Photoreflectance at operating bias links interface traps to photovoltaic performance in ZnSnP2 solar cells

We apply photoreflectance (PR) and electrically biased PR (EBPR) to quantify the electric field and interface trap density at the CdS/ZnSnP2 junction in ITO/CdS/ZnSnP2/Cu3P/Cu solar cells and relate them to device performance. PR spectra exhibit Franz–Keldysh oscillations above the ZnSnP2 band edge, yielding the surface electric field and surface potential. EBPR provides the surface-potential dependence on DC bias in the forward-bias range relevant to solar-cell operation. Applying the Terman formalism to this dependence yields the energy-resolved interface trap density (Dit), which is donor-like. A cell with a dominant Dit component at ∼0.3 eV above the ZnSnP2 valence band maximum shows a lower short-circuit current density. In addition, this component lies near the surface potential corresponding to the diode knee, where its occupancy changes during operation, suggesting that recombination via this state contributes to the higher dark ideality factor and reduced open-circuit voltage. PR/EBPR thus offers a practical layer-selective approach to link operational electric fields and interface traps to photovoltaic performance.