Solar Light Harvesting Research Articles

Mixed‐Organic‐Cation Perovskite Photovoltaics for Enhanced Solar‐Light Harvesting

Hybrid organic-inorganic lead halide perovskite APbX3 pigments, such as methylammonium lead iodide, have recently emerged as excellent light harvesters in solid-state mesoscopic solar cells. An important target for the further improvement of the performance of perovskite-based photovoltaics is to extend their optical-absorption onset further into the red to enhance solar-light harvesting. Herein, we show that this goal can be reached by using a mixture of formamidinium (HN=CHNH3 (+), FA) and methylammonium (CH3 NH3 (+), MA) cations in the A position of the APbI3 perovskite structure. This combination leads to an enhanced short-circuit current and thus superior devices to those based on only CH3 NH3 (+). This concept has not been applied previously in perovskite-based solar cells. It shows great potential as a versatile tool to tune the structural, electrical, and optoelectronic properties of the light-harvesting materials.

Regenerable Photovoltaic Devices with a Hydrogel-Embedded Microvascular Network

Light-driven degradation of photoactive molecules could be one of the major obstacles to stable long term operation of organic dye-based solar light harvesting devices. One solution to this problem may be mimicking the regeneration functionality of a plant leaf. We report an organic dye photovoltaic system that has been endowed with such microfluidic regeneration functionality. A hydrogel medium with embedded channels allows rapid and uniform supply of photoactive reagents by a convection-diffusion mechanism. A washing-activation cycle enables reliable replacement of the organic component in a dye-sensitized photovoltaic system. Repetitive restoration of photovoltaic performance after intensive device degradation is demonstrated.

Roles of Cocatalysts in Photocatalysis and Photoelectrocatalysis

Since the 1970s, splitting water using solar energy has been a focus of great attention as a possible means for converting solar energy to chemical energy in the form of clean and renewable hydrogen fuel. Approaches to solar water splitting include photocatalytic water splitting with homogeneous or heterogeneous photocatalysts, photoelectrochemical or photoelectrocatalytic (PEC) water splitting with a PEC cell, and electrolysis of water with photovoltaic cells coupled to electrocatalysts. Though many materials are capable of photocatalytically producing hydrogen and/or oxygen, the overall energy conversion efficiency is still low and far from practical application. This is mainly due to the fact that the three crucial steps for the water splitting reaction: solar light harvesting, charge separation and transportation, and the catalytic reduction and oxidation reactions, are not efficient enough or simultaneously. Water splitting is a thermodynamically uphill reaction, requiring transfer of multiple electrons, making it one of the most challenging reactions in chemistry. This Account describes the important roles of cocatalysts in photocatalytic and PEC water splitting reactions. For semiconductor-based photocatalytic and PEC systems, we show that loading proper cocatalysts, especially dual cocatalysts for reduction and oxidation, on semiconductors (as light harvesters) can significantly enhance the activities of photocatalytic and PEC water splitting reactions. Loading oxidation and/or reduction cocatalysts on semiconductors can facilitate oxidation and reduction reactions by providing the active sites/reaction sites while suppressing the charge recombination and reverse reactions. In a PEC water splitting system, the water oxidation and reduction reactions occur at opposite electrodes, so cocatalysts loaded on the electrode materials mainly act as active sites/reaction sites spatially separated as natural photosynthesis does. In both cases, the nature of the loaded cocatalysts and their interaction with the semiconductor through the interface/junction are important. The cocatalyst can provide trapping sites for the photogenerated charges and promote the charge separation, thus enhancing the quantum efficiency; the cocatalysts could improve the photostability of the catalysts by timely consuming of the photogenerated charges, particularly the holes; most importantly, the cocatalysts catalyze the reactions by lowering the activation energy. Our research shows that loading suitable dual cocatalysts on semiconductors can significantly increase the photocatalytic activities of hydrogen and oxygen evolution reactions, and even make the overall water splitting reaction possible. All of these findings suggest that dual cocatalysts are necessary for developing highly efficient photocatalysts for water splitting reactions.

