Materials For Solar Energy Conversion Research Articles

Construction of CuS/ZnS heterostructures with enhanced photocatalytic activities on dye degradation and Cr (VI) reduction

The CuS/ZnS nanocomposites with particle morphology were prepared by ion exchange method. The systemically characterizations were used to study the phase structure, morphology, heterojunction, surface chemical components and optical properties. Furthermore, photocatalytic properties were researched by using 20 mg/L methylene blue and 40 mg/L of potassium dichromate solution as photocatalytic ox-red indicators, respectively. The results showed that the ZnS/CuS composites degraded 86.1% methylene blue solution within 50 min, which was 2.2 times higher than that of bare CuS nanoparticles. Significantly, the degradation rate of Cr (VI) reached 96.6% only within 8 min, which was greatly higher than that of each single component. Therefore, selecting appropriate semiconductors to construct type I-type hetero-structured composite materials could also take into account the three aspects’ efficiency of photo-quantum absorption, photogenerated charge transfer separation and surface chemical reaction for modifying the photocatalytic property, which provided a reliable experimental basis for the design and development of efficient solar energy conversion materials.

Effects of Mono- and Bifunctional Surface Ligands of Cu-In-Se Quantum Dots on Photoelectrochemical Hydrogen Production.

Semiconductor nanocrystal quantum dots (QDs) are promising materials for solar energy conversion because of their bandgap tunability, high absorption coefficient, and improved hot-carrier generation. CuInSe2 (CISe)-based QDs have attracted attention because of their low toxicity and wide light-absorption range, spanning visible to near-infrared light. In this work, we study the effects of the surface ligands of colloidal CISe QDs on the photoelectrochemical characteristics of QD-photoanodes. Colloidal CISe QDs with mono- and bifunctional surface ligands are prepared and used in the fabrication of type-II heterojunction photoanodes by adsorbing QDs on mesoporous TiO2. QDs with monofunctional ligands are directly attached on TiO2 through partial ligand detachment, which is beneficial for electron transfer between QDs and TiO2. In contrast, bifunctional ligands bridge QDs and TiO2, increasing the amount of QD adsorption. Finally, photoanodes fabricated with oleylamine-passivated QDs show a current density of ~8.2 mA/cm2, while those fabricated with mercaptopropionic-acid-passivated QDs demonstrate a current density of ~6.7 mA/cm2 (at 0.6 VRHE under one sun illumination). Our study provides important information for the preparation of QD photoelectrodes for efficient photoelectrochemical hydrogen generation.

Noble-metal free plasmonic nanomaterials for enhanced photocatalytic applications—A review

Plasmonic nanomaterial catalysis is currently at the frontier of photocatalysis, overcoming the limitations of wide bandgap semiconductors for light absorption. Its localized surface plasmon resonance (LSPR) properties allow broad ultraviolet—visible—near infrared ray (UV—vis—NIR) absorption, making it an ideal material for solar energy conversion. Most plasmonic nanostructures rely on precious metals. Although noble metal plasmonic nanomaterials have proven to be one of the strategies for enhancing photocatalytic activity, their expensive cost and limitations in light absorption range have hindered their practical application. As a result, noble-metal free plasmonic nanomaterials have risen to the top of the research priority list. Therefore, this paper reviews the fundamental principles and classification of the LSPR effect of noble-metal free plasmonic nanomaterials in photocatalytic and their recent applications in hydrogen generation, carbon dioxide reduction, and pollutant degradation. Specific cases elucidate the possible working mechanism of enhanced photocatalysis by noble-metal free plasmonic nanomaterials. Finally, the challenges and future opportunities for noble-metal free plasmonic nanomaterials in energy conversion and storage are discussed and envisioned.

