Organic Donor-acceptor Research Articles

A highly emissive AIE-active luminophore exhibiting deep-red to near-infrared piezochromism and high-quality lasing.

Further development of high-efficiency and low-cost organic fluorescent materials is intrinsically hampered by the energy gap law and spin statistics, especially in the near-infrared (NIR) region. Here we design a novel building block with aggregation-induced emission (AIE) activity for realizing highly efficient luminophores covering the deep-red and NIR region, which originates from an increase in the orbital overlap and electron-withdrawing ability. An organic donor–acceptor molecule (BPMT) with the building block is prepared and can readily form J-type molecular columns with multiple C–H⋯N/O interactions. Notably, such synthesized materials can emit fluorescence centered at 701 nm with extremely high photoluminescence quantum yields (PLQYs) of 48.7%. Experimental and theoretical investigations reveal that the formation of the hybridized local and charge-transfer (HLCT) state and substantial C–H⋯N/O interactions contribute to a fast radiative decay rate and a slow nonradiative decay rate, respectively, resulting in high PLQYs in the solid state covering the NIR range. Remarkably, such BPMT crystals, as a first example, reveal strong-penetrability piezochromism along with a distinct PL change from the deep-red (λmax = 704 nm) to NIR (λmax = 821 nm) region. Moreover, such typical AIE-active luminophores are demonstrated to be a good candidate as a lasing medium. Together with epoxy resin by a self-assembly method, a microlaser is successfully illustrated with a lasing wavelength of 735.2 nm at a threshold of 22.3 kW cm−2. These results provide a promising approach to extend the contents of deep-red/NIR luminophores and open a new avenue to enable applications ranging from chemical sensing to lasing.

One-Dimensional Fullerene/Porphyrin Cocrystals: Near-Infrared Light Sensing through Component Interactions.

Recently, organic donor-acceptor (D-A) cocrystals have attracted special interest as functional materials because of their unique chemical and physical properties that are not exhibited by simple mixtures of their components. Herein, we report the preparation of one-dimensional novel D-A cocrystals from C60 and 5,10,15,20-tetrakis(3,5-dimethoxyphenyl)porphyrin (3,5-TPP); these cocrystals have near-infrared (NIR) light-sensing abilities, despite each of their component molecule individually having no NIR light-sensing properties. Micrometer-sized rectangular columnar C60-3,5-TPP cocrystals were produced by a simple liquid-liquid interfacial precipitation method. The cocrystals exhibit a new strong transition in the NIR region indicative of the existence of charge-transfer interactions between C60 and 3,5-TPP in the cocrystals. The C60-3,5-TPP cocrystals showed n-type transport characteristics with NIR light-sensing properties when the cocrystals were incorporated in bottom-gate/bottom-contact organic phototransistors, revealing that organic cocrystals with suitable charge-transfer interaction are useful as functional materials for the creation of novel NIR-light-sensing devices.

The Cost of Converting Excitons into Free Charge Carriers in Organic Solar Cells.

Efficient exciton dissociation and subsequent generation of free charge carriers at the organic donor-acceptor interface requires a number of electron-transfer processes. It is a common view that these steps result in an unavoidable energy loss in organic photovoltaic devices that is not present in other types of solar cells. The currently best performing organic solar cells with power conversion efficiencies over 16% challenge this view, and no interfacial charge-transfer states with energy significantly lower than the strongly absorbing singlet states are detected within the gap of the used donor and acceptor materials. This Perspective will discuss implications, the remaining sources of energy loss, and open questions to be solved to achieve power conversion efficiencies over 20%.

Determining Ultralow Absorption Coefficients of Organic Semiconductors from the Sub‐Bandgap Photovoltaic External Quantum Efficiency

AbstractEnergy states below the bandgap of a semiconductor, such as trap states or charge transfer states in organic donor–acceptor blends, can contribute to light absorption. Due to their low number density or ultrasmall absorption cross‐section, the absorption coefficient of these states is challenging to measure using conventional transmission/reflection spectrophotometry. As an alternative, the external quantum efficiency (EQE) of photovoltaic devices is often used as a representative of the absorption coefficient, where the spectral line shape of the EQE is considered to follow the absorption coefficient of the active layer material. Here, it is shown that the sub‐bandgap EQE is subject to thickness dependent low finesse cavity interference effects within the device—making this assumption questionable. A better estimate for the absorption coefficient is obtained when EQE spectra corresponding to different active layer thicknesses are fitted simultaneously for one attenuation coefficient using an iterative transfer matrix method. The principle is demonstrated for two model acceptor–donor systems (PCE12‐ITIC and PBTTT‐PC71BM) and accurate subgap absorption coefficients are determined. This approach has particular relevance for both understanding subgap states and their utilization in organic optoelectronic devices.

