Solar Energy Conversion Applications Research Articles

Morphological, structural, optical and photocurrent characteristics of Bi(TixMn(1-x))O3 perovskite nanorods grown by hydrothermal synthesis

This study focuses on the hydrothermal synthesis of Bi(TixMn(1-x))O3 (x = 0.3–0.9) perovskite nanorods on fluorine-doped tin oxide glass substrates and investigates the influence of Ti4+ on their morphological, structural, optical properties, and photocatalytic behavior. The high-resolution XPS spectra results have proved that Ti has been properly substituted into the Bi(TixMn(1-x))O3 nanorods. The characterization of morphology reveals a decrease in average diameter from 222 to 115 nm as the Ti concentration increases, while the band-gap energy experiences a slight reduction from 3.158 to 3.132 eV. X-ray and Raman spectroscopy analyses of the Bi(TixMn(1-x))O3 nanorods indicate that Ti ions substitute not only Mn3+ but also Bi3+ ions, leading to structural distortions in the crystal lattice. Moreover, the Bi(TixMn(1-x))O3 systems demonstrate enhanced photocurrent densities at 1.5 V potential, ranging from 3.19 to 5.54 mA/cm2, as the Ti concentration increases. This enhancement can be attributed to the combined effects of increased surface area, accelerating electron transfer at the interface of the electrode, and decreased optical band-gap energy. It is anticipated that this novel system will pave the way for further advancements in the field of photocatalytic water splitting and solar energy conversion applications.

Eco-friendly synthesis of chemically cross-linked chitosan/cellulose nanocrystal/CMK-3 aerogel based shape-stable phase change material with enhanced energy conversion and storage

Phase change materials (PCMs) have attracted numerous attention owing to their high energy storage density, cost-effective and operationally simple, however, the “solid-liquid” leakage and limited solar absorbance seriously hinder their widespread applications. Herein, an innovative chitosan/cellulose nanocrystal/CMK-3 (CS/CNC/CMK-3) aerogel based shape-stable PCM (SSPCM) was successfully synthesized, in which chemically cross-linked CS and CNC acted as three-dimensional supporting skeleton, CMK-3 endowed solar-to-thermal energy conversion ability and the impregnating polyethylene glycol (PEG) acted as the latent heat storage unit. The as-synthesized CS/CNC/CMK-3 aerogel/PEG (CCCA/PEG) showed ultrahigh melting/crystallization enthalpy of 178.5/171.1 J g−1 and excellent shape stability. The PEG was effectively embedded into the hierarchical porous architecture and the composite PCM could preserve its original shape without any leakage even compressed above the melting point of PEG. Meanwhile, the CCCA/PEG exhibited robust thermal reliability with an ultralow enthalpy fading rate of 0.030 ± 0.012 % per cycle over 100 thermal cycles. Intriguingly, the introduction of CMK-3 also significantly improved the solar-to-thermal energy conversion performance of CCCA/PEG, and a high solar-to-thermal conversion efficiency of 93.1 % could be realized. This work provided a potential strategy to design and synthesize high-performance sustainable SSPCM, which showed tremendous potential in the practical solar energy harvesting, conversion and storage applications.

Bandgap Engineering of Melon using Highly Reduced Graphene Oxide for Enhanced Photoelectrochemical Hydrogen Evolution.

The uncondensed form of polymeric carbon nitrides (PCN), generally known as melon, is a stacked 2D structure of poly(aminoimino)heptazine. Melon is used as a photocatalyst in solar energy conversion applications, but suffers from poor photoconversion efficiency due to weak optical absorption in the visible spectrum, high activation energy, and inefficient separation of photoexcited charge carriers. Experimental and theoretical studies are reported to engineer the bandgap of melon with highly reduced graphene oxide (HRG). Three HRG@melon nanocomposites with different HRG:melon ratios (0.5%, 1%, and 2%) areprepared. The 1% HRG@melon nanocomposite shows higher photocurrent density (71µA cm-2 ) than melon (24µA cm-2 ) in alkaline conditions. The addition of a hole scavenger further increases the photocurrent density to 630µA cm-2 relative to the reversible hydrogen electrode (RHE). These experimental results arevalidated by calculations using density functional theory (DFT), which revealed that HRG results in a significant charge redistribution and an improved photocatalytic hydrogen evolution reaction (HER).

