Light Harvesting Performance Research Articles

Enhanced visible light absorption performance of SnS2 and SnSe2 via surface charge transfer doping.

The layered two-dimensional (2D) SnS2 and SnSe2 have received intensive attention due to their sizable band gaps and potential properties. However, it has been shown that the visible light absorption of SnS2 and SnSe2 are restricted as photocatalysts and light-harvesting material absorbers for water splitting and high-performance optoelectronic devices. Herein, to enhance the visible light absorption performance of SnS2 and SnSe2, we performed a systematic investigation on tuning the electronic and optical properties of monolayers SnS2 and SnSe2via surface charge transfer doping (SCTD) with the adsorption of molybdenum trioxide (MoO3) and potassium (K) as surface dopants based on density functional theory. Our calculations reveal that MoO3 molecules and K atoms can draw/donate electrons from/to SnS2 and SnSe2 as acceptors and donors, respectively. The adsorption of MoO3 molecules introduces a new flat impurity state in the gap of the monolayers SnS2/SnSe2, and the Fermi level moves correspondingly to the top of valence band, resulting in a p-type doping of the monolayer SnS2/SnSe2. With the adsorption of K atoms, the electrons can transfer from K atoms to the monolayer of SnS2 and SnSe2, making K an effective electron-donating dopant. Meanwhile, the bandgaps of monolayers SnS2 and SnSe2 decrease after the MoO3 and K doping, which leads to the appearance of appreciable new absorption peaks at around ∼650/480 and ∼600/680 nm, respectively, and yielding an enhanced visible light absorption of SnS2 and SnSe2. Our results unveil that SCTD is an effective way to improve the photocatalytic and light-harvesting performance of SnS2 and SnSe2, broadening their applications in splitting water and degrading environmental pollutants under sunlight irradiation.

Structure-property relationships: Influence of number of anchoring groups in triphenylamine-carbazole motifs on light harvesting and photovoltaic performance for dye-sensitized solar cells

Three novel organic sensitizers were molecularly designed and synthesized based on triphenylamine (TPA), as the core donor (DTPA), linked to one, two or three carbazole (DCarb) moieties with different number of anchoring groups (AG) to generate: DTPA-π-DCarb-A (DE1), DTPA(π-DCarb-A)2 (DE-2) and DTPA(π-DCarb-A)3 (DE-3). The primary goal of this study was to investigate the influence of bridging different number of DCarb and AG, through π-spacer, with TPA on the photovoltaic performance in DSSCs. The molar extinction coefficient increased with increasing the number of DCarb and A. DE-3 showed the greatest light harvesting in solution and when anchored onto TiO2. Hence, the increase in the number of DCarb and AG led to better light harvesting and stronger electronic coupling with TiO2, which reflected positively on the photovoltaic properties, and this is the first systematic study of its kind involving TPA-π-carbazole motif. DE-3 achieved the best efficiency of 5.44% and showed IPCE of 74% at 470 nm. The Voc increased with increasing the number of anchoring groups. Equilibrium molecular geometries and frontier molecular orbitals studied by DFT and TD-DFT calculations were in excellent agreement with experiments, which showed enhanced electronic coupling with TiO2 and photovoltaic performance as the number of anchoring groups increased.

Hierarchical Graphene Foam for Efficient Omnidirectional Solar-Thermal Energy Conversion.

Efficient solar-thermal energy conversion is essential for the harvesting and transformation of abundant solar energy, leading to the exploration and design of efficient solar-thermal materials. Carbon-based materials, especially graphene, have the advantages of broadband absorption and excellent photothermal properties, and hold promise for solar-thermal energy conversion. However, to date, graphene-based solar-thermal materials with superior omnidirectional light harvesting performances remain elusive. Herein, hierarchical graphene foam (h-G foam) with continuous porosity grown via plasma-enhanced chemical vapor deposition is reported, showing dramatic enhancement of broadband and omnidirectional absorption of sunlight, which thereby can enable a considerable elevation of temperature. Used as a heating material, the external solar-thermal energy conversion efficiency of the h-G foam impressively reaches up to ≈93.4%, and the solar-vapor conversion efficiency exceeds 90% for seawater desalination with high endurance.

