Modeling Techniques for Multijunction Solar Cells

Multi-junction solar cell efficiencies far exceed those attainable with silicon photovoltaics. Recently, this technology has been applied to photonic power converters with conversion efficiencies higher than 65%. These devices operate well in part because of luminescent coupling that occurs in the multijunction device. We present modeling results to explain how this boosts device efficiencies by approximately 70 mV per junction in GaAs devices. Luminescent coupling also increases efficiency in devices with four more junctions, however, the high cost of materials remains a barrier to their widespread use. Substantial cost reduction could be achieved by replacing the germanium substrate with a less expensive alternative: silicon. Threading dislocations introduced by the lattice mismatch between silicon and other layers have a detrimental effect on performance. In this research, we seek to accommodate lattice mismatch by introducing a voided germanium interface layer on the silicon substrate to intercept dislocations and prevent them from reaching the active layers. We present simulation results exploring the effect of threading dislocations and substrate doping on device performance.