Challenge of replacing CdS in CuInSe2-based solar cells

This paper discusses some key issues concerning the replacement of CdS buffer layers in CIS solar cell structures, and describes investigations of alternative buffer layers deposited by MOCVD. One apparently unique property of CdS buffer layers grown by CBD is that a ZnO TCO can be deposited on top of a CdS/CIS structure without significantly degrading the photovoltaic properties of the CdS-CIS junction. Investigation of alternative buffer materials such as high resistance ZnO (i-ZnO), ZnSe and InSe have first identified MOCVD growth procedures that yield Al/X/CIS test structures (X=i-ZnO, ZnSe and InSe) with good properties, and then addressed the challenge of fabricating efficient, complete cells with conductive ZnO top contact layers. These studies have been conducted with Siemens CIS and CIGSS substrates, and with NREL CIGS substrates. A total area efficiency of 12.7% and estimated active area efficiency of 13.4% is reported for a CIGS cell with an i-ZnO buffer layer grown by MOCVD.