Fabrication of Orientation-Tunable Si Nanowires on Silicon Pyramids with Omnidirectional Light Absorption.

In this work, the different orientation of SiNWs on Si pyramids by a two step MACE method have been fabricated. By tuning the structure of Ag catalyst film and controlling the concentration of H2O2 or the etching temperature, the tunability of the orientation of SiNWs from <111> to <100> on Si pyramids was realized. Si structures composed of Si pyramids and SiNWs exhibit better omnidirectional light-trapping ability by multiple reflections. Si structures with structural tunability and enhanced light harvesting performance will find a wide variety of significant applications in solar cells, photodetectors, and optoelectronic devices.

Enhanced light harvesting of dye-sensitized solar cells with TiO2 microspheres as light scattering layer

Two kinds of TiO2 microspheres (TMS) with average diameter of 1500 nm but different surface were fabricated by solvothermal method from different Ti source. The effect of TMS on the light harvesting and photovoltaic performance of dye-sensitized solar cells (DSSCs)was investigated. The UV–Vis diffusion reflectance spectra and absorption spectra of N719 dye in detached solutions proved that the TMS showed dual functions of light scattering and dye-adsorption which was an important functional material in DSSCs. The results showed that the TMS made from titanium(IV) isopropoxide with rough surface (TMSR) exhibited better photovoltaic performance than that of TMS made from tetrabutyl titanate with smooth surface (TMSS). To further improve the photovoltaic performance, the double-layered DSSCs made of P25 as an underlayer and TMS as a light-scattering layer (P25-TMS) were fabricated. The photovoltaic performance of double-layered DSSCs was higher than that of the single-layered DSSCs with similar thickness. Especially, the DSSCs made of P25 as an underlayer and the TMSR as a light-scattering layer (P25-TMSR) had a highest power conversion efficiency of 7.62%. This was higher than that of single-layered TMSR-based cell (5.54%), P25-based cell (5.75%), and double-layered P25-TMSS-based cell (6.78%) with similar thickness. This was mainly attributed to the large specific surface area, superior light scattering ability, and fast electron transport of TMSR.

Improvement in light harvesting and device performance of dye sensitized solar cells using electrophoretic deposited hollow TiO2 NPs scattering layer

The concept of bi-layer structure in photoanodes is of great importance to prepare highly efficient dye-sensitized solar cells. In this work, Hollow TiO2 (H-TiO2) NPs are successfully produced through the etching of Au NPs in Au@TiO2 core-shell NPs which were synthesized by microwave hydrothermal route. Electrophoretic deposition was utilized to make H-TiO2 NP scattering layer on the top of small sized TiO2 NPs layer. The photoanodes based on H-TiO2 NPs scattering layer showed a photoconversion efficiency of 7.58% which is much higher than that of photoanode without H-TiO2 scattering layer (6.08%). The improved photovoltaic performance is mainly due to relatively light scattering effect, more surface area for effective dye adsorption and increased light harvesting capacity of H-TiO2 NPs in the entire spectral range.

Hydrogenated Bismuth Molybdate Nanoframe for Efficient Sunlight-Driven Nitrogen Fixation from Air.

Sunlight-driven dinitrogen fixation can lead to a novel concept for the production of ammonia under mild conditions. However, the efficient artificial photosynthesis of ammonia from ordinary air (instead of high pure N2 ) has never been implemented. Here, we report for the first time the intrinsic catalytic activity of Bi2 MoO6 catalyst for direct ammonia synthesis under light irradiation. The edge-exposed coordinatively unsaturated Mo atoms in an Mo-O coordination polyhedron can act as activation centers to achieve the chemisorption, activation, and photoreduction of dinitrogen efficiently. Using that insight as a starting point, through rational structure and defect engineering, the optimized Bi2 MoO6 sunlight-driven nitrogen fixation system, which simultaneously possesses robust nitrogen activation ability, excellent light-harvesting performance, and efficient charge transmission was successfully constructed. As a surprising achievement, this photocatalytic system demonstrated for the first time ultra-efficient (1.3 mmol g-1 h-1 ) and stable sunlight-driven nitrogen fixation from air in the absence of any organic scavengers.

