Development of CdS based stable thin film photo electrochemical solar cells

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Abstract

CdS semiconductor films have been prepared from chemical bath deposition from basic solutions. Electron microscopic studies have revealed the presence of polycrystalline mixed cubic and hexagonal phases of CdS in the deposits. When surface treated, these layers show good photo electro chemical behaviour in Ti/CdS/∼S/C cell configuration where as photo etching improves the stability of CdS photo anode in polysulphide electrolyte to a great extent. Optical absorption studies of these films have been carried out in the wavelength range 350–800 nm.

Introduction

The high production cost of conventional solar cells requires the search for cheaper methods and materials suitable for solar energy conversion. Recently there has been much interest in the application of semiconductor (SC) electrodes to photo electrochemical (PEC) solar cells for solar energy conversion [1]. Direct band gap SC with high absorption coefficients are favorite candidates for solar cells. CdS belonging to the II–IV group is one important SC used in photovoltaic and PEC solar cells [2], [3], [4], [5], [6]. The basic problem with CdS deposition is uniformity over large surface area, stoichiometry and reproducibility. The chemical bath deposition (CBD) technique appears to be relatively simple, inexpensive and convenient for large area deposition of polycrystalline CdS films. However, because of inherent defects present in the polycrystalline materials, a considerable portion of the photo generated charge carriers is lost due to recombination. Considerable efforts are currently been devoted towards removing these grain boundary effects by subjecting these films to surface treatments. In order to optimize growth conditions, we carried out a detailed investigation of the influence of parameters during deposition. Here, we present our results of the influence of annealing, PEC etching on the photovoltaic performance of CBD CdS photo anodes. Optical property analysis of these films was also carried out by measuring the optical absorption.

Section snippets

Experimental details

The cadmium sulphide films were deposited by CBD on titanium substrates at 80 °C from a solution containing 10 CC of 1 M solution of AR grade CdS04 mixed with 25 CC of 2 M ammonia solutions and 10 CC of 1 M solution of thiourea. The film thickness was of the order of 8300 Å. These films were then annealed in air at 350 °C for 2 min. These annealed films were photo-chemically etched with 10 times diluted mixture of 97% HCI and 3% HNO3) with graphite as counter electrode. These films were thoroughly rinsed

Results

Fig. 1 shows the scanning electron microscopic (SEM) pictures of surface of CdS photo anode. The surface was spongy and rough. The crystalline size was small but appears to be improved after annealing. Fig. 2 shows that in electrolyte consisting of 1 M NaOH and 1 M S, the photo current falls rapidly. Now by adding 0.5 M Na2S to the above solution, it was found that the photo current is more stable. Finally, taking the electrolyte having concentration of one mole each, the photo current was found

Discussion

SEM studies showed that the CdS exists in two crystallographic modifications, namely hexagonal (wurtzite) and cubic (zinc blend) phases which may arise from long exposure of the film in the deposition bath and is in good agreement with the reported results [7]. The crystalline size was small but appears to be improved after annealing. However, due to the spongy nature of the deposits, it is difficult to measure the crystal size. The clusters seen at the top surface are more likely to be either

Conclusion

SEM studies showed that the CdS exists in two crystallographic modifications, namely hexagonal (wurtzite) and cubic (zinc blende) phases. The crystalline size was small but appears to be improved after annealing. The clusters seen at the top surface are more likely to be either loosely attached S or colloidal CdS particles. The factors which accelerate the dissolution of sulphur in polysulphide solution, and which thus decreases the photo electrode polarization, should also increase the

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