NiO Modified CN Film As Photoanodes for Photoelectrochemical Water Oxidation

The utilization of solar energy, by far the most promising renewable energy resource, remains one of the hottest topics in the 21st century. Metal-free carbon nitride (CN) material emerged as a promising water splitting photocatalyst in 2009 due to its appropriate visible light absorption, suitable optical band gap energy (2.7 eV), and facile synthesis. Though CN is one of the leading materials for solar energy conversion, this material is limited by light absorption, rapid charge recombination, and low charge carrier mobility. Metal oxide are thus investigated for counterbalancing CN’s inherent drawbacks. Interfacial CN/metal oxide heterostructures facilitate charge transportation, resulting in improved sunlight-driven water splitting process. In this work, low-cost NiO transition-metal oxide material was applied to speed up the sluggish kinetics of the oxygen evolution reaction (OER). Typically, CN modification is achieved by traditional hydrothermal approach, while its disadvantages such as uneven coating particle size and heterogenous distribution are becoming increasingly apparent. Aiming at a conformal morphology, atomic layer deposition (ALD) is developed as the state-of-the-art technique. It has boosted the depositing accuracy by achieving precisely depositing thickness and extremely homogenous surface. In our process, a uniform CN film was deposited on FTO substrates using a dipping-drying technique with a hot saturated thiourea aqueous solution followed by a thermal treatment. Then plasma-enhanced atomic layer deposition (PEALD) was used to modify the CN film with a thin layer of NiO. PEALD controls the NiO modification on a fine-scale, allowing to with deposit a nanoscale NiO layer on the CN surface while exposing adequate CN photoreactive sites. According to our results, the modified NiO/CN heterostructure has the potential to improve photoelectrochemical water oxidation as a photoanode in alkaline solution. From the morphology side, the NiO loaded flake-like CN creates relatively high specific area for OER. Then we investigated the photo-process kinetic using a series of optical and spectroelectrochemcial techniques. Optical absorption is enhanced, showing stronger visible light absorption up to 700 nm. Fast charge separation is suggested in photoluminescence (PL) characterization. Superior (photo)electrochemical (PEC) activity is foreseen through PEC and EC measurements. To conclude, this is the first time we succeed to modify the CN film with the ALD technique for solar-driven water oxidation, and has the highly possibility of reaching the photo(electro)chemical performance new level.

Donor-acceptor carbazole-based conjugated microporous polymers as photocatalysts for visible-light-driven H2 and O2 evolution from water splitting

Although novel donor–acceptor conjugated microporous polymers (D-A CMPs) that absorb visible light broadly and allow the efficient isolation of photoinduced electrons and holes are being sought for water splitting photocatalysis, their elusive energy band structures have made it a challenge to produce both H2 and O2 simultaneously. In this paper, for the first time, we develop a series of semiconducting D-A CMPs with various sulfur and nitrogen contents and degrees of conjugation for effective photocatalytic H2 and O2 generation from water. Here, we used a carbazole (Cz) moiety as the donor, and pyrene (Py), triphenyltriazine (TPT), benzothiadiazole (BT), and thiazolylthiazole (TzTz) as the acceptors. The resulting D-A CMPs had large surface areas (up to 1530 m2 g–1) and high thermal stabilities (Td10: up to 623 °C; char yield: up to 83 wt%). Moreover, the Cz-TzTz CMP provided an excellent hydrogen evolution rate (HER), reaching 15.3 mmol g–1 h–1 in the presence of ascorbic acid as the sacrificial electron donor, but without the need for an additional Pt co-catalyst. Interestingly, the Cz-TPT CMP provided an excellent oxygen evolution rate (OER) of 3.38 mmol g–1 h–1 when using AgNO3 as a sacrificial electron acceptor; this value is competitive with those of most previously reported organic photocatalysts. These highly active CMP-based photocatalysts appear to be useful materials for solar energy harvesting and conversion.

Hierarchical Hollow Zinc Oxide Nanocomposites Derived from Morphology-Tunable Coordination Polymers for Enhanced Solar Hydrogen Production.

Infinite coordination-polymer particles (CPPs) are promising materials for solar energy conversion with high efficiency. However, the range of organic ligands that may be used to create CPPs is limited, as are strategies for modification, thereby hindering the applications of such material. In this paper, competitive evolution-morphological and structural change from Zn-based crystallites to amorphous particles is described. Controlled contribution of organic linkers selectively derived six Zn-CPPs with multivariate characters. Based on the diversity of these substructures, hollow zinc oxide particles were initially formed by self-pyrolysis of CPPs and effectively modified by ultrathin doped nanosheets. The obtained double-sided heterojunctions offer fully-covered active sites, bringing together efficient light-excited charge-transfer nanochannels, which exhibit an excellent solar H2 -releasing activity (e.g., 4512.5 μmol h-1 g-1 ) and stable cyclability.

