Drive Charge Separation Research Articles

On the Molecular Origin of Charge Separation at the Donor–Acceptor Interface

AbstractFullerene‐based acceptors have dominated organic solar cells for almost two decades. It is only within the last few years that alternative acceptors rival their dominance, introducing much more flexibility in the optoelectronic properties of these material blends. However, a fundamental physical understanding of the processes that drive charge separation at organic heterojunctions is still missing, but urgently needed to direct further material improvements. Here a combined experimental and theoretical approach is used to understand the intimate mechanisms by which molecular structure contributes to exciton dissociation, charge separation, and charge recombination at the donor–acceptor (D–A) interface. Model systems comprised of polythiophene‐based donor and rylene diimide‐based acceptor polymers are used and a detailed density functional theory (DFT) investigation is performed. The results point to the roles that geometric deformations and direct‐contact intermolecular polarization play in establishing a driving force (energy gradient) for the optoelectronic processes taking place at the interface. A substantial impact for this driving force is found to stem from polymer deformations at the interface, a finding that can clearly lead to new design approaches in the development of the next generation of conjugated polymers and small molecules.

Molecules interface engineering derived external electric field for effective charge separation in photoelectrocatalysis

Realizing efficient charge separation and transfer is a key challenge for solar energy conversion. Here, we proposed the creation of an external electric field (EEF) by molecules interface engineering with silane grafting to form strong positive electricity on surface in semiconductor photocatalyst, driving charge separation and transfer in both bulk and surface. Forced by the EEF, WO3 nanoplates anchored silane molecules (WO3-SiH2NH2) for efficient photoelectrochemical (PEC) water oxidation shows 44.8% photocurrent enhancement. Interestingly, the photocurrent onset on WO3-SiH2NH2 exhibited a 0.3V negative shift from 0.5 to 0.2 VRHE compared with pure WO3 caused by the change of band edge position. The strategy of creating EEF is proven effective using other semiconductors and opens up a potential application to construct numerous high performance photoelectrodes for water splitting.

The role of bandgap energy excess in surface emission of terahertz radiation from semiconductors

We use a Monte-Carlo model to simulate semi-classical photo-carrier dynamics of InAs, InGaAs, and GaAs that leads to terahertz emission. We compare the emission power of all three semiconductors as a function of excitation photon energy finding that the carrier excess excitation energy is more relevant to explain their performance difference than their mobilities. We conclude that ballistic transport after photoexcitation is the dominant mechanism for terahertz emission instead of diffusion driven or surface field driven charge separation, which were traditionally considered the most relevant mechanisms.

Semiconductor interfacial carrier dynamics via photoinduced electric fields.

Solar photoconversion in semiconductors is driven by charge separation at the interface of the semiconductor and contacting layers. Here we demonstrate that time-resolved photoinduced reflectance from a semiconductor captures interfacial carrier dynamics. We applied this transient photoreflectance method to study charge transfer at p-type gallium-indium phosphide (p-GaInP2) interfaces critically important to solar-driven water splitting. We monitored the formation and decay of transient electric fields that form upon photoexcitation within bare p-GaInP2, p-GaInP2/platinum (Pt), and p-GaInP2/amorphous titania (TiO2) interfaces. The data show that a field at both the p-GaInP2/Pt and p-GaInP2/TiO2 interfaces drives charge separation. Additionally, the charge recombination rate at the p-GaInP2/TiO2 interface is greatly reduced owing to its p-n nature, compared with the Schottky nature of the p-GaInP2/Pt interface.

Competition between diagonal and off-diagonal coupling gives rise to charge-transfer states in polymeric solar cells.

