A study of the correlation between dislocation and diffusion length in In49Ga51P solar cells
Williams, Scott Edward
Haegel, Nancy M.
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A quantitative, contact-free method for extracting minority carrier diffusion length is used to measure the relatively small variations in diffusion length associated with dislocation bands in mismatched epitaxy in the p-type region of a two dimensional heterostructure of a triple junction (InGaP/GaAs/Ge) solar cell sample. These measurements are taken using the line scan mode of a Scanning Electron Microscope coupled with an optical microscope. This technique allowed the variations in diffusion length in the In49Ga51P sample to be measured to within 0.1 microns. Also, the variations were not random but varied spatially with respect to the light and dark cathodoluminescence bands on the sample. However, there is an inverse relationship between the maximum luminescent intensity and the diffusion length. Since the radiative lifetime and non-radiative lifetime are on the same order of magnitude, a relationship between the maximum luminescent intensity and minority carrier diffusion length to the lifetimes were derived. With the radiative lifetime inversely dependent on the free hole concentration, a simulation was conducted to qualitatively reproduce the relationship between luminescent intensity and minority carrier diffusion length. The model simulated the non-radiative lifetime and free hole concentration decreasing across dislocation bands. This described the behavior of the non-radiative lifetime due to defect states associated with the dislocations. It also qualitatively illustrated the increase in radiative lifetime if the free hole concentration is reduced due to variations in Fermi level. Therefore, the simulation qualitatively described the spatial behavior of the diffusion length due to the presence of dislocations and reproduced the experimental anti-correlation between the diffusion length and maximum luminescent intensity. Areas of further research are offered to expand this work to other triple junction solar cell materials to include effects of lattice mismatched materials, varying mole concentrations, atomic ordering, and doping concentration.
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