About the efficiency limits of heterojunction solar cells

Abstract

We study n-type hydrogenated amorphous silicon/p-type crystalline silicon heterojunction solar cells focusing on the sensitivity of the open-circuit voltage to the presence of an intrinsic layer and to various recombination mechanisms. We describe our fabrication process based on low-cost steps, even for the metallization of the front side. Heterojunctions are characterized by quasi-steady-state photoconductance, capacitance vs. temperature and frequency, capacitance–voltage, quantum efficiency and current–voltage measurements. A new simulation code enables us to model the solar cells. Our best heterojunction solar cell exhibits an efficiency of 15.20% without texturization. We also report a record open-circuit voltage of 677mV for a heterojunction solar cell on p-type substrate and our first results for textured wafers. For our optimized solar cells, the role of the intrinsic layer and the recombination at the a-Si:H/c-Si is negligible. We highlight the importance of conduction band offset in the efficiency of heterojunction solar cells and exhibit a triple paradox in the comprehension of these structures, especially in the comparison between heterojunctions solar cells based on n-type or p-type substrates.