Fluorescence self-quenching of tetraphenylporphyrin in liquid medium

Self-quenching of the fluorescence emission of tetraphenylporphyrin at high concentrations in toluene at the ambient temperature (300K) is discussed in detail based on steady state and time-resolved fluorescence measurements. The fluorescence self-quenching is mainly attributed to re-absorption effect and the Förster type resonance energy transfer process (homotransfer). The re-absorption effect is found to deform the fluorescence emission spectra significantly in energy positions as well as relative intensities of different peaks at high concentrations. Nearly ideal fluorescence emission spectra are observed at a concentration ∼10−7mol/L. Moreover, there is an apparent enhancement of the fluorescence lifetime value of tetraphenylporphyrin in toluene at high concentrations, especially on the blue side of the fluorescence emission spectra. To the best knowledge of the authors, this is the first detail report on the fluorescence self-quenching of porphyrins in liquid medium. This finding carries great importance in view of the widespread research on porphyrins in the fields of solar light harvesting, artificial photosynthesis, photodynamic therapy, etc.

(Invited) Lessons From Nature about Solar Light Harvesting and Implications for Energy Harvesting

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Integration of multiple chromophores with native photosynthetic antennas to enhance solar energy capture and delivery

Native length bacterial light-harvesting peptides carrying covalently attached designer chromophores have been created that self-assemble with native bacteriochlorophyll a (BChl a) to afford stable antennas with enhanced spectral coverage. Native (or native-like) α- and β-peptides interact with each other and BChl a to form a heterodimeric (αβ-dyad) unit that can then oligomerize to form biohybrid analogs of the bacterial core light-harvesting complex (LH1). Pairs of distinct synthetic chromophores were incorporated in αβ-dyads at selected distances from the BChl a target site (position 0). Two designs were explored. One design used green-yellow absorbing/emitting Oregon Green at the −34 position (toward the N-terminus relative to the BChl a coordination site) of β and orange-red absorbing/emitting Rhodamine Red at the −20 position of α, which combine with BChl a to give homogeneous oligomers. A second design used two different β-peptide conjugates, one with Oregon Green at the −34 position and the second with a near-infrared absorbing/emitting synthetic bacteriochlorin at the −14 position, which combine with α and BChl a to give a heterogeneous mixture of oligomers. The designs afford antennas with ∼45 to ∼60 pigments, provide enhanced spectral coverage across the visible and near-infrared regions relative to native antennas, and accommodate pigments at remote sites that contribute to solar light harvesting via an energy-transfer cascade. The efficiencies of energy-transfer to the BChl a target in the biohybrid antennas are comparable to native antennas, as revealed by static and time-resolved absorption and emission studies. The results show that the biohybrid approach, where designer chromophores are integrated via semisynthesis with native-like scaffolding, constitutes a versatile platform technology for rapid prototyping of antennas for solar energy capture without the laborious synthesis typically required for creating artificial photosynthetic light-harvesting architectures.

SiO2–Ag–SiO2–TiO2 multi-shell structures: plasmon enhanced photocatalysts with wide-spectral-response

Tailorable synthesis of plasmon enhanced catalysts with high solar-light harvesting and energy-conversion efficiency has attracted wide interest due to its scientific and technological importance. In this paper, novel SiO2–Ag–SiO2–TiO2 multi-shell photocatalysts with wide-spectral-response were systematically designed and controllably synthesized, where the SiO2 spheres were used as the cores, and the SiO2 interlayers coated on the Ag nanoparticle (NP) shells were used to separate the Ag from the TiO2 shell. The structures of the SiO2–Ag–SiO2–TiO2 multi-shell photocatalysts can be tailored by changing the thickness of SiO2 interlayers from 1 to 2, 5, 8, 12, and 20 nm, while the anatase N-doped TiO2 shells with visible light response are maintained at a thickness of 20 nm. The photocatalytic activity tests show that the enhanced photocatalytic efficiency under both ultraviolet (UV) and visible light irradiation is related to the existence of Ag NP shells and the thickness of SiO2 interlayers. The complicated coupling mechanisms between TiO2 and a plasmon are systematically discussed, and a clear physical picture for the complicated coupling processes is presented. The main reasons for the enhancement of the photocatalytic activity of the SiO2–Ag–SiO2–TiO2 multi-shell structures are the localized surface plasmon resonance (LSPR) effect and scattering effect induced by Ag NPs.

