Determining the Effectiveness of Radiative Cooler-Integrated Solar Cells

Extensive research has been conducted in recent years to increase the power conversion efficiency of solar cells (SCs). A previous study proposed the improvement of intrinsic efficiency beyond the Shockley–Queisser (SQ) limits, by steering different wavelength bands of sunlight towards an SC using a multi-junction design. The efficiency of SCs can be further improved by enhancing the light absorption using light-trapping (LT) techniques, which include geometrical engineering of the SC, and the application of grating, random and plasmonic structures. However, an SC cannot practically achieve this efficiency due to its dependency on the operating temperature. It is stated that the high efficiency in various SCs can be achieved at an illumination of AM 1.5G and a temperature of 25 °C. However, the temperature of the SC typically exceeds this value in outdoor conditions, where it heats up by tens of degrees above the ambient temperature, which decreases the lifespan and efficiency of the SC.<br/>The passive radiative cooling method can potentially resolve the heating issue of the SC owing to its compact and cost-effective approach. It involves spontaneously cooling objects by emitting heat to the outer space without consuming energy through the transparent atmospheric transmittance window (λ ~ 8-13 µm). Recent studies have presented various types of radiative coolers (RCs) which have been demonstrated to successfully lower the temperature of SCs. Research has also been conducted to theoretically analyze the effectiveness of RCs in compensating for the reduced conversion efficiency of SCs due to elevated temperatures. However, these studies have evaluated or tested the potential of an RC on specific target SCs, such as silicon (Si), concentrated perovskite, or low-bandgap concentrated perovskite, which are limited to single-junction SCs. Further research is required to determine the type of SC that can most retain its original efficiency even at high environmental temperatures, when adapting the RC technique.<br/>The efficiency of SCs can be significantly improved through a comprehensive understanding of the practical operation of the RC on SCs since the SC industry encompasses various types of cells. This study theoretically proves that the multi junction solar cell (MJSC) is the most effective type of SC when an RC is applied. It also presents the limitations of the radiative cooling technique when sub-bandgap (sub-BG) absorption is considered. Consequently, the proposed MJSC is demonstrated to be immune to heating by sub-BG photons, which can lead to the development of novel SCs by reducing the burden of designing additional sub-BG filters or reflectors. A structure is then fabricated which performs both light trapping and radiative cooling based on pioneering SC research, and is applied to the InGaP/GaAs/Ge MJSC. Multiple outdoor experiments are conducted to demonstrate that radiative cooling can contribute to a temperature drop of ~6°C. The reduced temperature also results in an absolute increase of the open-circuit voltage of the MJSC by ~2%. Therefore, RCs are expected to be closely connected to the next generation of efficiency control methods through precise prediction in modeling SCs which are immune to heat from the external environment, thus reducing the design complexity.