Solar AM Research Articles

Solar spectral response of photochromic YHO films

Photochromic yttrium oxyhydride, YHO, has excellent properties as a responsive solar control material, in particular due to its color-neutral darkening. In this work, we have investigated YHO’s spectral response to the solar AM 1.5 spectrum to evaluate how the total photon activation flux changes as a function of thickness and wavelength. The calculations are based on ellipsometry measurements in the 240–2000 nm range. Our results show that the UV light absorption saturates at around 400 nm thick film, while blue light (>400 nm) continues to provide photoactivation well beyond 1 μm thickness. Understanding the spectral response and absorption per unit time of activating photons as a function of material thickness is important for evaluating the material or coating for applications.

A high-performance ultra-wideband metasurface absorber and thermal emitter for solar energy harvesting and thermal applications.

Solar radiation is the Earth's most plentiful renewable energy source. Metasurface-based nanostructures can store solar energy efficiently and exhibit consistent behavior when interacting with light waves. This study investigates an ultra-thin, ultra-wideband solar absorber and thermal emitter that operates in the 400-5000 nm spectrum. The proposed structure design consists of a thin MXene monolayer at the top, followed by a nickel-made fractal L-shaped resonator film mounted on a SiO2 substrate. This device achieves greater than 90% of the aggregative absorption over the 4133 nm ultra-wideband region ranging from 867 nm to 5000 nm. Within its operational band, the solar absorber exhibits excellent solar energy storage capabilities under the solar AM 1.5 model curve. Furthermore, the absorber structure maintains a stable thermal radiation efficiency of 94.5-95.5% over the temperature range of 300-700 K. In addition, the physical mechanism underlying the device's ultra-wideband high absorption characteristics is adequately explained using impedance matching theory and the distribution of surface current density at high absorption wavelengths. The proposed structure design's symmetry shows excellent resilience to polarization state variations as well as wide angular stability to maintain high absorption rate. Given all of these advantages, the proposed structure would be highly suitable for solar energy and thermal radiation applications.

Broadband and Efficient Metamaterial Absorber Design Based on Gold-MgF2-Tungsten Hybrid Structure for Solar Thermal Application.

We have presented a solar absorber design with gold-MgF2-tungsten materials. The solar absorber design is optimized with nonlinear optimization mathematical method to find and optimize geometrical parameters. The wideband absorber is made of a three-layer structure composed of tungsten, magnesium fluoride, and gold. This study analyzed the absorber's performance using numerical methods over the sun wavelength range of 0.25 μm to 3 μm. The solar AM 1.5 absorption spectrum is a benchmark against which the proposed structure's absorbing characteristics are evaluated and discussed. It is necessary to analyze the behavior of the absorber under a variety of various physical parameter conditions in order to determine the results and structural dimensions that are optimal. The nonlinear parametric optimization algorithm is applied to obtain the optimized solution. This structure can absorb more than 98% of light across the near-infrared and visible light spectrums. In addition, the structure has a high absorption efficiency for the far range of the infrared spectrum and the THz range. The absorber that has been presented is versatile enough to be used in a variety of solar applications, both narrowband and broadband. The design of the solar cell that has been presented will be of assistance in designing a solar cell that has high efficiency. The proposed optimized design with optimized parameters will help design solar thermal absorbers.

First-Principles Calculations to Investigate Direct-Band Novel Cobalt-Based Double Perovskite Materials for Optoelectronic Applications

A series of novel cobalt-based perovskite materials have been studied through density functional theory first-principles calculation. The cubic perovskite structure stability is demonstrated by tolerance factors and Born–Huang stability criteria in mechanics as well as formation energies in thermodynamics. The elastic constant analysis indicates that the fluoride perovskites are ductile and ionic compounds while the chloride perovskites are brittle and covalent. All the perovskites are direct semiconductors with band gaps as 1.644, 0.455, 2.136, and 0.683 eV for K2InCoF6, K2InCoCl6, Rb2InCoF6, and Rb2InCoCl6, respectively. The absorption coefficient reveals all the materials still have broad absorption spectra compared with solar AM1.5 irradiance despite photon unavailability in the small wavelength range. Taking into account other physical properties, these perovskites still exhibit great potential and deserve further experimental research. Due to a proper band gap value and high light absorption, K2InCoF6 is considered as the best candidate for solar materials.