On the Energetic Dependence of Charge Separation in Low-Band-Gap Polymer/Fullerene Blends

The energetic driving force required to drive charge separation across donor/acceptor heterojunctions is a key consideration for organic optoelectronic devices. Herein we report a series of transient absorption and photocurrent experiments as a function of excitation wavelength and temperature for two low-band-gap polymer/fullerene blends to study the mechanism of charge separation at the donor/acceptor interface. For the blend that exhibits the smallest donor/acceptor LUMO energy level offset, the photocurrent quantum yield falls as the photon excitation energy is reduced toward the band gap, but the yield of bound, interfacial charge transfer states rises. This interplay between bound and free charge generation as a function of initial exciton energy provides key evidence for the role of excess energy in driving charge separation of direct relevance to the development of low-band-gap polymers for enhanced solar light harvesting.

Inorganic–organic core–shell titania nanoparticles for efficient visible light activated photocatalysis

Nanostructured modified TiO2 (m-TiO2) was synthesized using the gel combustion method based on the calcination of an acidified alkoxide solution mixed with urea. The materials were characterized by Raman, FT-IR and UV–vis diffuse reflectance spectroscopies, transmission (TEM) and scanning electron microscopies (SEM), X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR), in comparison with reference material untreated with urea (ref-TiO2). The effect of both the urea content and calcination temperature were optimized, providing the optimal absorption threshold of 2.19eV for solar light harvesting. The photocatalytic performance of the m-TiO2 powder was tested for the degradation of methylene blue (MB) azo dye under UVA (350–365nm), visible (440–460nm), and daylight (350–750nm) illumination. The hybrid inorganic/organic material shows exceptional physicochemical properties and significant photocatalytic activity, especially in the visible, attributed to sensitization of the TiO2 by a thin porous layer of carbonacious species in controlled core–shell morphology.

Photocatalytic Activity Enhanced by Plasmonic Resonant Energy Transfer from Metal to Semiconductor

Plasmonic metal nanostructures have been incorporated into semiconductors to enhance the solar-light harvesting and the energy-conversion efficiency. So far the mechanism of energy transfer from the plasmonic metal to semiconductors remains unclear. Herein the underlying plasmonic energy-transfer mechanism is unambiguously determined in Au@SiO(2)@Cu(2)O sandwich nanostructures by transient-absorption and photocatalysis action spectrum measurement. The gold core converts the energy of incident photons into localized surface plasmon resonance oscillations and transfers the plasmonic energy to the Cu(2)O semiconductor shell via resonant energy transfer (RET). RET generates electron-hole pairs in the semiconductor by the dipole-dipole interaction between the plasmonic metal (donor) and semiconductor (acceptor), which greatly enhances the visible-light photocatalytic activity as compared to the semiconductor alone. RET from a plasmonic metal to a semiconductor is a viable and efficient mechanism that can be used to guide the design of photocatalysts, photovoltaics, and other optoelectronic devices.

