Bulk resistivity optimization for low-bulk-lifetime silicon solar cells

Guidelines are presented which are designed to achieve planar solar cell ef ficiencies as high as 17.5% using existing fabrication technologies and sil icon substrates with lifetimes as low as 20 mus. Device simulations are per formed to elucidate the need and impact of base doping optimization for dif ferent back-surface passivation schemes, cell thicknesses, emitter profiles , and degrees of dopant-defect interaction. Results indicate that optimal r esistivity is a function of back-surface passivation, with the aluminum bac k-surface field (BSF) requiting the highest resistivity, the oxide/nitride stack passivation excelling at an intermediate resistivity, and the ohmic c ontact needing the lowest resistivity. A comparison of simulated 300 and 10 0 mum cells shows that thinner cells magnify the differences in optimal res istivity for the three back-surface passivation schemes. A lifetime model i s used to account for dopant-defect interaction that can lower bulk lifetim e at higher doping levels. It is demonstrated that cell efficiency decrease s and optimal resistivity increases at higher levels of dopant-defect inter action. Simulated devices with an optimized base doping showed an efficienc y improvement of as much as 2% (absolute) compared with identical devices w ith a typical base doping level (1.6 or 1.8 Omega cm) and bulk lifetime of 20 mus. Copyright (C) 2001 John Wiley & Sons, Ltd.