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

Abstract

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 CdTe‐based solar cell characteristics such as first quadrant rollover in current–voltage (J‐V) versus temperature (JVT). Parasitic downward band bending also produces voltage‐dependent photocurrent collection, producing fill factor (FF) efficiency losses, J‐V dark/light non‐superposition (or “J‐V take‐off”), and irregularities in J sc –V oc and suns–V oc 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‐μm absorber thicknesses and low mobilities) it primarily affects FF rather than V oc , with removal of donor‐like interface defects increasing the simulated FF from 74.8% to 76.6%. 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.