Quantum-coherent energy transfer: implications for biology and new energy technologies

The important thing in science is not so much to obtain new facts as to discover new ways of thinking about them. Sir William Henry Bragg Nobel Prize For Physics, 1915 There is a growing body of scientific evidence that light-initiated reactions in the molecular components of photosynthetic

Fabrication and characterization of dye-sensitized solar cells using electrospun TiO2 nanofibre as a solar light harvesting layer

In this work, we investigated the dye-sensitized solar cells (DSSCs) using photoanode, which were fabricated directly from electrospinning of TiO2 nanofibres onto the substrate. The electrochemical impedance spectroscopy (EIS) and current–voltage diagram were used to analyse electron transport in electrospun nanofibres and determine their applicability in DSSCs. In order to improve the short-circuit photocurrent of fabricated cells, we treated the electrospun TiO2 electrode with the TiCl4 aqueous solution. The modification of the photoelectrode significantly improves the power conversion efficiency (over 26%) of the solar cells attributed to the higher electron lifetime (τ) and reduction in recombination processes as indicated by the EIS of the solar cells.

Graphene Nanoribbons as Low Band Gap Donor Materials for Organic Photovoltaics: Quantum Chemical Aided Design

Graphene nanoribbons (GNRs) are strips of graphene cut along a specific direction that feature peculiar electronic and optical properties owing to lateral confinement effects. We show here by means of (time-dependent) density functional theory calculations that GNRs with properly designed edge structures fulfill the requirements in terms of electronic level alignment with common acceptors (namely, C(60)), solar light harvesting, and singlet-triplet exchange energy to be used as low band gap semiconductors for organic photovoltaics.

Can computational approaches aid in untangling the inherent complexity of practical organic photovoltaic systems?

AbstractOrganic materials, in particular conjugated polymers, have recently become the subject of extensive research for photovoltaic device applications. This increase of interest is primarily the result of their potentially low manufacturing cost, compatibility with flexible substrates, diverse chemical tunability, scalability, and ease of processing currently available for suitable bulk heterojunction (BHJ) construction. However, to date, these materials have not been able to exceed power conversion efficiencies (PCE) beyond 5–9%, values short of those considered commercially viable. The deficit in PCE appears to derive from a combination of physicochemical and device complexities associated with inadequate hole transport mobility, solubility and miscibility with an appropriate acceptor, narrow electronic band gap for efficient solar light harvesting, appropriate highest occupied molecular orbital (HOMO) and lowest unoccopied molecular orbital (LUMO) energies to maximize the open‐circuit voltage (Voc) and electron transfer to the acceptor, and in particular the control of the multidimensional problem of BHJ morphology. In this review article, we provide an overview of some of the recent progress toward implementing theory, modeling, and simulation approaches in combination with results from precision synthesis, characterization, and device fabrication as a mean to overcome/understand the inherent issues that limit practical applications of organic photovoltaics. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

Solar light harvesting by energy transfer: from ecology to coherence

Over billions of years, evolutionary pressure has facilitated the development of sophisticated and diverse light-harvesting systems in photosynthetic organisms, enabling them to adapt to a variety of habitats and light conditions. Today, driven by the need for cheap and efficient solar power, we turn to photosynthetic organisms and their light-active supramolecular assemblies for bio-inspiration. By studying natural systems, we can learn more about the basic principles behind the fine-tuned functionalities of light capture and energy transfer on the molecular level. In this review, we use ideas from evolutionary ecology and quantum mechanics to elucidate the parameters that underpin the efficient and robust light-harvesting machinery of natural light-harvesting systems. The family of antenna proteins of cryptophyte algae serves as an example to illustrate the evolutionary diversification process on a structural and consequently a photophysical level. Two-dimensional electronic spectroscopy experiments reveal the existence of coherence among vibronic levels in the initial response of light-harvesting proteins to femtosecond optical excitation. We discuss what implications quantum transport processes might have for light harvesting.

Use of colloidal upconversion nanocrystals for energy relay solar cell light harvesting in the near-infrared region

Colloidal upconversion (UC) nanocrystals were explored as energy relay materials for dye-sensitized solar cells for the first time. The utilization of colloidal UC nanocrystals was found to significantly enhance the upconversion efficiency and improve the photocurrent of the cells for low infrared irradiation intensity. In addition, it was found that UC nanocrystals of small size favor infiltration into a TiO2 film and bring higher relay efficiency. Finally, we found that UC nanocrystals can serve as a scattering material to increase the light absorption capability of the cells and increase the overall photocurrent of the cells under simulated sunlight irradiation.

