Simulated Solar Illumination Research Articles

Donor–Acceptor–Acceptor's Molecules for Vacuum‐Deposited Organic Photovoltaics with Efficiency Exceeding 9%

AbstractThree vacuum‐deposited donor–acceptor–acceptor (d–a–a') small molecule donors are studied with different side chains attached to an asymmetric heterotetracene donor block for use in high efficiency organic photovoltaics (OPVs). The donor with an isobutyl side chain yields the highest crystal packing density compared to molecules with 2‐ethylhexyl or n‐butyl chains, leading to the largest absorption coefficient and short circuit current in an OPV. It also exhibits a higher fill factor, consistent with its preferred out‐of‐plane molecular π–π stacking arrangement that facilitates charge transport in the direction perpendicular to the substrate. A power conversion efficiency of 9.3 ± 0.5% is achieved under 1 sun intensity, AM 1.5 G simulated solar illumination, which is significantly higher than 7.5 ± 0.4% of the other two molecules. These results indicate that side chain modification of d–a–a' small molecules offers an effective approach to control the crystal packing configuration, thereby improving the device performance.

Significant enhancement of power conversion efficiency of dye-sensitized solar cells by the incorporation of TiO2–Au nanocomposite in TiO2 photoanode

In this report, the effect of incorporation of hydrothermally prepared TiO2–Au nanocomposites in the photoanode of dye-sensitized solar cells (DSSCs), prepared from commercially available TiO2 nanoparticles, has been investigated. Electrophoretic deposition technique has been utilized for nanocomposite-doped photoanode preparation. The formation of hydrothermally prepared TiO2–Au nanocomposites has been confirmed by the X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), UV–Vis spectroscopy. The HRTEM images establish that the particle size of Au nanoparticles dispersed in TiO2 matrix varies from 2 to 45 nm. TiO2–Au photoelectrode has been characterized by XRD, field emission scanning electron microscopy, Raman spectroscopy and photoluminescence spectroscopy in order to confirm the successful preparation of plasmonic photoanodes. Measurement of current–voltage characteristics of the plasmonic dye-sensitized solar cells under the solar simulator illumination (100 mW/cm2, AM 1.5) shows enormous enhancement of power conversion efficiency. The PCE of plasmonic DSSCs is 10.1%, which is 134% greater than the DSSCs with pristine TiO2 photoanode of the same thickness. Electro-impedance spectroscopy reveals that the back electron transfer from the conduction band of Au–TiO2 photoanode to either dye or electrolyte has been significantly suppressed in the DSSC with plasmonic photoelectrode.

Efficient and Selective Photocatalytic Oxidation of Benzylic Alcohols with Hybrid Organic–Inorganic Perovskite Materials

The impressive optoelectronic performance and low production cost of metal halide perovskites have inspired applications well beyond efficient solar cells. Herein, we widen the materials engineering options available for the efficient and selective photocatalytic oxidation of benzylic alcohols, an industrially significant reaction, using formamidinium lead bromide (FAPbBr3) and other perovskite-based materials. The best performance was obtained using a FAPbBr3/TiO2 hybrid photocatalyst under simulated solar illumination. Detailed optical studies reveal the synergetic photophysical pathways arising in FAPbBr3/TiO2 composites. An experimentally supported model rationalizing the large conversion enhancement over the pure constituents shows that this strategy offers new prospects for metal halide perovskites in photocatalytic applications.

Solar water splitting with nanostructured hematite: The role of annealing-temperature

The vertically aligned hematite (Fe2O3) nanorods fabrication has been considered as an effective method for photoelectrochemical (PEC) water splitting. To improve the activity, two-step annealing treatment in the temperature range from 550 °C to 800 °C was proposed for the Fe2O3 nanorods synthesized by hydrothermal method. This study reveals that the OER (oxygen evolution reaction) performance increases with the annealing temperature, which yields photocurrent density of 1.036 mA cm−2 and ABPC (applied bias photon-to-current) efficiency of 0.86% under simulated solar illumination of 100 mW cm−2 (AM 1.5G) condition at 1.23 V vs. RHE (reversible hydrogen electrode) at 800 °C. The second-step short time high temperature annealing treatment has a primary contribution on the fine-tune crystallite size and the diffusion and incorporation of Sn atoms. In addition, examining the corresponding resistance properties reveal a substantial improvement in the conductivity within the hematite film and at the conducting interface, and a suppressed charge carrier recombination process.

