Thickness Effect of Nanocrystal TiO2 Photoanodes on Dye Sensitized Solar Cells (DSSC) Performances

Yan Wang1,2, Zi-Qing Lu2, Guang-Wei Du1,2, Xu Wang2, De-Dong Han2, Li-Feng Liu2*,

Yi Wang2, Xiao-Yan Liu2, Jin-Feng Kang2*

1 Peking University Shenzhen Graduate School, Shenzhen 518055, China 2 Institute of Microelectronics, Peking University, Beijing 100871, China

*E-mail: kangjf@pku.edu.cn, lfliu@ime.pku.edu.cn Abstract

In this paper, the thickness effect of TiO2 photoanodes on Dye Sensitized Solar Cells (DSSCs) is investigated. The optimization thickness of 12~15m nanocrystal TiO2 is experimentally demonstrated and reproduced by simulation for the large-size DSSCs (4cm×4cm). The simulation shows that thinner TiO2 electrodes lead to smaller current due to the shortage of the absorbed dye molecular but thicker TiO2 electrodes also cause the same effect due to the enhanced dark reaction between electrons in the TiO2 and the electrolytes and the loss of the electrons during transportation in TiO2 electrodes. 1. Introduction

As one member of the solar cell family, the dye-sensitized solar cell shows its superiority during the fierce competition with the traditional silicon solar cell owning to its low cost and easy fabrication. First invented by M. Grätzel et al., it uses TiO2 and Pt on TCO substrates as electrodes, I-/I3

- redox couple in acetonitrile as the electrolyte, and Ru-bipyridyl dye N719 as the sensitizer.[1] The electrons induced from the photo-exited dye first inject into the TiO2, then flow to the counter-electrode and recombination with the I3

-. Till now, the highest solar energy conversion of DSSCs has reached over 10%.[2]

The TiO2 photoanode has played an important role in electron transportation and dye absorption. A. Listori et al. carefully explained the electron transfer dynamics in DSSC. They found that the thickness of TiO2 came to be a decisive factor besides architecture and porosity as a result of loss of the electrons while transferring in DSSC.[3] S. Ito et al. and E. Lancelle-Beltran et al. separately investigate the effects of the thickness of TiO2 in 0.25cm2 DSSC with liquid electrolyte and in 0.25cm2

all solid state DSSC. It is recognized that the efficiency first increases then falls down when the thickness becomes larger in both works.[4,5] However, there is still no research on the effect of the thickness of TiO2 on large area DSSCs.

In this paper, 4cm×4cm DSSCs with different thickness of TiO2 photoanodes were fabricated. We proposed the influence mechanism of the thickness of

TiO2 of DSSCs based on the results of our experiment. The effect is almost the same as the small area ones but needs to be adapted in consideration of the area. Due to the impact of the transportation along the surface (the lateral orientation), the optimum thickness has become larger. To demonstrate our theory, we also developed a model based on it and our group previous work. [6] The simulation results agree with the data of our experiment well.

2. Experiment

In this study, the architecture of fabricated DSSCs is the same as the earliest one. They were produced by the following steps. First, the FTO glass plates(Nippon Sheet Glass, Solar, 4 mm thick) were cleaned in three detergent solutions(water, acetone and ethanol) using an ultrasonic bath for 5 min each. Then the TiO2 paste which was made from TiO2 powder manufactured was screen printed on the substrate to form a 4cm×4cm assorted layer photo-electrode. After drying the nanocrystalline TiO2 layer at 120℃, another same TiO2 layer were coated using the same method on previous TiO2 layer. By repeating this process, we can get TiO2 electrodes with different thickness due to different numbers of layers. Afterwards, a paste containing 200nm anatase particles was coated by screen printing as scattering layer. Then the assorted layer was annealed in air under 450℃ for 1h. After that, the assorted electrode was immersed in a 0.5mM N719 dye solution adsorbing dye at room temperature for 24h. We used the surlyn film as sealing spacer to form the sandwich “structure”. Finally, the electrolyte was infused into the assembled DSSCs. The DSSCs were tested by Keithley 2400 under AM1.5 simulated solar power.

3. Results and Discussion

Figure 1 shows the scanning electron microscopy (SEM) of the screen printed TiO2 photoanode (without scattering layer). From the measured data, the thickness of each TiO2 layer (not the scattering one) is about 2.4um. This is controlled by the screen printing plate and particle size so we can assure that each layer’s thickness is about the same.

978-1-4673-2475-5/12/$31.00 ©2012 IEEE

(a) (b)

Figure 1. SEM image of TiO2 layer.a)surface view; b)sectional view.

Figure 2 shows the I-V curves of DSSCs with

different thickness of TiO2 layers. The results show that the energy conversion efficient of the DSSCs changes dramatically with the thickness.

0.0 0.2 0.4 0.6 0.80

5

10

15

Cu

rren

t D

ensi

ty /

mA

/cm

2

Voltage / V

number of layers of TiO2 3 4 6 7 9

Figure 2. Measured I-V curves of DSSCs with different

thickness of TiO2 electrodes

Figure 3 is the I-V curves of DSSCs under dark condition. The results indicate that the dark reaction increases with the thickness.

0.0 0.2 0.4 0.6 0.8 1.0-0.20

-0.15

-0.10

-0.05

0.00

Dar

k C

urr

ent

/ A

Voltage / V

number of TiO2 layers

4 6 9

Figure 3. Measured I-V curves of DSSCs in dark

Figure 4 shows the dependence of the energy

conversion, short circuit current density and open voltage on the thickness of TiO2 layer. The efficiency of

large area DSSCs first rises and then falls along with the increase of thickness, which is the same as the smaller ones. The open voltage is almost constant but the short circuit current density has the same behavior as the energy efficiency.

