Decoupling Reversible Interface Trapping and Irreversible Bulk Transitions in Solution-Processed Indium Zinc Oxide Thin-Film Transistors

In this study, we systematically decoupled reversible charge transitions via recombination and irreversible bulk trapping via ionization in solution-processed indium zinc oxide thin-film transistors (TFTs) under positive- and negative-bias-stress (PBS and NBS) conditions. We defined highly decoupled degradation behavior by completely evaluating time-dependent transfer characteristics and saturation leakage currents across a range of indium molarities (0.0125 M to 0.2 M). Results indicate that PBS-induced instability is likely governed by a reversible electrostatic neutralization process reducing total effective shallow and deep acceptor-like states, which are dynamically counteracted by interfacial recombination at the dielectric/semiconductor boundary. Conversely, severe degradation under NBS originated from irreversible bulk trapping triggered by the ionization of donor-like oxygen vacancies in a ZnO amorphous random network. Total effective trapped charges were calculated from threshold voltage shifts to clarify these defect kinetics quantitatively; these calculations demonstrated direct correlation with the integrated theoretical capacities of the deep and shallow acceptor-like gap-state distributions. Finally, we propose a comprehensive density of state–energy band alignment model incorporating thermal activation energies and flat-band voltages. This analytical framework proves that the composition-dependent Fermi level positioning rigorously limits and dictates complex bias-stress instabilities, offering profound insights for designing highly stable amorphous oxide semiconductor TFTs.