Photonics for Solar Energy Systems X

We present a critical review on optical research in photovoltaics guided by the question: Which topics do we regard as most relevant to accelerate the large-scale implementation of photovoltaics? The following areas were identified: 1) The development of high-performance earth-abundant solar cell materials and the reduction of indium and silver in the device. 2) Color concepts for an appealing optical appearance of solar modules as photovoltaic modules enter urban environments as well as agricultural areas on a large scale. 3) Optical optimization of multijunction solar cells based on perovskite, III-V semiconductors and silicon to overcome the single-junction efficiency limit. 4) Accurate energy yield predictions considering the full complex illumination conditions particularly regarding bifacial and multijunction solar cells. 5) Advanced concepts with strong momentum such as radiative cooling and spectral conversion.

To achieve optimal optical performance and minimal reflection losses, a textured interface between perovskite and silicon sub-cells in perovskite-silicon tandem solar cells is needed. However, the perovskite solar cells fabrication method yielding the highest efficiencies, spin coating, is not compatible with the conventional random micro-pyramidal texture on silicon. This study focuses on creating periodic inverted micro-pyramidal textures in silicon through photolithography, reactive ion etching, and wet-chemical etching, enabling the deposition of fully textured perovskite solar cells with low reflectance on the textured silicon via spin-coating. This breakthrough lays the foundation for the fabrication of optically and electronically optimized perovskite-silicon tandem solar cells.

Dedicated optical models are crucial for advancing the modelling of next-generation solar cells. Incorporating various textures of different shapes and sizes into solar cells significantly improves light management. This study optimizes the optical design and predicts the performance of a novel thin-film tandem solar cell device. The top cell features a hydrogenated amorphous silicon (a-Si:H) absorber layer, while the bottom cell incorporates a low-bandgap tin-lead-based perovskite (Sn-Pb PVK) absorber layer, all supported on a flexible aluminum substrate Optical simulations exhibit 24 mA/cm² as total implied photo-current density when the absorber layer thicknesses are current-matched. The maximum absorptance reaches 80% at 500 nm for a-Si:H and only 62% at 800 nm for Sn-Pb PVK sub-cells. Experimental results show open-circuit voltages of 0.9 V for a-Si:H and 0.85 V for Sn-Pb PVK solar cells. Based on the highest achieved fill factor of 0.77, the researchers estimated a power conversion efficiency exceeding 16%.

In this contribution we study numerically, how sinusoidal nanotextures would affect the (optical) performance of all-perovskite tandem solar cells. The simulations are conducted with the finite element method (FEM) and consider solar cells in glass superstrate configuration. We correct for the multiple interactions between light that is reflected from the solar cell stack and the glass-air interface with an iterative scattering-matrix approach. To achieve current matching, we optimize the perovskite thickness of the top cell using the Newton method. Results show that front texturing improves the photocurrent density with respect to the planar reference. Additional texturing between top and bottom cell hardly improves the performance. Full texturing leads to an increased photocurrent density, which can mostly be attributed to light trapping at the absorption edge of the bottom cell. Our study shows how texturing can help to further increase the efficiency of all-perovskite tandem solar cells.

Glass textures can fulfil various effects in photovoltaic (PV) modules: enhanced in-coupling for large incidence angles, glare reduction or color appearance with high angular tolerance. These effects are of particular interest for integrated PV (e.g. building or vehicle integration). This overview will describe the texture dependent effects, give design guidelines, describe industrial texturing technologies and show exemplary results.

We explore the ability of hyperuniform disordered structures to enhance light absorption in thin-film solar cells architectures, and perform full electromagnetic wave simulations that unveil light trapping techniques capable to attain large absorption enhancements up 85\% over the visible spectrum. We predicate this enhancement to the interplay between two key physical phenomena: ultimate control over the light diffraction via a hyperuniformly-patterned surface layer which results in a highly-efficient coupling of light to the quasi-guided modes of the absorbing silicon film and a concomitant minimisation of the reflection losses atop of the solar absorber. Our experimental results further validate this approach, showcasing an impressive 66.5% enhancement in solar light absorption in a freely suspended 1-μm c-Si membrane across the spectral range from 400 to 1050 nm. The absorption equivalent photocurrent stands at a remarkable 26.3 mA/cm^2, exceeding values reported in the literature for silicon layers of similar thickness.

