Impacts of Fermi Level Pinning at Hole-Selective Contacts in CdSeTe/CdTe Solar Cells

TL;DR

This paper develops a physics-based model to analyze how Fermi level pinning at hole contacts affects the efficiency of CdSeTe/CdTe solar cells, highlighting potential improvements with passivated layers.

Contribution

It introduces a comprehensive device physics model that explains the impact of Fermi level pinning on solar cell performance and explores strategies for efficiency enhancement.

Findings

Fermi level pinning primarily reduces fill factor (FF).

Passivated hole-selective layers can mitigate downward band bending.

Model aligns with experimental characterization of materials and devices.

Abstract

P-type doped CdTe free surfaces Schottky contacts, and even interfaces with isostructural p-ZnTe frequently exhibit downward band bending and moderate to high recombination velocities. Fermi level pinning by donor-like states can explain these band diagram features, as well as device response characteristics such as 1st quadrant rollover in current-voltage (JV) versus temperature (JVT). Parasitic downward band bending also produces voltage-dependent photocurrent collection, producing fill factor (FF) efficiency losses, JV dark/light non-superposition (or JV take-off), and irregularities in Jsc-Voc and Suns-Voc measurements. Herein, we develop a device physics model of state-of-the-art CdSeTe/CdTe solar cells consistent with known characterization of materials and devices, including the optical, thermalization, and trapping effects of band tail states and isolated defects. We use this model to demonstrate that Fermi-level pinning at the p-ZnTe/p-CdSeTe hole contact by donor-like defects reproduces the aforementioned observables, and conclude that (for contemporary few-um absorber thicknesses and low mobilities) it primarily affects FF rather than Voc. We investigate the performance gains possible from hypothetical passivated, hole-selective layers at the ZnTe/CdTe interface, which eliminate the downwards band bending caused by donor-like defects. For thinner devices and larger minority carrier diffusion lengths, these strategies will become more important for continued efficiency improvements.