Improved voltage response in III?V solar cells based on engineered spontaneous emission

In order to obtain a high photovoltaic (PV) efficiency, a solar cell must operate at both a high current and voltage. The current is determined by the semiconductor´s ability to convert above-bandgap photons into electron-hole pairs that can be collected, while the maximum achievable voltage depends on maximizing the carrier densities and minimizing recombination within the cell. For a high quality semiconductor like GaAs, which has been shown to have an internal fluorescence yield of 99.7%, non-radiative recombination can be minimized to the point where the PV efficiency is limited by radiative emission from the cell. Here we show an improvement in output voltage and efficiency by engineering the spontaneous emission rate using photonic crystal structures. The proposed device is composed of a GaAs PV cell that has been nano-patterned with photonic crystals in order to control carrier spontaneous emission and, as a result, increase device output voltage. In the proposed device, this emission control is achieved by tuning the bandgap of the photonic crystal structure near the semiconductor band edge. Under these operating conditions, the open circuit voltage is increased by a factor of -(kBT/q) ln[FP], where FP is defined as the ratio of the spontaneous emission rate in the nano-patterned solar cell to the spontaneous emission rate in bulk GaAs. By engineering small FP the voltage of the device can be significantly improved, leading to photovoltaic efficiencies of ~36% from a single junction device.