Enhanced Internal Reflectance of Emission in Solar Cells with Angle- and Energy-selective Reflectors

The open-circuit voltage of solar cells benefits from highly internally reflective surfaces on the front and rear of the cells that restrict radiative recombination current losses and lead to maximum device efficiency. While the back reflector reflects emitted photons broadly, an energy-selective front reflector is tailored to reflect photons with energies below the designed energy and can be energetically situated just above the solar cell absorber bandgap. Devices with an energy of restriction $E_{R} > E_{g}$ show a maximum V OC analogous to blue-shifting the absorption and emission spectra to that of a solar cell with a bandgap $E_{g}=E_{R}$ . Devices that incorporate both angle- and energy-selectivity benefit from reduced radiative recombination current losses due to etendue expansion loss reduction and reduced blackbody emission absorption, respectively. Resulting V OC gains of several hundred meV are expected for low-bandgap solar cells, corresponding with up to 15% absolute efficiency increase for $\mathrm{E}_{g} and $E_{R}$ between 1.1 and 1.2 eV.