A facile synthesis of goethite-modified g-C3N4 composite for photocatalytic degradation of tylosin in an aqueous solution

The goethite/g-C3N4 photocatalyst was successfully synthesized by a simple and convenient method. The properties and structure of the as-prepared catalysts were characterized by a variety of analytical methods. The results indicated goethite was uniformly accumulated on the surface of g-C3N4. Furthermore, a novel binary goethite/g-C3N4 photocatalyst was successfully constructed by loading goethite into the g-C3N4 surface as an electron medium. The photodegradation efficiency of goethite/g-C3N4 composites for TYL was studied under simulated sunlight irradiation. The results show that the photodegradation efficiency of goethite/g-C3N4 composites on TYL is much higher than that of goethite and g-C3N4 under simulated sunlight irradiation. The content of the goethite in the composites was analyzed and compared under simulated solar irradiation to further improve the photocatalytic activity. In all as-synthesized composites, the goethite/g-C3N4 with 50 wt% of goethite have the highest photodegradation efficiency and can decompose almost all TYL within 30 min under simulated solar illumination. A sample of goethite/g-C3N4 also exhibited favorable stability and durability. It is a promising green and non-toxic photocatalyst that can be used to treat water contaminated with antibiotics.

Mesoporous tungsten oxide modified by nanolayered manganese-calcium oxide as robust photoanode for solar water splitting

Described herein is the synthesis, characterization and photoelectrochemical behavior of a novel composite consisting of nanolayered manganese-calcium oxide (MCO) and mesoporous tungsten trioxide (WO3). The samples were characterized by transmission electron microscopy (TEM), and X-ray diffraction (XRD). The results demonstrated that superior interfacial contacts had been formed between WO3 and MCO. UV–vis diffuse reflectance spectroscopy (DRS), photoelectrochemical characterization, and incident photon-to-current efficiency (IPCE) revealed an enhanced light harvesting and effective electron-hole separation. A photoelectrochemical (PEC) cell composed of the n-type MCO/WO3 as a photoanode and platinum sheet as a counter electrode was assembled to estimate the feasibility for overall water splitting under a solar simulator illumination. The photocatalytic hydrogen and oxygen production from the photochemical cell with optimized photocatalyst (MCO/WO3-9) under 2 h simulated solar light irradiation was 1.9 μmol and 0.7 μmol, respectively, at low extra bias (0.90 V vs. RHE). Our investigation suggests that coupling MCO with n-type semiconductor WO3 as photoanode is a promising method to improve the activity of overall water splitting to generate oxygen and hydrogen.

Large area growth of MoTe2 films as high performance counter electrodes for dye-sensitized solar cells

A cost effective and efficient alternative counter electrode (CE) to replace commercially existing platinum (Pt)-based CEs for dye-sensitized solar cells (DSSCs) is necessary to make DSSCs competitive. Herein, we report the large-area growth of molybdenum telluride (MoTe2) thin films by sputtering-chemical vapor deposition (CVD) on conductive glass substrates for Pt-free CEs of DSSCs. Cyclic voltammetry (CV), Tafel curve analysis, and electrochemical impedance spectroscopy (EIS) results showed that the as-synthesized MoTe2 exhibited good electrocatalytic properties and a low charge transfer resistance at the electrolyte-electrode interface. The optimized MoTe2 CE revealed a high power conversion efficiency of 7.25% under a simulated solar illumination of 100 mW cm−2 (AM 1.5), which was comparable to the 8.15% observed for a DSSC with a Pt CE. The low cost and good electrocatalytic properties of MoTe2 thin films make them as an alternative CE for DSSCs.

