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