Color aesthetics in photovoltaic modules are essential, especially in design-sensitive applications like building integrated photovoltaics (BIPV). Distributed Bragg reflector-based color filters can modify the appearance of silicon solar cells. This study extends the aesthetic evaluation to emerging perovskite solar cells, typically gray-brown, by integrating them with a MorphoColor® color filter. We present simulated and measured angular resolved reflectance measurements and evaluate the color appearance from varied viewing angles. The used simulation environment is based on wave optics and raytracing. Next, we study the impact of individual layer parameters on the color appearance and the maximum achievable short circuit current density in the perovskite solar cell. Findings show that changes in color filter-perovskite interfacing layers influence the saturation and hue of the color impression as well as the angular color stability. Finally, we present initial concepts of optimizing the specific layer stack, demonstrating its potential to tailor a customized color design.

The MorphoColor concept makes use of multilayer and structure interaction for colored photovoltaic (PV) modules. The photonic effect, inspired by the Morpho butterfly's wing, is optimized to produce saturated colors, a high angular tolerance of the color appearance and to maintain a very high module efficiency. This concept, implemented on textured glass surfaces, has already been applied to integrated photovoltaic modules. As a further development, we investigated implementing the MorphoColor interface on a polymeric film which then can be integrated into a PV module. This MorphoFlex concept enables twofold flexibility: for the production process as well as for curved modules. We will present the process chain for the film structuring, coating and integration, first results on film and module level, and show demonstrators.

Dielectric metasurfaces with tailored disordered have recently emerged as a highly favored light-trapping approach for solar cells compared to traditional techniques like direct nanostructuring and antireflective (AR) coatings. The research interest is due to their superior light-scattering properties, ideal for thin solar cells. Moreover, they successfully overcome the detrimental impact on electrical performance due to texturing and can be produced on macroscopically large wafers using self-assembly nanofabrication methods. This work investigates a graded-index-based design strategy for such disordered metasurfaces where the disordered arrangement is tailored towards a correlated, hyperuniform disorder. Individual scattering elements are designed by drawing inspiration from transformation optics. We numerically and experimentally demonstrate that our disordered graded-index metasurface can significantly improve the light-trapping and antireflective performance of a solar cell stack, especially when compared to structures that implement only one of the two design strategies.

Computational design of perovskite based solar cell devices with enhanced power conversion efficiency. The designs reflect efficient trapping of incident electromagnetic waves.

Ultrathin Cu(In,Ga)Se2 (CIGSe) solar cells are promising for cost-effective large-scale production having a wide application portfolio including bifacial and tandem solar cells. However, incomplete absorption of the solar spectrum is the main drawback of these solar cells having only several hundred nanometers absorber layer thickness. In this contribution, we demonstrate functional back contacts for ultrathin CIGSe solar cells containing SiO2 nanostructures in combination with a metallic back reflector. External quantum efficiency measurements reveal that by implementing a functional back contact, an optical gain in the wavelength range from 800 nm to 1200 nm can be obtained. By means of optical simulations, it is shown that periodic nanostructures lead to locally enhanced absorption as a result of multi-resonant quasi-guided modes in addition to double-pass absorption due to the metallic reflector. Furthermore, different nanostructure geometries and cell architectures are investigated with the aim of further optimization of the solar cell performance.

The evolution of solar cell technology demands abundant, non-toxic materials like silicon. CZTSSe, emerging as a next-gen solar material, outperforms previous compounds, offering superior potential. Its natural, non-toxic nature combined with suitable bandgap properties positions it for remarkable solar cell performance. Our study investigates optimal CZTSSe solar cell features, including absorber and buffer layer thickness, temperature, and doping concentrations within the ITO/i-ZnO/CdS/CZTSSe device structure. Notably, our findings demonstrate a cell with PCE of 12.85%, VOC of 0.4599V, JSC of 40.540027mAcm−2, and FF of 68.92%. The proposed device architecture holds promise for experimental applications and resonates with fellow researchers in the field.