It has long been a puzzle on what drives charge separation in artificial polymeric solar cells as a consensus has yet to emerge among rivaling theories based upon electronic localization and delocalization pictures. Here we propose an alternative using the two-bath spin-boson model with simultaneous diagonal and off-diagonal coupling: the critical phase, which is born out of the competition of the two coupling types, and is neither localized nor delocalized. The decoherence-free feature of the critical phase also helps explain sustained coherence of the charge-transfer state. Exploiting Hamiltonian symmetries in an enhanced algorithm of density-matrix renormalization group, we map out boundaries of the critical phase to a precision previously unattainable, and determine the bath spectral densities inducive to the existence of the charge-transfer state.

Systems approach to excitation-energy and electron transfer reaction networks in photosystem II complex: model studies for chlorophyll a fluorescence induction kinetics

Photosystem II (PS II) is a protein complex which evolves oxygen and drives charge separation for photosynthesis employing electron and excitation-energy transfer processes over a wide timescale range from picoseconds to milliseconds. While the fluorescence emitted by the antenna pigments of this complex is known as an important indicator of the activity of photosynthesis, its interpretation was difficult because of the complexity of PS II. In this study, an extensive kinetic model which describes the complex and multi-timescale characteristics of PS II is analyzed through the use of the hierarchical coarse-graining method proposed in the authors׳ earlier work. In this coarse-grained analysis, the reaction center (RC) is described by two states, open and closed RCs, both of which consist of oxidized and neutral special pairs being in quasi-equilibrium states. Besides, the PS II model at millisecond scale with three-state RC, which was studied previously, could be derived by suitably adjusting the kinetic parameters of electron transfer between tyrosine and RC. Our novel coarse-grained model of PS II can appropriately explain the light-intensity dependent change of the characteristic patterns of fluorescence induction kinetics from O-J-I-P, which shows two inflection points, J and I, between initial point O and peak point P, to O-J-D-I-P, which shows a dip D between J and I inflection points.

Design rules for organic donor-acceptor heterojunctions: pathway for charge splitting and detrapping.

Organic solar cells rely on the conversion of a Frenkel exciton into free charges via a charge-transfer state formed on a molecular donor-acceptor pair. These charge-transfer states are strongly bound by Coulomb interactions and yet efficiently converted into charge-separated states. A microscopic understanding of this process, though crucial to the functionality of any solar cell, has not yet been achieved. Here we show how long-range molecular order and interfacial mixing generate homogeneous electrostatic forces that can drive charge separation and prevent minority carrier trapping across a donor-acceptor interphase. Comparing a variety of small-molecule donor-fullerene combinations, we illustrate how tuning of molecular orientation and interfacial mixing leads to a trade-off between photovoltaic gap and charge-splitting and detrapping forces, with consequences for the design of efficient photovoltaic devices.

A Solution‐Processed UV‐Sensitive Photodiode Produced Using a New Silicon Nanocrystal Ink

This article presents a simple and effective method of functionalizing hydrogen‐terminated silicon (Si) nanocrystals (NCs) to form a high‐quality colloidal Si NC ink with short ligands that allow charge transport in nanocrystal solid films. Si NCs fabricated by laser‐pyrolysis and acid etching are passivated with allyl disulfide via ultraviolet (UV)‐initiated hydrosilylation to form a stable colloidal Si NC ink. Then a Si NC‐based photodiode is directly fabricated in air from this ink. Only a solution‐processed poly(3,4‐ethylenedioxy‐thiophene):poly(styrene sulfonate) (PEDOT: PSS) electron blocking layer and top‐ and bottom‐contacts are needed along with the Si NC layer to construct the device. A Schottky‐junction at the interface between the Si NC absorber layer and aluminum (Al) back electrode drives charge separation in the device under illumination. The unpackaged Si NC‐based photodiode exhibites a peak photoresponse of 0.02 A W−1 to UV light in air, within an order of magnitude of the response of commercially available gallium phosphide (GaP), gallium nitride (GaN), and silicon carbide (SiC) based photodetectors. This provides a new pathway to large‐area, low‐cost solution‐processed UV photodetectors on flexible substrates and demonstrates the potential of this new silicon nanocrystal ink for broader applications in solution‐processed optoelectronics.

