Effect of Zn incorporation into CuInS2 solar cell absorbers on microstructural and electrical properties

Abstract

Zn incorporation into CuInS2 absorbers is found to increase the open-circuit voltage but decrease the short-circuit current of the corresponding thin-film solar cells. In this article, we study the effect of Zn incorporation into CuInS2 absorbers with a focus on the mechanisms leading to the measured changes in the electrical properties of the solar cells. Solar cells with varying Zn concentrations in their absorbers are characterized via the application of transmission electron microscopy, quantum efficiency, and current-voltage measurements, as well as admittance, x-ray photoelectron and photoluminescence spectroscopy. A Zn accumulation on the absorber side of the CuInS2–CdS interface and a higher structural defect density within the absorber are found after Zn incorporation. Capacitance, quantum efficiency, and current-voltage measurements in combination with device simulations suggest that Zn incorporation induces or enhances a shallow donor at the CuInS2–CdS interface. The interface defect pins the Fermi level close to the CdS conduction band, leading to an inversion at the heterointerface and thus reducing the recombination at interface defects and increasing the open-circuit voltage. A shallow bulk acceptor about 0.15 eV above the valence band edge is observed to increase with increasing Zn concentration in the CuInS2 absorbers and is responsible for a gradual decrease in the short-circuit current and the gain in the open-circuit voltage as the Zn concentration increases.