Coherent Real-Space Charge Transport Across a Donor-Acceptor Interface Mediated by Vibronic Couplings.

There is growing experimental and theoretical evidence that vibronic couplings, couplings between electronic and nuclear degrees of freedom, play a fundamental role in ultrafast excited-state dynamics in organic donor-acceptor hybrids. Whereas vibronic coupling has been shown to support charge separation at donor-acceptor interfaces, so far, little is known about its role in the real-space transport of charges in such systems. Here we theoretically study charge transport in thiophene:fullerene stacks using time-dependent density functional tight-binding theory combined with Ehrenfest molecular dynamics for open systems. Our results reveal coherent oscillations of the charge density between neighboring donor sites, persisting for ∼200 fs and promoting charge transport within the polymer stacks. At the donor-acceptor interface, vibronic wave packets are launched, propagating coherently over distances of more than 3 nm into the acceptor region. This supports previous experimental observations of long-range ballistic charge-carrier motion in organic photovoltaic systems and highlights the importance of vibronic coupling engineering as a concept for tailoring the functionality of hybrid organic devices.

Conjugated Polymers Containing Building Blocks 1,3,4,6-Tetraarylpyrrolo[3,2-b]pyrrole-2,5-dione (isoDPP), Benzodipyrrolidone (BDP) or Naphthodipyrrolidone (NDP): A Review.

π-Conjugated organic donor–acceptor (D–A) type polymers are widely developed and used in electronic device. Among which, diketopyrrolopyrrole (DPP)-based polymers have received the most attention due to their high performances. The novel chromophores named 1,3,4,6-tetraarylpyrrolo[3,2-b]pyrrole-2,5-dione (isoDPP), benzodipyrrolidone (BDP) and naphthodipyrrolidone (NDP) are resemble DPP in chemical structure. IsoDPP is an isomer of DPP, with the switching position of carbonyl and amide units. The cores of BDP and NDP are tri- and tetracyclic, whereas isoDPP is bicyclic. π-Conjugation extension could result polymers with distinct optical, electrochemical and device performance. It is expected that the polymers containing these high-performance electron-deficient pigments are potential in the electronic device applications, and have the potential to be better than the DPP-based ones. IsoDPP, BDP, and NDP based polymers are synthesized since 2011, and have not receive desirable attention. In this work, the synthesis, properties (optical and electrochemical characteristics), electronic device as well as their relationship depending on core-extension or structure subtle optimization have been reviewed. The final goal is to outline a theoretical scaffold for the design the D–A type conjugated polymers, which is potential for high-performance electronic devices.

Concurrent Optimization of Organic Donor–Acceptor Pairs through Machine Learning

AbstractIn this work an instance of the general problem occurring when optimizing multicomponent materials is treated: can components be optimized separately or the optimization should occur simultaneously? This problem is investigated from a computational perspective in the domain of donor–acceptor pairs for organic photovoltaics, since most experimental research reports optimization of each component separately. A collection of organic donors and acceptors recently analyzed is used to train nonlinear machine learning models of different families to predict the power conversion efficiency of donor–acceptor pairs, considering computed electronic and structural parameters of both components. The trained models are then used to predict photovoltaic performance for donor–acceptor combinations for which experimental data are not available in the data set. Data structure, and the usefulness of the trained models are critically assessed by predicting some donor–acceptor pairs that recently appeared in the literature, and the best combinations are proposed as worth investigating experimentally.

Superexchange Induced Charge Transport in Organic Donor–Acceptor Cocrystals and Copolymers: A Theoretical Perspective

Organic donor–acceptor (D–A) blending systems have displayed great potential in the application of organic optoelectronics. Conventionally, charge transport is not allowed along the stacking direct...

