Fabrication of FTO/CBD-CdS/ED-CdTe/Cu/Au solar cells and boosting its performance by CdCl2 treatment

Abstract

The thin film CdS/CdTe solar cells are promising cost-effective clean energy generating devices against the global energy crisis. Chemical bath deposition (CBD) and electrodeposition (ED) were recognized as being simple and low-cost techniques over a range of growth techniques available for development of CdS and CdTe thin films respectively. The use of aforesaid two techniques successively in fabrication of glass/FTO/CBD-CdS/ED-CdTe solar cells was not reported. This attempt is to do so and moreover, to assess the effect of CdCl2 treatment in performance enhancement of the device produced. In preparation of thin CBD-CdS layers on FTO glass substrate, a bath consisted of Cd(CH3COO)2 (0.033 mol/L), CS(NH2)2 (0.667 mol/L), CH3CO2NH4 (1.0 mol/L) and NH4OH (25%) was employed at 90 ℃. Annealed (375 ℃ for 30 min) CBD-CdS samples were subjected to CdTe deposition by ED system equipped with a three electrodes system. Herein, the CdS thin films were specifically developed enabling them to withstand in a highly acidic bath during the ED process. The ED bath used consisted of CdSO4 (1.0 mol/L) and TeO2 (1.0 mmol/L) at pH 2.3 and 65 ℃. The potential of -0.650 mV was maintained between the reference and working electrodes during each deposition (3 hrs). Samples were sprayed with CdCl2 solution (1.0 mol/L) for 2 s and then annealed (390 ℃ for 15 min). Back contacts (Cu/Au) were deposited on the CdCl2 treated glass/FTO/CBD-CdS/ED-CdTe devices by thermal evaporation. The devices were characterized under the illumination of AM 1.5 (100 mW/cm2). The efficiencies of the CdCl2 treated devices were found to be higher (6.23%) than untreated ones (2.66%). A significant variation in Jsc, Voc, and FF values was observed in CdCl2 treated devices (24.68 mA/cm2, 664 mV, and 38.0%) over untreated devices (14.95 mA/cm2, 531 mV, and 33.5%). The SEM analysis revealed remarkable increment in CdTe grain sizes (~140 nm to ~591 nm) with less grain boundaries in the CdTe sample upon CdCl2 treatment, thus leading to improved photovoltaic performance. This work demonstrated that CdS and CdTe can be synthesized using cost effective methods of CBD and ED respectively and, the FTO/CBD- CdS/ED-CdTe/Cu/Au device efficiency can be significantly improved by the CdCl2 treatment.