Development of thin polysilicon layers for application in monoPoly™ cells with screen-printed and fired metallization

Abstract

Passivating contacts are an important technological innovation for crystalline silicon (c-Si) solar cells that can deliver efficiencies of over 24% in mass production. In this study, we comprehensively analyze n-type bifacial c-Si solar cells with rear side SiOx/phosphorus-doped (n+) poly-Si passivating contacts (monoPoly™ cells) with varying thicknesses (50–250 nm) of the n+ poly-Si layer. The poly-Si layers are deposited by low-pressure chemical vapor deposition (LPCVD) and then phosphorus doped ex-situ in a diffusion furnace. The ex-situ doping is carefully optimized for each individual thickness to achieve a step-function-like doping profile. Excellent passivation properties are achieved with the SiOx/n+ poly-Si stack, with a dark saturation current density of only 1 fA/cm2 for 150 nm n+ poly-Si layers. Metallization is realized by screen printing using commercially available fire-through pastes. Very low dark saturation current densities of <50 fA/cm2 are achieved under the metal contacts for n+ poly-Si thicknesses ≥100 nm. Trade-offs in the cells’ current-voltage (I–V) parameters are analyzed as a function of n+ poly-Si thickness and efficiency limiting factors are identified for cells with 50 nm n+ poly-Si layers.