Ex-situ doping of polysilicon hole contacts for silicon solar cells via electron-beam boron evaporation

In this study, a novel method for doping of p⁺ polysilicon (poly-Si)/SiO_x passivated contacts is demonstrated. This is achieved by using a thin (∼3 nm) boron layer, deposited by electron beam evaporation, as a dopant source on top of an intrinsic poly-Si layer, which allows diffusion of boron into the structure at temperatures above 900 °C. Surface passivation, exemplified by the implied open circuit voltage (iV_oc), and contact resistance, represented by the specific contact resistivity (ρ_c), were studied as a function of activation parameters including the drive-in temperature/time. By optimising the activation condition, doping layer thickness, and hydrogenation process, an iV_oc of 709 mV and a ρ_c of 3.2 mΩcm² is achieved for a 180 nm poly-Si film. This technique was also demonstrated to allow simple patterning of p⁺ poly-Si regions via use of a shadow mask during the boron deposition process. These results highlight an alternative way to form patterned region doping for high performance p⁺ poly-Si/SiO_x passivated contacts, allowing advanced silicon solar cell architectures.