Energetic Stability and Band-Edge Orbitals of Layered Inorganic Perovskite Compounds for Solar Energy Applications.

Halide and oxide perovskite semiconductors (e.g., CsPbI3 and SrTiO3) have been widely studied for solar energy conversion applications. The optoelectronic properties and performance of these compounds can be tuned through the growth of layered perovskite superstructures. While oxides are quite varied in the compositions and geometries taken up by layered perovskites, halides have proven much more limited. In this paper, we use density functional theory calculations and chemical intuition to explore why this is the case. We show that, in general, the thermodynamic stability or instability of layered perovskite superstructures depends on the interplay of their ionic and covalent character and, just as importantly, on the features of other competing phases.

Influence of the oxidation on the optical properties of Mo-Si-Ti, Ta-Mo-Cr-Al and Ta-Mo-Cr-Ti-Al alloys

Three refractory element-based alloys were oxidized for 6 h at 1100 °C in air inside a muffle furnace and the way the oxidation, especially through the evolution of the surface composition and roughness, affects the optical properties of these materials is discussed here. The ternary Mo-20.0Si-52.8Ti (at%) serves as an example of Mo-based alloys, while the equimolar Ta-Mo-Cr-Al (TMCA) and Ta-Mo-Cr-Ti-Al (TMCTA) are typical of the class of oxidation-resistant refractory compositionally complex alloys. The normal spectral emissivity of these three samples was increased due to the oxidation process. Oxidized TMCA and TMCTA exhibit a solar absorptivity higher than that of state-of-the-art Ni-based alloys used for solar receivers in concentrated solar power plants. Thus, the two refractory compositionally complex alloys are considered promising candidates for solar thermal energy conversion application.

Energy and exergy analysis of a bidirectional solar thermoelectric generator combining thermal energy storage

In this paper, energy and exergy analysis of a bidirectional solar thermoelectric generator (STEG) coupled to a latent heat storage and cooling system (LHSCS) has been carried out. The effect of various parameters of LHSCS on energy and exergy efficiencies of STEG have been analysed under climatic conditions of Chile’s Atacama Desert. It is found that the most relevant design parameter to improve the energy and exergy efficiencies of the thermoelectric generator (TEG) is the container insulation, followed by heat sink at the TEG hot side, fin thickness and the aspect ratio of the container. The results showed that an optimally designed insulation container can improve the energy and exergy efficiencies of LHSCS by 30% and 200%, respectively, and the TEG conversion efficiency by 30% during nighttime. Further, inclusion of heat sink at TEG hot side during reverse operation of TEG at night can improve the TEG efficiency by 20%. The optimal fin thickness can improve the TEG conversion efficiency by 20% during the night and LHSCS energy and exergy efficiencies by 30% and 23%, respectively. The container geometry should have higher aspect ratios. This study may help in optimal design of LHSCS for solar energy conversion applications in the desert locations.

Ternary Oxide Semiconductors and Alloys: Hope and Reality

This perspective talk focuses on the history and status of new materials with targeted application in solar energy conversion. Specifically, photoelectrochemical energy conversion/solar water splitting and wide bandgap oxide semiconductors and alloys for solar windows and displays are the two targeted application areas. The Cu-M-V-O family of oxide semiconductors will be discussed in this poster. Both synthetic aspects and chemical composition-property-performance correlations will be presented.Acknowledgements. This work was primarily supported by the National Science Foundation UTA/NU Partnership for Research and Education in Materials (NSF DMR-2122128).

