Interface study of CuInSe2/ZnO and Cu(In,Ga)Se2/ZnO devices using ALD ZnO buffer layers

Abstract Solar cells based on CuInSe2 (CIS) and Cu(In,Ga)Se2 (CIGS), with a ZnO buffer layer deposited by atomic layer deposition (ALD), are compared to their reference cells; (CIS or CIGS)/CdS/ZnO. While the CIS/ZnO devices show only slightly lower efficiencies compared to their reference cells, the difference between the CIGS/ZnO devices and their reference cells is larger. In the latter case, the main difference is the lower open circuit voltage of approximately 200 mV of the direct ZnO devices. The valence band offset between CIS and ZnO is determined by ultraviolet photoelectron spectroscopy to −2.2 eV which gives a conduction band offset, ΔEc, of +0.1±0.2 eV. This slightly positive offset is in contrast to our previous result for the CIGS/ZnO interface of ΔEc=−0.2±0.2 eV, and is a possible explanation for the much lower voltage loss observed for the CIS/ZnO devices. Zn diffusion into the different absorbers is investigated by energy dispersive X-ray spectroscopy on transmission electron microscope cross-sections prepared from direct ZnO devices. These cross-sections also show very good coverage of the absorber surface by the (ALD)ZnO layer.