Bimodal porous TiO2 structures templated by graft copolymer/homopolymer blend for dye-sensitized solar cells with polymer electrolyte

Bimodal porous TiO2 (BP-TiO2) with large surface area, high porosity, good interconnectivity, and excellent light-scattering ability are synthesized via a facile one-step method using a self-assembled blend template consisting of an amphiphilic poly(vinyl chloride)-g-poly(oxyethylene methacrylate) (PVC-g-POEM) graft copolymer and a hydrophobic poly(vinyl chloride) (PVC) homopolymer. The hydrophilically surface-modified TiO2 nanoparticles selectively interact with the hydrophilic POEM chains, while the addition of the PVC homopolymer increases the hydrophobic domain size, resulting in the formation of dual pores (i.e., macropores and mesopores). The sizes and numbers of macropores can easily be controlled by changing the molecular weight and amount of the PVC homopolymer. The polymer electrolyte dye-sensitized solar cells (DSSCs) fabricated with BP-TiO2 photoanodes exhibited energy conversion efficiencies of up to 7.6% at 100 mW cm−2, which is much higher than those of mesoporous TiO2 (5.8%) with PVC-g-POEM only and conventional nanocrystalline TiO2 (4.9%) with commercial Dyesol paste. The enhanced energy conversion efficiencies mostly resulted from the light-scattering effects of the macropores, which increased the light-harvesting efficiencies. The improved light-harvesting and photovoltaic performances of the DSSCs were characterized by UV–vis spectroscopy, incident photon-to-current conversion efficiency analysis, electrochemical impedance spectroscopy, intensity-modulated photocurrent spectroscopy, and intensity-modulated photovoltage spectroscopy.

Synthesis of novel SnO2@TiO2 nanofibers as an efficient photoanode of dye-sensitized solar cells

Dye-sensitized Solar Cells (DSCs) have attracted much attention because they directly convert solar light into electrical power without any negative environmental effect. Experimentally, this work introduces a novel working electrode to enhance the light harvesting and photovoltaic performance of solar cell. The novel electrode composed of one-dimensional SnO2@TiO2 nanofibers which were synthesized by a facile and one-step technique of electrospinning. The prepared material was characterized by field emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM), X-ray diffractometer (XRD), Fourier transform infrared spectroscopy (FTIR), and UV–Visible spectrophotometric techniques. XRD revealed that the synthesized TiO2 nanofibers exhibit Anatase phase. SnO2@TiO2 nanofibers are employed as photoanode of DSCs. The results obtained have indicated that the presence of tin enhances the amount of dye adsorption which leads to high migration of electrons from the photosensitizer to the conduction band of the collection electrode. Accordingly, the novel photoanode was found to improve the short circuit current density from 3.02 mA/cm2 for pristine TiO2 nanofibers to 6.10 mA/cm2 as well as increasing the open circuit voltage from 815.0 mV to 828.5 mV at suitable concentration of SnO2, which was found in this study to be 10%. Moreover, remarkably improved photovoltaic efficiency of 3.41% was achieved in case of utilizing SnO2-incorporated TiO2 nanofibers as photoanode based DSCs which is higher than that of the conversion performance in case of pristine TiO2 nanofibers (i.e. 1.61%).

