally when

organic and inorganic components are combined into a hetero-

junction device, the polymers are used as donors to absorb

sunlight and transport holes, while the inorganic semiconductors

function as acceptors to transport electrons. In such devices, an

energy conversion efficiency exceeding 3% has been in reach. 22

More recently, solid state dye-sensitized solar cell (DSSC) struc-

ture has been adopted to fabricate organic (polymer) – inorganic

hybrid solar cells.23,24 In these devices, the inorganic semi-

conductors (e.g., porous TiO2) are sensitized by a traditional dye

or a light absorbinginorganic semiconductor (e.g., Sb2S3),andthe

Center for Advanced Photovoltaics, South Dakota State University,Brookings, SD, 57007, USA. E-mail: [email protected].; Fax:+1 605 688 4401; Tel: +1 605 688 6965

Broader context

Sunlight is an abundant and renewable energy resource, and converting sunlight into electricity has been regarded as one of the most

promising approaches to provide clean energy. Polymer–inorganic hybrid solar cells become a promising candidate for offering low

cost solar energy, since they can be fabricated by solution-based processing, such as screen printing, ink jet printing, and roll-to-roll

processing. They also have the advantages of being lightweight and mechanically flexible. Power conversion efficiencies exceeding

3% have been achieved for polymer–inorganic hybrid solar cells. However, these efficiencies are still lower than what is required for

commercial applications. To further improve device performance, several challenges need to be considered including morphology

control, polymer and inorganic material engineering, and donor–acceptor interface modification.

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polymers function as a hole transporter to reduce the dyes and/or

also work as an additional donor to absorb light. A power

conversion efficiency of 5.13% has recently been achieved in this

type of hybrid solar cells.24 The different functions of organic and

inorganic materials provide additional opportunities to improve

solar cell performance by taking advantages of organic solar

cells, inorganic semiconductor solar cells, and DSSCs.24

In general, polymer–inorganic semiconductor hybrid solar

cells can be fabricated in three configurations: 1) a planar bilayerhybrid solar cell formed by depositing a distinctive layer of donor

polymer on top of an inorganic acceptor layer; 2) a randomly

mixed bulk heterojunction hybrid solar cell made by blending an

inorganic nanoscale semiconductor with a polymer in solution,

and then depositing onto a substrate to form an active layer; 3)

an ordered heterojunction hybrid solar cell fabricated by infil-

trating an ex-situ synthesized polymer, or in situ polymerizing

a polymer into the nanopores of a pre-prepared inorganic

nanostructured template.

In this paper, the recent developments of polymer–inorganic

hybrid solar cells are reviewed. First, the mechanism, theory, cell

characterization and efficiency prediction of polymer–inorganic

solar cells are introduced. Second, polymer–inorganic semi-conductor hybrid solar cell materials and architectures are

described. Third, various approaches to improve cell performance

are discussed. These include morphology control, surface modifi-

cation at polymer–inorganic interface, filling or in situ polymeri-

zation of polymer into pre-synthesized nanostructures, and in situ

growth of inorganic semiconductors in the matrix of conjugated

polymers. Finally, the conclusion and outlook to further improve

polymer–inorganic hybrid solar cell performance are presented.

2. Mechanism and theory

2.1. Theory of photovoltaic process

Solar cell is a device that produces electricity from sunlight.

Upon illumination, photons are absorbed by an active layer in

a solar cell, and then electron-hole pairs are generated as exci-

tons. The excitons need to diffuse to a donor–acceptor (DA)

interface to dissociate into free charges. After that, electrons and

holes need to transport to electrodes through their corresponding

percolation pathway.25 During these processes, six main steps

affect device performance (Fig. 1): i) photon absorption (ha); ii)

exciton generation (hex); iii) exciton diffusion (hdiff ); iv) exciton

dissociation (hed); v) charge transport (htr); vi) charge collection

(hcc). The h is the yield of each process.These six processes determining a solar cell performance can

be better understood in a connection with external quantum

efficiency (EQE) of a device. EQE is defined as a percentage of

the number of charge carriers collected at the electrode under

short-circuit condition to the number of photons incident on the

device.26 EQE can be expressed as the product of the above steps.

EQE ¼ ha Â hex Â hdiff Â hed Â htr Â hcc (1)

Fig. 1 Energy level diagram of a polymer – inorganic hybrid solar cellwith six key processes. (LUMO: lowest unoccupied molecular orbital;

HOMO: highest occupied molecular orbital. hv: photon energy).

Tingting Xu

Tingting Xu received her bachelor

degree major in polymer material

science and engineering and

master degree in material science

from Northwestern Polytechnical

University, Xi’an of China in 2004

and 2007, respectively. She joined

the Ph. D. Photovoltaics program

in Electrical Engineering depart-ment at South Dakota State

University in 2007 August. She is

pursing her Ph. D. degree under

the supervision of Prof. Qiquan

Qiao in the filed of polymer–inor-

ganic semiconductor hybrid solar

cells.

Qiquan Qiao

Qiquan Qiao obtained the B.S.

degree from Hefei University of

Technology, M.S. degree from

the Shanghai Institute of Optics

and Fine Mechanics, Chinese

Academy of Sciences, and Ph.D.

degree from the Virginia

Commonwealth University. In

2006, he joined the group of Prof. John Reynolds at the

University of Florida as a Post-

doctoral Researcher. In 2007, he

joined the faculty at South

Dakota State University where

he established the Organic

Electronics Laboratory. Current

research focuses on organic photovoltaic materials and devices. He

received 2009 Bergmann Memorial Research Award from US-

Israel Binational Science Foundation and 2010 Early Career

(CAREER) Award from the National Science Foundation.

This journal is ª The Royal Society of Chemistry 2011 Energy Environ. Sci., 2011, 4, 2700–2720 | 2701

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The photon absorption yield ha is determined by the absorp-

tion spectral band, optical absorption coefficient, and thickness

of a photoactive layer, as well as internal reflection.27 Most of

semiconducting polymers (e.g., P3HT, MDMO-PPV, etc) have

a bandgap larger than 2 eV,28 which limits light absorption range

less than 650 nm. Therefore, only a small portion of sunlight can

be absorbed in polymer solar cells. Usually, the thickness of an

active layer is in the order of 100 nm to avoid exciton and charge

tranport loss. To well balance sufficient light absorption andefficient charge transport, both a wide absorption spectral band

and a high absorption coefficient become important for

absorbing enough sunlight. The absorption coefficients of

conjugated polymers were reported to be much higher than that

of silicon so that a thin layer (e.g., 100 nm) of polymer is enough

to absorb sufficient light.28

Unlike inorganic semiconductors such as silicon, conjugated

polymers have a relatively low dielectric constant, therefore after

photoexcitation, Coulomb interaction between electrons and

holes is so strong that they form excitons, instead of free

carriers.29 The binding energy is typically $0.1–0.4 eV. Conju-

gated polymers also have a strong electron-vibration coupling.

When photoexcited to an excited state, they will relax to thelowest excited state and reach to their equilibrium geometry,30

forming excitons.

The efficiency (hdiff ) of exciton diffusion to a DA interface is

related to its exciton diffusion length (LD) and distance (Li)

between photoexciton location and DA interface which serves as

a dissociation center.31 The LD is equal to (Ds)1/2, where D is

diffusion coefficient and s is exciton lifetime.30 Excitons can

diffuse to a DA interface with Li # LD, otherwise they may

recombine with a reduced hdiff .26 Typically, exciton diffusion

length is in the range of 4$20 nm for conjugated polymers.32–34

The efficiency (hed) of exciton dissociation into free holes and

electrons relies on DA LUMO energy offsets and internal electric

field at a DA heterojunction.26 From current understanding, theminimum energy required to dissociate an exciton in a conju-

gated polymer is what is needed to overcome the exciton binding

energy.35 This energy can be provided by the offset between the

LUMO energy levels of the donor and the acceptor.26,35

The htr is carrier transport efficiency. The holes are trans-

ported in a conjugated polymer, while the electrons are trans-

ported in an inorganic semiconductor. Both the donor and

acceptor materials are required to form highly efficient percola-

tion networks spanning the entire active layer to provide efficient

charge transport.36 The polymers need to have a higher degree of

planarity for efficient backbone stacking for a high hole mobility.

Through the treatments such as thermal and solvent annealing,

the polymers should also be able to self-assemble into a moreorganized structure. In addition, the inorganic nanocrystal

acceptors need to physically touch each other and form an effi-

cient percolation pathway for electron transport. The transport

efficiency is also influenced by the energy level and densities of

trap states in their respective transport materials.26 The trap

states that are typically caused by structure defects and impurity

species can be a recombination center leading to charge transport

losses.