Phase-controlled synthesis of bismuth oxide polymorphs for photocatalytic applications

Bismuth oxide polymorphs (β-Bi2O3 nanoparticles and α-Bi2O3 coral-like morphology) were synthesized and they show an enhanced efficacy for the degradation of rhodamine B and acid orange 7 as compared to that of methylene blue and methyl orange under simulated solar illumination.

Fabrication and photocatalytic property of magnetic SrTiO3/NiFe2O4 heterojunction nanocomposites.

Novel multifunctional SrTiO3/NiFe2O4 nanocomposites were successfully fabricated via a two-step route. The as-prepared samples were characterized by using X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), field-emission transmission electron microscopy (TEM), UV-visible diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy and vibrating sample magnetometry (VSM). The results indicate that the SrTiO3/NiFe2O4 heterostructures are composed of SrTiO3 spheroidal nanoparticles adhered to NiFe2O4 polyhedra. The heterojunction established in the composite material accelerates the process of electron–hole pair separation and boosts the photo-Fenton reaction. Among the samples, 15 wt% SrTiO3/NiFe2O4 nanocomposites exhibit a powerful light response and excellent room temperature ferromagnetism. Subsequently, the photocatalytic degradation of RhB over the as-prepared samples was investigated and optimized, revealing that the 15 wt% SrTiO3/NiFe2O4 nanocomposites exhibit the best photocatalytic activity and stability under simulated solar light irradiation. Furthermore, according to experimental results, the possible mechanism of improved photocatalytic activity was also proposed.

Electrochemical hydrogenation of mixed-phase TiO2 nanotube arrays enables remarkably enhanced photoelectrochemical water splitting performance

We first report that photoelectrochemical (PEC) performance of electrochemically hydrogenated TiO2 nanotube arrays (TNTAs) as high-efficiency photoanodes for solar water splitting could be well tuned by designing and adjusting the phase structure and composition of TNTAs. Among various TNTAs annealed at different temperature ranging from 300 to 700 °C, well-crystallized single anatase (A) phase TNTAs-400 photoanode shows the best photoresponse properties and PEC performance due to the favorable crystallinity, grain size and tubular structures. After electrochemical hydrogenation (EH), anatase-rutile (A-R) mixed phase EH-TNTAs-600 photoanode exhibits the highest photoactivity and PEC performance for solar water splitting. Under simulated solar illumination, EH-TNTAs-600 achieves the best photoconversion efficiency of up to 1.52% and maximum H2 generation rate of 40.4 µmol h−1 cm−2, outstripping other EH-TNTAs photoanodes. Systematic studies reveal that the signigicantly enhanced PEC performance for A-R mixed phaes EH-TNTAs-600 photoanode could be attributed to the synergy of A-R mixed phases and intentionally introduced Ti3+ (oxygen vacancies) which enhances the photoactivity over both UV and visible-light regions, and boosts both charge separation and transfer efficiencies. These findings provide new insight and guidelines for the construction of highly efficient TiO2-based devices for the application of solar water splitting.

Coupling copper and hydrogenated TiO2 to bare TiO2 structures for improved photocatalytic performance

AbstractCoupling of metals and hydrogenated TiO2 (HT) to bare TiO2 may improve synergistically the photocatalytic activity of TiO2 for pollutant decomposition. Herein, we address this issue by investigating the photocatalytic performances of Cu‐loaded HT (CuHT)/bare TiO2 (CuHTT) heterojunction nanocomposites for the degradation of harmful n‐butanol under simulated solar light illumination. CuHTT with a CuHT‐to‐TiO2 composition ratio of 0.1 showed a photocatalytic efficiency exceeding those of four reference photocatalysts, exhibiting the advantages of improved visible light absorption efficiency, charge carrier separation, and adsorption capacity. Increasing the CuHT‐to‐TiO2 ratio from 0.01 to 0.1 improved the photocatalytic efficiency of CuHTT, whereas a further increase to 0.9 resulted in a decreased photocatalytic efficiency. Moreover, the photocatalytic efficiency improved as relative humidity increased from 20% to 70%, decreasing upon its further increase to 95%. The efficiencies of n‐butanol to CO2 conversion over CuHTT were lower than the corresponding decomposition efficiencies. Incompletely oxidized CO and three organic vapors (butanal, propanal, and 1‐propanol) were determined as major intermediates. A possible mechanism for CuHTT‐catalyzed photodegradation under simulated solar illumination was proposed.