Luminescent solar concentrator (LSC) is an optoelectronic device which converts direct and diffused sunlight into electricity when coupled with photovoltaic (PV) cells around its edge. Large Stokes shift is beneficial to minimize the reabsorption loss and improve the overall efficiency of LSCs. In this study, the fabricated Gold-doped silver nanoclusters embedded in Polyvinylpyrrolidone (Au-AgNCs@PVP) matrix exhibited significantly large Stokes shift of about ~200 nm with spectral overlap integral of 0.03 and Urbach tail observed on the absorption spectra along ~480-600 nm. This is a typical behavior of emissions from self-trapped excitons (STEs) due to the lattice distortion caused by a strong exciton-phonon coupling. The strong coupling is evident by having small steepness constant around ~0.32 extracted from the temperature dependent Urbach energy, thus, proving the photoluminescence originating from STEs.

Motivated by porphyrin molecules excellent charge mobility and various optical transitions in the visible region, we introduce a simple zinc(II)β-tetra-bromo-meso-tetra-phenyl-porphyrin as an additive in the perovskite layer to improve the performance and stability of planar perovskite solar cells. Its compatible energy levels and high charge mobility lead to better electron-hole transport through the perovskite layer and defect passivation in the grain boundaries. Moreover, it also has a positive effect on the perovskite film formation quality. As a result, the efficiency of the best device is up to 18.5%, a factor of 15% increase to that of the reference cell with a value of 16.1%, which is superior in planar device structure with copper indium disulfide (CIS) as a hole transport material and carbon as back contact. Furthermore, enhanced hydrophobicity and crystalline quality improve the stability of devices, and the modified device maintained %96 of its initial efficiency after 40 days in comparison with the control device with %38 drop in its performance.

The high conversion efficiency of solar energy is achieved by means of photovoltaic Multi Junction Solar Cells (MJSC) exploiting III-V semiconductors on germanium (Ge) substrate. Intensive studies are conducted at present involving alternation of substrate material in order to lower solar energy costs. Studies suggest that III-V solar cells on silicon (Si) substrates could be less expensive than Ge substrates used in conventional MJSC technology. The study analyses the possibility and technological mode of growing GaP mono-crystalline thin films on the surface of the crystalline Si. Here we discuss multi-junction solar cell III-V/Si efficiency enhanced by sunlight-splitting equipment using suitable optical lenses.

Perovskite light-emitting diodes (PeLEDs) have attracted considerable attention because of their potential in display and lighting applications.

In this work, an innovative hexagonal concentrator was designed, optimized, and experimentally tested. The resulting PMMA optics achieved an 80% transmittance over the whole design range 300 – 1800 nm, with a concentration factor reaching 400X. The concentrator optics exploited the asymmetry of the innovative solar tracking system to match the acceptance angle ranges of ±17° and ±5°, in the two orthogonal directions, and improve the conversion efficiency. Consecutive to indoor testing, the experimental results provided good agreement with the simulated performances, making this a suitable design for high performance hybrid CPV/PV applications.

3D laser nanoprinting based on multi-photon absorption (or multi-step absorption) has become an established commercially available and widespread technology. Here, we focus on recent progress concerning increasing print speed, improving the accessible spatial resolution beyond the diffraction limit, increasing the palette of available materials, and reducing instrument cost.

Biological discovery is a driving force of biomedical progress. With rapidly advancing technology to collect and analyze information from cells and tissues, we generate biomedical knowledge at rates never before attainable to science. Nevertheless, conversion of this knowledge to patient benefits remains a slow process. To accelerate the process of reaching solutions for healthcare, it would be important to complement this culture of discovery with a culture of problem-solving in healthcare. The talk focuses on recent progress with optical and optoacoustic technologies, as well as computational methods, which open new paths for solutions in biology and medicine. Particular attention is given on the use of these technologies for early detection and monitoring of disease evolution. The talk further shows new classes of imaging systems and sensors for assessing biochemical and pathophysiological parameters of systemic diseases, complement knowledge from –omic analytics and drive integrated solutions for improving healthcare.

