Impact of copper back contact in CdTe solar cells: study of defects by temperature-dependent capacitance–voltage measurements

Abstract

Copper diffusion from back contact creates semishallow and deep-level defects in n+-CdS/p-CdTe solar cells. This study analyzes the impact of copper as a function of annealing temperatures during back contact formation. The observed defect levels, formed after copper diffusion, were identified by employing the temperature-dependent capacitance–voltage (C-V) characteristics at reverse bias in the dark. Theoretical background involving recombination centers, defect density and the role of impurity defects (copper-related defects) suggests that the temperature-dependent C-V profiling is a suitable technique to investigate the copper-related defects. Samples, annealed at 160°C for 30 min, show only one energy level, whereas samples annealed at 280ºC for 30 min reveal two distinct deep levels. From the reported observations, the trap levels were identified as copper-related defects. Variation in annealing temperature can impact the defect formation process: substitutional impurities of copper (160°C) and deep levels (280°C).