Long-range Charge Separation Research Articles

Long-Range Charge Separation Enabled by Intramoiety Delocalized Excitations in Copolymer Donors in Organic Photovoltaic Blends.

For over two decades, most high-performance organic photovoltaics (OPVs) have been made with donor:acceptor bulk heterojunctions with domain sizes limited by exciton diffusion, where charge separation mostly takes place through the dissociation of the interfacial charge-transfer (xCT) excitons. Recently, nonfullerene acceptor (NFA)-based OPVs have shown excellent compatibility to device structures with large domains in active layers. However, it remains elusive how the excitations that are distant from the interfaces are converted into free charges. Here, we report the identification of a new charge separation channel in model copolymer/NFA blends mediated by intra-moiety delocalized excitations in both planar heterojunctions and donor-enriched bulk heterojunctions. The delocalized excitations induced by interchromophore electronic interactions in copolymer donors mediate the long-range charge separation and dissociate into free charges without forming the bound xCT states first, releasing the constraints associated with the short exciton diffusion length in organic materials. The long-range charge separation mechanism uncovered in this work, in cooperation with the short-range xCT-mediated pathway, holds the potential to further optimize OPVs with diverse device structures.

Driving force and nonequilibrium vibronic dynamics in charge separation of strongly bound electron–hole pairs

Electron-hole pairs in organic photovoltaics efficiently dissociate although their Coulomb-binding energy exceeds thermal energy at room temperature. The vibronic coupling of electronic states to structured vibrational environments containing multiple underdamped modes is thought to assist charge separation. However, non-perturbative simulations of such large, spatially extended, electronic-vibrational (vibronic) systems remain an unmet challenge which current methods bypass by considering effective one-dimensional Coulomb potentials or unstructured environments where the effect of underdamped modes is ignored. Here we address this challenge with a non-perturbative simulation tool and investigate the charge separation dynamics in one, two and three-dimensional donor-acceptor networks to identify under what conditions underdamped vibrational motion induces efficient long-range charge separation. The resulting comprehensive picture of ultrafast charge separation differentiates electronic or vibronic couplings mechanisms for a wide range of driving forces and identifies the role of entropic effects in extended systems. This provides a toolbox for the design of efficient charge separation pathways in artificial nanostructures.

Why and how could an aliphatic bridge allow for a long-range photoinduced charge separation in Tröger’s bases derivatives

Tröger’s bases (TBs) have shown great potential to be used in different fields of science, such as biology, organic synthesis, photoelectronic applications, among others. As we have recently shown, a series of asymmetrically substituted Tröger’s base derivatives showed unexpected pull–push behavior. The aliphatic diazocine heterocycle which connect an electron donor and an electron acceptor moiety efficiently couples both electronic subsystems as if it were a typical π-conjugated linker. A thorough computational study was intended to shed light on the origin of the observed photophysical properties. A modified version of the CAM-B3LYP functional yielded an accurate prediction of the absorption and emission spectra of these species. In contrast, range-corrected and classical hybrid approaches showed too high and remarkably too low excited states energies, respectively.Unlike the typical π-linked donor/acceptor systems, a threshold was found in the redox gap of the centers in order to obtain a full charge separation. The role of the aliphatic bridge was found to be related to the unusual topology of the frontier orbitals involved, to the tension and particular molecular shape of the Tröger bicycle and to a contribution due to homoconjugation as well. Calculations on the actual Tröger derivatives and specific models were able to quantify the magnitude of the different contributions that make possible the charge separation.

