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

Chalcopyrite selenide single crystals and epitaxial layers (CuIn 1− x Ga x Se 2 , 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 E a , E b , and E c of both CuInSe 2 (CIS) and CuGaSe 2 (CGS) layers sensed by PR were higher than the energies of single crystals. CuInSe 2 and CuGaSe 2 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 E a , E b , E c energies of tensely stressed CuGaSe 2 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 CuGaSe 2 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 E FE , in the PL‐spectra, was lower than the E a transition energy, in the PR‐spectra, which is assigned to partial quenching of Δ CF with the increase of external tensile stress by gallium‐segregation at the chalcopyrite/GaAs‐interface. The stress dependence of Δ CF is negligible in CuInSe 2 and linear, with a rate of 9 meV/100 MPa, in CuGaSe 2 . It is revealed that the energy band‐gap of photovoltaic chalcopyrite absorbers can be tuned by simultaneous built‐in and external lattice‐tuning.