Space-charge-limited current in the Au/CdSe/Ni structure has been studied to determine the trap depth and density in electro-deposited n-CdSe films. A high density of shallow traps was observed. Donor concentration, band bending and minority-carrier diffusion length measurements were carried out by studying the photocurrent-voltage characteristics of an n-CdSe film/electrolyte rectifying barrier using a 5 mW He-Ne laser for illumination. These parameters have been correlated with the observed performance of solar cells. The influence of annealing on the physical properties is also discussed.
An electrodeposition process to grow high-purity CdSe films from low-purity materials is described. Purity control was achieved by optimising the deposition potential and impurity masking with EDTA. Auger depth profiles of CdSe films were used to examine their stoichiometry and impurity content. The photo-anodic performance of CdSe film depends on annealing history and etching conditions. Optimum annealing and etching conditions are reported to achieve the best efficiency. A typical n-CdSe/ Ni/(S22-,S2-)/Ni solar cell yielded an open-circuit voltage of 500 mV, short-circuit current of 10 mA cm-2 and 3.6% efficiency under 50 mW cm-2 illumination from a tungsten-halogen lamp.
Electrochemical photovoltaic cells, with open-circuit voltage = 530 mV, short-circuit current = 9 mA cm-*, fill factor = 0.55 and efficiency= 5% have been fabricated from chemically formed Cd, .,Zn,Se layers. The results of our measurements on donor concentration, minority carrier diffusion length, band bending and traps have also been reported. The performance of Cd,-.Zn,Se-film-based solar cells has been compared with the CdSe-film-based cells and the possible reasons for t h e improved performance of the former cells have been discussed.
A preparatory technique for Cd,-,Zn,Se films is described. The results of investigations on the composition, surface features and photocorrosion of the Zn-ion-treated CdSe films have also been reported. It is shown that the annealing of these films at 100°C for 30 min causes diffusion of Zn deep within the bulk. The resulting Cdl-,Zn,Se films have also been examined by a scanning electron microscope. It is shown that the Zn adsorption and diffusion is mediated mainly via the grain boundaries. Short-term stability tests indicate that the S e s e exchange as a result of photocorrosion in the sulphide/ polysulphide electrolyte is considerably reduced in Cdl-,Zn,Se films compared to the CdSe films.
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