A study of electrostatically sprayed CuInS2 and ZnS thin films

Abstract

The investigation of ternary compounds for the fabrication of thin film solar cells is well documented but the production of thin films by electrostatic spray deposition (ESD) is still limited. This thesis represents the first attempt to deposit photovoltaic thin films using this novel method. The lack of information regarding the deposition of CuInS2 by ESD required a statistical investigation of the effects of different deposition variables. To achieve this, a new image analysis method was developed to calculate the variation of the thickness of the as-deposited films using their optical density. The thickness variation across the sample was then used to define its uniformity. Once the conditions for the best uniformity were defined, different needle and substrate materials were investigated to try and improve the performance of the ESD method for potential large scale production. A formation of precipitates was observed during the preparation of the chloride precursor solutions (prepared from CuCl2, InCl3 and thiourea salts). A possible reaction mechanism was proposed and the chemical composition of precipitates analysed. An explanation of the absence of the precipitate during the preparation of a nitrate starting solution (from Cu(NO3)2, In(NO3)3 and thiourea salts) was reported. The behaviour of the aerosol cone for the two different starting solutions (nitrate and chloride) was established using the laser-based particle image velocimetry (PIV) measurement technique. The properties of as-deposited films sprayed by the two precursor solutions were thoroughly investigated using a number of techniques including X-ray diffraction (XRD), Rutherford backscattering (RBS), optoelectronic characterisation and miniSIMS. The effect of different precursor molar ratios on the properties of as-deposited films was studied for both the nitrate and chloride based solutions using the above techniques. ZnS films were also deposited using ESD for the first time. Using the information acquired during the deposition of the CuInS2, a fully comprehensive analysis of the effect of different deposition conditions and starting solution properties was performed. Finally, CdS/CuInS2 and ZnS/CuInS2 junctions were prepared and tested using EQE measurements in solution. Substrate and superstrate device configurations were used to prepare complete solar cells which were analysed using IV measurements