The hcc is charge collection efficiency. It is the fraction of the

charges transported from the active layer to the electrodes with

respect to the total free charges that are supposed to transport to

the electrodes. The hcc depends on the energy levels of the active

layer, the electrodes and the interface between them.26

2.2. Characterization of solar cell performance

Solar cell efficiency can be calculated from its current density-

voltage (J-V) characteristic curves. From such curves, open-

circuit voltage (Voc), short-circuit current density (Jsc) and fill

factor (FF) can be obtained. Then energy conversion efficiencycan be determined by

h ¼J scV ocFF

P s(2)

Where Ps is the incident light power density. A standard test

condition for solar cells is Air Mass 1.5 (AM 1.5) with an incident

power density of $100 mWcmÀ2 at a temperature of 25 C.37

Equivalent circuit of a solar cell is shown as Fig. 2. A series

resistance Rs originates from contact and bulk semiconductor,

and a shunt resistance Rsh comes from poor diode contact.38 The

J-V characteristics can be described as39

J ¼ J 0

expqðV À JRsAÞ

nk B T

À 1

þ

V À JRsA

RshA À J ph (3)

Where kB is Boltzmann’s constant, T is temperature, q is

elementary charge, A is device area, n is ideality factor of the

diode, J0 is reverse saturation current density, Jph is photocur-

rent, Rs is series resistance and Rsh is shunt resistance. The J-V

curves and photovoltaic parameters including Voc and FF

strongly depend on the n, J0, Rs, and Rsh. Fig. 3 shows typical

dark and illuminated current density-voltage (J-V) curves, in

which three distinctive regions can be seen.40 The first (I) is the

linear region in negative potentials and low positive potentials, in

which the current density is dominated by the shunt resistance

(Rsh). The second (II) is the region at mediate positive regions

where the curve shows an exponential behavior and the currentdensity is related to the diode. The third (III) is another linear

region in high positive potentials where the current density is

related to the series resistance (Rs).40

The ideality factor (n) is a figure which shows how closely

a diode behaves like an ideal diode and it is typically deviated

from the ideal by recombination in the junction.41 In polymer-

fullerene and polymer-inorganic hybrid solar cells that can be

pictured as an ‘‘extended pn junction’’, recombination can

happen at the DA interface (junction) when the separated elec-

trons and holes meet,42,43 causing n to deviate from 1. Thus, the

ideality factor (n) can be regarded as an indicator of DA

morphology, phase separation and their interfacial area.40

Waldauf et al. reported that bulk heterojunction solar cells with

Fig. 2 Equivalent circuit of a solar cell.


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their active layers made from the same production batch have

comparable diode idealities, indicated by similar slopes in the

exponential regime of their J-V curves.40 It was also found that

different solvents led to different morphologies in organic solarcells, resulting in different idealities.43

The reverse saturation current density (J0) is also an important

parameter affecting the J-V curves in the exponential regimes and

thus cell performance. The J0 is an indicator of how many

charges can overcome the energetic barrier in the reverse direc-

tion. These charges are regarded as minority charges at the

donor/acceptor interface. In a typical pn junction, J0 can be

described as40,43,44

J 0 ¼ J i exp

Àqf

nkT

(4)

Where J i depends on material purity and f is energetic barrier

voltage. f was found to be in good agreement with energydifference of the acceptor’s LUMO and the donor’s HOMO. In

the previous report by Waldauf et al., the fitted J0 was found to

follow J 0(P 3HT ) ¼ J i exp(À7347$T À1) in the P3HT:PCBM solar

cells and J 0(MDMO À PPV ) ¼ J i exp(À10895$T À1) in the

MDMO-PPV:PCBM devices, respectively.40 The J0 value

increases with increasing temperature (T), but decreases as the

material quality (purity) and energetic barrier improve.

The series resistance (Rs) is another parameter that affects the

J-V characteristics and solar cell performance. The Rs results

from limited conductivity of organic layer, contact resistance

between organic layer and its corresponding electrodes, and

connecting resistance between the electrodes and external circuit.

The Rs can reduce the FF and it can also reduce the J sc if it is toohigh. Generally the Rs has no impact on open circuit voltage

(Voc) since the entire current flows through the diode at the Voc

condition, but no current flows though the Rs. However, at the

points close to the Voc, the Rs greatly affects the J-V curves,

providing a simplified method to estimate the Rs by measuring

the slope of the J-V curves in the regime close to the Voc.41 The Rs

should be minimized to reduce the energy loss, especially in large

area solar cells.41,44

The shunt resistance (Rsh) is also a parameter affecting the J-V

characteristics and solar cell performance. The Rsh may be

related to the device structure and film morphology. For

example, Rsh can be lowered by the leakage current through the

pinholes and recombination of charge carriers in the devices.45

The morphology and thickness can be processed with care to

reduce the pinholes and recombination in the devices so that the

Rsh can be increased. The Rsh needs to be maximized to reduce

the power loss caused by the current that bypasses the solar cell

junction and load through an alternate current path from the low

Rsh. A small Rsh lowers the current flowing through the diode

(junction) and thus reduces the Voc. A simplified way toapproximately calculate the Rsh is tomeasure the slope of the J-V

curves in the regime close to the Jsc.41

The above discussed n, J0, Rs and Rsh can strongly affect the

photovoltaic parameters including Voc, FF, Jsc and cell efficiency

(h). Voc is defined as the voltage across the cell under illumina-

tion with a zero current at which the dark current and short

circuit photocurrent was exactly cancelled out.37 By solving the

current density verse voltage equation (eq 3) at J ¼ 0 and V ¼

Voc, Voc can be derived as:46

V oc ¼nk B T

qln

J ph

J 0þ 1 À

V oc

J 0Rsh

(5)

Fig. 4 shows the dependence of Voc on n and J0, calculatedusing eq 6 that is derived from eq. 5 by assuming an infinitely

large Rsh. In Fig. 4, we assume Jph ¼ 9 Â 10À3 A cmÀ2, which is

close to the experimental Jsc in a polymer/CdSe tetrapod hybrid

solar cell reported by Dayal et al.22 It can be seen that the

decrease of J0 and increase of n can significantly increase the Voc.

V oc ¼nk B T

qln

J ph

J 0þ 1

(6)

When the Rsh is not large enough, it can also affect the V oc of

polymer–inorganic hybrid solar cells. Fig. 5 shows the depen-

dence of Voc on the Rsh plotted using eq 5. In this calculation, we

assume n ¼ 1.4, J0 ¼ 1 Â 10À10 A cmÀ2, Jph ¼ 9 Â 10À3 A cmÀ2.

The FF is greatly affected by the Rs and Rsh. The relationship

has been reported previously.46 High FF can be achieved with

low Rs and high Rsh (ideally Rs ¼ 0, Rsh ¼N). Therefore, the Rs

needs to be minimized and the Rsh should be maximized to

ensure a high FF. The dependence of FF on Rs and Rsh can be

approximated as46

Fig. 3 Typical semi-logarithmic current density-voltage (J-V) curves

showing three distinctive regions.

Fig. 4 Dependence of open circuit voltage (Voc) on ideality factor (n)

and reverse saturation current density (J0).


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FF ðRs; RshÞzFF ð0;NÞ

1 À

J scRs

V oc

À

V oc

J scRsh

(7)

Fig. 6 shows the dependence of FF on Rs and Rsh, calculated

using eq 7. At the above mentioned assumption of n ¼ 1.4, J0 ¼ 1

Â 10À10 A cmÀ2, and Jsc ¼ 9 Â 10À3 A cmÀ2, the FF (0,N) was

obtained to be 0.796 from the simulated J-V curves with Rs ¼0 and Rsh ¼N at room temperature. Also, we assume Voc ¼ 0.65

V in Fig. 6. These assumptions are reasonable because the Jsc and

Voc are close to those in a recently published polymer/CdSe

hybrid solar cell.22 In addition, Novotny et al. found that n ¼

1.37 and J0 ¼ 1 Â 10À10 A cmÀ2 in a polymer/InP nanowire hybrid

photodiode.42 As seen in Fig. 6, the FF can be significantly

increased as the Rs decreases and Rsh increases.

2.3. Efficiency prediction of polymer–inorganic hybrid solar

cells

The Jsc and Voc are two basic factors to calculate solar cell effi-

ciency. The physical processes governing these two parameters

need to be well studied to design new materials and device

configurations for achieving higher conversion efficiency.

The Jsc is the photocurrent density generated by a photovoltaic

device under illumination at short circuit condition. It depends

on the incident light intensity and absorption spectrum of active

materials, which can be described as:35

J sc ¼

ð NE g

e,N phðE Þ,EQE ðE ÞdE (8)

Where Nph is the spectral photon flux of incident light, Eg is the

bandgap of active materials and E is the photon energy.