Organic ligand complementary passivation to Colloidal-quantum-dot surface enables efficient infrared solar cells

Lead sulfide colloidal quantum dot (PbS CQD) solar cells present great potential in infrared (IR) conversion due to high IR absorption coefficient and solution processability. The large-size IR QDs introduce specific nonpolar (100) facets that undisplay in small-size CQDs. The neutrality of (100) facets and reversible ligand exchange process are crucial challenges for IR QD surface passivation. Here, organic halide salt phenethylammonium iodide/bromine (PEAX, X = I/Br) additive ligands are adopted to cooperate with traditional lead halides for QD surface passivation especially for (100) neutral facets. Both nuclear magnetic resonance (NMR) and density functional theory (DFT) simulation reveal that the organic ligand PEAX can bind on PbS QD planes to enhance passivation without compromising transport property. Further device related photophysical studies confirm the suppressed bulk and interface defect concentration. The optimal PEABr based solar cells possess a power conversion efficiency (PCE) of 12.28 % under solar AM1.5G (certified value of 11.98 %). The crystalline Si filtered efficiency reaches 1.42 %. Both efficiencies are the record values for such absorption range QD (excitonic peak ∼1.3 μm) solar cells. Moreover, the supplementary PEAX ligand suppress the migration of halogen ions and elevate device stability.

Light harvesting coating design accelerated by deep learning for semi-transparent polymer solar cells

The reduction in optical loss in polymer solar cells (PSCs) plays a crucial role in the development of high-performance PSCs devices. Especially for the semi-transparent PSCs, high reflective transparent electrodes lead to low energy utilization. Optical multi-layer coating is proven to be an effective approach to reduce the reflection and transmission loss. In this work, a double-sided PSCs device coating strategy was used to reduce the device optical loss. Optical coating design on a multi-layer PSCs device is far more complex. The dispersion and thickness of each layer both have an impact on the optical property. Meanwhile, the illuminance spectrum is based on the solar AM1.5 spectrum rather than a common-used standard illuminance CIE-E spectrum. It brings many difficulties to the optical design, and the global optimization is generally time-consuming. To fast solve the optimization problem in optical design of the multi-layer coating for PSCs, we combine deep learning (DL) method with hybrid optimization algorithms. By designing a multi-layer device structure to achieve the highest light harvesting with tandem simplex simulated annealing and assisted simplex simulated annealing, we show unambiguously that DL is a powerful tool to minimize the computation cost and maximize the design efficiency for optical multi-layer design. The optical loss of the semi-transparent device is reduced from 52.71% to 27.95%, and the simulation time is reduced by a factor of 276 compared with standard simplex simulated annealing. This provides an efficient optical design strategy in multi-layer coating design for PSCs to achieve desired optical performance.

High performance of TiO2/CuxO photoelectrodes for regenerative solar energy storage in a vanadium photoelectrochemical cell

Photocatalysts for harvesting solar energy to either electricity or chemical fuels have attracted much attention recently, but they have big obstacles such as wide bandgaps and rapid charge recombinations to overcome for final applications. In this study, we investigates a useful method to utilize vanadium redox pairs, which are commonly applied for vanadium redox flow batteries, to diminish charge recombinations and thus to enhance photocurrent response in regenerative solar energy storage. The results reveal significant improvements in photocurrent density by forming cuprous and cupric oxides in TiO2/CuxO electrodes under solar AM 1.5 illuminations using the vanadium photoelectrochemical storage cell at 0.025 mol L−1 of vanadium redox species in the acid electrolytes. In addition, the stabilized photocurrent density of the copper content optimized TiO2/CuxO electrodes is almost tripled from the TiO2 only electrode because the charge recombinations can be mitigated with the content optimized TiO2/CuxO electrodes. Therefore, the optimized TiO2/CuxO electrode results in the highest charge storing performance in the catholyte chamber, and the roles of vanadium redox species are also clearly demonstrated.