Effect of molecular packing on modulation of electronic properties of organic donor–acceptor hybrid gels

Here we report gelation properties and molecular packing of phenyl alanine conjugated perylenebisimide (PPA, P) in dimethyl formamide (DMF) and tetrahydrofuran (THF). In both solvents, PPA self-assembles to crystalline nanofibers and exhibits solvent dependent packing polymorphism of PPA molecule. The gels reveal almost comparable optoelectronic behavior in solutions, but xerogels prepared from P3TAA-PPA hybrid(PP), (a donor-acceptor system) in THF exhibits four times higher conductivity than that in DMF and 12 times higher conductivity in former than the later on illumination with light of 1 sun. Spectroscopy, microscopy, X-ray scattering and electrical measurements suggest that the robust crystalline packing of PPA in THF compare to DMF results in better percolation of charges. Due to different packing arrangement, PP-THF hybrid xerogel exhibits negative differential resistance whereas PP-DMF xerogel acts as electronic memory.The impedance results indicate lower charge transfer resistance and higher capacitance in P-THF than in P-DMF xerogel for the difference of molecular packing and this is also greatly reflected in hybrid xerogels. This study proves that although different packing of PBI may have small effect in optical properties and conductivity of pristine assemblies, but such packing differences are critical in determining the efficiency of charge transport when incorporated into donor-acceptor composites.

Highly Efficient Thermally Activated Delayed Fluorescence via J-Aggregates with Strong Intermolecular Charge Transfer.

The development of high-efficiency and low-cost organic emissive materials and devices is intrinsically limited by the energy-gap law and spin statistics, especially in the near-infrared (NIR) region. A novel design strategy is reported for realizing highly efficient thermally activated delayed fluorescence (TADF) materials via J-aggregates with strong intermolecular charge transfer (CT). Two organic donor-acceptor molecules with strong and planar acceptor are designed and synthesized, which can readily form J-aggregates with strong intermolecular CT in solid states and exhibit wide-tuning emissions from yellow to NIR. Experimental and theoretical investigations expose that the formation of such J-aggregates mixes Frenkel excitons and CT excitons, which not only contributes to a fast radiative decay rate and a slow nonradiative decay rate for achieving nearly unity photoluminescence efficiency in solid films, but significantly decreases the energy gap between the lowest singlet and triplet excited states (≈0.3 eV) to induce high-efficiency TADF even in the NIR region. These organic light-emitting diodes exhibit external quantum efficiencies of 15.8% for red emission and 14.1% for NIR emission, which represent the best result for NIR organic light-emitting diodes (OLEDs) based on TADF materials. These findings open a new avenue for the development of high-efficiency organic emissive materials and devices based on molecular aggregates.

Effect of Charge-Transfer State Energy on Charge Generation Efficiency via Singlet Fission in Pentacene-Fullerene Solar Cells.

Singlet fission in pentacene creates two triplet excitons per absorbed photon. In a solar cell, each triplet can generate an electron–hole pair, and hence, external quantum efficiencies exceeding 100% have been reported for pentacene–fullerene solar cells. The energetics of this process are intriguing because the minimum photon energy loss, defined as the energy difference between the (triplet) exciton state and the open-circuit voltage, is less than 0.5 eV and distinctively smaller than that in most organic donor–acceptor solar cells. To investigate the energetics of this process, we analyze the effect of the energy of the lowest unoccupied molecular orbital (LUMO) for different fullerene derivatives. With the LUMO energy becoming less negative, the open-circuit voltage increases and charge generation decreases. For all but one of the fullerenes tested, the charge-transfer state energy is distinctively higher than the pentacene triplet energy, revealing that charge generation via singlet fission is actually endergonic. An elementary Marcus model for the rate of electron transfer provides a qualitative description of the experimental trends, in accordance with an endergonic charge transfer. Considering that charge generation from triplet states is endergonic, involvement of pentacene singlet states, either from direct photoexcitation or via triplet–triplet annihilation, cannot be excluded.

Organic LEDs and solar cells united.

Organic donor–acceptor heterojunctions can show efficient electroluminescence and at the same time generate charges under photovoltaic operation.

