Bandgap of thin film solar cell absorbers: A comparison of various determination methods

Highlights: • Comparison of different methods for the bandgap determination from EQE • Criteria for the choice of a method are given. • Effects of interference fringes on the bandgap determination are discussed. • Recommended methods: derivative (inflection point), (E EQE)2 and ERE integrand • Open source software released for the analysis of solar cells. -- Abstract: Knowledge of the absorber bandgap is often needed for assessing the junction quality of a thin film solar cell, for example when computing the open-circuit voltage deficit. The bandgap is typically estimated from the routine measurement of the external quantum efficiency (EQE) of finished devices. However the extraction becomes ambiguous in the case of very thin absorbers, or in presence of bandgap gradients or collection issues of charge carriers. This work reviews several methods for the determination of the bandgap from EQE measurements and discusses their validity conditions. The numerical results are compared based on experimental EQE of several different thin film solar cells such as CuInGaSe₂, CuInSe₂, CdTe, CuZnSnSe and perovskite, and systematic trends are identified. Numerical simulations are also performed to illustrate the behavior of the different bandgap extraction methods in presence of a bandgap gradient and different magnitudes of the exponential tail states.