ChemInform Abstract: Materials Design Considerations for Charge Generation in Organic Solar Cells

AbstractReview: 176 refs.

Driving charge separation for hybrid solar cells: photo-induced hole transfer in conjugated copolymer and semiconductor nanoparticle assemblies

This work reports on the use of an internal electrostatic field to facilitate charge separation at inorganic-organic interfaces, analogous to those in hybrid solar cells. Systematic charge transfer studies show that the donor-acceptor charge transfer rate is highly sensitive to the direction of the internal electric field.

Visible light active Ru-doped sodium niobate pervoskite decorated with platinum nanoparticles via surface capping

An efficient visible light active NaNb1−xRuxO3 perovskite was hydrothermally fabricated and characterized. Various amounts of ruthenium substitution (0–5%) for Nb5+ in the niobate framework caused structural evolution from nanocubes to nanowires. 1% Ru-doped niobates [NR(1)], which had a highly crystalline orthorhombic structure, adsorbed visible light most substantially with an optical band gap of ∼2.3eV. With the aid of 1H MAS NMR examination, the addition of protons into the niobate by acid treatment was convinced and its projected structure of [RuO6] plus [NbO6] octahedra with some defects was proposed. NR(1) itself was not effective at carrying out photocatalytic decomposition of phenol in water. To enhance visible light driven charge separation, platinum was dispersed onto NR(1) using surface capped Pt nanoparticles (PtNP) and Pt complexes (e.g., [Pt(NH3)4]Cl2). PtNPs laden NR(1) exhibited superior photocatalytic degradation of phenol in water. This increased photocatalytic activity was attributable to synergistic effects of facilitated electron transfer by PtNPs.

Materials Design Considerations for Charge Generation in Organic Solar Cells

This article reviews some of our recent progress on materials design guidelines for photoinduced charge generation in bulk-heterojunction organic solar cells. Over the last 7 years, our group has employed transient absorption measurement to determine the relative quantum yields of long-lived polaron pairs for over 300 different organic Donor/Acceptor blend films. We have shown that this optical assay of charge separation can be a strong indicator of photocurrent generation efficiency in complete devices. In this review, we consider the lessons that can be drawn from these studies concerning the parameters that determine efficiency of this photoinduced charge separation in such solar cells. We consistently find, from studies of several materials series, that the energy offset driving charge separation is a key determinant of the efficiency of this charge generation, and thereby photocurrent generation. Moreover, we find that the magnitude of the energy offset required to drive charge separation, and the st...

Electronic Structure of Semiconducting and Metallic Tubes in TiO2/Carbon Nanotube Heterojunctions: Density Functional Theory Calculations.

The electronic structure of the TiO2(110) surface interfaced with both a semiconducting and metallic carbon nanotube (CNT) was investigated by density functional theory. Our simulations rationalized visible light photocatalytic activity of CNT/TiO2 hybrid materials higher than that under ultraviolent irradiation and showed that the photoactivity of a semiconducting CNT decorating TiO2 is better than that of the metallic CNT/TiO2 system due to efficient charge separation across the interface. This suggests that semiconducting CNT/TiO2 could be a potential photovoltaic material. In contrast, strong interaction between a metallic CNT and TiO2 leads to large charge transfer. Such charge transfer reduces the built-in potential, in turn resulting in inefficient charge separation. Functionalizing the metallic CNT with a small platinum cluster can increase the built-in potential and drive charge separation. These observations indicate that the CNT/TiO2 interface can be a potential photovoltaic material by a metal cluster decorating a CNT despite a real tube being composed of the mixture of metallic and semiconducting CNTs.

