Fabrication and characterization of amorphous Si/crystalline Si heterojunction devices for photovoltaic applications

Abstract

Heterojunction diode and heterojunction photovoltaic cell structures are fabricated with (n+)a-Si/(i)a-Si and rf-sputtered indium-tin-oxide/Al films deposited on p-type crystalline Si using a plasma-enhanced chemical vapor deposition cluster tool system. Dark current–voltage characteristics of the heterojunction diodes are used to determine the carrier transport mechanisms. Experimental results showed the current is recombination-dominated at low forward bias (VA<0.25 V at 27 °C), multitunneling capture emission (MTCE)-dominated at medium bias (0.25<VA<0.45 at 27 °C), a combination of diffusion or thermionic emission and MTCE at high bias, and series resistance limited at very high bias ranges. Measurement results show that the boundary between each region is not fixed and is strongly dependent on temperature. High-frequency capacitance–voltage measurement has been used to extract band diagram parameters of the heterojunction diodes. Measured parameters, built-in potential, and conduction and valence band offsets, have been verified by Analysis of Microelectronic and Photonic Structures numerical device simulator. External quantum efficiency has been measured on photovoltaic cells and used for calculation of short-circuit current under AM1.5 illumination.