The tantalizing possibility of 31% solar-to-electric power conversion efficiency in thin film crystalline silicon solar cell architectures relies essentially on solar absorption well beyond the Lambertian light trapping limit (Bhattacharya and John in Nat Sci Rep 9:12482, 2019). Up to now, no solar cell architecture has exhibited above-Lambertian solar absorption, integrated over the broad solar spectrum. In this work, we experimentally demonstrate two types of photonic crystal (PhC) solar cells architectures that exceed Lambertian light absorption, integrated over the entire 300-1,200 nm wavelength band. These measurements confirm theoretically predicted wave-interference-based optical resonances associated with long lifetime, slow-light modes and parallel-to-interface refraction. These phenomena are beyond the realm of ray optics. Using two types of 10-μm thick PhC's, first an Inverted Pyramid PhC with lattice constant a = 2,500 nm and second a Teepee PhC with a = 1,200 nm, we observe solar absorption well beyond the Lambertian limit over λ = 950-1,200 nm. Our absorption measurements correspond to the maximum-achievable-photocurrent-density (MAPD), under AM1.5G illumination at 4-degree incident angle, 41.29 and 41.52 mA/cm 2 for the Inverted Pyramid and Teepee PhC, respectively, in agreement with wave-optics, numerical simulations. Both of these values exceed the MAPD (= 39.63 mA/cm 2) corresponding to the Lambertian limit for a 10-μm thick silicon for solar absorption over the 300-1,200 nm band. The efficiency and cost of photovoltaics has steadily improved in recent years in the effort to create a competitive renewable energy resource. Silicon solar cells have been the dominant driving force in photovoltaics due to the abundance and environmentally friendly nature of silicon. The maximum possible power conversion efficiency of a single junction, crystalline silicon (c-Si) solar cell under one sun illumination at room temperature is 32.33% 2. The highest efficiency real-world n-type silicon solar cell to date, by Kaneka Corp 3,4 , exhibits 26.7% conversion efficiency, followed closely by the p-type silicon solar cell, by the Institute for Solar Energy Research Hamelin (ISFH), Germany with 26.1% efficiency 5,6. An analysis of the Kaneka, 165 μm thick, c-Si cell shows that in the absence of any extrinsic loss mechanism, the limiting efficiency of such a cell is 29.1% 3. The competing factors responsible for this limit of the conversion efficiency are ray-optics light trapping 7,8 and intrinsic loss due to Auger charge-carrier recombination. Essentially, the thicker the cell, the more light is absorbed. However, this is accompanied by increased bulk non-radiative recombination loss of charge carriers. In the case of ideal Lambertian light-trapping and a state-of-the-art Auger recombination model 9 , the optimal silicon thickness is reduced to 110 μm and a theoretical limit to conversion efficiency (assuming no surface recombination losses) is increased to 29.43% 8. In traditional ray-optics based light trapping s...

In this work, a novel lithographic method is proposed to prepare three-dimensional (3D) photonic crystal (PC) that is different from conventional top-down and bottom-up approaches. The method combines a 2D optical mask and off-the-axis double optical exposures to create a desirable 3D PC structure. Since the method uses only two optical exposures of a photo-resist layer, it is inherently a low-cost, high throughput and wafer-scale lithographic method. The method is implemented to make a slanted post 3D PC having the SP2 lattice symmetry. Three types of SP2 3D PC structures were successfully fabricated with a minimum feature size of d=1.5 μm over a large scale of 8x10 mm2, without any observable fabrication defects. The SP2 PCs are: (i) SU8 posts in air background, (ii) air pores in CdS background and (iii) Pt coated on SU8 SP2 templates. A spectroscopic study of the SP2 PCs shows select spectral regions of high reflectance, indicating the existence of a photonic stop band. This low-cost and large-scale method could enable broader technological impacts of 3D PC materials in areas such as thermo-photovoltaics and above room-temperature Bose-Einstein Condensation. Furthermore, this off-axis method could lead to the creation of an entirely new class of slanted-rod based photonic crystals, such as topological photonic crystal in 3D.

A recent discovery of super-Planckian radiation in the far-field has attracted a great deal of interest in thermal radiation control using photonic crystals (PC) architecture. The finding is based on a careful comparison of radiation intensity emitted from a PC nano-filament and a blackbody at the same temperature and same wavelength at ~1.7m. Extending that finding, we determined the absolute spectral radiance of such a PC filament at ~1.7m at elevated temperature, T=445-723 º K, and compared its value to that predicted by Planck's blackbody radiation law. Our data confirmed that the PC spectral radiance surpassed that of a blackbody by an order-of-magnitude. We also determined the spectral emissivity of our PC filament and showed that its value far exceeded unity at resonance and fell below unity away from resonance. The observation of non-equality between absorptivity and emissivity suggests that a strong optical nonreciprocity takes place in our PC nano-filament.