Comparative operando XPS Studies of Quasi-Fermi Level Splitting and Open-Circuit Voltage in CZTSe/CdS and CIGS/CdS Junctions and Device Structures

This contribution describes operando x-ray photoelectron spectroscopy (opXPS) studies of Cu(In,Ga)Se₂ (CIGS) and Cu₂ZnSnSe₄ (CZTSe) absorber layers and device structures. X-ray-excited valence-band and core-level spectra were acquired on bare absorber surfaces and after CdS and ZnO:Al depositions in standard absorber/CdS/ZnO/ZnO:Al device stacks. OpXPS data sets were acquired at various x-ray fluxes, with and without white-light illumination. From these measurements, quasi-Fermi level splitting (ΔE_F) values in the absorber/CdS junctions were measured directly as a function of excitation conditions. For both CIGS and CZTSe, results show that ΔE_F proportional to the full open-circuit voltage (V_OC) of the completed devices is present after the deposition of the CdS layer - i.e., ΔE_F = qV_OC - demonstrating that the so-called V_OC deficit in CZTSe solar cells is also present at this stage of processing. The authors find that photoexcitation due to x-rays or stray visible light during XPS or similar measurements can produce measurable photovoltages in materials, absorber/CdS junctions, and finished devices. In situ current density versus voltage measurements on a typical CIGS device reveal that x-ray-induced photoexcitation associated with typical XPS measurement conditions is equivalent to ∼3 × 10^-3 suns, which in this study produced V_OC = 440 mV. These findings demonstrate that accounting for photoexcitation conditions during XPS or similar measurements can improve band-offset determinations and produce more reliable values for the junction built-in voltage. The implications of the study findings on the CZTSe V_OC deficit and device performance are discussed.