Detailed balance analysis of vertical GaAs nanowire array solar cells: exceeding the Shockley Queisser limit.

We performed detailed balance analysis using rigorous coupled-wave analysis (RCWA) on vertical GaAs nanowire (NW) arrays. Both freestanding NW arrays as well as NW arrays on a perfect back reflector are assessed. Both types of vertical NW arrays demonstrate efficiencies that exceed the Shockley Queisser (SQ) or radiative efficiency limit when the NWs are sufficiently long. The use of a back reflector enhances the efficiency of NW solar cells by increasing solar absorption and suppressing emission from the backside of the solar cell. We study the light trapping and material reduction advantages of NWs. Furthermore, we compare simulations that evaluate detailed balance efficiency with ultimate efficiency and show that ultimate efficiency studies can determine near-optimal solar cells while vastly reducing the number of simulations that need to be performed. While open circuit voltages above the radiative limit can be achieved, tradeoffs with short circuit current must be carefully considered. We also compare our simulation results to other claims in the literature that NWs are capable of exceeding the SQ limit.