6 8 10 12 14 16 18 20 220

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14

16

0

100

200

300

400

500

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En

erg

y C

on

vers

ion

Eff

icie

ncy

/ %

Thickness of TiO2 / m

energy conversion efficiency short circuit current density C

urr

ent

Den

sit

y / m

A/c

m2

open circuit voltage

Vo

ltag

e /

mV

Figure 4. Characteristic curves of parameters of DSSCs

Table I provides specific details of I-V parameters of

DSSCs. The Voc is open circuit voltage and Jsc is the short circuit density. FF is the filling factor of DSSCs andη is the energy conversion efficiency.

Table I. Measured I-V Parameters of DSSCs number of layers

Voc / mV Jsc / mA/cm2

FF η / %

3 619 6.56 0.45 1.84 4 728 11.85 0.46 3.96 6 682 15.1225 0.44 4.54 7 592 4.2 0.43 1.07 9 580 1.375 0.57 0.46

Based on these results, we proposed the influence

mechanism of TiO2 thickness. There are three main factors that change along with the thickness: the number of the absorbed dye molecular; the recombination between the electrolyte and electrons injected into the TiO2 photoanode; the loss of electrons during the transportation in TiO2 layer. We now consider each of them separately.

Firstly, the thicker the TiO2 layer is, the more absorbed dye molecules there are. The relationship between them is exponential, that is to say, Ndye∝exp(d)(d is the thickness). The electron injection rate into the TiO2 electrode can be expressed using this equation [3]:

2*

2 1 , exp4

OX

inj FB

E Ek A V f E E E dE

k T

(1)

In this equation, ρ(E) is the density of acceptor states of semiconductor at energy E. The effect of more

3μm 5μm

absorbed molecules equals to more density of states of acceptor states. We assume there relationship between them is linear, then the injection rate kinj and current density J also have positive correlation with exp(d).

Secondly, the recombination between the electrons in TiO2 layer and the I3

- intensifies with the increase of the thickness as shown in figure 3, causing the current to reduce. The current due to this recombination can be expressed as

0m

r et OXJ qk C n n (2)

where n and n0 is the density of electrons in TiO2 layer under light and dark condition. Thus when the layer becomes thicker, the recombination current becomes larger which contributes to the reduction of the current density.

Finally, the loss of the electrons during transportation in TiO2 increases with the thickness. The rate of electron capture of the trap states is:

t t tR k N n n (3)

where Nt is the density of trapping states. Obviously, there are more trapping densities in thicker TiO2 layers. Thus more trapping electrons mean less current.

Neither of the factors above has something to do with the area. That is why the characteristic curve of efficiency of the large area DSSC is almost the same as the smaller one, as shown in figure 4. However, the best thickness shifts from 8um to 15um when the area increases.

6 8 10 12 14 16 18 20 220

1

2

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50

100

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250

300

En

erg

y C

on

ver

sio

n E

ffic

ien

cy /

%

Thickness of TiO2 Layers / um

Efficiency Resistance

Eq

uiv

anle

nt

Res

ista

nce

of

TiO

2 /

Figure 5. Simulation results of the theory based model

This effect can be explained by the fact that the

transportation of electrons along the lateral orientation becomes more notable, the effect of which is equal to connecting a resistor to the main resistor which is related

to the transportation of electrons TiO2 along the vertical direction in parallel. Therefore, if we consider all these factors together, the main characteristics of the energy conversion efficiency depending on the thickness of TiO2 layer can be the same for whatever the area is and the optimum thickness will slightly increase for the larger area DSSC. Based on this mechanism, we use the simulation method to test our theory, the results of which are shown in figure 5. [6] The results demonstrate that our theory about the influence of the thickness of TiO2

electrode on DSSCs to be valid.

4. Conclusion In this study, we have investigated the thickness effect

of TiO2 photoanodes on the performance of DSSCs. The study of experiment and simulation indicates that a optimization thickness of 12~15m exists for TiO2 photoanodes of the large-sized DSSCs. Thinner or thicker thickness would cause the smaller current due to the insufficient dye molecules absorption on TiO2 or enhanced dark-current effect. Acknowledgments

This work was partly supported by Science and Technology Commission of Shanghai Municipality under Grant No. 11nm0500600.

References [1] Brian O’Regan and Michael Grätzel, Nature, 353,

p.737(1991) [2] Michael Grätzel, Journal of Photochemistry and

Photobiology C: Photochemistry Reviews, 4, p.145(2003)

[3] Andrea Listorti, Brian O’Regan and James R Durrant, Chemistry of Materials, 23, p.3381(2011)

[4] Seigo Ito, Shaik M. Zakeeruddin, Robin Humphry-Baker, Paul Liska, Raphaël Charvet, Pascal Comte, Mohammad K. Nazeeruddin, Peter Péchy, Masakazu Takata, Hidetoshi Miura, Satoshi Uchida and Michael Grätzel, Advanced Materials, 18, p.1202(2006)

[5] Emmanuelle Lancelle-Beltran, Philippe Prené, Christophe Boscher, Philippe Belleville, Pierrick Buvat and Clément Sanchez, Advanced Materials, 18, p.2579(2006)

[6] Ziqing Lu, Bao Wang, Haiyou Yin, Xu Wang, Lifeng Liu, Gang Du, Jinfeng Kang and Xiaoyan Liu, 5th Aseanian Conference on Dye-Sensitized and Organic Soalr Cells, p.92(2010)