Abstract
Energy band alignment at the heterointerface between CdS and kesterite Cu2ZnSnS4 (CZTS) and its alloys plays a crucial role in determining the efficiency of the solar cells. Whereas Ag alloying of CZTS has been shown to reduce anti-site defects in the bulk and thus rise the efficiency, the electronic properties at the interface with the CdS buffer layer have not been extensively investigated. In this work, we present a detailed study on the band alignment between n-CdS and p-CZTS upon Ag alloying by depth-profiling ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Our findings indicate that core-level peaks and the valence band edge of CdS exhibit a significant shift to a lower energy (larger than 0.4 eV) upon the etching of the CdS layer, which can be assigned due to band bending and chemical shift induced by a change in the chemical composition across the interface. Using a simplified model based on charge depletion layer conservation, a significantly larger total charge region depletion width was determined in Ag-alloyed CZTS as compared to its undoped counterpart. Our findings reveal a cliff-like band alignment at both CdS/CZTS and CdS/Ag-CZTS heterointerfaces. However, the conduction-band offset decreases by more than 0.1 eV upon Ag alloying of CZTS. The approach demonstrated here enables nanometer-scale depth profiling of the electronic structure of the p–n junction and can be universally applied to study entirely new platforms of oxide/chalcogenide heterostructures for next-generation optoelectronic devices.
Highlights
The Cu2ZnSn(SxSe1−x)[4] (CZTSSe) solar cells have currently reached a power conversion efficiency of 12.6%1, which is slightly more than half the efficiency of C uInxGa1−xSe2 (CIGS) solar c ells[2]
We perform a systematic study on the band alignment at the CdS/CZTS and CdS/ACZTS interfaces using X-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS)
XPS/UPS is a non-destructive tool to investigate the electronic properties of a semiconductor and can be used to determine heterojunction offsets, as demonstrated previously for CdS/ CIGS20
Summary
The Cu2ZnSn(SxSe1−x)[4] (CZTSSe) solar cells have currently reached a power conversion efficiency of 12.6%1, which is slightly more than half the efficiency of C uInxGa1−xSe2 (CIGS) solar c ells[2]. There is an optimum value for Ag alloying of CZTS because: i) a high Ag content reduces the p-type conductivity and leads to an n-type conductivity in the case of AZTS, and ii) the band gap increases from 1.5 eV for pure CZTS to 2.1 eV for AZTS, which is no longer an optimum value for a single junction solar cell. A positive conduction band offset (CBO) of ~ 0.25 eV has been experimentally estimated for the CdS/ CIGS device with a power conversion efficiency better than 18%9. On the other hand, when the CBO is negative, that is, the position of the CBM of the n-type semiconductor is lower than that of the p-type semiconductor (cliff-like band alignment), the cell performance is poor due to enhanced carrier recombination at trap states near the interface[5,13]. The samples were vacuum-sealed and transported to the beamline for measurements
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