Extensive work has been reported on the Voc of polymer-

fullerene solar cells. Early reports considered that the Voc was

determined by work function difference of the two electrodesoriginally based on metal-insulator-metal devices.47,48 Further

significant studies found that Voc was linearly correlated to the

DA effective bandgap (Eg, DA ¼ HOMOD-LUMOA).33,49–51

Scharber et al. reported Voc ¼ Eg,DA/e À 0.3V for multiple

polymers with different HOMO energy levels, in which 0.3 V is

an empirical value describing the difference between the

maximum built-in potential (Vbi) and the Voc.52 More recently,

Vandewal et al. studied the origin of relationship between the Voc

and Eg, DA.51,53,54 The Eg, DA was also regarded as a polaron pair

energy or a charge transfer state energy. They found that the

Eg, DA (a theoretical maximum of Voc), is proportionally related

to the experimental Voc but several 100 meVs larger than the Voc

(e.g., Voc ¼ Eg,DA/e À 0.43V). The DA properties, morphologies,and polymer ordering structure in the film can affect E g, DA.55

Vandewal et al. observed that the energy of the charge transfer

state (ECT) is linearly related to the crystalline nanofiber mass

fraction f as E CT ¼ E CT 0 – 0.2f , and the V oc follows the same

trend as E CT .55 Also, ECT was found to increase slightly as side

chain gets longer. The discrepancy between the Eg, DA and Voc is

possibly assigned to radiative and nonradiative recombination,

energetic disorders, band bending due to carrier diffusion, and

energy required to separate polaron pairs.51,53,56 To maximize the

Voc, this discrepancy between the Eg, DA and Voc needs to be

minimized.

The Voc in polymer–inorganic hybrid solar cells was also

found to depend on the difference between the polymer HOMOand the inorganic acceptor conduction band (CB).57,58 In addi-

tion, the Eg and CB of inorganic semiconductors differs with

nanocrystal diameters and lengths, which can also lead to vari-

ations in Voc.59 The Voc of polymer–inorganic hybrid solar cells

can be increased by either moving the polymer HOMO farther

away from vacuum level or pushing the inorganic acceptor

conduction band closer to vacuum level, but still having an

energy offset between the polymer LUMO and acceptor CB

larger than exciton binding energy (Eb).

Low bandgap polymers can be used to increase light absorp-

tion for higher photocurrent by broadening the absorption

spectrum, however, the decrease of conjugated polymer bandg-

aps will also affect the Voc. In a bulk-heterojunction solar cell, thetheoretical maximum Voc can be described as

V oc,max ¼ E g ,DA ¼ E g,D À (LUMOD À LUMOA) (9)

If the Eb is comparable in both the low and high bandgap

polymers, then the energy offset (LUMOD - LUMOA) needed to

dissociate excitons is also comparable. Thus, according to eq 9,

a lower bandgap (Eg) will lead to a reduced theoretical maximum

of Voc. In order to achieve significantly higher energy conversion

efficiency, a balance between donor bandgap (mainly deter-

mining Jsc) and donor LUMO (mainly determining Voc) needs to

Fig. 5 Dependence of open circuit voltage (Voc) on shunt resistance

(Rsh).

Fig. 6 Dependence of fill factor (FF) on series resistance (Rs) and shunt

resistance (Rsh).


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be achieved if a specific acceptor material is used with its LUMO

or conduction band fixed.

Fig. 7 illustrates the 3D contour plots of polymer LUMO,

polymer bandgap, and cell efficiency with three typical inorganic

acceptors of (a) TiO2; (b) ZnO; and (c) CdSe. They have different

conduction bands (CB): CB(TiO2) ¼ $ 4.2 eV, CB(ZnO) ¼ $ 4.4

eV, and CB(CdSe) ¼ $ 3.7 eV. To reach the highest achieveable

cell efficiencies in polymer–inorganic semiconductor hybrid solar

cells, the polymer bandgap and LUMO must be optimized with

a balance between current and voltage output. As depicted in

Fig. 7, by varying the LUMO level and bandgap of donor

polymer, the highest conversion efficiency can be anticipated. In

this calculation, we did not consider the current contributed byphotogeneration of inorganic acceptors. Although some accep-

tors (e.g., CdSe) may absorb light at certain wavelengths, it is

generally true that the majority of light absorption takes place in

the donor polymer. For example, CdSe has a bandgap of 2.1 eV

and photon absorption capability of up to 650 nm.60 However,

Dayal et al.22 report that in a polymer/CdSe hybrid film con-

taining about 90% weight of CdSe nanoparticles (nano-tetra-

pods), the contribution of light absorption from CdSe is only

34%, as opposed to 66% by PCPDTBT (poly[2,6-(4,4-bis-(2-

ethylhexyl)-4H -cyclopenta [2,1-b;3,4-b0]dithiophene)-alt-4,7-

(2,1,3-benzothiadiazole)]). In addition, light absorption in TiO2

and ZnO is only in the UV range less than 400 nm, while the

photon flux in the UV region is less pronounced compared tothose in the visible and near infrared range. Therefore, it is

reasonable to simplify the calculation by assuming that light

absorption only occurs in the polymers. The calculation also uses

the theoretical maximum Voc that is equal to the energy differ-

ence between the donor HOMO and the acceptor LUMO, which

has been reported by others.57,58 In order to overcome the exciton

binding energy (Eb), the polymer LUMO must be at least Eb

higher than the acceptor CB. By selectively designing the poly-

mers with an optimal Eg and LUMO, cell efficiencies beyond

10% are possible, as shown in Fig. 7. Regardless of the acceptor

materials, the donor polymers should have an Eg at $1.5–1.6 eV

in a single junction structure to achieve efficiencies higher than

10%. However, the polymer LUMOs differ according to thecorresponding acceptor CBs. When CB(TiO2) ¼ $ 4.2 eV,

CB(ZnO) ¼ $ 4.4 eV, and CB(CdSe) ¼ $ 3.7 eV, the polymer

LUMO energy levels need to be $ 3.8 eV, $ 4.0 eV and $ 3.3 eV

for the respective TiO2, ZnO and CdSe based hybrid solar cells in

order to obtain cell efficiency above 10%.

3. Polymer–inorganic semiconductor hybrid solar

cell materials and architectures

In polymer hybrid solar cells, a conjugated polymer is used as

a donor to harvest sunlight, generate excitons and transport holes.

With appropriate energy offsets, excitons are dissociated at thepolymer–inorganic interface. A general design rule for donor

polymers in a single junction hyrbid solar cell is to reduce its

bandgap to an optimal $1.5–1.6 eV and keep its LUMO level

above the inorganic acceptor CB by an amount of E b. Then, the

polymer absorbs a wide spectrum of light without sacrificing the

Voc. Fig. 8 is an energy band diagram of conjugated polymers and

nanoscale inorganicsemiconductors. Thisfigure givesguidanceto

choose the appropriate polymer and inorganic acceptor material

pairs with proper energy offsets. It is important to mention that

the energy levels shown in Fig. 8 are from isolated donor and

acceptor materials, but when they are in contact and form

Fig. 7 3D contour plots of polymer LUMO, polymer bandgap (Eg) and

cell efficiency (h) in a single junction solar cell structure with three

representative inorganic semiconductor acceptors of (a) TiO2; (b) ZnO;

and (c) CdSe. The conversion efficiencies of solar cells were calculated by

assuming IPCE ¼ 65%, FF¼ 60% under AM 1.5 with an incident light

intensity of 100 mW cmÀ2.


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junctions these levels may shift due to interface dipoles and other

effects. For simplicity, these effects are not considered here.

3.1 Conjugated polymers as donor

Various conjugated polymers have been used in hybrid solar

cells. Polythiophene (PT) and its derivatives, and poly(p-phe-

nylene vinylene) (PPV) and its derivatives are two major types of

polymers used in polymer hybrid solar cells. Water soluble

polymers have also been reported.3,61,62 These polymers typically

only absorb light up to 650 nm, showing a limited light har-

vesting capability. In addition, their HOMO/LUMO levels arenot optimal for high efficiencies. The key optical and electronic

properties of donor polymers are their LUMO/HOMO levels,

bandgaps, and carrier mobility. Further substantial increase in

device efficiency will partly depend on optimized HOMO/

LUMO levels, increased carrier mobility, and significant

broadening of polymer absorption spectra to harness low energy

photons.

The LUMO/HOMO energy levels need to be optimized to

achieve an efficient charge transfer and a high EDA, leading to

a high Voc. Specifically, the donor polymer LUMO levels need

to be close to acceptor LUMO or CB edge to minimize energy

loss during electron transfer from donor to acceptor, but still

higher by about Eb to provide enough energy offset for excitondissociation. The donor polymer HOMO should be as low as

possible to maximize the Voc.

