Buffer-less Cu(In,Ga)Se<inf>2</inf> solar cells with new transparent electrode for band offset control

Abstract

Cu(In,Ga)Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (CIGS) solar cells without buffer layers were fabricated. Typically, CdS, Zn(O,S,OH), or InS buffer layers are used in high efficiency CIGS solar cells to reduce interface recombination. One of the important parameters to obtain high quality interface is the conduction band offset (CBO) between the buffer and CIGS layers. Thus, the choice of the buffer layer material is important. In this study, we have proposed the use of transparent conductive oxide (TCO) which can control the CBO to reduce interface recombination and eliminate the buffer layers. We used Zn(O,S):Al (AZOS) prepared by co-sputtering of ZnO:Al (AZO) and ZnS targets as a novel TCO. The efficiency of a CIGS solar cell with AZO/CIGS/Mo/soda-lime glass structure, i.e. buffer-less structure, was significantly low because of severe shunting. The addition of sulfur into AZO increased shunt resistance, resulting in higher open circuit voltage and efficiency. The result is the first proof of concept of the CIGS solar cells without buffer layers.