Synergistic Enhancement of Photoresponse and Humidity Response in PPy/TiO2 Heterostructures

This study investigates the responses in the resistance of polypyrrole (PPy) and titanium dioxide (TiO2) single layers and PPy/TiO2 heterostructures to rectangular pulses of monochromatic LED illumination under controlled relative humidity. Exposure to moisture increases the resistance of PPy while rapidly decreases that of TiO2, reflecting opposite effects of electron donation from hydroxyl (–OH) groups in adsorbed H2O on the majority carrier densities of the two materials. Under monochromatic illumination, the resistance of PPy decreases, whereas that of TiO2 increases accompanied by a brief transient at excitation wavelengths near its optical edge (367 nm and 398 nm). This photoresponse is attributed to photoinduced modifications of carrier density through two opposing processes: photogeneration, which enhances charge carriers, and H2O photodesorption, which reduces them. A combination of illumination and moisture exposure results in an intensified photoresponse of PPy, thereby enhancing its humidity response. The PPy/TiO2 heterostructure demonstrates a mixed photoresponse arising from contrasting behaviors of its p-type PPy and n-type TiO2 components, coupled with charge exchange across the p–n junction. When simultaneously exposed to moisture and monochromatic light, the heterostructure undergoes contrasting photoinduced carrier generation in its two components, which in turn modulates the depletion region in inverse phase with carrier density, thereby intensifying the overall photoresponse. This complementary “push–pull” interaction synergistically enhances both the photoresponse and the humidity response of the PPy/TiO2 heterostructure.