Wastewater treatment and electricity generation from a sunlight-powered single chamber microbial fuel cell

A novel hybrid single chamber microbial fuel cell is described in which a TiO2 nanotube array (TNT) photoanode is coupled with a conventional bioanode to achieve simultaneous degradation of methylene blue (MB) dye with improved power generation. As compared to a conventional microbial fuel cell (MFC), the described hybrid-MFC exhibits enhanced power density (14%), current density (33%), and voltage (4%) while simultaneously degrading MB dye, 82.79% after 3.5h of operation under simulated solar light illumination. The key factor attributed to the enhanced performance is the addition of photogenerated electrons to the MFC external circuit. The effect of various design configurations is also investigated, such as the presence of an air cathode, anode type, and illumination. The hybrid-MFC strategy provides new directions for productive and economical utilization of microbial fuel cells.

Cobalt Porphyrin–Polypyridyl Surface Coatings for Photoelectrosynthetic Hydrogen Production

Hybrid materials that link light capture and conversion technologies with the ability to drive reductive chemical transformations are attractive as components in photoelectrosynthetic cells. We show that thin-film polypyridine surface coatings provide a molecular interface to assemble cobalt porphyrin catalysts for hydrogen evolution onto a visible-light-absorbing p-type gallium phosphide semiconductor. Spectroscopic techniques, including grazing angle attenuated total reflection Fourier transform infrared spectroscopy, confirm that the cobalt centers of the porphyrin macrocycles coordinate to pyridyl nitrogen sites of the organic surface coating. The cobalt porphyrin surface concentration and fraction of pyridyl sites coordinated to a cobalt center are quantified using complementary methods of ellipsometry, inductively coupled plasma mass spectrometry, and X-ray photoelectron spectroscopy. In aqueous solutions under simulated solar illumination the modified cathode is photochemically active for hydrogen production, generating the product gas with near-unity Faradaic efficiency at a rate of ≈10 μL min-1 cm-2 when studied in a three-electrode configuration and polarized at the equilibrium potential of the H+/H2 couple. This equates to a photoelectrochemical hydrogen evolution reaction activity of 17.6 H2 molecules s-1 Co-1, the highest value reported to date for a molecular-modified semiconductor. Key features of the functionalized photocathode include (1) the relative ease of synthetic preparation made possible by application of an organic surface coating that provides molecular recognition sites for immobilizing the cobalt porphyrin complexes at the semiconductor surface and (2) the use of visible light to drive cathodic fuel-forming reactions in aqueous solutions with no added organic acids or sacrificial chemical reductants.

Tuning the photoelectrochemical properties of hierarchical TiO2 nanostructures by control of pulsed laser deposition and annealing in reducing conditions

Nanostructured TiO2 films with hierarchical morphology were synthesized by Pulsed Laser Deposition (PLD) and tested as photoanodes for photoelectrochemical water splitting. The tuning of their photoresponse was addressed by employing oxygen-poor conditions both during the growth and the post-deposition annealing of the material: depositions were performed in different Ar/O2 background gas mixtures from both TiO2 and Ti targets, while thermal treatments, after standard air annealing for crystallization, were performed in a Ar/H2 mixture. By testing the double-annealed photoanodes in a three-electrode cell with solar simulator illumination, clear trends with optimal synthesis conditions for each target material appeared; for these conditions, also the effect of vacuum annealing was studied. The morphological, structural and optical properties were investigated by SEM, Raman spectroscopy and UV-visible-IR spectroscopy. From these observations, it emerged that the films deposited in the presence of oxygen do not show substantial differences in their morphology/structure, on the contrary of pure Ar-deposited films; thus, the trends in photoresponse can be related to differences in the defect concentration of the material, induced by depositions in the different Ar/O2 atmospheres and by annealing in Ar/H2 mixture or vacuum. In particular, the reported results suggest that some degree of oxygen shortage in the deposition process leads to a better photoelectrochemical performance, and a combination between improved charge transport and surface hydrogenation/reduction effect, leading to enhanced photoresponse, is suggested. This work elucidates the possibility of an accurate tuning of the material photoactivity by control of the deposition and annealing conditions.