Optimizing the bandgap energy for a perovskite cell typically requires the complex refractive index, which are obtained from spectroscopic ellipsometry measurements. While these measurements are usually available for a few discrete bandgap energies, it is desired to have them in continuous range to find the optimal bandgap energy. We propose a new method for generating the complex refractive index, based on the interpolation of a few measured data sets. The Forouhi-Bloomer model is used to fit the measured data. Then a linear regression is applied to the fit parameters with respect to the bandgap energy. From the interpolated parameters, the refractive index curve of perovskite with any desired bandgap energy is finally reconstructed. We validated our method by simulating the absorptance of a single junction perovskite cell and a perovskite/silicon tandem cell, showing that our method can accurately model the refractive index.

Solar cells based on perovskite compounds have displayed remarkable efficiency, with a focus on enhancing parameters like short-circuit current (Jsc) and open-circuit voltage (Voc) to maximize their performance. However, Voc is closely related to recombination and optical effects and is an important parameter affecting energy conversion efficiency. To exploit the full potential of perovskite solar cells, photonic management emerges as a key strategy. This study delves into the optical strategies for improving the open-circuit voltage (Voc) and overall efficiency of perovskite solar cells.

We report a light-induced oxyhydroxides-alloy heterostructure reconfigured from a nickel-iron alloy film as a highly catalytic and protective layer on photoanodes for solar water oxidation. The optimized photoanodes exhibit a high applied bias photon-to-current efficiency of 4.24% and long-term stability beyond 250 hours, outperforming the closest competitors by 330% in efficiency and 408% in stability, respectively. This self-generated catalytic-protective oxyhydroxides-alloy layer coating strategy opens the way to implementing large scalable photoelectrochemical devices for solar fuel production with high efficiency and device lifetime.

As solar cells from direct semiconductors improve, i.e. become more radiative, luminescent coupling becomes more and more relevant. This has a strong impact on artifacts in EQE measurements of multi-junction solar cells, which e.g. is a challenge, when characterizing III-V//Si multi-junction solar cells. By measuring the response of these cells under varying illumination conditions, this effect can be understood and EQE measurements can be corrected.

In multijunction solar cells, luminescent coupling describes when the luminescence of one absorbing material is absorbed by another material within the same device. In order to optimize the design of multijunction devices, this luminescence needs to be quantified and the internal coupling maximised compared to losses to the environment. We will present methods for calculating the emission of dipole sources within periodically nanostructured devices in a computationally efficient way. As a case study, we take perovskite silicon tandem solar cells. We investigate the effect that nanostructuring has on luminescent coupling from perovskite to silicon.

This study introduces an optical approach to enhance the open-circuit voltage (Voc) in organic solar cells (OSCs) characterized by high radiative recombination. By exploring various one-dimensional structurations of the OSC device, we reduce Boltzmann losses arising from the mismatch between absorption and emission cones. This innovative method leads to a significant Voc enhancement, achieving an increase of approximately 30 mV through optical means. Furthermore, our findings establish a novel route in the development of planar geometry single-junction solar cells, surpassing the power conversion efficiency values set by the Shockley-Queisser limit.

This work has carried out a theoretical study of luminescent solar concentrators based on stacked poly(methyl methacrylate) dye-doped polymer optical fibres by means of Monte Carlo simulations. Using lumogen red and lumogen yellow as dopants, different geometrical configurations and dopant distributions have been tested. The behaviour of these configurations has been analysed, focusing on the output irradiance and the physical and geometrical reasons for the results obtained.

The selection of polymers for the packaging of emerging PV technologies like organic or perovskite solar cells is a critical aspect of ensuring the long-term reliability and performance of PV modules. Careful consideration should be given to potential degradation products, permeation properties, and possible incompatibilities among different materials within the module. This holistic approach to material selection can contribute to the development of more durable and efficient PV modules, addressing challenges related to degradation and failure modes beyond just the solar cells themselves.