In addition, the polymers need to have a high degree of

planarity and be able to self-assemble into an organized structure

with enhanced packing and charge transport via treatment such

as thermal and solvent annealing. The donor polymer should

also properly phase separate from the acceptor, and the DA

interfaces need to be finely managed within the reach of exciton

diffusion length to increase exciton dissociation. The polymer

also needs to create a donor phase network with efficient hole

transport. The donor polymer hole mobility should be very

high, for example, P3HT has a hole mobility (mh) up to 0.2 cm2

VÀ1 sÀ1.63

The donor polymer bandgaps should be close to the optimal

$1.5–1.6 eV for a single junction solar cell. It is important to

mention that the optimal bandgaps of the subcells in polymer

multijunction solar cells are different from this value, as reported

previously.25 Recently, low bandgap polymers have been repor-

ted for broad spectral light harvesting.65–68 The factors affecting

polymer bandgaps include effective conjugation length, effects of

substituent groups, intermolecular (inter-chain) interaction, and

bond alternation.69 One typical strategy to make low bandgap

polymers is to use donor–acceptor (D–A) or donor–acceptor-

donor (D–A-D) alternating copolymers because the hybridiza-tion of the donor and acceptor moiety energy levels can cause the

donor HOMO and acceptor LUMO to induce bandgap

compression in the hybrid molecule.70,71 Ultralow bandgap

polymers including poly[2,6-(4,4-bis-(2-ethylhexyl)-4H -cyclo-

penta[2,1-b;3,4-b0]dithio phene-alt-4,7-(2,1,3-benzothiadiazole)]

(PCPDTBT) (Eg z 1.4 eV),68 alternating bithiophene and thia-

diazoloquinoxaline polymer (PBTTQ) (Eg z 0.94 eV)72

and poly(5,7-bis(4-decanyl-2-thienyl)thieno[3,4-]diathiazole-

thiophene-2,5) (PDDTT) (Eg z 0.8–1.12 eV)73,74 have also been

developed and used for polymer-fullerene solar cells. However,

cell efficiencies of these low bandgaps have been quite low. For

example, in the PBTTQ-fullerene solar cells, both the Jsc and Voc

were very low at 0.31 mA cmÀ2 and 0.37 V, respectively, leading

to a cell efficiency of $0.08%. The authors attributed the low

performance to two possible reasons: a low hole mobility (mh ¼

1.7 Â 10À6 cm2 VÀ1 sÀ1) and a low LUMO offset ($0.33 eV) for

a possible insufficient free carrier generation.72 This indicates

that the design of ultralow bandgaps needs to be combined with

high carrier mobility and optimized energy levels. In addition,

the optical interference caused by the active layer film thickness

and long wavelength photon absorption by ITO and

PEDOT:PSS caused some new challenges in developing ultralow

bandgap polymer solar cells.72 When reducing the bandgaps of

donor polymers, care must be taken to choose proper energy

Fig. 8 A non-exclusive list of conjugated polymers and nanoscale acceptor materials with the energy level matching in solar cells application.

Reproduced with permission from ref. 64.


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levels of the HOMO and LUMO, which are related to the hybrid

solar cell Voc, as discussed above. Compared to P3HT, the

LUMO energy levels of low bandgap polymers should be

reduced. Fortunately, a poly(2,7-carbazole)-derivative-based

polymer PCDTBT was successfully designed to possess both

a low bandgap (Eg ¼ 1.7 eV) for improved Jsc and a lower

HOMO level for a higher Voc.75 In a PCDTBT-PC70BM solar cell

structure, PCDTBT has achieved a cell efficiency of 6.1% with

a Jsc of 10.6 mAcmÀ

2, a Voc of 0.88 V, and a FF of 0.66. Morerecently, a new class of polymers based on thieno[3,4-b]-thio-

phene and benzodithiophene alternating units have exhibited low

bandgap (Eg z 1.7–1.8 eV) and efficient light absorption up to

750 nm, which have achieved a high cell efficiency at $ 8% so

far.76,77 The reports show that these polymers have exhibited

some advantages:76,77 (1) the incorporation of fluorine into the

thieno[3,4-b]-thiophene results in a lower HOMO level and

increased Voc; (2) the polymer backbone quinoidal structure can

be stabilized and the planarity is improved by the thieno[3,4-b]-

thiophene moiety, leading to a high hole mobility; (3) the poly-

mer forms an effective morphology with fullerene derivatives for

both efficient exciton dissociation and high charge transport,

leading to a high Jsc and FF.Recently, Dayal et al. mixed tetrapod CdSe with a low

bandgap polymer, poly[2,6-(4,4-bis-(2-ethylhexyl)-4H -

cyclopenta[2,1-b;3,4-b0]dithiophene)-alt-4,7-(2,1,3-benzo-thia-

diazole)] (PCPDTBT) and achieved a cell efficiency of $3.13%

under AM 1.5 illumination. This low bandgap polymer can

absorb up to 850 nm, indicating low bandgap polymers are

very promising for further increase in polymer–inorganic

hybrid solar cell efficiency.22 Table 1 lists chemical structures,

LUMO/HOMO energy levels and bandgaps of some polymers

used in organic or organic-inorganic solar cells.

3.2 Inorganic nanoscale semiconductors as acceptors

Inorganic nanoscale semiconductors like CdSe, TiO2, and ZnO

with different kinds of nanostructures were used as acceptors and

electron transport materials in hybrid solar cells. CdSe with

various sizes (e.g., diameters and lengths) and different shapes

such as nanoparticles, nanorods, and tetrapods can be synthe-

sized and prepared by controlling the growth conditions.84–86

TiO2 nanoparticles, nanorods87–89 and nanotubes,90,91 and ZnO

nanorods92,93 and nanowires13,94,95 can also be obtained by

chemistry synthesis methods. For heterojunction solar cell

applications, inorganic nanoscale semiconductors as electron

acceptors need to have a higher electron affinity than the donor

polymers. In other words, the CB of acceptor inorganic semi-

conductors is required to lie well below the related LUMO of various donor polymers, making them energetically favorable for

exciton dissociation and charge transfer at the interfaces. In

addition, high electron accepting ability and electron mobility

are also required for applications in solar cells. Table 2 lists the

commonly used inorganic semiconductors with their conduction

band (CB), valence band (VB), and bandgap (Eg), respectively.

3.3 Polymer–inorganic hybrid solar cell architectures

The polymer–inorganic hybrid solar cells can be classified as: (1)

bilayer heterojunction; (2) bulk heterojunction; (3) ordered

heterojunction. The schematics of these architectures are shown

in Fig. 9. The bilayer heterojunction devices can be fabricated by

depositing a donor polymer layer and an inorganic semi-

conductor acceptor layer separately. In order to sufficiently

absorb light, the donor layer thickness needs to be close to

polymer absorption depth at $ 100 nm.114 However, due to the

short exciton diffusion length of $4–20 nm, only a minority of

photogenerated excitons can reach DA interface for successful

dissociation into free charge carriers. The challenge of a bilayersolar cell is that the interfacial DA area is limited by the surface

of the distinct polymer–inorganic interface.

Similar to polymer–fullerene solar cells, polymer–inorganic

bulk heterojunction (BHJ) solar cells can overcome the limita-

tions of low DA interfacial area and inefficient exciton dissoci-

ation. The inorganic nanocrysals can also be processed from

organic solvents such as chloroform, toluene, and chloroben-

zene, which offers a possibility to blend them with polymers in

solution. This type of devices can be made by simply blending the

two components or in situ synthesis of one counterpart into the

other. For example, TiO2 can be hydrolyzed in the polymer

solution to form an interpenetrating network in a polymer

matrix, or the polymers can be in situ polymerized in inorganicsemiconductor nanopores.

Ordered heterojunction solar cells are generally considered as

an ideal configuration for solar cells, because they have direct

charge transport pathways and managed heterojunctions. Inor-

ganic semiconductors can be grown as vertically aligned nano-

tubes, nanorods and nanowires onto a substrate. Polymers can

then be physically infiltrated into the nanopores or in situ

synthesized by UV-light assisted polymerization,115 chemical

oxidative method116 or electrochemical polymerization

method.117

4. Approaches to improve cell performanceIn addition to polymer bandgaps, LUMO/HOMO energy levels

and acceptor CBs, device engineering and film morphology are

also important parameters needed to be optimized. Various

approaches have been studied and these include active layer

thickness, morphology control in active layer, inorganic nano-

crystal geometry, surface modification at polymer–inorganic

interface, depositing polymers into nanostructural pores, in situ

synthesis of inorganic nanocrystals in polymer matrix. These

aspects are presented and a comprehensive understanding of the

above perspectives can help to improve cell performance.

4.1 Optimization of active layer thickness

Conjugated polymers typically have a high absorption coefficient

above 105 cmÀ1, and therefore a thin layer of active material with

a thickness of 100–300 nm is enough for sufficient light har-

vesting.114 Within the absorption lengths, a thicker active layer

can help to absorb more light, but it may be inefficient for charge

transport and collection. If enough acceptor material is incor-

porated, and the DA interfaces are optimized to ensure the

photoexcitation sites to be within the exciton diffusion length to

the interfaces in a bulk-heterojunction device structure, then the

exciton dissociation might be efficient, but the charge transport

and collection will be hampered in a thick active layer. The


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Table 1 Chemical structures, LUMO/HOMO levels and bandgaps of some polymers used in organic or hybrid solar cells

Polymer structure HOMO (eV) LUMO (eV) Eg (eV) Ref.

