Investigations of emitter characteristics of LPE GaAs solar cells

GaAs solar cells of 22% AM1.5 efficiency were processed using the LPE etchback-regrowth method. In order to improve performance and yield of the cells a systematic study of the emitter characteristics was performed. By varying the most significant growth parameters, viz. the melt composition, the temperature and the time of growth, series of cells with emitter thicknesses between 0.5 and 4 microns and with various thicknesses of the AlxGa1-xAs window layer were grown. The emitter layers were investigated by X-ray rocking curve analysis, CV profiler and photoluminescence decay measurements. A detailed study of the Zn diffusion process was performed and interpreted in the framework of the interstitial-substitutional diffusion model. It was found that the solar cell performance is not critically dependent on the emitter thickness xj between xj equal 1 and 3 microns. At smaller values of xj, however, Jsc and Voc decrease rapidly with xj. This is consistent with higher recombination paramet ers as observed in the PL decay measurements.