Design of energy band alignment at the Zn1−xMgxO/Cu(In,Ga)Se2 interface for Cd-free Cu(In,Ga)Se2 solar cells

Abstract

The electronic band structure at the Zn_1−xMg_xO/Cu(In_0.7Ga_0.3)Se₂ interface was investigated for its potential application in Cd-free Cu(In,Ga)Se₂ thin film solar cells. Zn_1−xMg_xO thin films with various Mg contents were grown by atomic layer deposition on Cu(In_0.7Ga_0.3)Se₂ absorbers, which were deposited by the co-evaporation of Cu, In, Ga, and Se elemental sources. The electron emissions from the valence band and core levels were measured by a depth profile technique using X-ray and ultraviolet photoelectron spectroscopy. The valence band maximum positions are around 3.17 eV for both Zn_0.9Mg_0.1O and Zn_0.8Mg_0.2O films, while the valence band maximum value for CIGS is 0.48 eV. As a result, the valence band offset value between the bulk Zn_1−xMg_xO (x = 0.1 and x = 0.2) region and the bulk CIGS region was 2.69 eV. The valence band offset value at the Zn_1−xMg_xO/CIGS interface was found to be 2.55 eV after considering a small band bending in the interface region. The bandgap energy of Zn_1−xMg_xO films increased from 3.25 to 3.76 eV as the Mg content increased from 0% to 25%. The combination of the valence band offset values and the bandgap energy of Zn_1−xMg_xO films results in the flat (0 eV) and cliff (−0.23 eV) conduction band alignments at the Zn_0.8Mg_0.2O/Cu(In_0.7Ga_0.3)Se₂ and Zn_0.9Mg_0.1O/Cu(In_0.7Ga_0.3)Se₂ interfaces, respectively. The experimental results suggest that the bandgap energy of Zn_1−xMg_xO films is the main factor that determines the conduction band offset at the Zn_1−xMg_xO/Cu(In_0.7Ga_0.3)Se₂ interface. Based on these results, we conclude that a Zn_1−xMg_xO film with a relatively high bandgap energy is necessary to create a suitable conduction band offset at the Zn_1−xMg_xO/CIGS interface to obtain a robust heterojunction. Also, ALD Zn_1−xMg_xO films can be considered as a promising alternative buffer material to replace the toxic CdS for environmental safety.