À5.2 À3.2 2.0 78

À5.3 À2.9 2.4 79

À5.3 À3.0 2.3 80

À5.5 À3.1 2.4 81

À5.5 À3.6 1.9 60

À4.9 À3.5 1.4 82

À5.15 À3.35 1.8 52,83

À5.5 À3.6 1.9 75

À4.7 À3.75 0.94 72

À4.71 À3.59 1.12 73


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solution processable polymer–inorganic acceptor film is mainly

disordered without long range order. Thus the charge transport

(hole via donor and electron via acceptor) mainly occurs via

hopping from one localized state to the next, which is different

from the band transport in inorganic semiconductors.114 In

addition, when the active layer thickness becomes larger, the

built-in electric field is reduced at a given built-in voltage, but

a longer hopping path needs to be overcome to extract electrons

and holes to their corresponding electrodes. Then the chargetransport/collection efficiency is reduced. In a non-inverted solar

cell with a thick active layer, photons that are absorbed in the

region close to the transparent hole collecting electrode have less

contribution to the current because the photogenerated electrons

have more difficulty reaching the electron collecting electrode.

The same case applies for the holes that are photogenerated by

photons absorbed close to the electron collecting electrode.59

Therefore, the thickness of active layer is important to determine

polymer/inorganic hybrid solar cell performance. In a recent

PCPDTBT/CdSe hybrid solar cell, the active layer thickness is

$100–120 nm.22 In addition, in a polymer/fullerene solar cell,

internal quantum efficiency (IQE) was found to decrease when

the active layer thickness was higher than 150 nm.118 In polymer–

inorganic hybrid solar cells, Huynh et al. studied the thickness

dependence of EQE and Rs in a hybrid CdSe/P3HT solar cell

with 90% by weight CdSe in the composite (Fig. 10).59 They

found that the EQE at 515 nm wavelength reached maximum at

a film thickness of 212 nm and then reduced significantly at

a higher thickness. They also correlated the EQE with Rs, andfound Rs also increased substantially at a thickness larger than

200 nm. In 2009, Oosterhout et al. reported the thickness effects

on cell efficiency of a P3HT/ZnO hybrid solar cell.119 They varied

the active layer thickness by changing spin speed during the film

fabrication processing. They found that device performance was

improved with increment of film thickness, which caused an

increase in current density. They found that both Jsc and IQE

reached the maximum at a film thickness of $150 nm. They then

assigned lower efficiency in the thinner active layer devices to low

ZnO content, coarse phase separation, and exciton losses

quenched by electrodes.119

Table 1 (Contd. )

Polymer structure HOMO (eV) LUMO (eV) Eg (eV) Ref.

À5.15 À3.31 1.84 77

Table 2 Energy levels and bandgaps of some inorganic semiconductors

Name CB (eV) VB (eV) Eg (eV) Ref

CdSe À3.7 À5.8 2.1 96TiO2 À4.2 À7.4 3.2 97ZnO À4.4 À7.9 3.5 98PbSe À4.2 À5.0 0.8 99PbS À3.3 À3.71 0.4 100,101CdTe À3.9 À6.3 2.4 102

CdS À4.5 À6.92 2.42 103–106Si À4.0 À5.12 1.12 107–109SnO2 À4.3 À7.9 3.6 110,111CuInS2 À3.7$À4.1 À6.0$À5.6 1.5 112CuInSe2 À4.6 À5.6 1.0 113

Fig. 9 Three different configurations of polymer–inorganic hybrid solar

cells: (a) bilayer heterojunctions; (b) bulk heterojunctions and (c) ordered

heterojunctions.

Fig. 10 The relationship (a) between the EQE and the thickness of

P3HT/CdSe active layer; (b) between normalized EQE, series resistance

(Rs) and the thickness of P3HT/CdSe active layer. Reproduced with

permission from ref. 59.


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4.2 Morphology control in active layer

The aim of morphology control in bulk heterojunction solar cells

is to ensure not only a large DA interfacial area for efficient

exciton dissociation and charge transfer, but also an inter-

penetrating bicontinuous percolating pathway for effective

charge transport to their corresponding electrodes. If domain

size of donor polymer is within its exciton diffusion length, any

photoexcitation in the composite is capable to reach an interfacefor exciton dissociation and charge transfer. Also in order to

produce a useful current, these separated carriers must transport

via an unhampered pathway to the electrodes before recombin-

ing. Otherwise, the electrons and holes may be stuck or must pass

through each other phase before reaching the electrodes. Then

the likelihood of bimolecular recombination is greatly increased.

Therefore, provided phase separation is sufficient, it is of great

importance to form bicontinuous paths from the interfaces to the

respective electrodes. The nanoscale morphology of an active

layer mainly depends on film preparation and treatments, such as

selection of solvents, use of additives, mixture of multiple

solvents, control of solvent evaporation rate and drying time,

control of spin speed, thermal annealing, solvent annealing,acceptor nanocrystal shapes (e.g., spherical, rods, and hyper-

branched, etc), and donor polymer molecular weights.48,120–124

Sun et al. used different solvents with different boiling points

including chloroform, thiopehene, and 1,2,4-trichlorobenzene

(TCB) to study evaporation rate and drying time effects on

device performance.125 Among these three solvents, TCB has the

highest boiling point (219 C) compared to those of chloroform

(61 C) and thiophene (84 C). The higher boiling point led to

a longer drying time during film preparation and helped to form

a large-scale self-assembled P3HT nanofibers. The P3HT fibrils

with a length of several mm or longer were found in the films spin-

coated from TCB-based solutions, but were not seen in the

chloroform- or thiophene-based solutions. The P3HT nanofibersshowed a high hole mobility along their length and improved

hole transport in the devices, leading to an increased cell effi-

ciency at 2.9% for TCB-based cells compared to those from

chloroform- (1.8%) or thiophene- (2.4%) based solutions.125

Thermal annealing at or above polymer glass transition

temperature has been very successful to increase light absorption

and carrier mobility in polymer-fullerene solar cells due to an

improved inter-chain packing and enhanced degree of P3HT

crystallinity.126 Beek et al. found that thermal annealing treat-

ment increased Jsc, FF and Voc in P3HT/ZnO hybrid solar cells

with a lower ZnO concentration.127 The improved Jsc was

attributed to increased hole mobility and enhanced P3HT

packing. The highest cell efficiency (0.92%) was achieved witha ZnO concentration of 26% by volume. For ZnO volume above

26%, the peak-to-peak roughness was significantly increased,

leading to a higher possibility of shunting, evidenced by the

decrease of Voc.127 Boucle et al. applied post-annealing treatment

on P3HT/nc-TiO2 hybrid solar cells, but only observed an

improved local P3HT/nc-TiO2 interface for increased charge

transfer yield.11 Here the nc-TiO2 was capped with ligand such as

trioctylphosphine oxide (TOPO). The annealing might cause

some reformation of the local blended P3HT/nc-TiO2 film,

leading to a small ligand displacement from nc-TiO2 and

improved contact between donor and acceptor, thereby leading

to a charge transfer efficiency six times higher than the unan-

nealed one. However, the final hybrid solar cells did not achieve

a high photocurrent due to poor charge transport via hopping

between the TOPO capped nc-TiO2. The TOPO surfactant

molecules might prevent the electron hopping.

The donor polymer molecular weight (MW) was also found to

affect the morphology and efficiency of polymer–inorganic

hybrid solar cells. Wu et al. reported that high MW P3HT

showed a larger donor domain with continuous absorptionmapping regions, assigned to more efficient p – p stacking and

electronic delocalization.128 However, the low MW P3HT films

exhibited a lot of grain boundaries and a much less pronounced

continuous absorption mapping region. The hole mobility for

high MW P3HT/TiO2 nanorod composite film was measured at

5.0 Â 10À3 cm2 VÀ1 sÀ1, which was higher than that (7.6 Â 10À4

cm2 VÀ1 sÀ1) of low MW P3HT based composite films. The

resultant hybrid cell efficiency (0.98%) for high MW P3HT was

also higher than 0.2% for low MW P3HT cells.128

The visual study of polymer–inorganic hybrid 3D morphology

has been very challenging. Oosterhout et al. studied the nano-

scale P3HT/ZnO bulk heterojunction 3D morphologies using

electron tomography.119 The ZnO network in the P3HT matrixwas formed from a precursor of diethylzinc undergoing

a hydrolysis processing to form Zn(OH)2, followed by conden-

sation reaction and annealing at 100 C. The percolation

pathway and the spherical contact distance was then analyzed

from the 3D morphology. Fig. 11 shows the rebuilt volumes of

P3HT/ZnO active layer (a–c) and calculated exciton quenching

efficiency (e–f) at different thicknesses of 57 nm, 100 nm and 167

nm. The rebuilt volumes of P3HT and ZnO were drawn via

electron tomography and the exciton quenching efficiency was

obtained from the diffusion equation by applying the cyclic

boundary conditions. Fig. 11 shows that the thicker films have

a finer phase-separated domain for both P3HT and ZnO, and

a much higher efficiency of exciton quenching. It was also foundthat the exciton quenching at the electrode, which does not

contribute to the current generation, was significantly reduced in

a thicker film partly due to a more efficient phase separation

morphology and a higher thickness. The IQE was observed to

increase from20% for 50 nmthickactive layer to 50% for 150 nm

thick film, suggesting a substantial improvement in both photon

absorption and charge carrier generation in a thicker film. The

improved cell performance was attributed to a more favorable

donor–acceptor phase separation caused by increased ZnO

volume content. This P3HT/ZnO hybrid solar cell achieved an

efficiency of $ 2% and the authors expected that a higher effi-

ciency could be reached, provided that a more efficient

morphology for significantly increased charge collection could beobtained for a higher IQE than the current 50%.

