Thermodynamics of a nanowire solar cell towards the radiative 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. In our research we study the performance of a nanowire solar cell that can beat the S-Q limit and approach the 46.7% ultimate limit by placing a plano-convex lens on top of each nanowire. We have shown numerically that a 2 μm long InP tapered nanowire with the top radius of 83 nm and a tapering angle of 1.2 degrees shows a high photon escape probability of 42% due to an adiabatic expansion of the fundamental HE₁₁ mode which is then collimated using a plano-convex lens with a diameter of 8 μm. Both effects cause the increase of the open-circuit voltage of the solar cell by 159 mV above the radiative limit which is just 154 mV below the ultimate limit. The lens concept is also studied for a planar solar cell from the thermodynamics point view in terms of local entropy generation within the cell due to absorption/emission processes and is planned to be extended to a nanowire geometry.