High Short-Circuit Current Density in CIS Solar Cells by a Simple Two-Step Selenization Process With a KF Postdeposition Treatment

Abstract

In this work, we present a CuInSe2 (CIS) thin-film solar cell with 14.7% efficiency and 40.2-mA/cm(2) short-circuit current. The samples were fabricated by the selenization of stacked elemental layers under selenium vapor in atmospheric ambient within a simple graphite box. Some samples were treated with a postdeposition treatment (PDT) with potassium fluoride (KF), whereas some were not, thus allowing for useful comparison. The use of the KF PDT led to two interesting improvements on our samples: formation of a hole-blocking layer between CdS/CIS interface, which reduces the tunneling recombination and significantly increases the open-circuit voltage. In addition, the KF PDT clearly reduces the symmetry of the nonohmic shunt leakage at low forward bias and reverse bias, which can be explained by a space-charge-limited current model, indicative of metal-semiconductor-metal (MSM) shunts. Discussion is presented to understand possible defects introduced during our processing steps and the physical origin of the shunt paths and why they might be of the MSM type. New directions to improve the performance of CIS solar cells are presented for future work using this very simple fabrication technique.