Polyoxometalate-based materials for efficient solar and visible light harvesting: application to the photocatalytic degradation of azo dyes

This report reviews the state of the art concerning the photocatalytic degradation of azo dyes using polyoxometalate-based materials. Recent developments toward polyoxometalates (POMs) and their assemblies that match multiple criteria are described. These criteria include their switchability by visible light, the possibility of multi-electronic photo-reduction of the POMs, followed by reversible electron exchange with a substrate, the possibility of heterogenization without loss of properties and finally a low cost. This strategy is recent and is shown to open up new avenues close to green chemistry achievements for studying the degradation of organic pollutants, including azo dyes.

Observing the Multiexciton State in Singlet Fission and Ensuing Ultrafast Multielectron Transfer

Multiple exciton generation (MEG) refers to the creation of two or more electron-hole pairs from the absorption of one photon. Although MEG holds great promise, it has proven challenging to implement, and questions remain about the underlying photo-physical dynamics in nanocrystalline as well as molecular media. Using the model system of pentacene/fullerene bilayers and femtosecond nonlinear spectroscopies, we directly observed the multiexciton (ME) state ensuing from singlet fission (a molecular manifestation of MEG) in pentacene. The data suggest that the state exists in coherent superposition with the singlet populated by optical excitation. We also found that multiple electron transfer from the ME state to the fullerene occurs on a subpicosecond time scale, which is one order of magnitude faster than that from the triplet exciton state.

Extension of the spectral responsivity of the photocurrent in solution-processed small molecule composite via a charge transfer excitation

Incorporating [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in solution-processed composites comprising the two small molecular semiconductors 9,10-Diphenylanthracene (DPA) and 5,6,11,12-tetraphenylnaphthacene (Rubrene) extends significantly the onset wavelength of the steady-state photoconductivity from 610 nm in the pristine DPA:Rubrene composite to 900 nm in the DPA:Rubrene:PCBM composite. The experimental data indicate carrier generation in the near IR spectral region arising from inter-molecular charge transfer (IMCT) excitation that potentially could be useful for extending solar radiation light harvesting. Pump/probe photoinduced absorption (PIA) measurements indicate instantaneous carrier generation at sub-band-gap photon energies, confirming the viability of IMCT excitations as the underlying carrier generation mechanism at the near IR spectral region.

Molecular engineering of conjugated polymers for solar cells and field-effect transistors: Side-chain versus main-chain electron acceptors

Two model polymers, containing fluorene as an electron-donating moiety and benzothiadiazole (BT) as an electron-accepting moiety, have been synthesized by Suzuki coupling reaction. Both polymers are composed of the same chemical composition, but the BT acceptor can be either at a side-chain (i.e., S-polymer) or along the polymer main chain (i.e., M-polymer). Their optical, electrochemical, and photovoltaic properties, together with the field-effect transistor (FET) characteristics, have been investigated experimentally and theoretically. The FET carrier mobilities were estimated to be 5.20 × 10−5 and 3.12 × 10−4 cm2 V−1 s−1 for the S-polymer and M-polymer, respectively. Furthermore, polymeric solar cells (PSCs) with the ITO/PEDOT:PSS/S-polymer or M-polymer:PC71BM(1:4)/Al structure were constructed and demonstrated to show a power conversion efficiency of 0.82 and 1.24% for the S-polymer and M-polymer, respectively. The observed superior device performances for the M-polymer in both FET and PSCs are attributable to its relatively low band-gap and close molecular packing for efficient solar light harvesting and charge transport. This study provides important insights into the design of ideal structure–property relationships for conjugate polymers in FETs and PSCs. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012