Ultra-broadband and wide-angle perfect solar absorber based on TiN nanodisk and Ti thin film structure

At present, solar energy is widely used as a kind of clean energy. The main solar radiation range under AM 1.5 is about 300 ~ 3000 nm. In this paper, we designed an efficient, ultra-broadband perfect solar absorber to have as long absorption bands in this range as possible to help alleviate the energy problem. The simulation calculations and experiments of the solar absorber show that the absorption bandwidth with absorption greater than 90% is greater than 2100 nm. It is worth noting that the perfect absorption bandwidth with absorption greater than 99% has more than 1600 nm. The absorption rate over the whole wavelength range (300 nm–3000 nm) (weighted directly around the sun by solar AM 1.5) is more than 90%. We can effectively control the absorption spectrum by adjusting the structural parameters. In addition, the proposed solar absorber is polarization independent, both the transverse electrical (TE) mode and the transverse magnetic (TM) mode, Absorption remains above 80% when the wide incidence angle is as high as 50°. Our propose design has high broadband absorption and great potential for solar thermal energy harvesting, thermoelectrics, and thermal emitters applications.

Tailoring morphological characteristics of zinc oxide using a one-step hydrothermal method for photoelectrochemical water splitting application

In the present paper describe the zinc oxide (ZnO) with various morphologies have been synthesized using the one-step hydrothermal method, in which the growth of ZnO nanostructures are significantly tailored by adjusting the pH level between 9 and 12 using 0.1 M Sodium hydroxide (NaOH). Significant results reveal the morphological properties of ZnO nanostructures varied with different pH values with the formation of ZnO nanostructures have different morphological such as a baton, star, flower, and rod-like structures. The present results show the rod-like structure of ZnO nanostructures exhibits the highest photocurrent density of 746.61 μAcm−2 (at 1.23 V vs RHE) under simulated solar AM 1.5G illumination in Potassium hydroxide (KOH) medium, also the other morphologies. The dependent of the photoelectrochemical (PEC) water splitting properties on the different morphological of ZnO nanostructures are studied. Achieving the morphological evolution mechanism has become one of the method to obtain the production of the hydrogen growth regime used for solar energy conversion and their applied storage potentials. The application of the ZnO nanostructures for PEC water splitting was proposed with the adoption of screen-printed carbon electrodes (SPCEs). These are to quantify the best degree of the highest photocurrent density with one-step tailoring with an ideal modeling system to enhance PEC water splitting performances. Thus, the screen-printed carbon electrodes (SPEs) has been used as an alternative method for fabrication and photoelectrodes testings.

In-situ Cl− ions formation during photocatalytic reaction of platinized nanocomposite for hydrogen generation

Platinised Titania is emerging as a promising catalyst for its chemical stability, large surface area, and good electronic charge transfer. It is one of the most efficient materials and has been studied over past few decades for photocatalytic hydrogen generation. Photodeposition of Pt nanoparticles on TiO2 surface enhances trapping of photoexcited electrons and restricts recombination process. In this article we explored the influence of chloride ion formed during in-situ photocatalytic process, on photocatalytic hydrogen generation by scavenging holes. Illumination under UV–Visible light (400 nm < λ < 700 nm) improves interband excitation and transfer of photogenerated electron to the conduction band of TiO2. Transmission electron microscopy (TEM) results reveal average particle size of Pt nanoparticle in the range of 10 nm with uniform distribution on TiO2 surface. In-situ photodeposition of Pt nanoparticle on TiO2 resulted in incredibly active photocatalyst for hydrogen generation due to in-situ generated Cl− ions. Also, to one of the highest reported (6.72%). The hydrogen evolution rate of 17 mmol h−1 with apparent quantum yield (AQE). The AQE significantly higher than that for catalyst under solar AM (1.5 G) illumination (0.04%).

