Role of wafer resistivity in silicon heterojunction solar cells performance: Some insights on carrier dynamics

In this work, we show the n-type silicon wafer resistivity (doping concentration) variation effect on the power conversion efficiency (PCE) of silicon heterojunction (SHJ) solar cells aided by a detailed simulation study. Initially, to identify the recombination-induced loss mechanisms based on wafer resistivity, the effective minority carrier lifetimes (τ_eff) were analyzed. It revealed that τ_eff at the operating point of maximum power (MPP) is strongly influenced by the doping density in the wafer. The SRH recombination remains relatively invariant between the wafer resistivity range ∼ 0.8 to ∼ 4.5 Ω-cm with τ_SRH ∼ 7 ms, whereas Auger recombination is significantly enhanced (τ_Aug from ∼ 7 ms to ∼ 4 ms) in low resistivity wafers. Low wafer resistivity also has a positive impact on the built-in potential at the n-c-Si/p-a-Si:H interface, increasing MPP voltage (V_mpp), from ∼ 600 mV to ∼ 639 mV and a pFF from ∼ 81 % to ∼ 85 %. It is also verified with built-in potential variation estimation based on wafer doping concentration using the Mott-Schottky plot generated from capacitance-voltage measurements. TCAD 3D device simulations also clarify a narrow space charge region with field effect at the n-c-Si/p-a-Si:H interface with higher doping influencing V_mpp and pFF, which is verified by incorporating fixed bulk defect impurities in the wafer. Contact resistivity measurements also highlight reduced resistance contribution from wafer and electron-selective contacts from 0.114 Ω-cm² to 0.040 Ω-cm², leading to overall PCE of an SHJ cell from ∼ 21.8 % to ∼ 23.4 % with a low-resistivity wafer.