A Theoretical Model for Voltage-Dependent Photocurrent Collection in CdTe Solar Cells

Abstract

The classic solar cell model assumes that the photo-generated current is a constant, independent of the cell’s output voltage. Experimental data of CdTe solar cells, however, show that the photocurrent collection efficiency decreases with the increase of the cell’s output voltage. In this work, we proposed a theoretical model for the CdTe thin-film cell, which assumes that the loss of photocurrent in the CdTe absorber is primarily due to the minority carrier recombination in the neutral region and at the back contact. By solving the neutral region’s diffusion equation, with proper boundary conditions, we have obtained the analytical expressions of the photocurrent collection efficiency and the cell’s J-V performance. Our theoretical results agree well with the experimental data. According to our theoretical model, the CdTe thin-film solar cell has an optimized p-doping level. A higher doping density may not be always good for a CdTe solar cell due to the reduced depletion width and decreased photocurrent at normal operation voltage, although the higher doping density can improve the open-circuit voltage by increasing built-in voltage.

Voltage deficit in PV with suppressed recombination

“…4) is voltage independent, corresponding to the condition that all photogenerated carriers are collected. (The possibility of voltage dependent photocurrent [40,41] would assume some recombination.) We note that in the approximation j L = const, the charge carrier concentration n will change with electric field E to maintain constant j L = nEeµ = g where g is the carrier generation rate per area.…”