Design, fabrication, and characterization of a luminescent solar concentrator with optimized optical concentration through minimization of optical losses

We present an optimized luminescent solar concentrator (LSC) based on the combination of different organic dyes as luminescent species and silicon solar cells. As a first part of this work, a deep screening of organic dyes used since the 1980s in LSC research has been performed and 14 of them have been chosen and characterized in a polymethyl methacrylate (PMMA) host at a fixed concentration (1%). Departing from this initial study, an empirical optimization procedure has been implemented, aiming the highest photon collection at the edges of the LSC plate under a solar AM1.5G spectrum. An optimal three-dye system has been selected in order to achieve a high absorption range over the uv–vis spectrum. The incorporation of these molecules in the PMMA host has been made following two different strategies: (i) the stacking of three individual LSC plates (one dye in each) and (ii) the merging of three organic dyes in a single LSC plate allowing Forster resonance energy transfer (FRET) to occur. In addition, PMMA bulk and thin film on glass plates have been manufactured considering stacking and FRET strategies. Finally, a bulk FRET-based LSC has been manufactured and its optical and electrical performance evaluated and compared to the best reported values used for LSC characterization. To the best of our knowledge, our LSC possesses the highest reported concentration (C=0.80), surpassing Slooff et al. (C=0.73).

BiVO4 photoanodes for water splitting with high injection efficiency, deposited by reactive magnetron co-sputtering

Photoactive bismuth vanadate (BiVO4) thin films were deposited by reactive co-magnetron sputtering from metallic Bi and V targets. The effects of the V-to-Bi ratio, molybdenum doping and post-annealing on the crystallographic and photoelectrochemical (PEC) properties of the BiVO4 films were investigated. Phase-pure monoclinic BiVO4 films, which are more photoactive than the tetragonal BiVO4 phase, were obtained under slightly vanadium-rich conditions. After annealing of the Mo-doped BiVO4 films, the photocurrent increased 2.6 times compared to undoped films. After optimization of the BiVO4 film thickness, the photocurrent densities (without a catalyst or a blocking layer or a hole scavenger) exceeded 1.2 mA/cm2 at a potential of 1.23 VRHE under solar AM1.5 irradiation. The surprisingly high injection efficiency of holes into the electrolyte is attributed to the highly porous film morphology. This co-magnetron sputtering preparation route for photoactive BiVO4 films opens new possibilities for the fabrication of large-scale devices for water splitting.

Solar Driven Processes at Semiconducting Metal Oxide Electrodes, Assisted By Non-Noble Metal Catalysts

Attempts to improve performance of the semiconductor photo-electrodes used either for oxidation or reduction of species present in the electrolyte very often require presence of a catalysts However, identification of new molecular catalysts, active in often highly corrosive media or under unfavourable band energetics is a challenge. N-type mesoporous tungsten trioxide, WO3, is one of few stable semiconductor materials able to photo-electrochemically split water under solar light irradiation but it’s operational acidic environment restricts use of any catalyst to the precious metal based oxygen-evolution electrocatalysts. In an attempt to overcome this limitation, an addition to the acidic electrolyte of Keggin-type polyoxometalates (POMs) species was investigated in function of their electrocatalytic capabilities to drive oxygen evolution. The presence of this catalyst in the electrolyte makes it particularly efficient for use with nanoporous structure allowing deep penetration of the catalyst dissolved in the electrolyte. Moreover, moderate doping with metal cations of WO3 has been found to enhance visible blue light absorption of tungsten trioxide and, consequently, its photocurrent conversion efficiency. Combination of those two effects allowed achieving of record high water photooxidation currents attained under simulated solar AM 1.5G irradiation. The enhancement of anodic photocurrents will be discussed in the frame of functionalization of different mixed metal oxides with the POMs and correlated with the measurements of the amounts of oxygen evolved at the modified electrodes.

