Optimized Ga-Doped Cz Wafers for POLO IBC Solar Cells with High Efficiency and Minimal LeTID Degradation

The PV industry is using 0.4 to 0.8 Ωcm low-resistivity Ga-doped Cz-Si wafers with carrier lifetimes < 700 µs for mass production of PERC and bifacial PERC+ cells. Our Quokka3 simulations predict, that a Ga-doped Cz wafer resistivity around 1.5 Ωcm will increase Voc and Jsc and the POLO IBC solar cell efficiency from the presently best measured efficiency of 23.9% up to a simulated efficiency of 24.3%, because the charge carrier density in POLO IBC solar cells depends less on the doping concentration due to their much higher Voc. In this paper, we investigate Gadoped Cz wafers with higher resistivities ranging from 0.7 to 4.8 Ωcm from Cz-Si ingots supplied by NorSun and Kalyon PV. POLO IBC lifetime precursors processed at ISFH without metal contacts show, that the effective carrier lifetime linearly increases with higher wafer resistivity up to 3 ms at 4.0 Ωcm. The measured implied Voc of up to 740 mV and the implied FF of up to 86% correspond to a calculated implied efficiency of 26.7% for a wide wafer resistivity range. The measured IV parameters of POLO IBC solar cells processed with different Ga-doped Si wafer resistivities agree qualitatively with our Quokka 3 simulations. During LeTID degradation conditions at 0.5 sun, 80°C, the POLO IBC lifetime samples show a moderate degradation in implied Voc and implied FF and reveal, that the LeTID-related wafer bulk defect density is independent of the Ga doping concentration. The POLO IBC solar cells show a maximum LeTID degradation of the efficiency of 4%rel which fully recovers during extended testing. The LeTID degradation could be further minimized by adjusting the firing temperature profile. In addition, we present our near-term POLO IBC efficiency roadmap towards 25% by optimizing the POLO IBC screen-printed Ag and Al metallization.