Charge Photogeneration for a Series of Thiazolo‐Thiazole Donor Polymers Blended with the Fullerene Electron Acceptors PCBM and ICBA

AbstractPhotoinduced charge separation in bulk heterojunction solar cells is studied using a series of thiazolo‐thiazole donor polymers that differ in their side groups (and bridging atoms) blended with two acceptor fullerenes, phenyl‐C71‐butyric acid methyl ester (PC71BM) and a fullerene indene‐C60 bisadduct (ICBA). Transient absorption spectroscopy is used to determine the yields and lifetimes of photogenerated charge carriers, complimented by cyclic voltammetry studies of materials energetics, wide angle X‐ray diffraction and transmission electron microscopy studies of neat and blend film crystallinity and photoluminescence quenching studies of polymer/fullerene phase segregation, and the correlation of these measurements with device photocurrents. Good correlation between the initial polaron yield and the energetic driving force driving charge separation, ΔECS is observed. All blend films exhibit a power law transient absorption decay phase assigned to non‐geminate recombination of dissociated charges; the amplitude of this power law decay phase shows excellent correlation with photocurrent density in the devices. Furthermore, for films of one (relatively amorphous) donor polymer blended with ICBA, we observe an additional 100 ns geminate recombination phase. The implications of the observations reported are discussed in terms of the role of materials' crystallinity in influencing charge dissociation in such devices, and thus materials design requirements for efficient solar cell function.

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.

Energy versuselectron transfer in organic solar cells: a comparison of the photophysics of two indenofluorene: fullerene blend films

In this paper, we compare the photophysics and photovoltaic device performance of two indenofluorene based polymers: poly[2,8-(6,6,12,12-tetraoctylindenofluorene)-co-4,7-(2,1,3-benzothiodiazole] (IF8BT) and poly[2,8-(6,6,12,12-tetraoctylindenofluorene)-co-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiodiazole] (IF8TBTT) blended with [6,6]-phenyl C61 butyric acid methyl ester (PCBM). Photovoltaic devices made with IF8TBTT exhibit greatly superior photocurrent generation and photovoltaic efficiency compared to those made with IF8BT. The poor device efficiency of IF8BT/PCBM devices is shown to result from efficient, ultrafast singlet Förster energy transfer from IF8BT to PCBM, with the resultant PCBM singlet exciton lacking sufficient energy to drive charge photogeneration. The higher photocurrent generation observed for IF8TBTT/PCBM devices is shown to result from IF8TBTT's relatively weak, red-shifted photoluminescence characteristics, which switches off the polymer to fullerene singlet energy transfer pathway. As a consequence, IF8TBTT singlet excitons are able to drive charge separation at the polymer/fullerene interface, resulting in efficient photocurrent generation. These results are discussed in terms of the impact of donor/acceptor energy transfer upon photophysics and energetics of charge photogeneration in organic photovoltaic devices. The relevance of these results to the design of polymers for organic photovoltaic applications is also discussed, particularly with regard to explaining why highly luminescent polymers developed for organic light emitting diode applications often give relatively poor performance in organic photovoltaic devices.

An investigation of slow charge separation in a Tyrosine M210 to Tryptophan mutant of the Rhodobacter sphaeroides reaction center by femtosecond mid-infrared spectroscopy

