Tailored emission to boost open-circuit voltage in solar cells

Recently, a lot of research focus has been on how to make solar cells more efficient. One direction is to enhance the open-circuit voltage V_oc by optimizing the emission of photons in the cell, where emission is a necessary loss process due to the reciprocity between absorption and emission of light. Here, we performed a Shockley-Queisser detailed balance analysis to predict the benefit of managing emitted photons in a single-junction solar cell. First, at low internal luminescence efficiency η_int, non-radiative recombination dominates, and management of emitted photons plays negligible role for V_oc . Similarly, for an external luminescence efficiency η_ext<10%, externally emitted photons play negligible role, and V_oc is set either by non-radiative recombination; or parasitic absorption of internally emitted photons. For higher η_ext, the V_oc can be boosted, maximally by 15%, by restricting the external emission to match the incidence cone of the AM1.5D sun light spectrum. Such emission restriction corresponds to lower escape probability of internally emitted photons, enhances photon recycling, drops η_ext, and actually makes the solar cell into a worse LED. Finally, for partly diffuse incident light, by restricting the angular emission for photons in a 130 nm wavelength range around the bandgap, we predict a maximum 14% relative boost in solar cell efficiency. The results of this paper are intended to serve as a general guideline on how to utilize emission-tuning possibilities to develop highly efficient photovoltaic devices.