Boosting electrocatalytic activities of plasmonic metallic nanostructures by tuning the kinetic pre-exponential factor

The electrocatalytic activity of metallic nanostructures is commonly tuned by changing their structure, composition, size and morphology to decrease the activation energy of a rate-determining elementary step. Here, we report that the electrocatalytic activity of the excited plasmonic model Pd nanostructured electrocatalyst for oxygen reduction reaction (ORR) can be enhanced significantly by increasing the kinetic pre-exponential factor. The localized surface plasmon resonance (SPR) effect promotes a two-orders of magnitude increase in the apparent pre-exponential factor in the Arrhenius equation. Combined experimental and theoretical analyses of the kinetic H/D isotope effect indicate that the observed increase in the apparent pre-exponential factor is attributable to an increased quantum–mechanical proton tunneling arising from the local electric field enhancement upon SPR excitation. Intriguingly, the ORR activity enhancement can be observed under simulated solar light illumination, implying potential for fuel cell applications. The methodology developed in this study is applicable to other proton-coupled electron-transfer reactions, thus opening a promising route to manipulating electrocatalytic reactivity by tuning the kinetic pre-exponential factor and suggesting potential for fuel cell applications.

Plasma processed carbon thin films applied to dye-sensitized solar cells

Plasma-treated carbon thin films are investigated as counter electrodes for dye-sensitized solar cells. The films were grown onto fluorine-doped tin oxide (FTO) substrates by magnetron sputtering using pure graphite target and argon atmosphere and subsequently annealed at 600 °C for 30 min in vacuum. These films were then submitted to a plasma texturing process in a reactive ion etching reactor using three different gas combinations: sulfur hexafluoride/argon (SF6 + Ar), sulfur hexafluoride/hydrogen (SF6 + H2), and sulfur hexafluoride/oxygen (SF6 + O2). The morphology and structure of the obtained films were characterized by scanning electron microscopy and Raman spectroscopy. Cyclic voltammetry technique allowed accessing the improvements in their catalytic properties, while the photocurrent-voltage curves under simulated solar illumination AM 1.5G (100 mW/cm2) evaluated the performance of the respective assembled solar cells. The results show that photovoltaic performance is significantly affected by the different plasma texturing conditions used. The carbon counter electrode obtained after SF6 + O2 plasma texturing achieved the best power conversion efficiency of 2.23%, which is comparable to the 2.31% obtained using the commercial platinum counter electrode.

Impact of Thermal Annealing on Organic Photovoltaic Cells Using Regioisomeric Donor-Acceptor-Acceptor Molecules.

We report a promising set of donor-acceptor-acceptor (D-A-A) electron-donor materials based on coplanar thieno[3,2-b]/[2,3-b]indole, benzo[c][1,2,5]thiadiazole, and dicyanovinylene, which are found to show broadband absorption with high extinction coefficients. The role of the regioisomeric electron-donating thienoindole moiety on the physical and structural properties is examined. Bulk heterojunction (BHJ) organic photovoltaic cells (OPVs) based on the thieno[2,3-b]indole-based electron donor NTU-2, using C70 as an electron acceptor, show a champion power conversion efficiency of 5.2% under AM 1.5G solar simulated illumination. This efficiency is limited by a low fill factor (FF), as has previously been the case in D-A-A systems. In order to identify the origin of the limited FF, further insight into donor layer charge-transport behavior is realized by examining planar heterojunction OPVs, with emphasis on the evolution of film morphology with thermal annealing. Compared to as-deposited OPVs that exhibit insufficient donor crystallinity, crystalline OPVs based on annealed thin films show an increase in the short-circuit current density, FF, and power conversion efficiency. These results suggest that that the crystallization of D-A-A molecules might not be realized spontaneously at room temperature and that further processing is needed to realize efficient charge transport in these materials.

