Optimisation of pH of the CdCl2+Ga2(SO4)3 activation step of CdS/CdTe based thin-film solar cells

In order to produce high efficiency solar cells based on CdTe, CdCl2 post-growth treatment is an essential processing step. This treatment can be further improved by adding elements such as Fluorine and Gallium into the CdCl2 solution. Through systematic experimentation, it has been found that the pH value of the treatment solution also affect the conversion efficiency of the solar cells. This work therefore focuses on the effect of pH value of CdCl2+Ga2(SO4)3 aqueous solution on the device efficiencies. The graded bandgap device structure, glass/FTO/n-ZnS/n-CdS/n-CdTe/Au was used in this work. The pH values of 1.00, 2.00 and 3.00 for CdCl2+Ga2(SO4)3 solutions were utilised for the activation of glass/FTO/n-ZnS/n-CdS/n-CdTe layers and its effects were explored for both the CdTe material and device properties. It has been found that both CdTe material properties and solar cell device properties are superior when the pH value of 2.00 is used for post-growth treatment. The best conversion efficiency observed in this work for the above graded bandgap device is 12.2%.