4.3 Engineering of inorganic nanocrystal geometry

The geometry of inorganic semiconductors has a strong effect on

solar cell performance.36 The inorganic nanocrystals can be

synthesized with various geometries including spherical nano-

particles, nanorods, and tetrapods, etc. In addition, the bandgap

can be tuned by changing nanocrystal sizes, i.e. the bandgap can

be increased by decreasing nanocrystal diameters. The


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absorption coefficient of nanosize materials can be enhanced by

quantum confinement compared to their bulk counterparts.

The effect of CdSe geometry on the performance of polymer

hybrid solar cells has been extensively studied. In 1996, Green-

ham et al. reported the first CdSe/MEH-PPV solar cells. They

blended 5 nm diameter CdSe nanocrystals with MEH-PPV as anactive layer, and achieved a power conversion efficiency of

$0.25% under a monochromator illumination at 514 nm.129 The

low efficiency was assigned to poor electron transport in the

nanocrystal networks. In order to improve electron transport,

elongated nanocrystals were made to form an improved direct

charge transport pathway. In 1999, Huynh et al. used longer

CdSe nanorods with dimensions of 8 Â 13 nm and 4 Â 7 nm to

mix with P3HT, respectively.130 They reported quantum effi-

ciency was increased by four times with the increase in crystal size

from 4 Â 7 nm to 8 Â 13 nm. The increased efficiency might be

caused by improved directed electron transport chains from

elongated nanocrystals. The solar cells made by the 8 Â 13 nm

sized CdSe obtained a Jsc of 0.031 mAcmÀ2, a Voc of 0.57 V, and

a h of 2% under 514 nm monochromator illumination. This

efficiency was one order of magnitude increase in h compared

with previous 5 nm diameter nanocrystals CdSe/MEH-PPV

devices reported earlier.129 The improvement in FF was attrib-

uted to P3HT with a more efficient hole transport than MEH-

PPV. Then in 2002, the authors increased CdSe nanorod size to 7

Â 60 nm with P3HT and achieved a h of 1.7% under AM1.5G

illumination.131 The authors varied nanorod lengths from 7 nm,

30 nmto 60 nmat a fixeddiameter of 7 nm, and observed that the

EQE strongly depended on the nanorod lengths, as shown in

Fig. 12. The 7 Â 60 nm CdSe nanorods achieved the highest

EQE, which was attributed to an improved electron transport in

the elongated nanorods. Electron transport is dominated by

a hopping process in the short nanocrystals, while band

conduction is prevalent in the elogated nanorods.131 The increase

of aspect ratio in nanorods enhanced electron transport and thus

led to a high EQE. The authors also studied solar cell perfor-mance dependence on diameter; they varied the diameter from

3 nm to 7 nm at a fixed 60 nm length. It was found that the Voc

and FF using these two diameters were comparable.131

After that, extensive work has been reported using various

geometries from nanorods, tetrapods to hyberbranced nano-

crystals (see Table 3).14,22,36,60,125,132,133 Gur et al. reported

Fig. 11 Reconstructed volumes of P3HT/ZnO active layer measured by electron tomography (image size: 700 nm Â 700 nm) at different thicknesses of

(a) 57 nm; (b) 100 nm, and (c) 167 nm, respectively. ZnO presents the yellow and P3HT appears transparent. The exciton quenching efficiency in the

horizontal cross-section of bulk materials of differentthicknesses: (a) 57 nm; (b) 100 nm, and(c) 167 nm, respectively. Reproduced with permission from

ref. 119.

Fig. 12 EQE spectra of P3HT/CdSe nanorod solar cells with varying

lengths at 7, 30, and 60 nm, respectively. Reproduced with permission

from ref. 131.


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surface in a hybrid bulk heterojunction solar cell.11 Both Jsc andVoc was improved in the Z907-capped TiO2 nanorod hybrid solar

cells, leading to an increase of cell efficiency by about three times

from 0.02% for TOPO-capped cells to 0.07% for Z907-capped

cells. The stronger transient absorption spectroscopy (TAS)

signals at 1 ms (Fig. 15) in Z907-capped samples compared to that

in TOPO-capped devices suggests that charge separation in the

former was significantly improved. However, the overall effi-

ciency was low and this was mainly limited by poor charge

transport and charge trapping in the TiO2 nanorods.11

In 2009, Lin et al. reported the interface modification results in

P3HT/TiO2 nanorods solar cells.139,140 They reported that device

performance could be improved by removing the insulating

ligand oleic acid (OA), and replacing it with pyridine (PYR),

anthracene 9-carboxylic acid (ACA), CuPc, and N3 dye. The

surface modifier ligands enhanced charge separation and

hindered back recombination. The transient open circuit voltage

decay (TOCVD) measurements showed that the recombination

Table 4 Surface modification molecules

Polymer–inorganic interfaceengineering molecules Name Ref

PYR 139

ACA 139

CuPc 140

Z907 11,12,58

N3 140

N719 58

Fig. 14 Polymer/TiO2 band diagram showing the shift of TiO2 CB edge

compared to polymer HOMO: (a) the dipole direction is towards TiO 2,

(b) no surface modification, and (c) the dipole direction is towards the

polymer. Reproduced with permission from ref. 58.

Fig. 15 Transient absorption spectroscopy (TAS) signals of TOPO-

capped and Z907-capped cells. The excitation is at 520 nm. Reproduced

with permission from ref. 11.

Fig. 16 The recombination rate (krec) vs. light intensity at open circuit

voltage (Voc) measured by transient open circuit voltage decay

(TOCVD). The inset is a diagram showing the recombination mecha-

nism: the transient pulse generated charges decays at open circuit

condition. Reproduced with permission from ref. 140.


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rate (krec) was with the order of PYR > ACA > CuPc > N3

(Fig. 16). The resultant hybrid P3HT/TiO2 nanorod solar cells

using N3 surface modifier achieved an efficiency of 2.2% with

a Jsc of 4.33 mAcmÀ2, a Voc of 0.78 V, and a FF of 0.65 under the

AM 1.5 illumination.140

In addition, various other approaches have also been reported

to improve the interface and electronic interaction between the

polymers and inorganic nanocrystals. Milliron et al. enhanced

electronic interaction in the phosphonic acid (PO3) functional-ized oligothiophene and CdSe nanocrystals.141 Beek et al.

reported an efficient photoinduced charge transfer in hetero-

supramolecular assemblies consisting of carboxylic acid func-

tionalized TiO2 nanoparticles and conjugated oligomers.142,143

Goodman et al. reported their work by tethering P3HT directly

to CdSe quantum dot (QD) surface.10

4.5 Depositing polymers into ex-situ prepared inorganic

nanopores

Physically blending the polymers with inorganic nanocrystals

and then depositing onto a substrate is a simple processing

method to make an active layer, which also helps to form anintimate mixture of donor and acceptor.20 However, the nano-

crystals tend to aggregrate,26 which may prevent the blend from

forming an effective interface for efficient charge transfer. Also in

some cases, the nanocrystals need to be capped with a ligand to

prevent aggregation, which can block both charge transfer from

polymers to inorganic nanocrystals and charge transport via the

latter, leading to low solar cell performance.11 In addition, the

donor polymers and acceptor nanocrystals are randomly

dispersed in the physically blended films, which may lead to non-

continuous pathways for charge transport. To solve these issues,

an acceptor nanostructured template can be made first and then

infiltrate the polymers into the nanostructured pores. By

controlling the pore size to be within twice that of the excitondiffusion length, all photo-generated excitons have a high

possibility to diffuse to a nearby interface for dissociation. Also,

the polymers and inorganic nanostructures can provide a direct

continuous pathway for both hole and electron transport,

reducing carrier transport time and suppress back charge trans-

fer probability.15 Three methods including direct infiltration of

polymers, direct penetration of inorganic nanostructures into

wet polymer film, and in situ polymerization of donor polymers

in inorganic nanostructures have been used to make the hyrbid

films.

(a) Direct infiltration of polymers into inorganic nano-

structural pores. Due to loss of conformational entropy in a porewith a size less than the polymer radius of gyration, it is chal-

lenging to infiltrate a polymer into the nanopores.144 The poly-

mer penetration into the nanopores can be affected by: pore size

and depth, polymer molecular weight, solvent, and drying

conditions.21 In 2003, Coakley et al. used a mixture solution of

titania sol–gel precursor and a structure-directing block copoly-

mer agent to form a porous TiO2 film with a uniform pore size,

depth, and film thickness.144 The authors then filled P3HT into

the TiO2 pores by spin coating a regioregular (rr) P3HT solution

onto TiO2 porous film, followed by heating at $ 100–200 C.

