Detailed-balance assessment of radiative cooling for multi-junction solar cells under unconcentrated and low-concentrated light

Multi-junction solar cells are the best technology to achieve high-efficiency photovoltaics. Yet, their thermal management is crucial to ensure high performance and reliability, particularly in concentrating photovoltaic systems. Recent studies have proposed radiative cooling as an innovative, passive, cost-effective, and scalable technique to cool down solar cells. In this study, we analyze its impact on multi-junction solar cells under different illumination conditions by means of a detailed-balance model. First, we demonstrate that radiative cooling can provide greater efficiency gain in multi-junction devices than in single-junction ones despite the fact that the former heat up less than the latter. In fact, in multi-junction cells, the lower heating is more than compensated for by the stronger efficiency degradation with increasing temperature, due to their wider radiative recombination spectrum. Then, we explore two possible strategies to effectively use radiative cooling in low-concentration photovoltaic systems, such as building integrated concentrating photovoltaics. The first one is to combine the radiative cooler with a nonradiative cooling system, which then has relaxed performance requirements. The second one is to increase the radiative cooler area relative to that of the solar cell. Both approaches can provide significant performance benefits, whose magnitude depends on the selected design and application. For an optimal triple-junction cell under 10-sun concentration, we find that a radiative cooler having 5$\times$ the area of the solar cell reduces by 90% the nonradiative cooling power required to maintain the cell temperature at 60 ${}^o$C and achieves +2% absolute efficiency gain over 1-sun operation.