Hierarchical Hollow Zinc Oxide Nanocomposites Derived from Morphology‐Tunable Coordination Polymers for Enhanced Solar Hydrogen Production

AbstractInfinite coordination‐polymer particles (CPPs) are promising materials for solar energy conversion with high efficiency. However, the range of organic ligands that may be used to create CPPs is limited, as are strategies for modification, thereby hindering the applications of such material. In this paper, competitive evolution–morphological and structural change from Zn‐based crystallites to amorphous particles is described. Controlled contribution of organic linkers selectively derived six Zn‐CPPs with multivariate characters. Based on the diversity of these substructures, hollow zinc oxide particles were initially formed by self‐pyrolysis of CPPs and effectively modified by ultrathin doped nanosheets. The obtained double‐sided heterojunctions offer fully‐covered active sites, bringing together efficient light‐excited charge‐transfer nanochannels, which exhibit an excellent solar H2‐releasing activity (e.g., 4512.5 μmol h−1 g−1) and stable cyclability.

Large-Area, High-Specific-Power Schottky-Junction Photovoltaics from CVD-Grown Monolayer MoS2.

The deployment of two-dimensional (2D) materials for solar energy conversion requires scalable large-area devices. Here, we present the design, modeling, fabrication, and characterization of monolayer MoS2-based lateral Schottky-junction photovoltaic (PV) devices grown by using chemical vapor deposition (CVD). The device design consists of asymmetric Ti and Pt metal contacts with a work function offset to enable charge separation. These early stage devices show repeatable performance under 1 sun illumination, with VOC of 160 mV, JSC of 0.01 mA/cm2, power conversion efficiency of 0.0005%, and specific power of 1.58 kW/kg. An optoelectronic model for this device is developed and validated with experimental results. This model is used to understand loss mechanisms and project optimized device designs. The model predicts that a 2D PV device with ∼70 kW/kg of specific power can be achieved with minimum optimization to the current devices. By increasing the thickness of the absorber layer, we can achieve even higher performance devices. Finally, a 25 mm2 area solar cell made with a 0.65 nm thick MoS2 monolayer is demonstrated, showing VOC of 210 mV under 1 sun illumination. This is the first demonstration of a large-area PV device made with CVD-grown scalable 2D materials.

Electronic defects in metal oxide photocatalysts

A deep understanding of defects is essential for the optimisation of materials for solar energy conversion. This is particularly true for metal oxide photo(electro)catalysts, which typically feature high concentrations of charged point defects that are electronically active. In photovoltaic materials, except for selected dopants, defects are considered detrimental and should be eliminated to minimise charge recombination. However, photocatalysis is a more complex process where defects can play an active role, for example, by stabilising charge separation and mediating rate-limiting catalytic steps. Here, we review the behaviour of electronic defects in metal oxides, paying special attention to the principles underpinning the formation and function of trapped charges in the form of polarons. We focus on how defects alter the electronic structure, statically or transiently upon illumination, and discuss the implications of such changes in light-driven catalytic reactions. Finally, we consider the applicability of lessons learned from oxide defect chemistry to new photocatalysts based on carbon nitrides, polymers and metal halide perovskites.

Efficient Solar Thermal Energy Conversion and Utilization by a Film of Conductive Metal–Organic Framework Layered on Nanocellulose

Developing materials for efficient solar thermal energy conversion (STEC) is currently a promising field in energy research. Traditional STEC materials such as carbon and plasmonic nanomaterials have limited efficiency of solar heat utilization, despite their high photothermal conversion efficiency. This paper describes a film composed of hybrid nanofibers of a metal–organic framework layered on cellulose (MC film), resulting in both high photothermal conversion and heat utilization efficiency. The mechanically strong and flexible film can be designed as a solar-driven actuator, enabling large-angle actuation and high contractile power up to 2.5 times greater than that of human muscle. Furthermore, the gathered heat by a MC film-based apparatus can be manipulated to drive solar steam generation for highly efficient seawater desalination, generating clean water at rate of 2.25 kg m–2 h–1 under one-sun irradiation without surface salt accumulation. This work may provide a design rule for developing high-performance STEC systems.

Tip-induced directional charge separation on one-dimensional BiVO4 nanocones for asymmetric light absorption

One dimensional (1D) semiconductor is a class of extensively attractive materials for many emerging solar energy conversion technologies. However, it is still of shortage to assess the impact of 1D structural symmetry on spatial charge separation and understand its underlying mechanism. Here we take controllably-synthesized 1D BiVO4 nanocones and nanorods as prototypes to study the influence of 1D symmetry on charge separation. It is found that the asymmetric BiVO4 nanocones enable more effective charge separation compared with the symmetric nanorods. The unexpected spatial charge separation on the nanocones is mainly ascribed to uneven light absorption induced diffusion-controllable charge separation due to symmetry breaking of 1D nanostructure, as evidenced by spatial and temporal resolved spectroscopy. Moreover, the promotion effect of charge separation on the nanocones was quantitatively evaluated to be over 20 times higher than that in BiVO4 nanorods. This work gives the first demonstration of the influence of 1D structural symmetry on the charge separation behavior, providing new insights to design and fabricate semiconductor materials for efficient solar energy conversion.