Enhanced light harvesting of TiO2/La0.95Tb0.05PO4 photoanodes for dye-sensitized solar cells

With an aim of enhancing light harvesting for dye adsorption and therefore photovoltaic performances of dye-sensitized solar cells (DSSCs), we present here an employment of La0.95Tb0.05PO4 incorporated TiO2 nanocrystallites as photoanodes. The preliminary results demonstrate that the dye absorption and therefore electron generation have been markedly enhanced, arising from the conversion of ultraviolet to visible light by La0.95Tb0.05PO4. The crystal structure and light harvesting performances of photoanodes are optimized by adjusting La0.95Tb0.05PO4 dosage. The power conversion efficiency is enhanced from 6.52% for pristine TiO2 based DSSC to 7.27% for the device employing TiO2/0.5 wt% La0.95Tb0.05PO4, yielding an efficiency enhancement by 11.35%. This study provides a new strategy for the fabrication of highly efficient DSSCs.

Enhancing the Optoelectronic Performance of Perovskite Solar Cells via a Textured CH3NH3PbI3 Morphology

Perovskite‐based solar cells are generally assembled as planar structures comprising a flat organoammonium metal halide perovskite layer, or mesoscopic structures employing a mesoporous metal‐oxide scaffold into which the perovskite material is infiltrated. To present, little attention has been directed toward the texturing of the perovskite material itself. Herein, a textured CH3NH3PbI3 morphology formed through a thin mesoporous TiO2 seeding layer and a gas‐assisted crystallization method is reported. The textured morphology comprises a multitiered nanostructure, which allows for significant improvements in the light harvesting and charge extraction performance of the solar cells. Due to these improvements, average short‐circuit current densities for a batch of 28 devices are in excess of 22 mA cm−2, and the maximum recorded power conversion efficiency is 16.3%. The performance gains concomitant with this textured CH3NH3PbI3 morphology provide further insights into how control of the perovskite microstructure can be used to enhance the cell performance.

Realizing omnidirectional light harvesting by employing hierarchical architecture for dye sensitized solar cells.

To improve the omnidirectional light-harvesting in dye-sensitized solar cells (DSSCs), here we present a dandelion-like structure composed of ZnO hemispherical shells and nanorods. Uniformly distributed hemispherical shells effectively suppress the reflection over the broadband region at incident angles up to 60°, greatly improving the optical absorption of the DSSCs. In addition, modulating the length of the ZnO nanorods controls the omnidirectional characteristics of DSSCs. This phenomenon is attributed to the degree of periodicity of the ZnO dandelion-like structures. Cells with shorter rods exhibit a high degree of periodicity, thus the conversion efficiencies of the cells show specific angle-independent features. On the other hand, the cells with longer lengths reveal angle-dependent photovoltaic performance. Along with the simulation, the cells with dandelion-like ZnO structures can couple incident photons efficiently to achieve excellent broadband and omnidirectional light-harvesting performances experimentally, and the DSSCs enhanced the conversion efficiency by 48% at large incident angles. All these findings not only provide further insight into the light-trapping mechanism in these complex three-dimensional nanostructures but also offer efficient omnidirectional and broadband nanostructured photovoltaics for advanced applications.

TiO2 hollow spheres as light scattering centers in TiO2 photoanodes for dye-sensitized solar cells: the effect of sphere diameter

Mono-dispersive TiO2 hollow spheres (THS) of 400, 500, 600, 750 and 950 nm in diameter are mixed into TiO2 nanoparticle films, respectively, as light scattering centers for optical absorption enhancement in dye-sensitized solar cells (DSSCs). The effect of THS diameter on light harvesting and photovoltaic performance of DSSCs has been investigated. At a THS content of 15% by weight, all the DSSCs with THS exhibit improved power conversion efficiency compared to that based on P25 TiO2 film. The conversion efficiency enhancement of the DSSCs is mainly attributed to the tradeoff between the enhanced light scattering effect and the loss of internal surface area, both of which are highly dependent on the diameter of THS. The DSSC based on 600 nm THS-embedded photoanode exhibits the highest conversion efficiency of 5.84%, which is 68.8% higher than that based on P25 TiO2 photoanode.

Packaging Glass with a Hierarchically Nanostructured Surface: A Universal Method to Achieve Self-Cleaning Omnidirectional Solar Cells.