Energy and electron transfer in a tyrosine M210 to tryptophan (YM210W) mutant of the Rhodobacter sphaeroides reaction center (RC) were investigated through time-resolved visible pump/mid-infrared (mid-IR) probe spectroscopy at room temperature, with the aim to further characterize the primary charge separated states in the RC. This mutant is known to display slow and multi-exponential charge separation, and was used in earlier work to prove the existence of an alternative route for charge separation starting from the accessory bacteriochlorophyll in the active branch, B(L). The mutant RCs were excited at 860 nm (direct excitation of the primary donor (P) BChls (P(L)/P(M))), 600 nm (unselective excitation), 805 nm (direct excitation of both accessory bacteriochlorophyll cofactors B(L) and B(M)) and 795 nm (direct excitation of B(L)). Absorption changes associated with carbonyl (C=O) stretch vibrational modes of the cofactors and protein were recorded in the region between 1600 and 1775 cm(-1), and both a sequential analysis and simultaneous target analysis of the data were performed. The decay of P* in the YM210W mutant was multi-exponential with lifetimes of 29 and 63.5 ps. The decay of P(+)B(L)(-) state was approximately 10 times longer in the YM210W RC than in the R-26 RC (approximately 7 ps vs. approximately 0.7 ps), and in the mid-IR difference absorption spectrum of P(+)B(L)(-) the stretching frequency of the 9-keto C=O group of B(L) in the ground state was located around 1675-1680 cm(-1), consistent with the presence of a hydrogen bond donated by an adjacent water molecule. Excitation at 795 nm produced a small amount of B(L)*-driven charge separation, as assessed from the excitation wavelength dependence of the raw difference spectra recorded during the first few ps after excitation. This process led to the formation of P(+)B(L)(-). Only the relaxed form of the P(+)H(L)(-) radical pair was observed in the YM210W mutant, and the mid-IR difference absorption spectra of P(+)H(L)(-) and P(+)B(L)(-) showed a change in the relative amplitude of the P(L)(+) and P(M)(+) bands when compared to equivalent spectra for the R-26 RC. This indicates that the YM210W mutation causes an increased localization of the electron hole on the P(M) half of the dimer. The absorbance difference spectrum of P(+)H(L)(-) in the R-26 RC contains a feature attributable to a Stark shift of one or more amide C=O oscillators. This feature was shifted to lower frequency by approximately 5 cm(-1) in the YM210W RC, and consideration of the limited structural changes in this RC indicates that this feature arises from an amide C=O group in the immediate vicinity of the M210 residue, most probably that of the adjacent M209 amino acid.

Acoustically driven charge separation in semiconductor heterostructures sensed by optical spectroscopy techniques

Abstract We demonstrate a method of using a two-layer sandwich structure, which includes a LiNbO3 plate and a semiconductor heterostructure to create an inhomogeneous stress and piezoelectric harmonic potential in the semiconductor. Both the GaAs/AlGaAs quantum well (QW) structures and SiGe/Si heterostructures are attempted, working with and without using a piezoelectric field in the semiconductor layer. The standing-wave fields generated in the semiconductor and the electron and hole distributions driven by the piezoelectric field are computed by finite element method (FEM) techniques. It is experimentally shown that, in a GaAs/AlxGa1-x As asymmetric double quantum well structure, the resonance enhancement of the narrower QW photoluminescence band is observed, which may be explained by the resonant charge transfer between the wider and narrower QWs. It is also shown that the piezoelectric fields quench the pure LO-phonon lines in the Raman spectra, whereas the coupled LO-phonon-plasmon mode strengthens. Experimental results indicate that the charge separation occurs in the plane of the QWs due to the piezoelectric fields. The recombination of carriers in the SiGe/Si heterostructures can be effectively enhanced by the presence of ultrasonic stress, displaying features consistent with varying electrical activity at dislocations.

Photosystem I: both sides now

Light absorbed by the chlorophylls of photosystem (PS) I drives charge separation and electron transfer through a chain of cofactors arranged along one half of the quasi-symmetrical heterodimeric core of the reaction centre. The other branch of the reaction centre is switched off though the influence of the protein, as is the case with the PSII and purple bacterial reaction centres. At least, that is the way the textbooks see it. New data 1 Guergova-Kuras M. et al. Evidence for two active branches for electron transfer in photosystem I. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 4437-4442 Crossref PubMed Scopus (261) Google Scholar show that this view is probably wrong and that, in fact, electron transfer occurs on both sides of the reaction centre in PSI.

Effect of Finite Resistivity on Gravitational Instability

It is shown that the finite resistivity of plasma has a stabilizing effect in a configuration in which a plasma is supported by a magnetic field against a gravitational force. The finite resistivity allows a discharge current to flow across magnetic lines of force, and the instability driving charge separation due to the gravitational drift is reduced.