An Organic Emitter Displaying Dual Emissions and Efficient Delayed Fluorescence White OLEDs

AbstractThe design and synthesis of one‐compound‐based white light organic solids that can be employed to fabricate efficient organic white light‐emitting devices (OLEDs) still remains a challenge. In this contribution, a classic organic donor–acceptor (D‐A) type molecule (PTZ‐BP) is presented, which is composed of phenothiazine (PTZ) as an electron donor and benzophenone (BP) as an electron acceptor. Intriguingly, PTZ‐BP displays dual emission and thermally activated delayed fluorescence properties. Its dual emission originates from locally excited state and intramolecular charge transfer excited state, respectively. PTZ‐BP‐doped thin film is used to fabricate efficient white light OLEDs with external quantum efficiency of 6.2%, which is among the highest values reported for the single‐emitter‐based small molecules white fluorescence OLEDs with similar Commission Internationale de L' Eclairage (CIE) coordinates so far.

Organic Chiral Charge Transfer Magnets.

In this work, through designing organic helix donor-acceptor complexes, one type of room-temperature chiral magnet was reported. Within these chiral charge transfer magnets, circularly polarized light could induce a larger saturation magnetization compared to linearly polarized light illumination with identical intensity. Moreover, the transmission light polarization from chiral magnets could be tuned via applying the magnetic field. Overall, room-temperature organic chiral magnets with optomagnetic effects will enhance the function of organic magnetochiral materials.

Spectroscopic Investigation of the Effect of Microstructure and Energetic Offset on the Nature of Interfacial Charge Transfer States in Polymer: Fullerene Blends.

Despite performance improvements of organic photovoltaics, the mechanism of photoinduced electron–hole separation at organic donor–acceptor interfaces remains poorly understood. Inconclusive experimental and theoretical results have produced contradictory models for electron–hole separation in which the role of interfacial charge-transfer (CT) states is unclear, with one model identifying them as limiting separation and another as readily dissociating. Here, polymer–fullerene blends with contrasting photocurrent properties and enthalpic offsets driving separation were studied. By modifying composition, film structures were varied from consisting of molecularly mixed polymer–fullerene domains to consisting of both molecularly mixed and fullerene domains. Transient absorption spectroscopy revealed that CT state dissociation generating separated electron–hole pairs is only efficient in the high energy offset blend with fullerene domains. In all other blends (with low offset or predominantly molecularly mixed domains), nanosecond geminate electron–hole recombination is observed revealing the importance of spatially localized electron–hole pairs (bound CT states) in the electron–hole dynamics. A two-dimensional lattice exciton model was used to simulate the excited state spectrum of a model system as a function of microstructure and energy offset. The results could reproduce the main features of experimental electroluminescence spectra indicating that electron–hole pairs become less bound and more spatially separated upon increasing energy offset and fullerene domain density. Differences between electroluminescence and photoluminescence spectra could be explained by CT photoluminescence being dominated by more-bound states, reflecting geminate recombination processes, while CT electroluminescence preferentially probes less-bound CT states that escape geminate recombination. These results suggest that apparently contradictory studies on electron–hole separation can be explained by the presence of both bound and unbound CT states in the same film, as a result of a range of interface structures.

Supramolecular Design of Donor-Acceptor Complexes via Heteroatom Replacement toward Structure and Electrical Transporting Property Tailoring.

A feasible strategy relies on using heteroatom replacement, namely, chemical modification of an organic compound. Here we present this design concept for donor-acceptor complexes, which involves introducing nitrogen atoms to the middle ring of donor molecules to promote short contacts and reduce steric effect of the mixed framework. These nitrogen-modified complexes can possess a shorter molecular distance besides the mixed-stacking pathway, enlarged π-π interactions, or even a scarce 1:2.5 molar ratio through extra acceptor insertion. As a result, the unique 1:2 complex with nitrogen atoms on the different sides demonstrated stable electron field-effect mobility performance, whereas the binary system with no nitrogen replacement or N atoms on the identical sides displayed poor ambipolar properties. These results confirmed that heteroatom replacement was a powerful molecular design tool to fine-tune the molecular packing of organic donor-acceptor complexes and their corresponding electronic properties.

Effect of Electron-Nuclear Hyperfine Interactions on Multiple-Quantum Coherences in Photogenerated Covalent Radical (Qubit) Pairs.

