Light-Bias-Dependent External Quantum Efficiency of Kesterite Cu2ZnSnS4 Solar Cells

The light-bias-dependent behavior of external quantum efficiency (EQE) in kesterite Cu2ZnSnS4 solar cells utilizing different buffers has been reported. Under red light bias, the blue EQE can be enhanced significantly, exceeding unity in magnitude due to photoconductivity of buffers increasing depletion region width and thereby increasing collection efficiency. Under blue light bias, the red EQE increases only in devices with a hybrid buffer. It stays constant with a CdS buffer due to saturated photoconductivity and even decreases with an In2S3 buffer due to optical injection of blue photons through the more transparent In2S3 into the absorber. Under white illumination, the enhancement can be observed only with unsaturated buffers such as In2S3 and the hybrid buffer, while the red EQE reduces due to optical injection. This light-bias-dependent behavior in EQE (including EQE exceeding unity) can be attributed to two key factors: photoconductivity of the buffer layers combined with low minority carrier lifetime of absorber. The effect leads to disagreement between Jsc measured under a simulator and that calculated from the integral of the EQE, revealing the necessity for light bias dependence investigation when verifying the consistency between them. Improving the minority carrier lifetime in absorber and majority carrier concentration in buffer to boost the device electrical property and performance is suggested based on this investigation.