Design and Modeling of High-Efficiency GaAs -Nanowire Metal-Oxide-Semiconductor Solar Cells beyond the Shockley-Queisser Limit: An NEGF Approach

The present work proposes a $\mathrm{Ga}\mathrm{As}$-nanowire-based vertical metal-oxide-semiconductor (MOS) solar cell of quantum scale to achieve very high efficiency beyond the Shockley-Queisser (SQ) limit. Photogeneration and carrier transport in such devices are analytically modeled by adopting nonequilibrium Green's function formalism based on second quantization field operators for the incident photons and generated photocarriers. The study suggests that the utilization of photogenerated light and heavy holes to harvest solar energy is capable of providing significantly higher power conversion efficiency above the SQ limit. Such superior efficiency is achieved due to the resonance of incident photon modes with the energy gap between three-dimensional-quantized electron states and two-dimensional-quantized hole subbands. The power conversion efficiency, along with other relevant solar-cell-performance parameters, such as open-circuit voltage, short-circuit current, fill factor, external quantum efficiency, and responsivity, is observed to depend significantly on the nanowire diameter and top-oxide thickness, which, in turn, controls the quantization effect in such MOS devices. The results show that the power conversion efficiency of 50% and above can be achieved in the present tosylate-modified poly(3,4-ethylenedioxythiophene) (PEDOT-Tos)/${\mathrm{Si}\mathrm{O}}_{2}/\mathrm{Ga}\mathrm{As}$-nanowire MOS solar cell for a combination of nanowire diameter and oxide thickness in the range of 18--14 nm and 4--2 nm, respectively. Thus, the proposed device scheme offers an alternative design route for next-generation solar cells with superior efficiency by engineering the quantization effect.