Modelling of two- and four-terminal thin-film silicon tandem solar cells

We have performed semi-empirical modelling of amorphous silicon/microcrystalline silicon thin-film solar cells in tandem optical configuration, under series (two-terminal) and independent (four-terminal) electrical connection. The four-terminal connection relaxes the constraint of current matching between the cells. Computer simulations indicate an increase in maximum initial efficiency from 10.0% (two-terminal) to 10.8% (four-terminal). Following degradation of the amorphous silicon top-cell the figures are 8.5% and 9.8% respectively, with an optimum top-cell absorber layer thickness in four-terminal connection of 150 nm. Changes in solar spectral quality have been simulated by applying weighted spectra, and improvements
in efficiency favouring four-terminal connection occur under red- and blue-rich conditions. However, these typically convey only a small fraction of the total annual insolation, and gains of 5% or less in annual electrical output are predicted. Possible increased optical and electrical losses due to additional contact layers in four-terminal connection are not taken into account.