Optimization of LPE-grown high efficiency GaAs solar cells
AlGaAs/GaAs heteroface solar cells were processed using the LPE etchback-regrowth method. Cells of 1 qcm area with a single AR coating exhibit an average efficiency of 21% AM 1.5 and peak values of 22.3 % AM 1.5. A series of 30 cells with emitter thickness varying between 0.2 and 4 microns was analyzed with respect to doping concentration by CV-profiling measurements. The influence of Al and Zn concentration on the depth and profile of the emitter was investigated with the help of a concentration dependent diffusion model. The homogeneity of the layers with respect to thickness and minority carrier recombination was analyzed by photoluminescence mapping and photoluminescence decay measurements. It was found that the optimal emitter thickness for our LPE etchback-regrowth process lies between 1.5 and 2 microns. For emitter thicknesses below 1 micron Isubsc and Vsuboc decrease strongly with decreasing thickness due to increased surface recombination velocity.