Dhruv Sharma

Photosensitization of TiO2 with Protoporphyrin IX:

Role of immersion solvent and time

Adsorption kinetics and aggregation are expected to be strongly dependent

on the properties of solvent.

By selection of solvent one can control aggregation of dye and absorption

kinetics of sensitizing dye. The presence of solvent can speed up or slow

down the electron transfer and recombination rate.

Change from one solvent to another can bring million fold change in

reaction rate.

Solvent effect is more powerful than any other factor in DSSC

performance.

The kinetics of adsorption and aggregation are expected to be strongly

dependent on the properties of solvents.

Importance of Solvent

The sensitizing dye plays an important role in the absorption of photons and

generation of electrons and hole pairs.

Most of the sensitized dye often aggregate on TiO2 surface. The excited electrons

from the aggregated dye molecule can not be efficiently transferred to the photo

electrode. Therefore, control of dye aggregation and absorption kinetics of

sensitizing dyes on the photo electrode is critical factor that affect the DSSC

performance.

In first paper, we discussed the effect of solvent on the aggregation of organic

sensitizing dyes and the adsorption mode of the dye-adsorption process for DSSCs.

They use organo-dendritic photo sensitizers Triplet-PTZ as dye and DMF only,

EtOH : DMF (90:10), EtOH : DMF (50:50) as solvent.

We found that the adsorption and aggregation of sensitizing dyes on the photo

electrode are strongly dependent on the solvent conditions used in the dye-

adsorption processes. The mechanism of adsorption of the Triplet PTZ dye under

different solvent conditions was evaluated to determine the photovoltaic properties

of DSSC.

In third paper, we will discuss the effects of porphyrin substituent and adsorption

condition (i.e immersing solvent and immersing time) on the photovoltaic properties

of porphyrin sensitized TiO2 cells.

Photovoltaic performances of DSSCs containing Triple-PTZ dyes adsorbed in

different solvent conditions.

Solvent conditions Amount of dye Jsc (mA cm−2) Voc (V) FF (%) (%)

(mol/cm2) ( 10−8)

EtOH:DMF 90:10 % 7.07 4.39 0.682 57.58 1.73

EtOH:DMF 50:50 % 4.61 11.14 0.676 61.08 60

DMF 3.80 11.96 0.681 60.15 4.90

UV–vis absorption spectra of Triple-PTZ in DMF solution (black circle) and on TiO2 surface

adsorbed using different dye solvent conditions (red, blue, and green circle).

The wavelength of maximum absorption (max) of Triple-PTZ adsorbed on TiO2 from a DMF

solution of the dye appeared at 425 nm, which was red-shifted from DMF solution λmax =

410 nm.

In another paper, DSSC using ethanol as a solvent show a higher efficiency than that

of a using water, reported at 0.71%, 0.52% respectively.

It is well known that water is dipolar and amphiprotic solvent with a high dielectric

constant.

The adsorption capacities determined by measuring the dye concentration before

and after adsorption using a UV-vs spectrophotometer at 487(for distill water),

498(for ethanol) and 501(for DMSO).

After immersing the TiO2 photo anode in the red dye, the TiO2 films turned a dark

red with DI water and ethanol but a light red color with DMSO solvent.

The UV-vs absorption peak can be seen for DI water solvent at a wavelength of about

487nm, of ethanol at about 498nm, and DMSO at roughly 501nm.

the absorbance intensity of Monascus red dye with DMSO solvent was very

high.

The polar aprotic solvent such as DMSO was found to be more effective for dye

diffusion, where as ethanol being polar protic in nature was not suitable for dye

diffusion.

Photovoltaic parameters of DSSC sensitized by adsorbing Monascus red dye on TiO2

films. Error range of samples was 0.01%

Solvent HOMO LUMO JSC VOC FF η[%]

(eV) (eV) [mA/cm2] [V]

Water 5.43 -2.41 1.07 0.67 0.73 0.52

Ethanol 5.39 -2.42 1.13 0.68 0.69 0.54

DMSO 4.98 -2.44 1.23 0.75 0.72 0.66

DSSC using natural Monascus red dissolved in DMSO solvent showed a higher

photovoltaic performance compared to other solvents of water and ethanol. This result

may come from the fact that a good dispersion of Monascus dye molecule dissolved in

DMSO solvent and the HOMO-LUMO gap obtained was found to be comparatively

narrower than other solvents, leading to 0.66% of energy conversion efficiency.

