Tandem solar cells involving III-V AND IV semiconductor junctions

In this study, we investigated double-junction PV devices based on III-V semiconductor materials, namely GaAs and AlGaAs for the top sub-cell, and group IV materials, i.e. Si and Ge, for the bottom sub-cell. Unlike the conventional and widely used monolithic series-connected two-terminal device configuration that was shown to deliver a slightly lower output, these double-junction cells were assembled with three terminals and operated independently. The main advantages of choosing these materials and device configuration are: (i) cheapest, largest and easiest group IV substrates; (ii) extended spectral coverage leading to more photons being converted; and (iii) no current matching or the associated tunnel junctions between the two sub-cells. We have undertaken a comprehensive modeling analysis for the device optimization and performance prediction. n/n/p as well as p/p/n device structures were investigated and optimized with regard to the thicknesses and doping levels of both top and bottom active junctions that lead to the highest device performance. Due to the split of the incident solar spectrum between the top and bottom sub-cells, the latter only receives the light to which the former is transparent (mainly in the near infrared) and therefore behaves differently from the single-junction cell counterpart. Optimal current-voltage and power-voltage characteristics were generated for individual cells. The output PV parameters were predicted as a function of the simulated operating conditions.