TiO2 nanocrystalline solar cells

PII: S0968-5677(98)00109-6

Supramolecular Science 5 (1998) 709—711  1998 Elsevier Science Limited Printed in Great Britain. All rights reserved 0968-5677/98/$19.00

CdSe/TiO2 nanocrystalline solar cells J.H. Fang, X.M. Lu, X.F. Zhang, D.G. Fu* and Z.H. Lu National Laboratory of Molecular and Biomolecular Electronics, Southeast University, Nanjing 2100096, People’s Republic of China

CdSe sensitized TiO nanocrystalline solar cells were made with the participation of silicotungstic acid (STA) during the deposition of CdSe, the resulting Voc and Isc were 0.23 V cm\ and 10 mA cm\, respectively. The doping, time and microporous membrane effects were also discussed.  1998 Elsevier Science Limited. All rights reserved. (Keywords: silicotungstic acid (STA); CdSe/TiO2; nanocrystalline solar cells)

1. INTRODUCTION Solar energy being pollution free and infinite energy resource, more and more attention is paid to its utilization. Till now, Si is the most extensively used solar energy material. To decrease cost and save expensive materials, researchs have been trying to improve the technology, searching alternative materials and making thin layer solar cells. One important aspect is to form sensitized solar cells by organizing a small gap material on another large gap material. The main advantage of this sensitization structure is that the region in which light excites electrons is separated from that in which charge is transferred, the recombination rate is then largely decreased and the photoelectric conversion efficiency is increased. TiO is a very important semiconductor material: cheap, safe, no environmental problem and stable. Light of wavelength shorter than 375 nm can be absorbed since the energy gap of TiO is 3.2 eV. Since the 1980s, people have been working on sensitizing TiO films, but its energy conversion efficiency is much lower than the present commercial level. The situation changed after the Gra¨tzel group fabricated Ru dye sensitized TiO porous nanocrystalline electrodes with energy conversion efficiency reaching 7.1% in 1991. Many researcheres were then attracted to this field encouraged by this result. The new type of solar cells differed from previous attempts in that (a) TiO electrodes had high surface area; (b) only monolayer dyes were adsorbed on the rough TiO surface. After several years, the efficiency reached 10%, but dye sensitizers were often accompanied by problems of longterm stability. CdSe is a traditional semiconductor material with energy gap E"1.7 eV. Kamat et al. reported the photovoltaic behavior of coupled CdSe/TiO films. They got

*Corresponding author

the relatively lower results of I "0.55 mA cm\, » "   0.3 V under 100 mV cm\ incident light (Gra¨tzel obtained I '18 mA cm\, » '0.7 V). Savadogo et al.   prepared CdSe thin films on conducting glass substrates using silicotungstic acid (STA) (H [Si(W O ) ]) in the    deposition bath, and the conversion efficiency was increased from 2% to 12%. Here, we report the introduction of STA into CdSe/TiO nanocrystalline solar cells,  and compared the photovoltaic effects with that of CdSe/TiO coupled solar cells without STA.  2. EXPERIMENTAL PROCEDURE TiO porous films were prepared on transparent con ducting glass (indium tin oxide coated, OTE) using the method descried elsewhere. 2.1. Deposition of CdSe layers The deposition of CdSe layers onto bare OTE and TiO /OTE electrodes was carried out following the  method of Savadogo et al. The original mixture contained an aqueous solution of 10 ml 1 M cadmium acetate, 5 ml triethanol amine (99%), 10 ml 25% ammonia and 15 ml 0.45 M sodium selenosulphate (prepared by refluxing 9 g Se power with 10 g anhydrous sodium sulphite in 150 ml water for about 4 h: the small amount of Se that may remain undissolved was separated through a G 3-4 glass filter). The final PH of the chemical bath was adjusted to 11.2 by adding NH OH dropwise.  The deposition of CdSe was made at temperatures of 40$2°C by dipping cleaned bare OTE and TiO /OTE  plates and holding them vertically on the walls of a 100 ml beaker. The formation of the film depended on the slow release of the Cd> and Se\ ions in aqueous basic media and their subsequent condensation on the substrates. For the deposition of CdSe layers with STA, 5 ml 10\ M STA was mixed into the original mixture and the other bath compositions were kept constant.

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CdSe/TiO2 nanocrystalline solar cells: J.H. Fang et al. After several hours the deposited films were thoroughly washed with distilled water and dried in air. Annealing of the films was done in air at 430°C for 1h. 2.2. Measurements and apparatus The morphology of the CdSe/TiO electrode was  examined with an Atomic Force Microscope (AFM, DICO, Nanoscope 3). The absorption spectra were recorded with a Shimadzu UV-2201 spectrometer. A two-electrode photoelectrochemical (PEC) cell was used in photovoltaic study. Before PEC measurements, the CdSe/TiO electrodes were etched using dilute aqua  regia (HCl : HNO : H O of 1 : 3 : 32) for 10S and then   washed by water. This treatment can remove the unsaturated bond and improve the surface activity. The counterelectrode (1 lm sputtered Pt coated conducting glass) and the CdSe/TiO electrodes were separated by  the 0.1 M Na S/0.01 M Na SO electrolyte (85 ll cm\).    The cell had an area of 0.5 cm. The short-circuit photocurrent was measured by a potentiostat model CMBP-1. Monochromatic illumination was obtained using a 500 W xenon arc lamp (to stimulate solar radiation) in combination with a grating monochromator (WPG3D). Intensity of the light was calibrated by a model OM-1001C radiometer/photometer. The irradiation area was 2.7 cm under white light, and 0.44 cm under monochromatic illumination.

