Impact of Photon Recycling on GaAs Solar Cell Designs

Radiatively dominated III-V semiconductor solar cells are strongly influenced by the effects of photon recycling. From a modeling standpoint, the semiconductor transport equations must account for this to predict accurate open-circuit voltages, even for cells on substrate. Using Shockley-Read-Hall (SRH) lifetimes based on internal quantum efficiency measurements, the current-voltage characteristics of cells on substrate and thin-film cells (where the substrate is removed) are predicted to good accuracy. Using this calibrated photon recycling model, the influences of base thickness and SRH lifetimes on device performance are studied in order to determine an optimal cell design using a conventional silver back reflector and a near-perfect back reflector. The result is efficiencies of > 28% under AM1.5g standard testing conditions for thin-film cells with electron and hole SRH lifetimes of 1 mu s and 100 ns, respectively. Finally, the doping concentration in the emitter is found to be unimportant in dictating the cell's open-circuit voltage as the cell approaches the radiative limit, whereas for nonradiatively dominated cells, the open-circuit voltage is found to depend on the cell's built-in voltage via emitter doping.