Thermodynamics of a nanowire solar cell: towards the ultimate limit

A lossless solar cell operating at the Shockley-Queisser (S-Q) limit generates an open-circuit voltage (V_OC) equal to the radiative limit. At V_OC, the highly directional beam of photons from the sun is absorbed and subsequently externally reemitted into a 4π solid angle, providing a large photon entropy loss. Moreover, due to many total internal reflections and low internal radiative efficiency, a lot of light is lost in nonradiative recombination events. In our research, we perform a nanophotonic optimization of a semiconductor nanowire geometry with a top microlens in order to decrease the photon entropy loss and to increase the photon escape probability for the nanowire, therefore increasing the output voltage. The optimization leads us to a maximum V_OC of 1178 mV which is 141 mV higher than the radiative limit and 172 mV lower than the ultimate limit. The photon entropy loss is also studied fundamentally from the thermodynamics point of view to better understand where the entropy is generated during the absorption-emission processes.