Nanoantennas for nanowire photovoltaics

We consider the use of plasmonic nanoantenna elements, hemispherical and cylindrical, for application in semiconductor nanowire (NW) vertical arrays. Using Mie theory and a finite element method, scattering and absorption efficiencies are obtained for the desired enhancement of interaction with light in the NWs. We find an optimal mixture of nanoantennae for efficient scattering of solar spectrum in the NW array. Spectral radiation patterns of scattered light are computed, and, for representing the total response of the nanoantenna-equipped NWs to the solar AM1.5G spectrum, the weighted average of scattering patterns for unpolarized normal incidence is obtained showing an advantageous overall directivity toward the NWs.

Silver/titania nanocomposite-modified photoelectrodes for photoelectrocatalytic methanol oxidation

Silver deposited titania (Ag/TiO2) nanocomposite thin films were fabricated by the simple sonochemical deposition of Ag on preformed aerosol-assisted chemical vapor deposited TiO2 thin films. The photelectrocatalytic performance of a newly fabricated Ag/TiO2-modified photoelectrode was studied for methanol oxidation under simulated solar AM 1.5G irradiation (100 mW/cm2). The Ag/TiO2-modified photoelectrode showed a photocurrent density of 1 mA/cm2, which is four times that of an unmodified TiO2 photoelectrode. The modification of Ag on the TiO2 surface significantly enhanced the photoelectrocatalytic performance by improving the interfacial charge transfer processes, which minimized the charge recombination. Density functional theory (DFT) calculation studies revealed that methanol could be easily adsorbed onto the Ag surfaces of Ag/TiO2 via a partial electron transfer from Ag to methanol. The newly fabricated Ag/TiO2-modified photoelectrode could be a promising candidate for photoelectrochemical applications.

Photo annealing effect on p-doped inverted organic solar cell

We report the transient positive photo annealing effect in which over 600% boost of power conversion efficiency was observed in inverted organic photovoltaic devices (OPV) made from P3HT/PCBM by spray method, after 2 hrs of constant solar AM 1.5 irradiation at low temperature. This is opposite to usual photodegradation of OPV, and cannot be explained by thermal activation alone since the mere temperature effect could only account for 30% of the enhancement. We have investigated the temperature dependence, cell geometry, oxygen influence, and conclude that, for p-doped active layer at room temperature, the predominant mechanism is photo-desorption of O2, which eliminates electron traps and reduces space charge screening. As temperature decreases, thermal activation and deep trap-state filling start to show noticeable effect on the enhancement of photocurrent at intermediate low temperature (T = 125 K). At very low temperature, the dominant mechanism for photo annealing is trap-filling, which significantly reduces recombination between free and trapped carriers. At all temperature, photo annealing effect depends on illumination direction from cathode or anode. We also explained the large fluctuation of photocurrent by the capture/reemit of trapped electrons from shallow electron traps of O2- generated by photo-doping. Our study has demonstrated the dynamic process of photo-doping and photo-desorption, and shown that photo annealing in vacuum can be an efficient method to improve OPV device efficiency.

Optical and Electronic Simulation of Silicon/Germanium Tandem Four Terminal Solar Cells

A tandem solar cell architecture of silicon and germanium solar cells in a mechanical (stack like) arrangement is evaluated to increase the efficiency of light absorption in the far infrared region from 1107 nm to 1907 nm wavelength, which constitutes about 14.5% of the power intensity in the solar AM 1.5 spectrum. In this work, the technical feasibility of tandem solar cells is investigated. Here, we report on detailed electrical and optical simulations of this structure quantifying the various theoretical and practical loss mechanisms in the encapsulation, interfaces, and in the device and indicate that a relative efficiency improvement of 20% may be attainable with silicon and germanium solar cells in this configuration. The optical and electrical parameters for silicon and germanium simulation models were extracted from experimental devices and material vendors. The developed simulation models were validated by comparing the performance of stand-alone silicon and germanium solar cells with experimental devices reported in the literature.

