Theory of high efficiency (Cd,Zn)S/CuInSe 2 thin film solar cells

Abstract

A theoretical model is presented which allows the fitting of calculated results to both dark and illuminated current versus voltage characteristics at several temperatures above 300 K for two relatively high efficiency Boeing (Cd,Zn)S/CuInSe 2 thin film solar cells. The model is based on the assumptions that (1) the device is an n-(Cd,Zn)S/p-CuInSe 2 heterojunction, (2) interface recombination is the dominant loss current mechanism both in the dark and under illumination and (3) the interface recombination centers are negatively charged and their density increases under illumination. The model naturally explains the large A factors observed (about 1.4 – 2.2), the temperature dependence of the current-voltage characteristics, the non-translation between the illuminated and dark current-voltage characteristics and the observed low open-circuit voltages. In addition, results of parametric variation studies are presented which show that a maximum open-circuit voltage of about 550 mV is possible in these devices if the effective dopings in the selenide and sulfide are greater than 5 × 10 15 cm −3 and greater than 5 × 10 16 cm −3 respectively, while the negatively charged interface recombination level density is below 6 × 10 10 cm −2 .