Coupling plasmonic effect into stable AgCl/NH2-MIL-68In Z-scheme heterojunction system for efficient photodegradation of levofloxacin

Unstable contact interface, inferior charge transmission efficiency and low redox capability significantly inhibit the photocatalytic property of heterojunctions. Herein, a stable AgCl/NH2-MIL-68In Z-scheme heterojunction that couples with plasmonic Ag nanoparticles (NPs) to promote efficient long-range charge separation is rationally designed. This heterostructure is successfully fabricated by the in-situ growth of AgCl and Ag NPs on the surface and framework of NH2-MIL-68In via a facile ion exchange precipitation approach. Benefiting from the surface plasmon resonance (SPR) effect of Ag NPs and the interfacial heterojunction, the visible light response is enhanced and the recombination rate of photogenerated charges is weakened. Impressively, the artificial Z-scheme heterojunction exhibits superior photocatalytic activity for levofloxacin degradation. Furthermore, the adjustment of the AgCl and Ag NPs loading ratio of NH2-MIL-68In further improves the photocatalytic degradation activity of levofloxacin (0.0222 min−1), which is much higher than that of pure NH2-MIL-68In (k = 0.00121 min−1) and AgCl (k = 0.00093 min−1). This work not only develops an effective photocatalyst for the degradation of organic pollutants, but also provides a promising sight for feasible designing the more stable Z-scheme heterojunction with efficient charge separation efficiency.

Charge Transport Dynamics of Quasi-Type II Perovskite Janus Nanocrystals in High-Performance Photoconductors

CsPbBr3-Pb4S3Br2 Janus nanocrystals (NCs) are the only nanomaterial where the epitaxial structure of perovskite and chalcogenide materials has been realized at the nanoscale, but their exciton dynamics mechanism has not yet been thoroughly investigated or applied in photodetection applications. This work reports an attractive device performance of perovskite photoconductors based on epitaxial CsPbBr3-Pb4S3Br2 Janus NCs, as well as the carrier relaxation and transfer mechanism of the heterojunction. By a combination of transient optical absorption and quantum dynamics simulation, it is demonstrated that the photogenerated holes on CsPbBr3 can be successfully extracted by Pb4S3Br2, while the hole transfer proceeds about three times faster than energy loss and remains "hot" for about 300 fs. This feature has favorable effects on long-range charge separation and transport; therefore, the Janus NCs photoconductors exhibit an exceptional responsivity of 34.0 A W-1 and specific detectivity of 1.26 × 1014 Jones.

Coupling Long-Range Facet Junction and Interfacial Heterojunction via Edge-Selective Deposition for High-Performance Z-Scheme Photocatalyst.

The construction of photocatalytic systems that have strong redox capability, effective charge separation, and large reactive surfaces is of great scientific and practical interest. Herein, an edge‐connected 2D/2D Z‐scheme system that combines the facet junction and the interfacial heterojunction to achieve effective long‐range charge separation and large reactive surface exposure is designed and fabricated. The heterostructure is realized by the selective growth of 2D‐layered MoS2 nanoflakes on the edge‐sites of thin TiO2 nanosheets via an Au‐promoted photodeposition method. Attributed to the synergetic coupling of the facet junction and the interfacial heterojunction that assures the effective charge separation, and the tremendous but physically separated reactive sites offered by layered MoS2 and highly‐exposed (001) facets of TiO2, respectively, the artificial Z‐scheme exhibits excellent photocatalytic performance in photodegradation tests. Moreover, the junctional plasmonic Au nanoclusters not only act as electron traps to promote the edge‐selective synthesis but also generate “hot electrons” to further boost photocatalytic performance. The Z‐scheme charge‐flow direction in the heterostructure and the roles of electrons and holes are comprehensively studied using in situ irradiated X‐ray photoelectron spectroscopy and photodegradation tests. This work offers a new insight into designing high‐performance Z‐scheme photocatalytic systems.

Why and How Could an Aliphatic Bridge Allow for a Long-Range Photoinduced Charge Separation in Tröger's Bases Derivatives

Why and How Could an Aliphatic Bridge Allow for a Long-Range Photoinduced Charge Separation in Tröger's Bases Derivatives

Deoxyribonucleic Acid Encoded and Size-Defined π-Stacking of Perylene Diimides.

