A theoretical model for the front region of cadmium telluride solar cells

Abstract

Recently, the modelling investigation on thin film chalcogenide solar cells has been focused on the front region of the devices as a possible source of fill factor losses through a light dependent barrier modulation. We present a model based on a modulated barrier photodiode in series with the main junction, which consistently explains apparent quantum efficiency measurements and, light and dark IV characteristics of CdTe devices. This bulk barrier forms in the front region of the device as a consequence of an increased compensation of donors in CdS, which can induce crossover and in extreme cases rollover in the IV curves. The internal resistance due to this majority carrier barrier, which depends on the irradiance level, is an important limiting factor for cell efficiency, inherent to the cell structure. Numerical simulation gives consistent results with the observed measurement characteristics.