Structural and photoelectrochemical investigation of boron-modified nanostructured tungsten trioxide films

We report a modification of nanostructured WO3 films by doping with boron. The films were obtained by a direct one-step sol–gel route involving tungstic acid/polyethelene glycol precursor. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) showed that the incorporation of boron results in the retention of a substantial amount of water and/or hydroxyl groups in the WO3 lattice and at the surface of nanoparticles occurring despite high temperature (550°C) annealing of the films. Another consequence of boron doping is the largely increased roughness factor revealed by atomic force microscopy (AFM) imaging. Both kinds of films are highly porous and consist of partly sintered particles with sizes in the range of tens of nanometers. The photoelectrochemical (PEC) studies performed under simulated solar AM 1.5 illumination showed significantly enhanced water oxidation photocurrents for B-WO3 photoanodes, by about 25% higher than those for the undoped WO3 films of similar thickness. The low extent of recombination of photogenerated charges was confirmed by incident photon-to-current conversion efficiencies (IPCEs) reaching 70% in the region of visible wavelengths at 420nm. The improved PEC properties were attributed to the increased surface hydroxylation of B-WO3 nanoparticles favoring water photo-oxidation reaction and to the larger surface area of the film exposed to the electrolyte.

Absorption Enhancement in Lossy Transition Metal Elements of Plasmonic Nanosandwiches

Combination of catalytically active transition metals and surface plasmons offers a promising way to drive chemical reactions by converting incident visible light into energetic electron–hole pairs acting as a mediator. In such a reaction enhancement scheme, the conversion efficiency is dependent on light absorption in the metal. Hence, increasing absorption in the plasmonic structure is expected to increase generation of electron–hole pairs and, consequently, the reaction rate. Furthermore, the abundance of energetic electrons might facilitate new reaction pathways. In this work we discuss optical properties of homo- and heterometallic plasmonic nanosandwiches consisting of two parallel disks made of gold and palladium. We show how near-field coupling between the sandwich elements can be used to enhance absorption in one of them. The limits of this enhancement are investigated using finite-difference time-domain simulations. Physical insight is gained through a simple coupled dipole analysis of the nanostructure. For small palladium disks (compared to the gold disk), total absorption enhancement integrated over the near visible solar AM1.5 spectrum is 8-fold, while for large palladium disks, similar in size to the gold one, it exceeds three.

Towards realization of a large‐area light‐emitting diode‐based solar simulator

ABSTRACTSolar simulators based on light‐emitting diodes (LEDs) have shown great promise as alternative light sources for indoor testing of photovoltaic cells with certain characteristics that make them superior to the traditional solar simulators. However, large‐area uniform illumination more suitable for larger cells and module measurements still remain a challenge today. In this paper, we discuss the development and fabrication of a scalable large‐area LED‐based solar simulator that consists of multiple tapered light guides. We demonstrate fine intermixing of many LED light rays and power delivery in the form of a synthesized AM 1.5 spectrum over an area of 25 cm × 50 cm with better than 10% spatial nonuniformity. We present the spectral output, the spatial uniformity, and the temporal stability of the simulator in both the constant current mode and the pulsed‐mode LED operation, and compare our data with the International Electrotechnical Commission standards on solar simulators for class rating. Although the light intensity with our current design and settings falls short of the standard solar AM 1.5 intensity, this design and further improvements open up the possibility of achieving large‐area, high‐power indoor solar simulation with various desired spectra. Copyright © 2011 John Wiley &amp; Sons, Ltd.