Design of energy band alignment at the Zn 1-xMg xO/Cu(In,Ga)Se 2 interface for Cd-free Cu(In,Ga)Se 2 solar cells

Abstract

The electronic band structure at the Zn 1-xMg xO/ Cu(In 0.7Ga 0.3)Se 2 interface was investigated for its potential application in Cd-free Cu(In,Ga)Se 2 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)Se 2 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)Se 2 and Zn 0.9Mg 0.1O/Cu(In 0.7Ga 0.3)Se 2 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)Se 2 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. © 2012 the Owner Societies.