Competition-induced bipolar transient photocurrent as a probe of quasi-Fermi level dynamics

Transient photocurrents are promising for applications in intelligent sensing and high-performance photovoltaics due to their fast response and high efficiency. However, the physical origin of transient spike photocurrents in semiconductor devices remains debatable. Here, we design a hybrid Mn-doped n-ZnO/p-Si and p-Si/Al Schottky structure, which exhibits a bipolar transient photocurrent due to junction competition. This provides an ideal platform for quantifying quasi-Fermi level dynamics during instantaneous illumination. By tuning the Mn doping concentration in ZnO and the illumination wavelength, we control the photogenerated carrier distribution in the Si interlayer, modulating the quasi-Fermi level profile and splitting at each junction. This approach allows the precise regulation of the amplitude and even polarity of the net transient photocurrent. This bipolar photoresponse enables a quantitative and predictive model that correlates macroscopic transient photocurrent with the microscopic quasi-Fermi level splitting, providing a smart way to tailor transient photocurrent via dynamic band alignment engineering.