Impact of lifetime variations and secondary barriers on cadmium telluride solar-cell performance

The thin-film CdTe solar cell (generally n-CdS/p-CdTe) is one of the leading candidates for terrestrial photovoltaic applications due to its low cost and high efficiency. However, compared with single-crystal cells of comparable band gap, there remains a significant voltage difference. Therefore, the fabrication of high-voltage CdTe solar cells is one of the major and critical challenges in recent years. From a device-physics point of view, variations in carrier lifetime, carrier (hole) density, and other aspects such as a back electron reflector, should be able to improve the voltage and efficiency.;This dissertation systematically studies the impact of lifetime variations and secondary barriers on CdTe solar-cell performance. Numerical simulation is used to evaluate how combinations of lifetime, carrier density, interfacial recombination, and back barriers affect cell behaviors. Strategies to improve voltage and cell performance are explored. The experimentally observed characteristics with significant back-contact barrier (back-hole barrier) are explained. Current-voltage distortion which would result from a front barrier is also discussed.;In the absence of secondary barriers, both higher lifetime and higher carrier density are needed to obtain significantly higher voltage. The combination of a significant back-hole barrier and a typical CdTe carrier density leads to two competing mechanisms that can alter the J-V characteristics in two different ways depending on the lifetime. One is a hole limitation on current in forward bias, and the second is a high electron contribution to the forward diode current. Simulated J-V curves illustrating the two major effects are in good agreement with experimental curves that have been observed in recent years. When an effective electron reflector is present at the back contact, the voltage should be increased because of the reduced voltage-limiting back recombination, and the lifetimes for high efficiency need not to be particularly high. Numerical simulations also show that a high front barrier caused by a dipole CdS may result in fill-factor and efficiency losses, thus J-V distortion.