Evolution of the Charge Carrier Lifetime Characteristics in Crystalline Silicon Wafers During Processing of Heterojunction Solar Cells

All process steps in the fabrication of amorphous / crystalline (a-Si:H/c-Si) silicon heterojunction solar cells affect interface recombination and thus have an impact on the open circuit voltage of the final solar cell. Transient photoconductance decay is a convenient method to assess the charge carrier lifetime which is a direct measure for recombination and thus the interface quality. In this contribution we present a step-by-step analysis and evaluation of the charge carrier lifetime throughout the processing of the solar cells starting from the saw-damage etched Si wafer to the complete solar cell device. Particular emphasis is put on the optimization of the random pyramid texture. For this, surface morphologies and a-Si:H/c-Si interface properties were quantitatively evaluated and optimized with respect to low recombination and large carrier lifetimes. Based on this optimization a-Si:H/c-Si heterojunction solar cells with conversion efficiencies exceeding 20% were prepared on c-Si wafers textured in isopropanol(IPA)-free alkaline solution.