One of the key approaches for achieving very-high efficiency for photovoltaic (PV) devices is to use, instead of single junctions, double or multi-junction devices due to an extended solar spectral coverage leading to an enhanced overall power output from the multi-junction PV devices.

 

In this study, we investigated two-junction solar cells based on group IV materials, i.e. Si and Ge, for top and bottom sub-cell, respectively. 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 [1, 2].

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 structure configurations 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 together with the corresponding quantum efficiency spectra. The applications of these devices in conventional PV, concentrator PV and thermal PV were assessed and the output PV parameters were predicted as a function of the simulated operating conditions.

 

References:

[1] M. Emziane, R. J. Nicholas, Journal of Applied Physics, 102 (2007) 074508.

[2] M. Emziane, Mater. Res. Soc. Symp. Proc., Vol 1165 (2009) M08-13.