Observation of Electronic and Structural Interaction between Small Polaron and Host Material Using Femtosecond XUV Reflection Spectroscopy

Small polaron formation in transition metal oxides is believed to be a reaction bottleneck in many solar energy conversion applications. While polaron formation has been previously confirmed, the microscopic interaction between a small polaron and its host material is largely unexplored. Here, using femtosecond XUV reflection spectroscopy, we report the evidence of electronic and structural interaction between the small polaron and its host material in CuFeO2, a photoelectrode material for CO2 reduction. Initial small polaron formation is observed as a spectral blue shift occurring within the first 100 fs. After polaron formation, we observe an increased coherent oscillation signal around the polaron sites, which is attributed to polaron-induced optical phonons. This observation suggests that the polaron-associated local lattice distortion can launch optical phonons in neighboring unit cells. In addition to structural coupling, the electronic states in the host materials can also be modified during polaron formation. As an example, we report an increase of Fe oxidation states after photoexcitation. The population of these highly oxidized Fe atoms strongly correlates with polaron dynamics, suggesting that polaron can alter its surrounding electronic states in host materials.

(Invited) Energy Level Alignment and Hot Carrier Extraction in Monolayer Semiconductor Photoelectrochemical Cells

The fundamental problem that limits the solar energy conversion efficiency of conventional semiconductors such as Si is that all absorbed photon energy above the band gap is lost as heat. The critical question that our research addresses is: Can we avoid energy losses in semiconductors? Hot-carrier systems that avoid such losses have tremendous potential in photovoltaics and solar fuels production, with theoretical efficiencies of 66% (well above the detailed-balance limit of 33%). Ultrathin 2D semiconductors such as monolayer (ML) MoS2 and WSe2 have unique physical and photophysical properties that could make hot-carrier energy conversion possible. The specific knowledge gap in the field is how the energy levels of 2D semiconductors move with applied potential and/or illumination, making the driving force for charge transfer (DG 0´) unclear. Since DG 0´ governs the hot-carrier extraction rate (k ET), understanding how and why DG 0´ changes under solar fuel generation conditions is critical to controlling k ET relative to the cooling rate. Absence of this critical information is limiting our ability to perform hot-carrier photochemistry. Our research team has employed photocurrent spectroscopy, steady-state absorption spectroscopy, and in situ femtosecond transient absorption spectroscopy as a function of applied potential to characterize underlying steps in a ML MoS2 photoelectrochemical cell. The rich data set informs us on the timescales for hot-carrier generation/cooling and exciton formation/recombination, as well as the magnitudes of changes in exciton energy levels, exciton binding energies, and the electronic band gap. These findings open the possibility of tuning the hot-carrier extraction rate relative to the cooling rate to ultimately utilize hot-carriers for solar energy conversion applications.

Ferroelastic twin domain patterns and polar domain walls of BiVO4 thin films via phase-field simulations

Polar domain walls in centrosymmetric ferroelastics (e.g., BiVO4 films) give rise to inhomogeneity and offer promises for solar energy conversion applications via expediting charge-carrier separation. However, the origin of polar domain walls and the relationship between them and spontaneous strain from structural phase transition remain elusive at the mesoscale in BiVO4 films. Here, from the perspective of phenomenological thermodynamics, the relationship between four variants of monoclinic (M) phase and the corresponding lattice parameters during the tetragonal-monoclinic (T-M) phase transition is revealed. Using phase-field simulations, it suggests that the formation of four variants of ferroelastic twin domains arises from spontaneous strain from T-M phase transition. Moreover, it is demonstrated that (i) polar domain walls in BiVO4 films originate from the flexoelectric effect induced by strain gradient across the domain walls during phase transition process; (ii) shear strain gradient dominates the contribution to the polarization vectors at the domain walls; (iii) 180° head-to-head or tail-to-tail in-plane polarization configurations emerge at the adjacent domain walls. The stability of several types of ferroelastic domain walls is also analyzed from the point of view of total free energy. These results not only provide a fundamental understanding of the origin of polar domain walls and the formation mechanisms of ferroelastic twin patterns, but also stimulate potential applications of polar domain walls of non-polar BiVO4 films in energy conversion.

Modulating electronic structure of two-dimensional AsP via external strain and electric field

In this paper, the density functional theory calculations show that the electronic structure of monolayer honeycomb AsP can be effectively tailored by both applied strain and electric field. By applying external strains, the band gap of monolayer honeycomb AsP can be monotonous reduced by bigger strains. More interestingly, by applying 10% tensile strains along biaxial and uniaxial zigzag directions, monolayer honeycomb AsP becomes a direct band gap semiconductor with the band gaps of 1.28 and 1.73 eV, respectively, which are in the visible light region and suggest solar energy conversion applications. This work provides a feasible way of making monolayer β-AsP a direct semiconductor and a good candidate for photocatalysts.

