Low temperature characteristic of ITO/SiO_x/c-Si heterojunction solar cell

Based on the temperature-dependent measurements and the numerical calculation, the temperature response of the photovoltaic parameters for a ITO/SiO_x/c-Si heterojunction solar cell have been investigated in the ascending sorting of 10-300 K. Under unique energy concentrated photon irradiation with the wavelength of 405 nm and power density of 667 mW cm^-2, it was found that the short-circuit current (I_SC) was nonlinearly increased and the open-circuit voltage (V_OC) decreased with temperature. The good passivation of the ITO/c-Si interface by a concomitant SiO_x buffer layer leads to the rare recombination of carriers in the intermediate region. The inversion layer model indicated that the band gap of c-silicon was narrowed and the Fermi level of n-type silicon (E_Fⁿ) tended to that of the intrinsic Fermi level (E_Fⁱ) (in the middle of band gap) with the increase of the temperature, which lessened the built-in voltage (V_D) and thus the V_OC. However, the reduction by 90% of V_OC is attributed to the shift of E_Fⁿ in c-silicon rather than the energy band narrowing. Through the analysis of the current-voltage relationship and the data fitting, we infer that the series resistance (R_s) is not responsible for the increase of I_SC, but the absorption coefficient and the depletion-width of c-silicon are the causes of the enhancing I_SC. Mostly, the interaction of the photon-generated excess 'cold hole' and the acoustic phonon in n-Si would influence the variation of I_ph or I_SC with temperature.