Exploring Cd-Zn-O-S alloys for improved buffer layers in thin-film photovoltaics

Abstract

To compete with existing and more mature solar cell technologies such as crystalline Si, thin-film photovoltaics require optimization of every aspect in the device heterostructure to reach maximum efficiencies and cost effectiveness. For absorbers like CdTe, ${\mathrm{Cu}(\mathrm{In},\mathrm{Ga})\mathrm{Se}}_{2}$ (CIGSe), and ${\mathrm{Cu}}_{2}{\mathrm{ZnSn}(\mathrm{S},\mathrm{Se})}_{4}$ (CZTSSe), improving the $n$-type buffer layer partner beyond conventional CdS is one avenue that can reduce photocurrent losses and improve overall performance. Here, we use first-principles calculations based on hybrid functionals to explore alloys spanning the Cd-, Zn-, O-, and S-containing phase space to identify compositions that may be superior to common buffers like pure CdS or Zn(O,S). We address issues highly correlated with device performance such as lattice-matching for improved buffer-absorber epitaxy and interface quality, dopability, the band gap for reduced absorption losses in the buffer, and the conduction-band offsets shown to facilitate improved charge separation from photoexcited carriers. We supplement our analysis with device-level simulations as parameterized from our calculations and real devices to assess our conclusions of low-Zn and O content buffers showing improved performance with respect to CdS buffers.