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. (C) 2015 Published by Elsevier Ltd.