In another paper they show that, the variation in molecular structure and the adsorption

condition will have a large impact on molecular packing, geometry, and aggregation of the

porphyrin molecules on the TiO2 electrodes, eventually affecting the photovoltaic properties.

In UV-vis absorption spectra, all porphyrin show approximately the same peak and the same

shape in Soret and Q band region.

From the absorption, emission an electrochemical data, the excited state redox potential (Eox*)

are approximated by extracting the zero excitation energy (E0-0 ) from the potentials of the ground

state couples. Driving force for electron injection (ΔGinj) from the porphyrin excited singlet state

to the conduction band of TiO2.

Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy is known to

be a tool for gain the information on the binding mode of the molecule ads orb on TiO2 surface .

The surface coverage value is also increased with increasing the immersing time to become

saturated for 12 hour (2 X 10-10mol cm-2).

The value with 2,4,6 – Me for immersing time of 12h increases in the order of DMF

(0.55%), CH2Cl2(1.1%), tBuOH:CAN (1:1)(2.1%), EtOH (3.2%) and MeOH (3.7%).

The surface coverage values for 2,4,6 Me (1.2 X 10-10 mol cm-2 for EtOH and MeOH,

1.3 X 10-10 mol cm-2 for tBuOH:ACN) are virtually the same when protic solvents are

used.

On the other hand, they(surface coverage= 5.3 X 10-11 mol cm-2 for DMF, 8.7 X 10-11 mol

cm-2 for CH2Cl2) are considerably lower in the case of non-protic solvents.

The high porphyrin density for the adsorption in the protic solvents indicates the significant

contribution of the protic solvents for indicates the significant contribution of the protic solvents

for the formation of a densely packed porphyrin monolayer on the TiO2 surface.

Further increase in the cell performance of porphyrin-sensitized TiO2 cells may be possible

by improving the device fabrication as well as the light harvesting property.

The photovoltaic properties are investigated as a function of the adsorption condition (i.e

immersing solvent and immersing time). For all porphyrins with an increase in the immersing

time, the value is increased rapidly to reach the maximum 0.5 to 1h and then decreased. Such

behavior may be rationalized by an increase of the porphyrin aggregation with increasing the

immersing time.

For first paper, the photovoltaic performances of the DSSCs were investigated from the I-V

curves of the DSSCs were measured using a computerized digital tungsten lamp as the light

source.

It is interesting that although a larger amount of Triple PTZ was adsorbed on the TiO2

surfaces in DMF solution with 90% ethanol the performance of the DSSC was worse than that of

the DSSC fabricated via dye deposited in DMF solution, as shown in above table.

To clarify the origin of reduced performance at higher dye loading with the

use of a poor solvent, the ATR-FT-IR spectra were acquired to evaluate the

adsorption mechanism of triplet PTZ on TiO2 surface.

it show that there is no interaction between cyano moieties and TiO2 surface.

Conclusion

The dye adsorbed on the TiO2 surface under a good solvent (i.e only DMF)

condition should be uniformly adsorbed over the entire area of the TiO2 surface

without aggregation, thus all the dye act as photovoltaic active species.

Under a poor solvent (EtOH:DMF) condition, the dyes were

aggregate on the TiO2 surface during the dye adsorption process.

Scope of Project

To study the effect of different solvent to control the aggregation and adsorption kinetics for

increase the efficiency of DSSC performance.

To make the highly transparent TiO2/PPIX films.

To study the light harvesting efficiency with immersing solvent and time.

To characterize the prepared TiO2/PPIX films by using optical technique.

To study the electron injection and charge recombination study by using Steady state and time

resolved fluorescence spectroscopy.