Natural photosystems use protein scaffolds to control intermolecular interactions that enable exciton flow, charge generation, and long-range charge separation. In contrast, there is limited structural control in current organic electronic devices such as OLEDs and solar cells. We report here the DNA-encoded assembly of π-conjugated perylene diimides (PDIs) with deterministic control over the number of electronically coupled molecules. The PDIs are integrated within DNA chains using phosphoramidite coupling chemistry, allowing selection of the DNA sequence to either side, and specification of intermolecular DNA hybridization. In this way, we have developed a “toolbox” for construction of any stacking sequence of these semiconducting molecules. We have discovered that we need to use a full hierarchy of interactions: DNA guides the semiconductors into specified close proximity, hydrophobic–hydrophilic differentiation drives aggregation of the semiconductor moieties, and local geometry and electrostatic interactions define intermolecular positioning. As a result, the PDIs pack to give substantial intermolecular π wave function overlap, leading to an evolution of singlet excited states from localized excitons in the PDI monomer to excimers with wave functions delocalized over all five PDIs in the pentamer. This is accompanied by a change in the dominant triplet forming mechanism from localized spin–orbit charge transfer mediated intersystem crossing for the monomer toward a delocalized excimer process for the pentamer. Our modular DNA-based assembly reveals real opportunities for the rapid development of bespoke semiconductor architectures with molecule-by-molecule precision.

Long-range light-modulated charge transport across the molecular heterostructure doped protein biopolymers.

Biological electron transfer (ET) across proteins is ubiquitous, such as the notable photosynthesis example, where light-induced charge separation takes place within the reaction center, followed by sequential ET via intramolecular cofactors within the protein. Far from biology, carbon dots (C-Dots) with their unique optoelectronic properties can be considered as game-changers for next-generation advanced technologies. Here, we use C-Dots for making heterostructure (HS) configurations by conjugating them to a natural ET mediator, the hemin molecule, thus making an electron donor–acceptor system. We show by transient absorption and emission spectroscopy that the rapid intramolecular charge separation happens following light excitation, which can be ascribed to an ultrafast electron and hole transfer (HT) from the C-Dot donor to the hemin acceptor. Upon integrating the HS into a protein matrix, we show that this HT within the HS configuration is 3.3 times faster compared to the same process in solution, indicating the active role of the protein in supporting the rapid light-induced long-range intermolecular charge separation. We further use impedance, electrochemical, and transient photocurrent measurements to show that the light-induced transient charge separation results in an enhanced ET and HT efficiency across the protein biopolymer. The charge conduction across our protein biopolymers, reaching nearly 0.01 S cm−1, along with the simplicity and low-cost of their formation promotes their use in a variety of optoelectronic devices, such as artificial photosynthesis, photo-responsive protonic–electronic transistors, and photodetectors.

The Role of Delocalization and Excess Energy in the Quantum Efficiency of Organic Solar Cells and the Validity of Optical Reciprocity Relations.

The photon energy dependence of long-range charge separation is studied for two prototypical polymer:fullerene systems. The internal quantum efficiency (IQE) of PCDTBT:PC61BM is experimentally shown to be independent of the excitation energy. In contrast, for TQ1:PC71BM the IQE is strongly energy-dependent for excitation energies close to charge transfer (CT) electroluminescence peak maximum while it becomes energy-independent at higher excitation energies. Kinetic Monte Carlo simulations reproduce the experimental IQE and reveal that the photon energy-dependence of the IQE is governed by charge delocalization. Efficient long-range separation at excitation energies corresponding to the CT electroluminescence peak maximum or lower requires an initial separation of the hole-electron pair by ∼4-5 nm, whereas delocalization is less important for charge separation at higher photon energies. Our modeling results suggest that a phenomenological reciprocity between CT electroluminescence and external quantum efficiency does not necessarily prove that commonly employed reciprocity relations between these spectra are valid from a fundamental perspective.

Ferroelectric Polarization Suppresses Nonradiative Electron-Hole Recombination in CH3NH3PbI3 Perovskites: A Time-Domain ab Initio Study.

