Low-temperature photovoltaic effect and current transport characteristics of YBa₂Cu₃O_7−x/Nb:SrTiO₃ heterojunction

In this study, a YBa₂Cu₃O_7−x (YBCO) superconducting film was prepared on Nb:SrTiO₃ (NSTO) substrate via a sol–gel method, forming a YBCO/NSTO heterojunction. The YBCO/NSTO heterojunction was irradiated with UV light, and the photovoltaic characteristics of YBCO/NSTO heterojunction were studied within the temperature range 50–300 K. The results show that YBCO/NSTO/In has a significant photovoltaic effect with a high open-circuit voltage (V_OC). At 300 K, the V_OC of YBCO/NSTO/In was ~ 0.31 V (light intensity ~ 10 mw/mm²). The V_OC of YBCO/NSTO/In increased with the decrease in temperature and reached 0.79 V when the temperature decreased to 50 K. The V_OC measurement results under different light intensities indicate that the V_OC decreased with the decrease in light intensity. The current density–voltage (J–V) curves were investigated at different temperatures (no light). The YBCO/NSTO/In heterojunction showed clear rectifying features above 200 K; with a decrease in temperature, the asymmetry of positive and negative current decreased. When YBCO underwent superconducting transition, because of the opening of YBCO superconducting energy gap, the V_OC and turn-on voltage (V_t) of J–V curves changed suddenly, and the variations in V_OC (when the light intensity is sufficient) and V_t were consistent with the superconducting energy gap of YBCO. With a decrease in the light intensity, the variations in V_OC decreased; for example, the variation was only 2 mV when the light intensity decreased to ~ 20%.