Calcium contacts to n-type crystalline silicon solar cells

Abstract

Direct metallization of lightly doped n-type crystalline silicon (c-Si) is known to routinely produce non-Ohmic (rectifying) contact behaviour. This has inhibited the development of n-type c-Si solar cells with partial rear contacts, an increasingly popular cell design for high performance p-type c-Si solar cells. In this contribution we demonstrate that low resistance Ohmic contact to n-type c-Si wafers can be achieved by incorporating a thin layer of the low work function metal calcium (ϕ ~2.9 eV) between the silicon surface and an overlying aluminium capping layer. Using this approach, contact resistivities of ρc ~2mΩcm2 can be realised on undiffused n-type silicon, thus enabling partial rear contacts cell designs on n-type silicon without the need for a phosphorus diffusion. Integrating the Ca/Al stack into a partial rear contact solar cell architecture fabricated on a lightly doped (ND = 4.5 × 1014 cm-3 ) n-type wafer resulted in a device efficiency of η = 17.6% where the Ca/Al contact comprised only ~1.26% of the rear surface. We demonstrate an improvement in this cell structure to an efficiency of η = 20.3% by simply increasing the wafer doping by an order of magnitude to ND = 5.4 × 1015 cm-3