Toward non-precious nanocomposite photocatalyst: An efficient ternary photoanode TiO2 nanotube/Co9S8/polyoxometalate for photoelectrochemical water splitting

At present the photoelectrochemical (PEC) water splitting achieves a very high efficiency, but the development of the efficient and inexpensive photoanode materials is still desired for practical application. Herein, we fabricate a composite photoanode consisting of TiO2 nanotube arrays modified with polyoxometalate (POM) and Co9S8, in which POM aims to facilitate charge separation and Co9S8 acts as good electron transport mediator. It is the first introduction of Co9S8 into PEC water oxidation system. Although these three components are all non-precious inorganic materials, the photocurrent density of the TiO2/Co9S8/POM composite photoanode reaches up to 1.12mA/cm2 at 1.23V vs RHE under simulated solar light illumination, which is among the highest PEC water oxidation photoanodes. Particularly, the TiO2/Co9S8/POM composite photoanode shows an 8.5-fold improvement in photocurrent density compared to the pristine TiO2 nanotube array photoanode. The measurements of EIS and Mott-Schottky plots revealed that the TiO2/Co9S8/POM composite photoanode held the best efficiency of charge separation and transport at the interface as well as the highest carrier density. The aforementioned results demonstrate a feasible approach to improving the PEC water splitting efficiency by dual modification of TiO2 nanotube arrays with low-cost materials.

Photovoltaic performance of an alternating cold–hot method deposited CdSe thin films

Cadmium selenide (CdSe) thin films were prepared on indium tin oxide substrate by an alternating cold–hot method, in which cadmium nitrate solution was used as a cold deposition solution, while sodium selenite and potassium borohydride mixed solution was used as a hot deposition solution. The influences of the preparation conditions such as the concentration of Cd(NO3)2, the number of deposition cycle and the cycle time, on the photoelectric performance of the sample under simulated sunlight were explored. The results show that the CdSe thin film prepared under the reaction conditions of 0.06 mol/l Cd(NO3)2 at the tenth deposition cycle (30 s per cycle) reaches the highest photovoltage of 0.285 V. Under the simulated solar illumination, the open-circuit voltage and short-circuit currents are 0.419 V and 5.57 mA/cm2, respectively. X-ray diffraction indicates that the strongest diffraction peak at 42.215° of the (111) crystal plane is corresponding to 15.15 nm CdSe nanocrystals. Scanning electron microscopy observation shows that the thickness of the CdSe film is about 200 nm and the size of the spherical and uniformly dispersed nanocrystals is around 50 nm.

Hydrogen-treated hierarchical titanium oxide nanostructures for photoelectrochemical water splitting

Hierarchical titanium oxide nanostructures were synthesized by Pulsed Laser Deposition (PLD) and investigated as photoanodes for photoelectrochemical water splitting. An explorative combined approach to enhance TiO2 performance was based, on the one hand, on the employment of hydrogenation treatments with the aim of improving quantum efficiency and extending light absorption to the visible range; on the other hand, on the optimization of morphology and structure, to increase light harvesting and charge separation/transport. This approach was pursued by depositing at a fixed background pressure with variable oxygen content (to control the growth morphology and structure) and by annealing in a Ar/H2 mixture (in substitution of or in combination with air annealing), in order to induce crystallization to the anatase structure and reduction/hydrogenation of the material. Morphology, structure and optical properties were investigated by SEM, Raman spectroscopy, X-ray diffraction and UV–visible-IR spectroscopy. An optical absorption tail towards the visible range appeared after Ar/H2 annealing, without any significant modification of the nanoscale structure after the different thermal treatments. Photocurrent measurements under solar simulator illumination showed a noteworthy increase of photoresponse for Ar/O2-deposited samples with air annealing followed by Ar/H2 annealing. These findings can be ascribed to the combination between an improved charge transport of TiO2 deposited in low-O2 atmosphere and a hydrogenation effect on the nanostructures surface layers, leading to improved quantum efficiency.