Effects of indium concentration on the efficiency of amorphous In-Zn-O/SiO_x/n-Si hetero-junction solar cells

Semiconductor-insulator-semiconductor (SIS) hetero-junction solar cells comprising of the amorphous indium zinc oxide (a-IZO) layer directly deposited onto the n-type Si substrates by pulsed laser deposition were fabricated. Characterizations on the physical properties of the a-IZO layer and the a-IZO/SiO_x interface as a function of In/(Zn+In) ratio were carried out to delineate their influences on the photovoltaic performance of SIS solar cells. The optical and electrical analyses indicated that the resistivity of a-IZO films decreased with increasing In concentration, reaching 4.5×10^-4 Ω-cm for In/(Zn+In)=0.5, which also exhibited a transmittance higher than 80% in the visible-light wavelength range. Moreover, combining with an optimally controlled insulating SiO_x layer (about 2.0 nm), the device exhibited excellent SIS solar cell performance with open-circuit voltage of 0.38 V, short-circuit current density of 45.1 mA cm ^-2, fill factor of 49.7% and a conversion efficiency of 8.4% under the AM1.5 illumination condition. The dramatic performance enhancement was attributed to the reduction of effective interface trap densities at the a-IZO/SiO_x interface and the increase of carrier mobility in the a-IZO layer resulted from the increase of In/(In+Zn) ratio.