Infrared-reflective ultrathin-metal-film-based transparent electrode with ultralow optical loss for high efficiency in solar cells.

In this work we study in-depth the antireflection and filtering properties of ultrathin-metal-film-based transparent electrodes (MTEs) integrated in thin-film solar cells. Based on numerical optimization of the MTE design and the experimental characterization of thin-film perovskite solar cell (PSC) samples, we show that reflection in the visible spectrum can be strongly suppressed, in contrast to common belief (due to the compact metal layer). The optical loss of the optimized electrode (~ 2.9%), composed of a low-resistivity metal and an insulator, is significantly lower than that of a conventional transparent conductive oxide (TCO ~ 6.3%), thanks to the very high transmission of visible light within the cell (> 91%) and low thickness (< 70 nm), whereas the reflection of infrared light (~ 70%) improves by > 370%. To assess the application potentials, integrated current density > 25 mA/cm2, power conversion efficiency > 20%, combined with vastly reduced device heat load by 177.1 W/m2 was achieved in state-of-the-art PSCs. Our study aims to set the basis for a novel interpretation of composite electrodes/structures, such as TCO-metal-TCO, dielectric-metal-dielectric or insulator-metal-insulator, and hyperbolic metamaterials, in high-efficiency optoelectronic devices, such as solar cells, semi-transparent, and concentrated systems, and other electro-optical components including smart windows, light-emitting diodes, and displays.

Highly ordered laser imprinted plasmonic metasurfaces for polarization sensitive perfect absorption.

We present polarization-sensitive gap surface plasmon metasurfaces fabricated with direct material processing using pulsed laser light, an alternative and versatile approach. In particular we imprint laser induced periodic surface structures on nanometer-thick Ni films, which are back-plated by a grounded dielectric layer with TiO2 and ZnO deposition followed by Au evaporation. The procedure results in a metal-insulator-metal type plasmonic metasurface with a corrugated top layer consisting of highly-ordered, sinusoidal shaped, periodic, thin, metallic nanowires. The metasurface sustains sharp, resonant gap surface plasmons and provides various opportunities for polarization control in reflection, which is here switched by the size and infiltrating material of the insulating cavity. The polarization control is associated with the polarization sensitive perfect absorption and leads to high extinction ratios in the near-IR and mid-IR spectral areas. Corresponding Fourier-transform infrared spectroscopy measurements experimentally demonstrate that the fabrication approach produces metasurfaces with very well-defined, controllable, sharp resonances and polarization sensitive resonant absorption response which, depending on the insulating cavity size, impacts either the normal or the parallel to the nanowires polarization.

Combined nano and micro structuring for enhanced radiative cooling and efficiency of photovoltaic cells.

Outdoor devices comprising materials with mid-IR emissions at the atmospheric window (8-13 µm) achieve passive heat dissipation to outer space (~ - 270 °C), besides the atmosphere, being suitable for cooling applications. Recent studies have shown that the micro-scale photonic patterning of such materials further enhances their spectral emissivity. This approach is crucial, especially for daytime operation, where solar radiation often increases the device heat load. However, micro-scale patterning is often sub-optimal for other wavelengths besides 8-13 µm, limiting the devices' efficiency. Here, we show that the superposition of properly designed in-plane nano- and micro-scaled periodic patterns results in enhanced device performance in the case of solar cell applications. We apply this idea in scalable, few-micron-thick, and simple single-material (glass) radiative coolers on top of simple-planar Si substrates, where we show an ~ 25.4% solar absorption enhancement, combined with a ~ ≤ 5.8 °C temperature reduction. Utilizing a coupled opto-electro-thermal modeling we evaluate our nano-micro-scale cooler also in the case of selected, highly-efficient Si-based photovoltaic architectures, where we achieve an efficiency enhancement of ~ 3.1%, which is 2.3 times higher compared to common anti-reflection layers, while the operating temperature of the device also decreases. Besides the enhanced performance of our nano-micro-scale cooler, our approach of superimposing double- or multi-periodic gratings is generic and suitable in all cases where the performance of a device depends on its response on more than one frequency bands.

Passive radiative cooling and other photonic approaches for the temperature control of photovoltaics: a comparative study for crystalline silicon-based architectures.

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 and how the radiative cooling impact is compared with the impact of other photonic strategies for reducing heat generation within PVs, such as ultraviolet and sub-bandgap reflection. Employing our modeling, which takes into account all the major intrinsic processes affected by the temperature variation in a PV device, we additionally 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 the requirements related to the reduction of heat generation, and demonstrated their associated impact on the temperature reduction and PV efficiency. Consistent with previous works, we showed that combining radiative cooling with sub-bandgap reflection proves to be more promising for increasing PVs' efficiency. Providing the physical mechanisms and requirements for reducing PV operating temperature, our study provides guidelines for utilizing suitable photonic structures for enhancing the efficiency and the lifetime of PV devices.

Efficient and environmental-friendly perovskite solar cells via embedding plasmonic nanoparticles: an optical simulation study on realistic device architectures.

Solution-processed, lead halide-based perovskite solar cells have recently overcome important challenges, 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 perovskite crystalline structures) makes it necessary that the thickness of the absorber layers to be further reduced. Here we address these issues via embedding spherical plasmonic nanoparticles of various sizes, composition, concentrations, and vertical positions, in realistic halide-based perovskite solar cells. We theoretically show that plasmon-enhanced near-field effects and scattering leads to a device photocurrent enhancement up to ∼7.3% when silver spheres are embedded inside the perovskite layer. An even further enhancement, up to ∼12%, is achieved with the combination of silver spheres in perovskite and aluminum spheres inside the hole transporting layer (PEDOT:PSS). The proper involvement of nanoparticles allows the employment of much thinner perovskite layers (up to 150 nm), reducing thus significantly the toxicity. Providing the requirements related to the design parameters of nanoparticles, our study establishes guidelines for a future development of highly-efficient, environmentally friendly and low-cost plasmonic perovskite solar cells.

Biomimetic Omnidirectional Antireflective Glass via Direct Ultrafast Laser Nanostructuring.

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 (SiO2 ). 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 antireflection properties found on the wings of the glasswing butterfly, Greta oto, and various Cicada species. The artificial structures exhibit impressive antireflective properties, both in the visible and infrared frequency ranges, which are remarkably stable over time. Accordingly, the laser-processed glass surfaces show reflectivity smaller than 1% for various angles of incidence in the visible spectrum for s-p linearly polarized configurations. However, in the near-infrared spectrum, the laser-textured glass shows higher transmittance compared to the pristine. It is envisaged that the current results will revolutionize the technology of antireflective transparent surfaces and impact numerous applications from glass displays to optoelectronic devices.

Perfect optical absorption with nanostructured metal films: design and experimental demonstration.

Structuring metal surfaces on the nanoscale has been shown to alter their fundamental processes like reflection or absorption by supporting surface plasmon resonances. Here, we propose metal films with subwavelength rectangular nanostructuring that perfectly absorb the incident radiation in the optical regime. The structures are fabricated with low-cost nanoimprint lithography and thus constitute an appealing alternative to elaborate absorber designs with complex meta-atoms or multilayer structuring. We conduct a thorough numerical analysis to gain physical insight on how the key structural parameters affect the optical response and identify the designs leading to broad spectral and angular bandwidths, both of which are highly desirable in practical absorber applications. Subsequently, we fabricate and measure the structures with an FT-IR spectrometer demonstrating very good agreement with theory. Finally, we assess the performance of the proposed structures as sensing devices by quantifying the dependence of the absorption peak frequency position on the superstrate material.