The ordered alignment of polar organic cations in hybrid organic-inorganic perovskites causes a ferroelectric phase, which is believed to be benign for photovoltaic devices. Using a combination of time-domain density functional theory and nonadiabatic molecular dynamics, we study the influence of the interactions of polar MA (CH3NH3+) cations and between their inorganic counterparts on the nonradiative electron-hole recombination in the room-temperature ferroelectric MAPbI3 perovskite. We show that ferroelectric alignment of the polar C-N bonds favors charge separation and reduces nonadiabatic coupling. Symmetry breaking enhances low-frequency collective motions and introduces additional high-frequency vibrations, thus accelerating quantum decoherence. Both factors contribute to suppressing the nonradiative electron-hole recombination, extending the charge carrier lifetimes to several nanoseconds and showing a factor of 3 increase compared to the pristine system. Consequently, ferroelectric engineering provides an excellent route to improve hybrid perovskite device performance as a result of long-range charge separation and slow charge recombination.

Long-range photoinduced charge separation in tröger bases D/A dyads

Tröger´s base (and its derivatives) are compounds comprised of two aromatic (or polyaromatic) rings bridged by a diazocine aliphatic cycle. We report herein the photophysical properties of two series of novel Tröger´s bases (TB) asymmetrically substituted by electron donor (D) and electron acceptor (A) substituents. In TB series 3, a carbonitrile group (ACN) lies at the position 2 of the heterocycle, while position 8 is occupied by a series of D with increasing reductant capacity: H (3a), CH3 (3b), OCH3 (3c) or N(CH3)2 (3d). A novel TB series (5a-5d) which comprise the same D, but a 2,2-dicyanovinyl group (A = CHC(CN)2) as electron acceptor, was synthesized and fully characterized. TB absorption (νAmax) and emission energies (νFmax), fluorescence quantum yields (ΦF) and emission lifetimes (τF) were determined in a series of aprotic solvents covering a wide range of medium polarity (ε∼2-38). νFmax, ΦF and τF largely depend on the polarity of the medium and the nature of D/A pair. From the solvatochromic study on νFmax, it is concluded that upon excitation TB´s develop large degrees of charge separation (CT). Photophysically, 3a-3c resembles 4-(N,N-dimethylamino)benzonitrile derivatives showing internal CT state dipole moments (μ1*) of ∼ 15–17 D. For 3d and the entire series 5, CT occurs throughout the diazocine ring giving rise to giant μ1* (> 25 D). This is indeed an unusual result, because it strongly suggests that the aliphatic diazocine ring can couple the D/A redox centers as a π bridge would do.

Non-Markovian Quantum-Classical Ratchet for Ultrafast Long-Range Electron-Hole Separation in Condensed Phases

In organic photovoltaic systems, a photogenerated molecular exciton in the donor domain dissociates into a hole and an electron at the donor-acceptor heterojunction, and subsequently separates into free charge carriers that can be extracted as photocurrents. The recombination of the once-separated electron and hole is a major loss mechanism in photovoltaic systems, which controls their performance. Hence, efficient photovoltaic systems need built-in ratchet mechanisms, namely, ultrafast charge separation and retarded charge recombination. In order to obtain insight into the internal working of the experimentally observed ultrafast long-range charge separation and protection against charge recombination, we theoretically investigate a potential ratchet mechanism arising from the combination of quantum delocalization and its destruction by performing numerically accurate quantum-dynamics calculations on a model system. We demonstrate that the non-Markovian effect originating from the slow polaron formation strongly suppresses the electron-transfer reaction back to the interfacial charge-transfer state stabilized at the donor-accepter interface and that it plays a critical role in maintaining the long-range electron-hole separation.

Field-Assisted Exciton Dissociation in Highly Efficient PffBT4T-2OD:Fullerene Organic Solar Cells

Understanding the photophysics of charge generation in organic semiconductors is a critical step toward the further optimization of organic solar cells. The separation of electron–hole pairs in systems with large energy offsets is relatively well-understood; however, the photophysics in blends with low driving energy remains unclear. Herein, we use the material system PffBT4T-2OD:PC71BM as an example to show that the built-in electric field plays a critical role toward long-range charge separation in high-performance devices. By using steady-state and time-resolved spectroscopic techniques, we show that in neat films an energetic barrier impedes polymer exciton dissociation, preventing charge transfer to the fullerene acceptor. In complete devices, this barrier is diminished due to the built-in electric field provided by the interlayers/contacts and accompanying space-charge distribution. The observed behavior could also be relevant to other systems with low driving energy and emphasizes the importance of...