Fused-silica packaging glass fabricated with a hierarchical structure by integrating small (ultrathin nanorods) and large (honeycomb nanowalls) structures was demonstrated with exceptional light-harvesting solar performance, which is attributed to the subwavelength feature of the nanorods and an efficient scattering ability of the honeycomb nanowalls. Si solar cells covered with the hierarchically structured packaging glass exhibit enhanced conversion efficiency by 5.2% at normal incidence, and the enhancement went up to 46% at the incident angle of 60°. The hierarchical structured packaging glass shows excellent self-cleaning characteristics: 98.8% of the efficiency is maintained after 6 weeks of outdoor exposure, indicating that the nanostructured surface effectively repels polluting dust/particles. The presented self-cleaning omnidirectional light-harvesting design using the hierarchical structured packaging glass is a potential universal scheme for practical solar applications.

Enhancing light-harvesting power with coherent vibrational interactions: A quantum heat engine picture.

Recent evidence suggests that quantum effects may have functional importance in biological light-harvesting systems. Along with delocalized electronic excitations, it is now suspected that quantum coherent interactions with certain near-resonant vibrations may contribute to light-harvesting performance. However, the actual quantum advantage offered by such coherent vibrational interactions has not yet been established. We investigate a quantum design principle, whereby coherent exchange of single energy quanta between electronic and vibrational degrees of freedom can enhance a light-harvesting system's power above what is possible by thermal mechanisms alone. We present a prototype quantum heat engine which cleanly illustrates this quantum design principle and quantifies its quantum advantage using thermodynamic measures of performance. We also demonstrate the principle's relevance in parameter regimes connected to natural light-harvesting structures.

Observation of hybrid artificial photosynthetic membranes using peripheral and core antennae from two different species of photosynthetic bacteria by AFM and fluorescence micro-spectroscopy

The primary process of photosynthesis involves the absorption of sun-light by antenna complexes and subsequent transfer of excitation energy toward the reaction center where a charge separation takes place. This process is usually performed in photosynthetic membranes. The photosynthetic apparatus from purple photosynthetic bacteria contains the reaction center (RC) and two types of antenna complexes, the core antenna light-harvesting 1 (LH1) complex and the peripheral light-harvesting 2 (LH2) complex, and is embedded in these membranes. The arrangement of these light-harvesting pigment–protein complexes in the photosynthetic membranes does have an influence on the light-harvesting performance in the primary process of photosynthesis. In this study, we have constructed hybrid artificial photosynthetic membranes by introducing both LH2 complexes (from Rhodopseudomonas acidophila) and LH1-RC core complexes (from Blastochloris viridis) to a lipid bilayer system composed of Egg PC. The architecture of the artificial photosynthetic membranes can be varied by controlling the ratio of added LH2 to LH1-RC core complexes (LH1-RC/LH2 = 1/1, 1/2, 1/5, 1/12). Previous studies using transmission electron microscopy (TEM) showed that the packing of LH1-RC in a trigonal lattice was changed to orthorhombic by increasing the ratio of LH2 to LH1-RC. We further investigated the local arrangement of the pigment–protein complexes using high-resolution atomic force microscopy and postulate some types of arrangement depending on the condition of the sample preparation. LH2 to LH1 energy transfer was studied using confocal laser fluorescence microscopy with 400 nm spatial resolutions. We found a difference in the ratio of the fluorescence intensity emitted from LH2 and LH1 depending on the local arrangement of these antenna complexes in the hybrid artificial photosynthetic membranes. The effect of the arrangement of the pigment–protein complexes on the light harvesting efficiency is discussed.

A comparison of light-harvesting performance of silicon nanocones and nanowires for radial-junction solar cells.

Silicon nanorod based radial-junction solar cells are competitive alternatives to traditional planar silicon solar cells. In various silicon nanorods, nanocone is always considered to be better than nanowire in light-absorption. Nevertheless, we find that this notion isn’t absolutely correct. Silicon nanocone is indeed significantly superior over nanowire in light-concentration due to its continuous diameters, and thus resonant wavelengths excited. However, the concentrated light can’t be effectively absorbed and converted to photogenerated carriers, since its propagation path in silicon nanocone is shorter than that in nanowire. The results provide critical clues for the design of silicon nanorod based radial-junction solar cells.

