aiming to design and fabricate biomimetic structures. [7] Research in this field indicated several methodologies to develop bioinspired surfaces, exhibiting hierarchical structuring at the nano-and micro-lengthscales. [8][9][10][11] Laser fabrication is a maskless process allowing material modifications with a high precision over size and the shape of the fabricated features. [12] However, due to optical diffraction, the feature size resolution is limited to the order of wavelength (i.e., microscale); therefore, the challenge in biomimetic laser processing is to beat the diffraction limit and realize the structural complexity of natural surfaces, also, at the nanoscale. Materials' structuring using ultrashort (less than 1 ps) laser pulses, in particular, proved to be a precise and highly versatile tool to realize artificial surfaces that quantitatively mimic the morphological features and functionalities of their natural archetypes. [13][14][15][16][17][18][19] This capability comes as the outcome of the optimal combination of the ultrafast laser field and material properties that enable the production of features with sizes beyond the diffraction limit (i.e., nanoscale). A prominent example is the formation of self-organized subwavelength, laser-induced periodic surface structures (LIPSS), which have been proven an important asset for the fabrication of nanostructures with a plethora of geometrical features. [13,[20][21][22][23][24][25] This work is the first report on direct laser nanofabrication of biomimetic omnidirectional antireflective glass surfaces. It was inspired from the unique antireflection properties of the wings of the glasswing butterfly, Greta oto, and the Cicada Cretensis species. [2,3] This property is due to the presence of arrays (with periodicity in the range of 150-250 nm) of nonreflective nanosized (sub-100 nm size) pillars on both the top and the bottom surface of the wing. The current state-of-the-art technologies employed for the production of antireflection surfaces require either complex multiple steps and time-consuming procedures or chemical processes, [8,[26][27][28][29][30][31][32] which, in some cases, produce hazardous wastes. At the same time, the chemical coatings' quality tends to degrade with time. [27,33,34] Here, we demonstrate a single-step laser texturing approach for the structuring of biomimetic antireflective nanopillars, on fused silica glass (SiO 2 ) surfaces. The overall properties of the produced surfaces were found remarkably similar to the natural butterfly and Cicada archetypes, both in terms of the surface morphology Here, a single-step, biomimetic approach for the realization of omnidirectional transparent antireflective glass is reported. In particular, it is shown that circularly polarized ultrashort laser pulses produce self-organized nanopillar structures on fused silica (SiO 2 ). The laser-induced nanostructures are selectively textured on the glass surface in order to mimic the spatial randomness, pillar-like morphology, as well as the remarkable antir...
The radiative cooling of objects during daytime under direct sunlight has recently been shown to be significantly enhanced by utilizing nanophotonic coatings. Multilayer thin film stacks, 2D photonic crystals, etc. as coating structures improved the thermal emission rate of a device in the infrared atmospheric transparency window reducing considerably devices' temperature. Due to the increased heating in photovoltaic (PV) devices, that has significant adverse consequences on both their efficiency and life-time, and inspired by the recent advances in daytime radiative cooling, we developed a coupled thermal-electrical modeling to examine the physical mechanisms on how a radiative cooler affects the overall efficiency of commercial photovoltaic modules. Employing this modeling, which takes into account all the major processes affected by the temperature variation in a PV device, we evaluated the relative impact of the main radiative cooling approaches proposed so far on the PV efficiency, and we established required conditions for optimized radiative cooling. Moreover, we identified the validity regimes of the currently existing PV-cooling models which treat the PV coolers as simple thermal emitters. Finally, we assessed some realistic photonic coolers from the literature, compatible with photovoltaics, to implement the radiative cooling requirements, and demonstrated their associated impact on the temperature reduction and PV efficiency. Providing the physical mechanisms and requirements for cooling radiatively solar cells, our study provides guidelines for utilizing suitable photonic structures as radiative coolers, enhancing the efficiency and the lifetime of PV devices.
Solution-processed, lead halide-based perovskite solar cells have overcome important challenges over the recent years, offering low-cost and high solar power conversion efficiencies. However, they still undergo unoptimized light collection due mainly to the thin (~350 nm) polycrystalline absorber layers. Moreover, their high toxicity (due to the presence of lead in the perovskite crystalline structure) makes it necessary that the thickness of the absorber layers to be further reduced, for their future commercialization, without reducing the device performance. Here we aim to address these issues via embedding spherical plasmonic nanoparticles of various sizes, composition, concentrations, and vertical positions, for the first time in realistic halide-based perovskite solar cells architecture, and to clarify their effect on the absorption properties and enhancement. We theoretically show that plasmon-enhanced near-field effects and scattering leads to a device photocurrent enhancement of up to ~7.3% when silver spheres are embedded inside the perovskite layer. Interestingly, the combination of silver spheres in perovskite and aluminum spheres inside the hole transporting layer (PEDOT:PSS) of the solar cell leads to an even further enhancement, of up to ~12%. This approach allows the employment of much thinner perovskite layers in PSCs (up to 150 nm) to reach the same photocurrent as the nanoparticles-free device and reducing thus significantly the toxicity of the device. Providing the requirements related to the size, shape, position, composition, and concentration of nanoparticles for the PSCs photocurrent enhancement, our study establishes guidelines for a future development of highly-efficient, environmentally friendly and low-cost plasmonic perovskite solar cells.
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