When the substrate was heated at 200 C for less than 1 h, it was

possible to get an absorbance of 0.5 by filling rr-P3HT into a 180

nm thick TiO2 film. In 2004, Ravirajan et al. reported the

penetration of poly(9.9-dioctylfluorene-co-bithiophene) (F8T2)

into a porous TiO2 film by dip coating.145 It was found that the

F8T2 could be infiltrated as deep as 100 nm into the pores. A

peak external quantum efficiency up to 13% and a mono-

chromatic energy conversion efficiency of 1.4% was achieved.

Later, Ravirajan et al. used a Z907 dye to cover the ZnO polar

surface as an amphiphilic monolayer so that the polar part of Z907 is attached onto ZnO while the non-polar part is facing

outwards the pores.12,21 The Z907 capped ZnO was then used to

penetrate P3HT by dip coating, and its non-polar surface would

be more compatible with P3HT, which might improve polymer

infiltration and charge separation.12

(b) Direct penetration of inorganic nanostructures into a wet

polymer film. Another approach to form a hybrid film is to push

the inorganic nanostructures into a wet film of donor polymer

(Fig. 17). Lin et al. reported silicon nanowires (SiNWs)/poly(3,4-

ethylenedioxy-thiophene):poly(styrene-sulfonate) (PEDOT:

PSS) solar cells, in which PEDOT functions as donor.146 SiNWs

were prepared by an etching method in an aqueous solution of AgNO3 and HF acid at room temperature. In order to get an

efficient interface with PEDOT:PSS, a hydrophilic SiNW surface

was created by exposing it a 60% humidity environment.

Different from the typical processing where a polymer solution

was deposited onto the top of an acceptor nanostructure, the

PEDOT:PSS aqueous solution was spin-coated onto an ITO

substrate with a thickness of 9 mm as a wet film. Then the SiNWs

were pushed into this wet film before they dried. After that, the

PEDOT:PSS film was only 200 nm. Then, the PEDOT:PSS/

SiNWs film was annealed at 140 C for 10 mins in a N2 envi-

ronment. These solar cells showed a very broad spectrum light

harvesting from 400–1100 nm with a peak EQE of $32% at

700 nm and a h of 5.09% under an AM1.5G illumination with anintensity of 100 mW cmÀ2.

(c) In situ polymerization of polymers in inorganic acceptor

nanopores. Due to the difficulty in filling donor polymers into

Fig. 17 Diagrams representing cell configuration and fabrication

procedures. Reproduced with permission from ref. 146.


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inorganic nanopores, another approach tried by various groups

is to in situ polymerize the donor materials directly in the

nanopores. Lu et al. used 2-thiophenecarboxylic acid as a func-

tional monomer for in situ polymerization of P3HT onto TiO2

nanoparticles.147 Zhang et al. in situ grew regiorandom P3HT

onto a nanoporous TiO2 surface116 and the procedures are shown

in Fig. 18. The hydrophilic TiO2 surface was first modified by

covalently attaching an organic monolayer via a siliane linker,

which reacted with the hydroxyl function groups of TiO2. Thesiliane linker also had a thiophene capping group, which could

serve as a polymerization initiator. Then, P3HT was polymerized

by a chemical oxidized method onto the TiO2 surface. Through

this method, a larger amount of P3HT was filled into the TiO 2

pores than that of the conventional infiltration in which the

P3HT was synthesized outside the pores. The in situ polymerized

P3HT/TiO2 sample also exhibited a stronger photoluminescence

quenching and more efficient charge separation than that made

by physical infiltration.

In 2009, Tepavcevic et al. reported a UV-assisted in situ

polymerization of polythiophene inside TiO2 nanotubes.115 They

immersed a TiO2 nanotube substrate into a 2,5-diiodothiopehene

solution and then treated it with UV irradiation in an argonenvironment. The C-I bond in 2,5-diiodothiopehene was then

photodissociated under the UV illumination at 250–300 nm, and

then led to monomer radicals. The continuous photodissociation

of the C–I bonds of the resultant reaction coupling products

helped to form a thiophene oligomer or polymer, which could be

coupled with or self-assembled onto the TiO2 nanotube surface.

The efficient exciton dissociation at the TiO2-polythiophene

interface indicated a strong coupling between TiO2 and poly-

thiophene, leading to an improved polythiophene crystallity and

p – p packing. The authors compared the in situ UV polymerized

polythiopehene/TiO2 nanotube solar cells with those made by

penetrating the pre-synthesized polythiopehene into the TiO2

nanotubes. The solar cell device structure is shown in Fig. 19.

They found that the photocurrent density of in situ UV poly-

merized solar cells was 5 mAcmÀ2, which was more than 1000

higher than the that of the infiltrated ex-situ polythiopehene

when back-illuminated (Ag side, transmission <10%) under 38

mA cmÀ2 illumination at 620 nm monochromator light.115

4.6 In situ synthesis of acceptor inorganic nanoscystals in the

matrix of donor polymers

The acceptor inorganic nanocrystals can also be in situ synthe-

sized in the donor polymer matrix to form an intimate DA

mixture for efficient bulk heterojunction devices. In 2003, van

Hal et al. reported a simple procedure to fabricate organic-

inorganic hybrid solar cells via in situ synthesis of TiO2.148

Titanium isopropoxide (Ti(OC3H7)4) and MDMO-PPV were

blended and dissolved in dry THF, and then the mixture solutionwas spin-coated onto a substrate to form a film with a thickness

$ 50–70 nm. At least 65% TiO2 was formed by the hydrolysis of

Ti(OC3H7)4 precursor in air. A continuous TiO2 interpenetrating

network was obtained in the MDMO-PPV matrix, and phase

separation took place in the nanometre scale. The device with the

configuration of glass/ITO/PEDOT:PSS/MDMO-PPV:TiO2/

LiF/Al achieved an EQE up to 11% with TiO2 volume fraction of

20% in the hybrid film. The reason for relatively low cell

performance was possibly that the TiO2 phase was amorphous as

it was not annealed into a crystalline structure at high tempera-

ture (e.g., 400–500 C) because the high temperature annealing

could decompose the polymer.

ZnO can also be in situ synthesized in the blended film.119,149

The ZnO network in the P3HT matrix was formed from

a precursor of diethylzinc undergoing a hydrolysis processing to

form Zn(OH)2, followed by condensation reaction and annealing

at 100 C. Compared with TiO2, ZnO exhibits some advantages

in hybrid solar cells: (1) it can be prepared in high purity and

form crystalline phase at a low temperature (e.g., 100 C); (2)

ZnO nanocrytals can be well dispersed without additional

surfactant or ligands in a nonpolar solvent, such as dichloro-

methane, chloroform, and chlorobenzene. Solar cells were

fabricated by blending ZnO precursor diethylzinc with MDMO-

PPV or P3HT from a common solution at a certain percentage of

Fig. 18 Procedures showing in situ polymerization of P3HT covalently

linked onto TiO2 nanostructures.

Fig. 19 Cell configuration of backside-illuminated UV-assisted in situ

polymerized polythiopehene/TiO2 nanotube solar cells. Reproduced with

permission from ref. 115.


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humidity and annealing at 100 C. A 2% energy conversion

efficiency was achieved under AM 1.5 illumination (100 mW

cmÀ2) witha Voc of 0.75V, a Jsc of 5.2 mAcmÀ2, a FF of 0.52, and

an EQE of 44% at 520 nm wavelength.

4.7 Polymer as both hole transporter and light absorber in solid

state dye-sensitized solar cells (sometimes also regarded as

polymer–inorganic hybrid solar cells)

A conjugated polymer can also be used as hole transporter or

collector in solid state DSSCs. Although polymers can absorb

light and work as donor, their major function is to transport

holes in solar cells. They are typically called solid state dye-

sensitized solar cells. However, a few groups also called such

devices as polymer–inorganic hybrid solar cells. Thus we will give

a brief discussion on this topic. A metal-free dye D102 (chemical

structure shown in Fig. 20) was used to modify and sensitize the

TiO2 surface, reported by Zhu et al.23 The device configuration is

shown in Fig. 20. In this device, the nanoporous TiO2 layer with

a thickness of $1.8 mm was first surface-modified by D102, fol-

lowed by being treated with bis(trifluoromethylsulfonyl)amine

lithium salt (Li(CF3SO2)2N) and 4-tert-butylpyridine (TBP).Afterwards, a layer of P3HT was spin-coated on top of the

modified and treated TiO2 surface, and then a metal electrode

was deposited to complete the entire device. N719 dye was also

used for comparison. The Li-salt and TBP treatment caused red-

shift in the absorption of D102 on TiO2, but a blue-shift in the

absorption of N719 on TiO2. Such treatments also led to

a positive shift of the oxidation onset potential, Eonset(ox), of

D102 on TiO2, but little effect on that of N719 on TiO2. The

positive shift of Eonset(ox) caused the HOMO and LUMO energy

levels of D102 to be 0.2 eV lower than those of P3HT, energet-

ically favorable for P3HT to function as both electron trans-

porter and light absorber. A solar cell efficiency of 2.63% was

achieved with a Voc of 0.83 V.23

Recently, Chang et al. reported a high efficiency nano-

structured inorganic-polymer hybrid solar cell made by

depositing Sb2S3 as an inorganic light absorber and P3HT as

both a hole transporter and an organic light absorber onto the

mesoporous TiO2 surface.24 The crystalline Sb2S3 has a great

potential as an inorganic semiconductor sensitizer for hybrid

solar cells, because it exhibits a high absorption coefficient (1.8 Â105 cmÀ1 at 450 nm) and an efficient bandgap at $1.7 eV. From

the energy level diagram and device structure shown in Fig. 21, it

can be clearly seen that the energy levels of TiO2, Sb2S3 and

P3HT match very well, where P3HT can be used as both a hole

transporter and light absorber. The resultant hybrid solar cell

achieved a h of 5.13% with a Jsc of 13.02 mA cmÀ2, a Voc of

0.65 V and a FF of 61% under AM 1.5G illumination at 100 mW

cmÀ2. This device also showed a high stability under room light in

air without cell sealing.

