Voc Degradation in TF-VLS Grown InP Solar Cells

Here we consider two hypotheses to explain the open-circuit voltage (Voc) degradation observed in thin-film vapor-liquid-solid (TF-VLS) grown p-type InP photovoltaic cells: bandgap narrowing and local shunting. First, a bandgap (Eg) narrowing effect is hypothesized, based on the surface inhomogeneity of VLS InP captured by the photoluminescence (PL) image. The PL data was used to estimate a spatially-resolved active Voc across surface of the InP sample. Combining this data with the effective Jsc allowed an assessment of the I-V characteristics of individual unit cells. Next, an H-SPICE diode compact model was utilized to reproduce the I-V characteristics of the whole sample. We find a good fit to the I-V performance of TF-VLS grown InP solar cell. Second, a local shunting effect was also considered as an alternative explanation of the Voc degradation effect. Again, PL image data was used, and small local shunt resistance was added in arbitrary elementary unit cells to represent certain dark spots seen in the PL image and dictate the Voc degradation occurred in the sample.