Designing multifunctional silica coatings for enhanced broadband antireflection and microfiber contamination sensing

Multifunctional silica coatings find diverse applications in solar energy conversion and pollution monitoring sensors due to their suitability for mass manufacturing. However, the challenge lies in producing robust silica coatings on a large scale with tunable refractive index. This work presents a two-step acid/base catalyzed sol–gel process for synthesizing multifunctional silica coatings with adjustable refractive index. The coatings' porous microstructure allows them to be used as double-layer broadband antireflective coatings and enhances the sensitivity of contamination sensors. The optimized double-layer coatings, designed using computer-aided techniques, demonstrate excellent broadband antireflective performance in the visible regions and high environmental durability. They achieve a maximum transmittance of 99.87% at 591 nm, with an average transmittance of 99.40% in the visible region (400–800 nm) and 98.63% from 400 nm to 1100 nm. Additionally, the coatings exhibit robust abrasion resistance and enhanced surface hydrophobicity with a water contact angle increased from 30° to 126° through HMDS treatment. This work investigated the particle growth mechanism and the relationship between silica nanoparticle microstructure and antireflective coating properties. Furthermore, the coatings' presence of abundant micropores allows successful application onto microfiber contamination sensors, the additional loss increases from 26.40 dB to 37.69 dB over 140 min and decreases to 31.55 dB within one minute, demonstrating rapid adsorption and desorption rates. The work successfully achieves a wide range of adjustable refractive index while maintaining excellent mechanical and optical properties, addressing the traditional trade-off between high porosity and poor mechanical stability. These rationally designed multifunctional silica coatings hold great potential for high-quality broadband antireflection and high sensitivity in microfiber contamination sensing.

Effect of Annealing and Etching Times on Anatase TiO2 Hollow Sphere

The development of high-efficiency photocatalysts plays an important role in the application of solar energy conversion. Titanium dioxide (TiO2) with an anatase crystalline phase is well-known as semiconductor thin layers for solar cells. This work has constructed a novel TiO2 hollow sphere (HST) in a 2-step synthesis. The first step is coating the SiO2 template with TiO2 to build a core-shell of SiO2@TiO2 (CSST). The second step is etching via sonication to elute the SiO2 from CSST and construct the HST. The annealing of CSST for 1 to 6 hours and etching for 1 to 7 hours has resulted in the HST with different crystallite sizes and microstrains. The HST of ~90 nm has been fabricated with crystallite size of 9.53 to 20.54 nm and microstrain from 0.34 to 3.42. It was found that the optimum crystallite size and microstrain of HST obtained via annealing the CSST for 2 hours and etching for 5 hours has the best photooxidation of I- under UV irradiation. The optimum crystallite size and microstrain of HST via annealing and etching times can be recommended for the future of solar cell fabrication and applications.

Stepping toward Portable Optoelectronics with SnO2 Quantum Dot-Based Electron Transport Layers.

With a power conversion efficiency (PCE) of more than 25%, perovskite solar cells (PSCs) have shown an immense potential application for solar energy conversion. Owing to lower manufacturing costs and facile processibility via printing techniques, PSCs can easily be scaled up to an industrial scale. The device performance of printed PSCs has been improving steadily with the development and optimization of the printing process for the device functional layers. Various kinds of SnO2 nanoparticle (NP) dispersion solutions including commercial ones are used to print the electron transport layer (ETL) of printed PSCs, and high processing temperatures are often required to obtain ETLs with optimum quality. This, however, limits the application of SnO2 ETLs in printed and flexible PSCs. In this work, the use of an alternative SnO2 dispersion solution based on SnO2 quantum dots (QDs) to fabricate ETLs of printed PSCs on flexible substrates is reported. A comparative analysis of the performance and properties of the obtained devices with the devices fabricated employing ETLs made with a commercial SnO2 NP dispersion solution is carried out. The ETLs made with SnO2 QDs are shown to improve the performance of devices by ∼11% on average compared to the ETLs made with SnO2 NPs. It is found that employing SnO2 QDs can reduce trap states in the perovskite layer and improve charge extraction in devices.

