Cu‐rich CuInSe2 solar cells with a Cu‐poor surface

We present the effects of an In‐Se surface treatment of Cu‐rich ([Cu]/[In] > 1) CuInSe₂ absorbers in order to utilize the superior transport properties of Cu‐rich absorbers for high‐efficiency solar cells. A Cu‐poor surface was successfully formed on a Cu‐rich absorber by co‐evaporation of indium and selenium under appropriate conditions. The aim is to suppress the interface recombination, which is generally observed at the interface between CdS and Cu‐rich CuInSe₂. A surface‐treated device achieved an efficiency of 13.1%, improved from 9.5% obtained with an untreated Cu‐rich device. The open‐circuit voltage of In–Se treated devices reach the same level as devices made from Cu‐poor absorbers grown by a three‐stage process. By comparing the results of the experiments and simulations, it is shown that the role of the Cu‐poor surface on Cu‐rich absorbers is to prevent interface recombination, mainly due to a lower doping level of the Cu‐poor surface layer than in the Cu‐rich CuInSe₂. Thus, it becomes possible to uncouple the interface from the bulk properties in Cu‐rich solar cells. Copyright © 2014 John Wiley & Sons, Ltd.