Standard solar cells heat up under sunlight, and the resulting increased temperature of the solar cell has adverse consequences on both its efficiency and its reliability. We introduce a general approach to radiatively lower the operating temperature of a solar cell through sky access, while maintaining its sunlight absorption. We present first an ideal scheme for the radiative cooling of solar cells. For an example case of a bare crystalline silicon solar cell, we show that the ideal scheme can passively lower the operating temperature by 18.3 K. We then show a microphotonic design based on realistic material properties, that approaches the performance of the ideal scheme. We also show that the radiative cooling effect is substantial, even in the presence of significant non-radiative heat change, and parasitic solar absorption in the cooling layer, provided that we design the cooling layer to be sufficiently thin.
Reflection occurs at an air-material interface. The development of antireflection schemes, which aims to cancel such reflection, is important for a wide variety of applications including solar cells and photodetectors. Recently, it has been demonstrated that a periodic array of resonant subwavelength objects placed at an air-material interface can significantly reduce reflection that otherwise would have occurred at such an interface. Here, we introduce the theoretical condition for complete reflection cancellation in this resonant antireflection scheme. Using both general theoretical arguments and analytical temporal coupled-mode theory formalisms, we show that in order to achieve perfect resonant antireflection, the periodicity of the array needs to be smaller than the free-space wavelength of the incident light for normal incidence, and also the resonances in the subwavelength objects need to radiate into air and the dielectric material in a balanced fashion. Our theory is validated using first-principles full-field electromagnetic simulations of structures operating in the infrared wavelength ranges. For solar cell or photodetector applications, resonant antireflection has the potential of providing a low-cost technique for antireflection that does not require nanofabrication into the absorber materials, which may introduce detrimental effects such as additional surface recombination. Our work here provides theoretical guidance for the practical design of such resonant antireflection schemes.
We consider light trapping in photonic crystals. Using temporal coupled-mode theory and assuming that the active material is weakly absorbing, we show that the upper bound of the angle-integrated light trapping absorption enhancement is proportional to the photonic density of states. The tight bound can be reached if all the modes supported by the structure are coupled to external radiation. We discuss the roles of van Hove singularity, effective medium theory, and periodicity. By appropriate design, the angle-integrated absorption enhancement could surpass the conventional limit substantially in two dimension and marginally in three dimension.
We present a practical and robust concept to bypass the typical trade-off between optical transparency and electrical
conductivity of transparent conducting electrodes. A transparent conducting electrode serves to transmit photons and
conduct electrons, and the frequencies of the corresponding optical and dc electric fields differ by at least 12 orders of
magnitude. Therefore, we could engineer the optical electric field to influence the optical property, which is not intrinsic,
of the transparent electrode without sacrificing its electrical performance. For a given light power input, the optical
impedance transformer reduces the loss in a transparent electrode by raising the refractive index of its surrounding
medium. The concept of optical impedance transformer can be realized by nanocone arrays, and we use it to design
nanophotonic structures that provide broadband and omnidirectional reduction of optical loss in an ultrathin graphene
electrode. In addition, the concept applies to thicker or nanostructured transparent electrodes. The results are verified
against first-principles full-field electromagnetic simulations.