A comparative study on the quantum-dot-sensitized, dye-sensitized and co-sensitized solar cells based on hollow spheres embedded porous TiO2 photoanodes

TiO2 hollow spheres (THS) of 400nm in diameter are mixed into TiO2 nanoparticles film as light scattering centers to improve the light harvesting and photovoltaic performance of solar cells based on CdS quantum-dot sensitized TiO2 (TiO2/CdS), N719 dye-sensitized TiO2 (TiO2/N719) and CdS/N719 co-sensitized TiO2 (TiO2/CdS/N719) photoanodes, respectively. The light harvesting and photovoltaic performance of the three types of solar cells are highly dependent on the THS content in TiO2 films due to: (1) the significant light scattering among or within THS, (2) the extended pore size distribution, and (3) the modulated electron transport in the TiO2 films. The optimal content of THS is around 10% by weight for TiO2/CdS and TiO2/CdS/N719 solar cells, and is around 15% for TiO2/N719 solar cell. It is suggested that, in TiO2/CdS and TiO2/CdS/N719 solar cells, high content THS can facilitate the aggregation of large CdS nanocrystals, which limits the injection of photo-generated electrons from CdS to TiO2 due to serious internal recombination loss, and thus decreases the power conversion efficiency. The co-sensitized solar cell with the optimal THS content exhibits higher power conversion efficiency compared to the single-component sensitized solar cells.

Nano carbon conformal coating strategy for enhanced photoelectrochemical responses and long-term stability of ZnO quantum dots

A conformal coating strategy with nanocarbon to enhance photoelectrochemical responses and the long-term stability of ZnO quantum dots is described. Strong anchoring bonds between a ZnO core and nanocarbon shell ameliorate the poor electrochemical stability of ZnO (such as photocorrosion) in liquid electrolyte. The conjugation of the graphene QD and C60 to the ZnO QDs leads to 71% and 99% quenching of the UV photoluminescence (PL) emission, respectively. Also, the decay time of the nanocomposites at UV wavelengths measured much faster than that for the reference of bare ZnO QDs. The moderate energy states and good charge conductance of the nanocarbons result in ultrafast charge transport from the ZnO core to the nanocarbon shell. Thereby, the ZnO core–nanocarbon shell quantum dots shows significantly improved light harvesting performance. The PEC cell test for water oxidation and conventional degradation test using organic dyes exhibited that the photoelectrochemical activities could be significantly improved. At 1.23V (vs. RHE) in pH 6.9 electrolyte, 6 times enhanced photocurrent density was achieved by the conformal coating with C60 (0.235mA/cm2 for ZnO–C60 photoanodes). In particular, the strong Zn–O–C bond structures on the ZnO surface prevented photoinduced holes from being consumed by the photocorrosion reaction of ZnO, thereby improving long-term stability.

Influence of the number of phenylethynyl units present in porphyrin sensitizer on its light harvesting and cell performance

Four phenylethynyl-porphyrins 4a, 4b, 5a, and 5b with either one (4a and 4b) or two (5a and 5b) phenylethynyl groups attached at C10 or/and C20 position of the porphyrin ring (defined as the axial direction), and with phenyl (4a and 5a) or 2,6-dialkoxylphenyl group (4b and 5b) at C5 position of porphyrin in a push–pull framework (defined as the push–pull direction) were designed and synthesized. The synthesized porphyrins were applied in dye-sensitized solar cells (DSSCs). Unexpectedly, although the light harvesting capability of the dye is enhanced with the increase of the number of phenylethynyl group as indicated in the UV–Vis spectra, the light to electricity conversion efficiency (η) of the corresponding solar cell drops. The influence of the number of the phenylethynyl group present along the axial direction of the porphyrin on the photovoltaic performance of DSSCs is well explained by the condensed Fukui function obtained via DFT calculations.