Nanophotonic engineering holds great promise for photovoltaics: the record conversion efficiencies of nanowire solar cells are increasing rapidly, and the record open-circuit voltages are becoming comparable to record planar equivalents. Furthermore, several authors have suggested that nanophotonic effects such as large absorption cross sections can reduce cost and increase efficiencies with respect to planar solar cells. These effects are particularly pronounced in single nanowire devices, where two out of the three dimensions are subwavelength. Therefore, single nanowire devices provide an ideal platform to study how nanophotonics affects photovoltaics. However, for these devices the standard definition of power conversion efficiency no longer applies, because the nanowire can absorb light from an area much larger than its own size. Additionally, the thermodynamic limit on the photovoltage is a priori unknown and may be very different from that of a planar solar cell. This complicates characterization and optimization of such nanoscale devices. Here we analyse an InP single nanowire solar cell using intrinsic metrics to place its performance on an absolute thermodynamic scale and pinpoint performance loss mechanisms. Determining these metrics requires integrating sphere microscopy, which enables simultaneous and spatially resolved quantitative absorption, internal quantum efficiency (IQE), and photoluminescence quantum yield (PLQY) measurements. For our record device we measure a photocurrent collection efficiency of >90% and an opencircuit voltage (850 mV) that is 73% of the thermodynamic limit (1.16 V).

Nature Nanotechnol.
Nanoscale Solar Cells

Mann, S., Oener, S., Cavalli, A., Haverkort, J., Bakkers, E., & Garnett, E. (2016). Quantifying Losses and Thermodynamic Limits in Nanophotonic Solar Cells. Nat. Nanotechnol., 11(12), 1071–1075. doi:10.1038/nnano.2016.162