3. RESULTS AND DISCUSSION 3.1. ¹he comparison between electrodes with and without S¹A Figure 1 is the absorption spectra of the CdSe film deposited directly on conducting glass with and without STA for 6 h. It can be seen that the absorption of the films with STA is higher than that without STA, and the absorption peak of the former has shorter waves than the latter. It seems that the STA acts as a catalyst or nucleator during the deposition of CdSe. On the one hand, more CdSe can be deposited onto the surface of TiO with the participation 

Figure 2 The photocurrent density spectra of coupled electrodes (a) with and (b) without STA (6 h)

Table 1 The data of CdSe/TiO /OTE system under stimulated sun light Condition

» (V) 

I (mA) 

I (lA) 

With STA Without STA

0.55 0.53

17.0 12.3

0.51 0.58

of STA, so as to absorb more sunlight. On the other hand, CdSe deposition with STA has more nuclei, and particles may cease growing when they meet each other, so the CdSe layer with STA has smaller particle size, which may be the reason for the difference in absorption peak position. By now, it is revealed that the deposition of CdSe layer with the participation of STA can result in fast deposition and small particle size. Small particle size is the precondition for the uniform modification of TiO with rough  surface, which in turn leads to a high photocurrent and hence a high quantum efficiency. This deduction is testified by photocurrent measurements of the coupled CdSe/TiO /OTE system. One can  see easily by a look at Figure 2 (the photocurrent density spectra) that the sensitized electrode with STA has higher photocurrent density. By the way, the dark current (I ) has been subtracted  from the measured short-circuit photocurrent in the calculation of photocurrent density. Table 1 gives the data under stimulated sunlight. In our experiments described in the following sections the CdSe layers were deposited with the participation of STA if not specified otherwise. 3.2. ¹he S¹A doped CdSe/¹iO nanocrystalline  solar cells

Figure 1 The absorption spectra of CdSe/OTE with the CdSe deposited (a) with and (b) without STA for 6 h

CdSe layer was deposited onto TiO /OTE electrodes  for different time periods under the same conditions. Figure 3 are the photocurrent density spectra of the coupled electrodes with different modification times. Table 2 gives the data under stimulated sunlight. It can be seen from the above diagrams that the photocurrent of the sensitized electrodes does not always increase with the increase of the amount of sensitizer CdSe,

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CdSe/TiO2 nanocrystalline solar cells: J.H. Fang et al. potential between the anode and cathode, since the photovoltage and photocurrent are very small. 3.4. An alternative approach

Figure 3 The photocurrent density spectra of the coupled electrodes with CdSe deposited for different times

Table 2 The results of PEC measurements of CdSe/TiO electrode  with CdSe deposited for different times Time (h)

» (V) 

I (mA) 

I (lA) 

2 3 4.5 6

0.59 0.61 0.58 0.55

24 27.1 22.7 17

0.37 0.41 0.43 0.51

but has a maximum. The maximum may appear at the deposition time that TiO is just covered uniformly by  a CdSe monolayer. With the further increase of time, the additional CdSe particles cannot effectively absorb light and inject electrons, but act as a potential barrier for charge transfer. The photocurrent spectra have maxima in the wavelength range 400—450 nm, consistent with that which appear in the light absorption spectra of CdSe indicating that CdSe can effectively sensitize TiO elec trodes.

To attain better modification state, we tried to cover the TiO surface with a microporous polycarbonate  filtration membrane during the deposition of CdSe (without STA). It can be found from SEM observation that there are large grains up to 350 nm of CdSe in the surface of TiO  electrode after 1.5 h deposition without the filtration membrane, while the particles have uniform size near 100 nm after 2.5 h deposition with the participation of the filtration membrane. The reason lies in that only particles smaller than 100 nm could pass through the membrane. The pores in the TiO surface were also close to 100 nm.  Therefore we anticipated that it may be possible to modify TiO with CdSe particles having similar size, which  may lead to better sensitization effects. (In this circumstance, it may be easier to attain a uniform CdSe modification layer on TiO microporous surface.) But, the  as-deposited CdSe layer had large resistance. After annealing, with the aim of decreasing the resistance, the CdSe particles grew to larger grains. 4. CONCLUSION We made a series of experiments on CdSe sensitized TiO  nanocrystalline solar cells. The best result we got was under CdSe deposition for 3 h with the participation of STA. The maximum photocurrent density reached 28.66 lA cm\ at 430 nm, » and I were 0.61 V and   27.1 mA, respectively (irradiated area 2.7 cm) under white light. The photocurrent and so the photo-to-electric conversion efficiency IPCE can be further increased by improving the modification technology.

3.3. ¹ime effect When light illuminated the solar cell, during the first several seconds, the photovoltage and photocurrent were higher than that under stable state. In solar cells with liquid electrolyte, conducting matter comprised large amounts of electrolyte molecules and ions. At the beginning of light illumination, the conducting electrolyte molecules and ions seem surplus. The electrolyte on the surface of electrodes can attract rapidly the photoexcited holes, the photoexcited electron—hole pairs can then be separated fast, resulting in larger photovoltage and photocurrent. Under stable state, the rate of electron transfer is limited by the speed of the molecules and ions in the electrolyte, the latter in turn are affected by the electric

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (No. 59582005) and China Postdoctoral Science Foundation. REFERENCES 1 Tributsch, H. and Calvin, M. Photochem. Photobiol. 1971, 14, 95 2 O’Regan, B. and Gratzel, M. Nature 1991, 353, 24 3 Nazeeruddin, M.K., Kay, A., Rodicio, I., Humphry-Baker, R., Mu¨ller, E., Liska, P., Vlachopoulos, N. and Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382 4 Liu, D. and Kamat, P.V. J. Phys. Chem. 1993, 97, 10,769 5 Mandal, K.C. and Savadogo, O. J. Mater. Sci. ¸ett. 1991, 10,1446

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