Adenine-functionalized graphene oxide as a charge transfer layer to enhance activity and stability of Cu2O photocathode for CO2 reduction reaction

Cuprous oxide (Cu2O) is one of the most promising light-absorbing materials for solar energy conversion, attracting extensive attention in photoelectrochemical carbon dioxide reduction reaction (PEC CO2RR). However, the inefficient collection of photogenerated charge carriers limits the application of Cu2O photocathodes. Herein, adenine biomolecules were introduced into graphene oxide (denoted as A-GO) as a charge transfer layer for coating Cu2O photocathode. The A-GO charge transfer layer can improve photoelectrochemical activity and stability of the Cu2O photocathode by facilitating the electrons transfer. Under AM 1.5 G illumination, the photocurrent density of the A-GO/Cu2O photocathode is 2.74 mA cm−2 at −1.1 V (vs Ag/AgCl), which is 2.23 and 1.56 times higher than that of bare Cu2O and GO/Cu2O photocathodes, respectively. The Lewis-basic and pyrimidine N sites in adenine biomolecules can assist the Cu2O for PEC CO2RR. During PEC CO2RR, the A-GO/Cu2O photocathode shows a high Faradaic efficiency of 69.25% for menthol production. Coating biomolecules functionalized GO as a charge transfer layer was a crucial strategy for building effective and stable Cu2O-based photocathodes for PEC CO2RR, as illustrated in this study.

Titanium‐Based Vacancy‐Ordered Double Halide Family in Perovskite Solar Cells

The toxicity and stability risk of perovskite structured materials have raised concerns in respective to utilization as a solar energy conversion material. The most common perovskite structured material is lead (Pb)‐based, which is an element that is knowingly toxic to humans and the environment. Although the stability issue has been well allayed with several optimizations, the ruinous Pb remains a future challenge for perovskite solar cells. Compositional and structural derivatives of the perovskite family, specifically vacancy‐ordered double halide perovskites (DHPs), have attracted the attention of researchers in terms of efficiency and toxicity issues subjugation. Although tin (Sn)‐based vacancy‐ordered DHPs have been widely explored, the intrinsic property conduces low performance output. Titanium (Ti) is a potential substituting candidate of Sn in a vacancy‐ordered DHPs structure. It is an environment‐friendly element ideal for sustainable perovskite structured compositions. Rudimentary studies of Ti‐based vacancy‐ordered DHPs emphasized its potential development as an eco‐friendly and stable solar cell. In promoting the development of Ti‐based vacancy‐ordered DHPs as potential absorbers, we summarized herein the recent progress of experimental and theoretical studies of this perovskite material.

Density Functional Theory Estimate of Halide Perovskite Band Gap: When Spin Orbit Coupling Helps

The description of the band gap of halide perovskites at the level of density functional theory (DFT) has been subject of several studies but still presents significant problems and deviations from experimental values. Various approaches have been proposed, including the use of system-specific hybrid functionals with a variable amount of exact exchange or the explicit inclusion of spin–orbit coupling (SOC) effects. In this work, we present a pragmatic recipe to compute the band gap of halide perovskites with a minimum average error. The recipe is tested on a set of 36 halide perovskites of the type ABX3 [A = Cs, methyl-ammonium (MA), and formamidinium (FA); B = Ge, Sn, and Pb; and X = Cl, Br, and I] for which experimental estimates of the band gap have been reported in the literature. Upon assessment of the accuracy of commonly used DFT functionals and the analysis of their performances based on error and statistical analysis, we suggest a strategy to compute band gaps in halide perovskites with a single functional. This is based on the use of the hybrid HSE06 functional where SOC is included exclusively for Pb-containing compounds. The results are rationalized in terms of the materials’ chemical nature and are corroborated by the prediction of their expected efficiencies in solar cells. The calculated efficiencies from band gaps obtained with the proposed approach closely follow the experimental trend, demonstrating the importance of adopting a reliable but material-independent computational strategy to screen new halide perovskite materials for solar energy conversion.

