Thin film solar cells based on Cu(In,Ga)Se2, called CIGS, is one of the most promising technologies for low cost, high
efficiency photovoltaics. The CIGS device is composed of four layers; molybdenum back contact, CIGS p-type absorber,
n-type buffer layer and doped ZnO top contact. The most common buffer layer is CdS, however it is desirable to find a
Cd-free, large band gap alternative. In this paper, the use of ZnO-based buffer layers deposited by atomic layer
deposition, ALD is described. Efficiencies of over 18% are shown by using Zn(O,S) or (Zn,Mg)O by ALD followed by
sputtered ZnO:Al. The role of the conduction band alignment across the heterojunction is discussed, and results for large
band gap CuGaSe2 absorbers are presented. In addition, light-soaking effects for devices with (Zn,Mg)O-based buffer
layers are related to measurements of persistent photoconductivity of ALD-(Zn,Mg)O thin films.