Spectral correction in yield forecasts for two-terminal perovskite–silicon tandem modules

The electrical efficiency of two-terminal tandem (2TT) solar cells depends strongly on the incident spectrum. This makes it difficult to forecast the energy yield (EY) of a system containing 2TT modules because the solar spectrum varies continuously, both for direct and diffuse sunlight. The impact of spectrum is often quantified by a spectral correction factor f<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</inf>, which describes how a module’s efficiency in the field varies from its efficiency under the AM1.5g spectrum. We use sophisticated EY software to predict f<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</inf> for bifacial perovskite–silicon 2TT modules installed on a single-axis tracker (the most prevalent power-plant configuration installed in 2024) and show that f<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</inf> can deviate from unity by up to 5% for the annual yield, and by up to 20% under some conditions. The implications for cell design are discussed. We also describe how f<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</inf> depends on atmospheric conditions, how 2TTs compare to Si modules, how the spectral dependence decreases once DC/AC inverters are included, and how f<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</inf> can be incorporated into PVSyst. Finally, we calculate the bifacial gain, which is necessarily smaller for 2TT than for Si modules.