Cadmium sulphide/cadmium selenide quantum dot solar cells with inexpensive electrodeposited silver/polyaniline composite counter-electrode

Abstract

The optical, electrical, morphological, and structural properties of low cost indium tin oxide (ITO)/TiO2/CdS/CdSe/ZnS quantum dot (QD) solar cells with the inexpensive silver/polyaniline counter electrode (CE) are reported in this study. The composition of these devices was verified by Energy Dispersive X-Ray Analysis (EDX). The spin coated mesoporous TiO2 layers on ITO glass substrates were sensitized with cadmium sulphide/cadmium selenide quantum dots using different number of SILAR (successive ionic layer adsorption and reaction) cycles. The ZnS film was also deposited using the same procedure. The low cost counter electrodes were prepared separately on stainless steel substrate by electro-chemical polymerization. The solar cells were tested by pouring the polysulphide electrolyte between the two electrodes. The devices showed an increase in PCE (power conversion efficiency) up to the 3 SILAR cycles, and decreases in PCE were observed for further SILAR cycles. The power conversion efficiency is as high as 5.0% for 3 SILAR cycles. UV-Vis-Spectroscopy measurements of the devices showed an increase in absorption spectra starting from 1.7 eV, which is well matched with the band gap of CdSe QDs. The enhancement in absorbance was found to linearly scale with the number of SILAR cycles up to 5 C and saturating for further SILAR cycles. The absorbance window continued up to 2.4 eV, the band gap of CdS QDs and the absorbance due to the Titania layer was found to start at 3.2 eV (band gap value of Titania). X-ray Diffraction Patterns showed that the particle size of CdS and CdSe QDs increased with the number of SILAR cycles. However, the intensity peak of CdS QDs was not observed for 5 C and higher SILAR cycles. The scanning electron microscope images of devices revealed capping of CdS QDs by CdSe QDs for 5 C and higher SILAR cycles. The observations revealed that CdS QDs were capped by CdSe QDs for 5 C and higher SILAR cycles, resulting decrease in PCE of these devices. The decrease in PCE is attributed to the poor charge collection of the charges contributed by CdS QDs due to its capping by CdSe QDs.