Effect of uniaxial strains on electronic and optical properties of SnSe from first-principles calculations

In this work, we systematically investigate the uniaxial strain dependence of the electronic and optical properties of SnSe using first-principles calculations and the Bethe−Salpeter equation (BSE) approach. The calculated results show that uniaxial strains significantly affect the main peaks of density of states (DOS) in the valence band and conduction band through the influence on the densities of Sn s/p-states and Se p-state. The main peaks of the real ε1(ω) and imaginary ε2(ω) parts of the dielectric function can be enhanced by suitable uniaxial strains. In particular, the peaks of ε1(ω) and ε2(ω) along the a-axis increase by 34.8 % and 54.8 % with a red shift under the strain εa = −6 %, respectively. The high absorption coefficient in a wide energy range induced by uniaxial strains suggests a bright prospect for its applications in solar energy conversion. Our work indicates that strain engineering can be an efficient method to modulate the optical properties of SnSe and opens up a new avenue to its practical application.

Ultrafast Deposition of NiFe Metal-organic Framework Catalysts Boosts BiVO4 Photoanode for Efficient Solar Water Splitting.

The severe photocorrosion of BiVO4 limits its application in solar energy conversion in the long-term photoelectrochemical stability test. Herein, we synthesized a Fe@Ni-MOFs/BiVO4 photoanode by a simple ultrasonic method and ultrafast deposition, which avoids the problems of damaging the surface structure of photoelectrode. The Fe@Ni-MOFs/BiVO4 shows a photocurrent density of 4.89 mA cm-2 at 1.23 VRHE with an onset potential of 0.25 VRHE under one sun illumination. Importantly, a stability over 30 h at 0.7 VRHE can be obtained, which is so far the best stability character for MOFs-based cocatalysts decorated on BiVO4. During operation, Fe@Ni-MOFs transforms into a homogeneous and active hydroxide layer covering the surface, which exposes more active sites for OER reactions. This work demonstrates a simple strategy for MOFs co-catalysts to obtain fast hole transfer capability and reduce carrier recombination, thereby improving PEC performance.

Microwave-assisted synthesis of edge-grafted carbon nitride by triazole rings for efficient photocatalytic hydrogen evolution

Copolymerization of conjugated nitrogen heterocycles within carbon nitride nowadays is proven to be a successful strategy to promote charge separation. Here, a novel edge-grafted carbon nitride nanosheets by triazole rings were synthesized from the copolymerization of dicyandiamide and 3-amino-1, 2, 4-triazole via microwave-assisted heating with improved yield. Based on the building of a feasible donor-acceptor (D-A) system by such edge grafting, the rapid charge transfer pathways were formed. Benefiting from the thin-layer structure of carbon nitride via microwave-assisted heating and abundant π electrons by intramolecular doping, the system showed enhanced visible light absorption from n-π∗ electrons transition, facile charge separation and transfer, and then an increased H2 evolution rate (637.58 μmol h−1 g−1), which is 17.43 times than the triazole modified sample from conventional thermal condensation method. This work enlightens an innovative pathway for the regulation of carbon nitride photocatalysts by conjugated nitrogen heterocycles for efficient solar energy conversion applications.

Fluorinated Antimony(V) Tetraarylporphyrins as High-Valent Electron Acceptors with Unparalleled Reduction Potentials