Here, is presented the impact of pristine graphene, used as additive in the active layer or as substrate, in Lead Halide Perovskites based Solar Cells and LEDs. A small concentration of graphene nanoplatelets, well above the percolation threshold, either in the PEDOT:PSS hole transport layer or in the MAPbI3 active layer of a SC produces structural changes that improve both the efficiency and the stability of the device. Similarly, adding graphene in MAPbBr3 active layer of LEDs lead to larger grain domain sizes and increased luminance. Finally, for MAPbBr3 thin films grown directly on substrates of graphene/graphene oxide, the concentration of the precursor solution and the nature of the substrate lead to a continuous film, or dispersed particles of nano-to-micrometer sizes. The crystal size and microstrain determines the emission properties, showing hugely enhanced photostabilities compared to MAPbBr3 on glass, capable of enduring light densities up to > 100 kW/cm2 for several minutes. These results open new opportunities for the design of nanoengineered photovoltaic devices based on combined graphene/perovskites with improved stability and performance.

Hybrid perovskite is one of the most prominent materials with photovoltaic property in the area of optoelectronics. According to its benefits, perovskite is favored by the scientists and has a various application in the third generation of solar conversion facility due to its extraordinary optoelectronic properties, including tunable bandgap, large absorption coefficient with high charge carrier mobilities, solution-processable and low-fabrication cost. Unfortunately, perovskite faces challenges from crucial aspects, including stability against degradation, cost reduction and hysteresis problems. Consequently, it is necessary to incorporate some additives, which turns to the carbon nanotubes due to its high conductivity and stable chemical property. However, traditional conduction heat method will lead to the temperature gradient and be harmful to the nucleation of crystal. Therefore, a new reliable microwave-based method is introduced to synthesize the mixture of carbon nanotubes with hybrid perovskite to improve carrier migration.

Here, a triple cation perovskite precursor solution was prepared using the microwave-assisted synthesis (MAS) method and fabricated as a perovskite transistor in a “fully-ambient air” environment. The stability of perovskite films was evaluated by measuring their optical properties and comparing the characteristics of MAS-perovskite solution to the conventional stirring method (CSM). Photoluminescence (PL) profiles and X-ray diffraction (XRD) patterns of the MAS-perovskite films were maintained over time during the exposure to air, showing enhanced stability compared to the CSM-perovskite films even without encapsulation. The MAS-prepared perovskite films showed anomalous photoluminescence blue-shift and line-width broadening with increasing temperature, emphasizing the unique properties of the triple cation perovskite fabricated. Aside from 7-fold PL enhancement, a 10-fold increase in carrier mobility compared to the CSM-perovskite devices was measured for MAS-perovskite devices. The success in preparing the ambient-fabricated perovskite transistor is beneficial for pushing the large-scale and efficient optoelectronic devices.

The exceptional magnetic, electrical, and optical properties exhibited by metal halide perovskites (MHPs) have garnered significant interest from the optoelectronic research community in recent decades. Perovskite solar cells (PeSCs) have gained substantial attention due to their inherent advantages, such as solution processability, tunable band gaps, high absorption coefficients, and long carrier diffusion lengths. To optimize the performance and stability of PeSCs, it is crucial to incorporate appropriate charge transport layers (CTLs) and/or interlayers. These CTLs need to possess suitable energy levels for efficient charge injection and transport, while effectively blocking opposite charges. Additionally, the CTL located beneath the perovskite layer plays a critical role as it significantly influences the crystal growth of the perovskite layer and affects the presence of interfacial defects. In this presentation, a new series of conjugated polyelectrolytes (CPEs) will be introduced as ideal interfacial layers and CTLs for various PeSCs devices.

Solar cell technology is pivotal in the sustainable energy landscape, offering a renewable energy avenue. This study presents a TOPCon solar cell structure simulation using SILVACO TCAD, aiming to enhance efficiency. The solar cell structure includes the absorber layer, contacts, and anti-reflection coating. SILVACO TCAD models carrier processes, and we explore parameters like absorber thickness, doping concentration, and contact materials to optimize efficiency. Adjusting absorber thickness enhances light absorption, while doping concentration influences carrier properties. Different contact materials reduce resistive losses and enhance charge extraction. The research contributes to advancing solar cell technology, improving conversion efficiency, and offers practical design insights. It lays the foundation for future investigations on innovative materials and device architectures, facilitating efficient and cost-effective solar cells.