5. Summary and outlook

Polymer–inorganic hybrid solar cells have gained extensiveattention and recent research progress is summarized in Table 5.

While power conversion efficiencies exceeding 3% have recently

been achieved for polymer–inorganic hybrid solar cells,22,137

these efficiencies are still lower than those of polymer-fullerene

solar cells, which have recently reached as high as 8.13%. 6 In

physically blended polymer–inorganic hybrid solar cells,

although charge separation has improved,11,21 it is still difficult to

control the composite morphology to provide an efficient charge

transport via inorganic nanocrystals. The visual study of poly-

mer–inorganic hybrid 3D morphology has also been challenging;

Oosterhout et al.’s recent report using electron tomography is

very promising.119 To resolve poor electron transport, pre-

synthesized inorganic nanostructures have been used to eitherinfiltrate the polymers or in situ polymerize donor materials

within the nanopores. Among these approaches, ordered nano-

structures are considered an ideal configuration for achieving

high efficiency utilizing one dimensional (1-D) inorganic semi-

conductors to realize continuous charge transport pathways.150

Conventional methods of fabricating ordered heterojunction

polymer inorganic solar cells (including filling inorganic

templates with ex-situ polymers) are quite challenging due to

inefficient polymer filling caused by frequent clogging of the

polymer in the nanostructure pores. To further improve device

performance of polymer–inorganic hybrid solar cells, several

Fig. 20 The chemical structure of P3HT, N719 and D102; The solar cell

configuration of FTO/TiO2 (compact layer)/P3HT/dye/TiO2 (nano-

porous)/Au/Ag, and a SEM image of the nanoporous TiO2. Reproduced

with permission from ref. 23.

Fig. 21 Energy level diagram and device configuration of P3HT/Sb2S3/

TiO2 hybrid solar cells. reproduced with permission from ref. 24.


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challenges need to be considered including morphology control,

polymer and inorganic material engineering, and donor–

acceptor interface modification. A more efficient morphology

control of polymer–inorganic active layer is needed to achieve

not only an effective charge separation, but also an efficient

charge transport and collection. In addition, the polymers and

inorganic semiconductors need to be engineered so that the offset

between the polymer LUMO and the inorganic conduction band

(CB) is optimized to reduce energy loss for improved Voc during

exciton dissociation and charge separation. Regardless of

acceptor materials, the donor polymer should have an Eg at

$1.5–1.6 eV to achieve efficiency beyond 10% in single junction

solar cells, assuming that only the polymer absorbs light and the

polymer–inorganic morphology is well optimized to support the

calculation assumption in Fig. 7. The interface between polymer

and inorganic semiconductor is also important for achieving

higher device performance. If the inorganic nanocrystals are

capped with ligands, such ligands may be removed or exchanged

Table 5 Donor and acceptor materials, device configurations and their performance in polymer–inorganic hybrid solar cells

Inorganic Polymers Device Configuration Voc (V) Jsc (mA cmÀ2) FF (%) h (%) Year Ref.

CdSe PCPDTBT ITO/PEDOT:PSS/PCPDTBT:CdSe/LiF/Al

0.674 9.02 51.47 3.13 2010 22

OC1C10-PPV ITO/PEDOT:PSS/OC1C10-PPV:CdSe/LiF/Al

0.76 9.1 44 2.8 2005 133

APFO-3 ITO/PEDOT:PSS/APFO-3:CdSe/Al

0.95 7.23 38 2.6 2006 60

P3HT ITO/PEDOT:PSS/P3HT:CdSe/Al 0.62 8.79 50 2.6 2006 125P3HT ITO/PEDOT:PSS/P3HT:CdSe/Al 0.6 7 - 2.18 2007 151OC1C10-PPV ITO/PEDOT:PSS/OC1C10-

PPV:CdSe/Al0.65 7.30 35 1.8 2003 132

P3HT ITO/PEDOT:PSS/P3HT:CdSe/Al 0.7 5.7 40 1.7 2002 131TiO2 P3HT FTO/TiO2/Sb2S3/P3HT/Au 0.556 12.3 69.9 5.06 2010 24

P3HT FTO/c-TiO2/nanoparticle-TiO2/D102/Li+ + TBP/P3HT/Au

0.83 5.19 61 2.63 2009 23

P3HT ITO/PEDOT:PSS/P3HT:TiO2(nanorods)/TiO2(nanorods)/Al

0.78 4.33 65 2.2 2009 140

P3HT ITO/PEDOT:PSS/P3HT:TiO2(nanorods, ACA)/TiO2(nanorods)/Al

0.75 3.49 65 1.7 2008 139

P3HT Ti/TiO2 (nanotubes)/P3HT:PCBM:P3HT-COOH/PEDOT:PSS/ITO

0.45 6.5 — 1 2007 152

P3HT ITO/PEDOT:PSS/P3HT:TiO2(nanorods, oleicacid)/TiO2(nanorods)/Al

0.64 2.73 56 0.98 2008 128

P3HT ITO/PEDOT:PSS/P3HT:TiO2/Al 0.52 2.97 54 0.83 2009 153P3HT ITO/TiO2 (nanorod array)/P3HT/

Au0.32 3.886 41.2 0.512 2008 154

ZnO P3HT ITO/ZnO (nanorod array)/P3HT:PCBM/VOx/Ag

0.58 10.4 65 3.9 2008 155

P3HT ITO/ZnO (nanorod array)/P3HT:PCBM/Ag

0.57 9.6 50 2.7 2007 156

P3HT ITO/ZnO fibers/P3HT:PCBM/Ag 0.475 10 43 2.03 2006 157P3HT ITO/ZnO(nanorod array)/N719

dye/P3HT:PCBM/Ag0.57 8.89 41 2 2009 158

MDMO-PPV ITO/PEDOT:PSS/MDMO-PPV:ZnO/Al

0.81 2.4 59 1.6 2004 7

P3HT ITO/PEDOT:PSS/P3HT:ZnO/LiF/

Al

0.83 3.3 50 1.4 2007 159

P3HT ITO/ZnO (nanorod array)/TiO2

(nanorods)/P3HT:TiO2

(nanorods)/PEDOT/Au

0.49 2.67 45 0.59 2007 160

P3HT ITO/ZnO (nanorod array)/P3HT/Ag

0.443 1.33 48.4 0.28 2007 161

P3HT ITO/ZnO/CdS/P3HT/Ag 0.604 0.39 48.2 0.11 2009 162P3HT-PQT-P Al/ZnO/P3HT-P/Au 0.40 0.32 28 0.036 2010 150

Al/ZnO/QT-P/Au 0.35 0.29 32 0.033Si P3HT ITO/PEDOT:PSS/

P3HT:PCBM:SiNW/Al0.425 11.61 39 1.93 2009 107

PEDOT ITO/PEDOT:SiNW/Ag 0.47 19.28 61 5.09 2010 146P3OT SiNWs (array)/P3OT + oxygen

plasma treated MWNTs/Au0.353 7.85 22 0.61 2009 163

P3HT ITO/PEDOT:PSS/P3HT:Si(nanocrystals)/Al

0.75 3.2 46 1.15 2009 164


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with a more efficient surface modification molecule to improve

phase separation, facilitate charge transfer, and hinder back

recombination. Although the current overall power conversion

efficiency of polymer–inorganic hybrid solar cells is lower that

those of polymer-fullerene solar cells and DSSCs, the trend of

efficiency increase in these cells is still apparent, indicating

another promising approach to achieve cost effective solar

energy.

Acknowledgements

The authors sincerely thank the financial support from the NSF

(ECCS-0950731), NASA EPSCoR (NNX09AP67A), ACS

Petroleum Research Funds DNI (48733DNI10), US-Israel

Binational Science Foundation (2008265), and US-Egypt Joint

Science &Technology Funds (913).

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