Ultrafast Long-Range Charge Separation in Nonfullerene Organic Solar Cells.

Rapid, long-range charge separation in polymer-fullerene organic solar cells (OSCs) enables electrons and holes to move beyond their Coulomb capture radius and overcome geminate recombination. Understanding the nature of charge generation and recombination mechanisms in efficient, nonfullerene-acceptor-based OSCs are critical to further improve device performance. Here we report charge dynamics in an OSC using a perylene diimide (PDI) dimer acceptor. We use transient absorption spectroscopy to track the time evolution of electroabsorption caused by the dipolar electric field generated between electron-hole pairs as they separate after ionization at the donor-acceptor interface. We show that charges separate rapidly (<1 ps) and that free charge carriers are generated very efficiently (∼90% quantum yield). However, in the PDI-based OSC, external charge extraction is impaired by faster nongeminate decay to the ground state and to lower-lying triplet states.

Ferroelectric Alignment of Organic Cations Inhibits Nonradiative Electron-Hole Recombination in Hybrid Perovskites: Ab Initio Nonadiabatic Molecular Dynamics.

Hybrid organic-inorganic perovskites show impressive potential for photovoltaic applications and currently give rise to one of the most vibrant research areas in the field. Until recently, the electrostatic interactions between their organic and inorganic components were considered mostly for stabilization of the fragile perovskite structure. We study the effect of local interactions of polar C-N bonds in the organic layer on the nonradiative electron-hole recombination in the recently reported room-temperature ferroelectric hybrid perovskite, (benzylammonium)2PbCl4. Using nonadiabatic molecular dynamics and real-time time-dependent density functional theory, we show that ferroelectric alignment of the polar groups weakens the electron-phonon nonadiabatic coupling and inhibits the nonradiative charge recombination. The effect is attributed to suppression of contributions of higher frequency phonons to the electron-phonon coupling. The coupling is dominated in the ferroelectric phase by slower collective motions. We also demonstrate the importance of van der Waals interactions for the charge-phonon relaxation in the hybrid perovskite systems. Combined with the long-range charge separation achievable in the ferroelectric phase, the weakened electron-phonon coupling indicates that ferroelectric order in hybrid perovskites can lead to increased excited-state lifetimes and improved solar energy conversion performance.

Ultrafast Long-Range Charge Separation in Organic Photovoltaics: Promotion by Off-Diagonal Vibronic Couplings and Entropy Increase.

The exciton dissociation in a model donor/acceptor heterojunction with electron-phonon couplings is simulated by a full quantum dynamical method, in which ultrafast long-range charge separation is observed. Such a novel scenario does not undergo short-range interfacial (pinned) charge transfer states, but can be mainly ascribed to the quantum resonance between local Frenkel excited states and a broad array of long-range charge transfer (LRCT) states assisted by the moderate off-diagonal vibronic couplings. The entropy-increasing effect associated with the very dense density of states for LRCT states is also found to be beneficial for lowering the free energy barrier for charge generation in organic solar cells.

Molecular Packing Determines Charge Separation in a Liquid Crystalline Bisthiophene-Perylene Diimide Donor-Acceptor Material.

