MECHANISM OF CARRIER TRANSPORT IN HIGHLY EFFICIENT SOLAR-CELLS HAVING INDIUM TIN OXIDE SI JUNCTIONS

The carrier transport mechanism of the Si solar cells having n-Si/indium tin oxide (ITO) junctions has been studied by use of the current-voltage and capacitance-voltage measurements and x-ray photoelectron spectroscopy. An 11-angstrom-thick nonstoichiometric Si oxide layer is formed when ITO is deposited by spray pyrolysis on a Si electrode etched with hydrofluoric acid. In this case, the tunneling probability of majority carriers through the oxide layer is high, and the thermionic emission current over the energy barrier in Si takes a dominant part of the dark current. On the other hand, for a Si electrode where a Si oxide layer is intentionally interposed between ITO and Si, the thermionic emission current is suppressed, and trap-assisted multistep tunneling through the depletion layer becomes dominant. By making a mat-structure treatment on the Si surface, a solar energy conversion efficiency of 13% and the photocurrent density of 42.5 mA cm-2 were attained under AM 1 100 mW cm-2 illumination.