MECHANISM OF CARRIER TRANSPORT THROUGH A SILICON-OXIDE LAYER FOR [INDIUM-TIN-OXIDE/SILICON-OXIDE/SILICON] SOLAR-CELLS

The mechanism of carrier transport through a thin silicon-oxide layer for (spray-deposited indium-tin-oxide (ITO)/silicon-oxide/Si) solar cells has been studied by measurements of the dark current density as a function of the thickness of the silicon-oxide layer, together with the observation of transmission electron micrographs. Cross-sectional transmission electron micrography shows that a uniform. silicon-oxide layer with the thickness of similar to 2 nm is present between ITO and Si when the ITO film is deposited on a flat Si(100) surface at 450 degrees C. The dark current density under a depletion condition strongly depends on the thickness of the silicon-oxide layer. It is concluded from these results that quantum mechanical tunneling is the dominant mechanism for the charge carrier transport through the silicon-oxide layer. On the other hand, when the ITO film is deposited on a mat-textured Si surface at the same temperature, a nonuniform silicon-oxide layer is formed, with ITO penetrating into the silicon-oxide layer in the top and valley regions of the pyramidal structure. By raising the deposition temperature of the ITO film on the flat Si(100) surface to 500 degrees C, the silicon-oxide layer becomes also nonuniform. For these diodes with the nonuniform silicon-oxide layer, the carrier transfer probability is less dependent on the thickness of the silicon-oxide layer, leading to the conclusion that minute channels of ITO are present in the silicon-oxide layer and charge carriers transfer through the channels. The photovoltage is decreased by the presence of the minute channels, with its magnitude depending on the density of the channels. The conversion efficiency of the [ITO/silicon-oxide/n-Si(100)] solar cells is unchanged upon illumination for 1000 h. The good cell stability is attributed to the well-crystallized ITO film which effectively suppresses diffusion of oxygen from the air and to low reactivity of ITO with Si at room temperature. (C) 1995 American Institute of Physics.