A series of fluorinated antimony(V) porphyrins, SbTPP(OMe)2·PF6, SbTPP(OTFE)2·PF6, SbT(4F)PP(OMe)2·PF6, SbT(35F)PP(OMe)2·PF6, SbT(345F)PP(OMe)2·PF6, SbT(4CF3)PP(OMe)2·PF6, SbT(35CF3)PP(OMe)2·PF6, and SbT(35CF3)PP(OTFE)2·PF6, have been synthesized with phenyl [P], 4-fluorophenyl [(4F)P], 3,5-difluorophenyl [(35F)P], 3,4,5-difluorophenyl [(345F)P], 4-trifluoromethylphenyl [(4CF3)P], and 3,5-bis(trifluoromethyl)phenyl [(35CF3)P], in the meso-positions. Additionally, the SbTPP(OTFE)2·PF6 and SbT(35CF3)PP(OTFE)2·PF6 carry trifluoroethoxy units in their axial-positions. The fluorination on the porphyrin peripherals ranges from zero fluorine atoms in SbTPP(OMe)2·PF6 to 30 fluorine atoms in SbT(35CF3)PP(OTFE)2·PF6. X-ray crystallography confirmed the structures of the investigated antimony(V) porphyrins. The absorption spectra depend on the number of fluorine atoms as it is blue-shifted with increasing fluorination. The series also exhibited rich redox chemistry with two reduction processes and one oxidation process. Remarkably, these porphyrins manifested the lowest reduction potentials reported among the main-group porphyrins, which are as low as -0.08 V vs SCE for SbT(35CF3)PP(OTFE)2·PF6. On the contrary, the oxidation potentials were found to be very large, that is equal to 2.20 V vs SCE or even higher for SbT(4CF3)PP(OMe)2·PF6 or SbT(35CF3)PP(OMe)2·PF6 and SbT(35CF3)PP(OTFE)2·PF6, respectively. These unprecedented potentials are due to a combination of two factors: (i) the +5-oxidation state of antimony in the porphyrin cavity and (ii) the presence of the strong electron-withdrawing fluorine atoms on the porphyrin peripherals. Density functional theory (DFT) calculations were used to support the experimental results. The systematic study of antimony(V) porphyrins, especially their high potentials, make them ideal for the construction of photoelectrodes and excellent electron acceptors for photoelectrochemical cells and artificial photosynthetic systems, respectively, for solar energy conversion and storage applications.

Regulation on the electrical and optical properties of two-dimensional stacked graphene quantum dots/gold quantum dots/tungsten sulfide heterostructures

Two-dimensional (2D) transition metal dichalcogenides (TMDCs), like WS2, have been particularly promising in the application of solar energy conversion. Nevertheless, how to enhance the absorption capacity and photogenerated dcharge transfer is yet challenging. Herein, 2D stacked heterostructures were firstly constructed by 2D Au quantum dots (QDs), graphene quantum dots (GQDs) and WS2 monolayer. Compared with WS2 monolayer, the absorption abilities of the Au QDs/WS2 heterostructures in visible-near infrared light region of 450–2300 nm increase evidently. Simultaneously, charge redistribution occurs at the WS2–Au interfaces and generates the built-in electric field with the direction from Au QDs to the WS2 monolayer. This built-in electric field is conducive to boosting photo-induced charge separation and transfer. Notably, more obvious charge transfer and stronger built-in electric field can be achieved in WS2–Au6 interface after introducing the GQDs. Therefore, this work provides unique insights for designing new WS2-based heterostructures to improve the solar energy conversion.

Single-atom cobalt-incorporating carbon nitride for photocatalytic solar hydrogen conversion: An X-ray spectromicroscopy study

The use of carbon nitride-based materials and light to drive catalytic water splitting has enormous potential for the production of hydrogen. Revealing the processes of molecular conjugation, nucleation, and crystallization in crystalline carbon nitride is expected to enhance the photocatalytic activity through the creation of isotype heterojunctions and active sites. In this work, the addition of cobalt salts in ionothermal synthesis was found to promote the phase transition of heptazine-based crystalline carbon nitride (CCN) to triazine-based poly (triazine imide) (PTI), resulting in the formation of a single-atom cobalt-doped coordinated isotype CCN/PTI heterojunction. The new hybrid orbital modulates the atomic/electronic structure and the band gap of the CCN/PTI heterojunction, and synergistically increases the absorption of visible light, accelerating the separation and transfer of photoexcited electrons and holes. Synchrotron-based X-ray spectroscopy and microscopy are used to identify the origin of the improved performance of the single-atom cobalt-doped CCN/PTI heterojunction in the photocatalytic hydrogen evolution reaction. This work demonstrates that synchrotron X-ray spectroscopy is a promising tool for designing materials aimed at enhancing photocatalytic activity in solar energy conversion applications.