High Efficiency Undoped Silicon Heterojunction Solar Cells

Unlike conventional solar cells, modern high efficiency passivated contacts solar cells like silicon heterojunction (SHJ) cells have excellent surface passivation and use high bulk lifetime wafers which increase the operating injection level of these devices. These solar cell architectures can benefit from having lower doped substrates, with undoped solar cells becoming an attractive option. There has been very limited literature on high bulk resistivity substrates (>>10 Ωcm). This thesis work provides a comprehensive assessment of the potential of high resistivity/undoped substrates for high performance and more reliable silicon solar cells by demonstrating the results from modeling as well as characterization of SHJ solar cells fabricated with high resistivity/undoped substrates under real-world illumination and temperature conditions that the cells/modules experience in the field. In this work, the results from the analytical model demonstrated the effects of various defects, variation in doping and temperature on the performance of silicon solar cells. Experimentally, SHJ cells with bulk resistivities in the range of 1 Ωcm to >15k Ωcm were fabricated, and cell efficiencies over 20% were measured at standard testing conditions (STC) across the entire range of bulk resistivities. The illumination response (0.1-1 sun) and temperature coefficients (25-90 °C) were shown to be independent of the bulk resistivity. No light induced degradation was observed in the n-type SHJ cells of all resistivity ranges whereas high resistivity p-type SHJ cells showed less degradation compared to that of commercial resistivity range (<10 Ωcm). Very high reverse breakdown voltages (over 1 kV) were demonstrated for SHJ cells fabricated with high resistivity wafers. Using simulation, the importance of having cells in the modules with breakdown voltage higher than the series string voltage for safe and reliable operation of the photovoltaic (PV) system was highlighted. The ingot yield can be improved by moving towards high resistivity ranges to manufacture high efficiency reliable solar cells by utilizing the entire ingot and eliminating the need to adhere to narrow resistivity range. Thus, the novel findings from this work can have profound impact on ingot and module manufacturing resulting in significant cost savings as well as improvement in the system reliability.