Modelling performance of two- and four-terminal thin-film silicon tandem solar cells under varying spectral conditions

In an amorphous silicon/microcrystalline silicon or ‘micromorph’ tandem solar cell, variations in solar spectral quality alter the proportion of photo-generated current developed by each sub-cell. This imbalance may reduce long-term energy conversion efficiency, compared with operation under a constant AM1.5G spectrum. Using semi-empirical modelling, we contrast the usual series-connected two-terminal tandem cell, constrained by current-matching, with a four-terminal tandem cell, in which both sub-cells are electrically independent. The model reveals that reduction in extracted power due to current mismatch in two-terminal connection is compensated somewhat by an increase in fill-factor either side of the current-matched point. The efficiency-mismatch curve is compared with spectral distributions of annual insolation in terms of average photon energy. Provided the two-terminal cell is matched to the prevailing spectral distribution, long-term benefits in electrical energy output in four-terminal connection are predicted to be modest. However departures from the AM1.5G spectrum occur worldwide, and this plus the flexibility to match to seasonal degradation/annealing cycles suggests that four-terminal connection may offer greater benefit.