Stretchable Photodetectors Based on Electrospun Polymer/Perovskite Composite Nanofibers

Polymer/lead halide perovskite composites are a promising platform for wearable optoelectronic devices. In particular, one-dimensional nanocomposites are considered core materials for solar energy conversion textiles and optical sensors. Herein, we designed stretchable photodetectors incorporating thin and uniform polymer/perovskite composite nanofibers produced by electrospinning of a polymer/perovskite precursor solution. During the reaction of precursors in the as-spun nanofiber, protruding perovskite nanoplates on the fiber surface and embedded nanoparticles within the fiber were formed, showing double photoluminescence emissions. Furthermore, the mechanical behavior of the nanofibers on a rubbery substrate strongly depends on their orientation angle (α) with respect to the applied tensile force. As α decreases, the loaded stress is significantly reduced, which contributes to a stretchability of up to 15% strain for any orientation. The excellent percolation of perovskite nanoparticles leads to high photocurrents under green and red light illumination. At 15% strain, the responsivity and detectivity of the stretchable photodetector at λ = 550 nm were 51.2 mA W–1 and 2.23 × 1011 Jones, respectively. The device performance was well retained during mechanical deformation and stability tests. This thin and stretchable device was also applied to a wearable photodetector to demonstrate its great potential as a wearable image sensor.

Concluding remarks: emerging inorganic materials in thin-film photovoltaics.

A personal perspective is given on the major results, themes and trends from the Faraday Discussion on emerging materials for solar energy technologies. This covers research progress into adamantine semiconductors, the narrowing divide between materials modelling and measurements of solar cells, as well as the control of defects in novel absorber materials that include Cu2ZnSnS4, Zn3P2, Se, GeSe, Sb2Se3 and BaZrS3. This paper is adapted from a transcript of the closing lecture.

Energy Conversion and Storage at the Centre of a Global Transition

Energy Conversion and Storage at the Centre of a Global Transition

Perspective Perovskites

Today the term ‘perovskite’ is used to designate the crystalline structure of a wide variety of ABX3 ionic compounds. However, historically this name was coined specifically for a mineral, calcium titanate (CaTiO3). The paper describes crystallographic structure of perovskite materials in connection with geometry of dual polyhedra and applications of one class of such materials (metal halides perovskites) for solar energy conversion. History of discovery of mineral perovskite is also discussed.

Thiol-Amine-Based Solution Processing of Cu2 S Thin Films for Photoelectrochemical Water Splitting.

Cu2S is a promising solar energy conversion material owing to its good optical properties, elemental earth abundance, and low cost. However, simple and cheap methods to prepare phase‐pure and photo‐active Cu2S thin films are lacking. This study concerns the development of a cost‐effective and high‐throughput method that consists of dissolving high‐purity commercial Cu2S powder in a thiol‐amine solvent mixture followed by spin coating and low‐temperature annealing to obtain phase‐pure crystalline low chalcocite Cu2S thin films. After coupling with a CdS buffer layer, a TiO2 protective layer and a RuO x hydrogen evolution catalyst, the champion Cu2S photocathode gives a photocurrent density of 2.5 mA cm−2 at −0.3 V vs. reversible hydrogen electrode (VRHE), an onset potential of 0.42 VRHE, and high stability over 12 h in pH 7 buffer solution under AM1.5 G simulated sunlight illumination (100 mW cm−2). This is the first thiol‐amine‐based ink deposition strategy to prepare phase‐pure Cu2S thin films achieving decent photoelectrochemical performance, which will facilitate its future scalable application for solar‐driven hydrogen fuel production.

Conjugated Porous Polymers: Ground‐Breaking Materials for Solar Energy Conversion

AbstractSolar energy conversion plays a very important role in the transition to a more sustainable energy system. In this sense, so many systems have been proposed to drive artificial photosynthesis, most of them based on inorganic semiconductors, and the achievements performed continue every day. However, most of these systems present well‐known shortcomings as low light absorption, fast charge recombination, and lack of tunability, thus limiting their efficiency. The use of organic polymers in general and conjugated porous polymers (CPPs) in particular, opens the door to a multitude of new possibilities when it comes to design and selection of a suitable photocatalyst. CPPs exhibit improved light harvesting, charge conduction properties, high photochemical stability, and high surface area that make them ideal for photocatalytic applications. Here, the use of CPPs and hybrids as photocatalysts for solar energy conversion in the hydrogen evolution reaction and the CO2 reduction reaction and its use as a photoelectrode are reviewed. The photocatalytic properties are focused upon here. The photophysics related to these applications is also discussed. Finally, a perspective is provided, proposing an in situ and operando characterization methodology that is expected to allow improvements to their optoelectronic properties and thereby their future design and applicability.