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High efficiencies of >30% are predicted for series-connected tandem solarcells when current-matching is achieved between the wide-bandgap top cell andsilicon bottom cell. Sub-cells are typically optimised for current-matchingbased on the standard AM1.5G spectrum, but in practice, the incident radiationon a solar cell can be very different from this standard due to the effects ofthe sun's location in the sky, atmospheric conditions, total diffuse elementetc. The resulting deviations in spectral content from optimum conditions leadto current mismatch between tandem cell layers that adversely affects thedevice's performance. To investigate the impact of this issue the energy yield(%) of tandem solar cells comprising a III-V wide-bandgap solar cell connectedelectrically and optically in series with a silicon bottom cell was simulatedover a full year using measured spectral data from Denver, CO. Top cells withbandgaps from 1.5-1.9 eV were modelled using an external radiative efficiencymethod. The predicted annual energy yields were as high as 31% with an optimum1.8 eV top cell, only 2.8% lower (absolute) than the AM1.5G predictedefficiency. The annual energy yield of tandem cells with no current-matchingconstraint, i.e. parallel-connected devices, was also simulated. Here thedifference between series and parallel connections were only significant fornon-optimum bandgap combinations. Our results indicate that AM1.5G basedoptimization of sub-cells can be effectively employed to achieve high energyyields of >25% for III-V/Si tandem solar cells in mid-latitude US locations,despite the continuous variation in spectra throughout a calendar year.

Predicted annual energy yield of III-V/c-Si tandem solar cells: modelling the effect of changing spectrum on current-matching