Dependence of Crystal‐Field Energy on Strain/Stress Sensed by Temperature Variation of Chalcopyrite Semiconductor (Optical) Band‐Gap for Efficient Band‐Gap Tuning in the CIS/CIGS Photovoltaic

Abstract

Chalcopyrite selenide single crystals and epitaxial layers (CuIn1−xGaxSe2, x = 0.00, 0.08, 0.19, 1.00) were characterized by temperature‐dependent photoreflectance (PR), photoluminescence (PL), photoluminescence–excitation (PLE), and variable excitation‐energy photoluminescence (VEPL) spectroscopy. The transition energies Ea, Eb, and Ec of both CuInSe2 (CIS) and CuGaSe2 (CGS) layers sensed by PR were higher than the energies of single crystals. CuInSe2 and CuGaSe2 grown on GaAs(001) underlie compressive and tensile stresses, respectively, which lead to band‐gap broadening in CIS and band‐gap narrowing in CGS. The increase of the Ea, Eb, Ec energies of tensely stressed CuGaSe2 layers to energies higher than those of the bulk originates from the stress dependence of the non‐cubic crystal field. Band‐gap scanning of the CuGaSe2 layer with continuous‐wave Ti:sapphire‐laser confirmed the absence of correlation between band‐gap readjustment and intrinsic defects. The energy of the band‐edge exciton EFE, in the PL‐spectra, was lower than the Ea transition energy, in the PR‐spectra, which is assigned to partial quenching of ΔCF with the increase of external tensile stress by gallium‐segregation at the chalcopyrite/GaAs‐interface. The stress dependence of ΔCF is negligible in CuInSe2 and linear, with a rate of 9 meV/100 MPa, in CuGaSe2. It is revealed that the energy band‐gap of photovoltaic chalcopyrite absorbers can be tuned by simultaneous built‐in and external lattice‐tuning.