Modeling spectral matching in two- and four-terminal thin-film silicon tandem solar cells

Tandem thin-film silicon solar cells offer the prospect of improved spectral matching, and hence increased solar conversion efficiencies, over comparable single cells. The deposition process naturally results in the constituent sub-cells being connected electrically as well as optically in series, with each therefore constrained to operate at the same load current. By careful
adjustment of absorber layer thicknesses, conversion efficiency may be optimised under this constraint for a given spectral distribution, typically the AM1.5G spectrum. However, variations in spectral quality arising from seasonal and diurnal effects alter the proportion of photogenerated current from each cell, causing imbalance and consequent reductions in efficiency. Here we compare the series-connected two-terminal tandem cell with a four-terminal
tandem cell, in which both cells remain optically in series but each may be operated independently at its maximum power point appropriate to the conditions of service. A semiempirical model is used, where realistic variations in average photon energy between 1.8 and 2.0 eV are introduced by a wavelength-dependent linear scaling factor applied to the AM1.5G spectrum. A two-terminal tandem cell optimised for AM1.5G operation reaches a peak efficiency of 11.2%, falling to 9.6% and 10.3% respectively at the spectral extremes. For the same subcell parameters, the model predicts a progressive increase in four-terminal conversion efficiency, from 10.6% to 11.7% over the same range. We also compare tandem-cell with single-cell performance, and discuss the practical difficulties of developing a four-terminal tandem module.