Combined electronic structure and quantum dynamical calculations are employed to investigate charge separation in a novel class of covalently bound bisthiophene-perylene diimide type donor-acceptor (DA) co-oligomer aggregates. In an earlier spectroscopic study of this DA system in a smectic liquid crystalline (LC) film, efficient and ultrafast (subpicosecond) initial charge separation was found to be followed by rapid recombination. By comparison, the same DA system in solution exhibits ultrafast resonant energy transfer followed by slower (picosecond scale) charge separation. The present first-principles study explains these contrasting observations, highlighting the role of an efficient intermolecular charge-transfer pathway that results from the molecular packing in the LC phase. Despite the efficiency of this primary charge-transfer step, long-range charge separation is impeded by a comparatively high Coulomb barrier in conjunction with small electron- and hole-transfer integrals. Quantum dynamical calculations are carried out for a fragment-based model Hamiltonian, parametrized by ab initio second-order Algebraic Diagrammatic Construction (ADC(2)) and Time-Dependent Density Functional Theory (TDDFT) electronic structure calculations. Simulations of coherent vibronic quantum dynamics for up to 156 electronic states and 48 modes are performed using the Multi-Layer Multi-Configuration Time-Dependent Hartree (ML-MCTDH) method. Excellent agreement with experimentally determined charge separation time scales is obtained, and the spatially coherent nature of the dynamics is analyzed.

Highly Active Supported Plasmonic Photocatalyst Consisting of Gold Nanoparticle-Loaded Mesoporous Titanium(IV) Oxide Overlayer and Conducting Substrate

We prepared mesoporous films consisting of gold nanoparticle-loaded titanium dioxide nanoparticles on various conducting substrates including FTO, ITO, Ti, Au, and Pt (Au/mp-TiO2/conducting substrate), and a glass plate for comparison. Electrochemical characterization of Au/mp-TiO2/FTO substrate clarified that the mesoporous structure of the Au/TiO2 overlayer permits the permeation of solution into the mesopores to reach the FTO surface. The visible-light activity (λ > 430 nm) of the immobilized “plasmonic photocatalysts” was evaluated for the oxidations of benzylamine and cinnamyl alcohol. The visible-light activity strongly depended on the kind of the substrates. Visible-light irradiation of Au/mp-TiO2/FTO in an aqueous solution of benzylamine selectively produced benzaldehyde with ∼100% yield at 16 h, while the yields for the Au/mp-TiO2/glass and Au/TiO2 particle systems were ∼75% under the same conditions. In the selective oxidation of cinnamyl alcohol to cinnamaldehyde, the Au/mp-TiO2/conducting substrate photocatalysts using Ti, Au, and Pt as the substrate showed significantly higher activities than even Au/TiO2 particles. By the technique for labeling and visualizing the reduction sites (J. Phys. Chem. B 2000, 104, 4585), this remarkable substrate effect was shown to originate from the long-range charge separation by the successive interfacial electron transfer from Au nanoparticle to the substrate through TiO2. A good positive correlation was found between the visible-light activity of Au/mp-TiO2/conducting substrate and the electrocatalytic activity of the conducting substrate for O2 reduction.

Distance distributions of photogenerated charge pairs in organic photovoltaic cells.

Strong Coulomb interactions in organic photovoltaic cells dictate that charges must separate over relatively long distances in order to circumvent geminate recombination and produce photocurrent. In this article, we measure the distance distributions of thermalized charge pairs by accessing a regime at low temperature where charge pairs are frozen out following the primary charge separation step and recombine monomolecularly via tunneling. The exponential attenuation of tunneling rate with distance provides a sensitive probe of the distance distribution of primary charge pairs, reminiscent of electron transfer studies in proteins. By fitting recombination dynamics to distributions of recombination rates, we identified populations of charge-transfer states and well-separated charge pairs. For the wide range of materials we studied, the yield of separated charges in the tunneling regime is strongly correlated with the yield of free charges measured via their intensity-dependent bimolecular recombination dynamics at room temperature. We therefore conclude that populations of free charges are established via long-range charge separation within the thermalization time scale, thus invoking early branching between free and bound charges across an energetic barrier. Subject to assumed values of the electron tunneling attenuation constant, we estimate critical charge separation distances of ∼3-4 nm in all materials. In some blends, large fullerene crystals can enhance charge separation yields; however, the important role of the polymers is also highlighted in blends that achieved significant charge separation with minimal fullerene concentration. We expect that our approach of isolating the intrinsic properties of primary charge pairs will be of considerable value in guiding new material development and testing the validity of proposed mechanisms for long-range charge separation.