Ultrafast photodriven electron transfer reactions starting from an excited singlet state in an organic donor-acceptor molecule can generate a spin-correlated radical pair (RP) with an initially entangled spin state that may prove useful as a two-qubit pair in quantum information protocols. Here we investigate the effects of modulating electron-nuclear hyperfine coupling by rapidly transferring an electron between two equivalent sites comprising the reduced acceptor of the RP. A covalent electron donor-acceptor molecule including a tetrathiafulvalene (TTF) donor, a 4-aminonaphthalene-1,8-imide (ANI) chromophoric primary acceptor, and an m-xylene bridged cyclophane having two equivalent pyromellitimides (PI2), TTF-ANI-PI2, as a secondary acceptor was synthesized along with an analogous molecule having one pyromellitimide (PI) acceptor, TTF-ANI-PI. Photoexcitation of ANI within each molecule results in sub-nanosecond formation of TTF+•-ANI-PI-• and TTF+•-ANI-PI2-•. The effect of reducing electron-nuclear hyperfine interactions in TTF+•-ANI-PI2-• relative to TTF+•-ANI-PI-• on decoherence of multiple-quantum coherences has been measured by pulse-EPR spectroscopy. This contribution is especially relevant in the absence of modulation of exchange or dipolar interactions, as with the RP at a fixed distance in the molecules in this work. The theoretical prediction of the contribution from an ensemble of hyperfine interactions to decoherence in these RPs is shown to be less than the full width at half-maximum of the quantum beat frequencies measured experimentally. Pulse bandwidth and off-resonant excitation by square microwave pulses are proposed as larger contributors to decoherence in these molecules than the hyperfine interactions, and specific pulse shapes relevant to arbitrary waveform generation are introduced.

Functional Conjugated Polymers for CO2 Reduction Using Visible Light.

The reduction of CO2 with visible light is a highly sustainable method for producing valuable chemicals. The function-led design of organic conjugated semiconductors with more chemical variety than that of inorganic semiconductors has emerged as a method for achieving carbon photofixation chemistry. Here, we report the molecular engineering of triazine-based conjugated microporous polymers to capture, activate and reduce CO2 to CO with visible light. The optical band gap of the CMPs is engineered by varying the organic electron-withdrawing (benzothiadiazole) and electron-donating units (thiophene) on the skeleton of the triazine rings while creating organic donor-acceptor junctions to promote the charge separation. This engineering also provides control of the texture, surface functionality and redox potentials of CMPs for achieving the light-induced conversion of CO2 to CO ambient conditions.

Revealing the Importance of Energetic and Entropic Contributions to the Driving Force for Charge Photogeneration.

Despite significant recent progress, much about the mechanism for charge photogeneration in organic photovoltaics remains unknown. Here, we use conjugated block copolymers as model systems to examine the effects of energetic and entropic driving forces in organic donor-acceptor materials. The block copolymers are designed such that an electron donor block and an electron acceptor block are covalently linked, embedding a donor-acceptor interface within the molecular structure. This enables model studies in solution where processes occurring between one donor and one acceptor are examined. First, energy levels and dielectric constantsthat governthe driving force for charge transfer are systematically tuned and charge transfer within individual block copolymer chains is quantified. Results indicate that in isolated chains, a significant driving force of ∼0.3 eV is necessary to facilitate significant exciton dissociation to charge-transfer states. Next, block copolymers are cast into films, allowing for intermolecular interactions and charge delocalization over multiple chains. In the solid state, charge transfer is significantly enhanced relative to isolated block copolymer chains. Using Marcus Theory, we concludethat changes in the energetic driving force alone cannot explain the increased efficiency of exciton dissociation to charge-transfer states in the solid state. This implies that increasing the number of accessible states for charge transfer introduces an entropic driving force that can play an important role in the charge-generation mechanism of organic materials, particularly in systems where the excited state energy level is close to that of the charge-transfer state.

Rational Construction of Highly Tunable Donor–Acceptor Materials Based on a Crystalline Host–Guest Platform

Organic donor-acceptor systems have attracted much attention due to their various potential applications. However, the rational construction and modulation of highly ordered donor-acceptor systems could be a challenge due to the complicated self-assembly process of donor and acceptor species. Considering the well-defined arrangement of species at the molecule level, a crystalline host-guest system could be an ideal platform for the rational construction of donor-acceptor systems. Herein, it is shown how the rational construction of highly tunable donor-acceptor materials can be achieved based on a crystalline host-guest platform. Within the well-established metal-organic framework NKU-111 as the crystalline host enabled by the relatively stable coordination-directed assembly, the introduction and arrangement of guest molecules in the crystals allow the rational construction of the NKU-111⊃guest donor-acceptor system. The donor-acceptor interaction in the systems can be readily modulated with different guest molecules, which can be justified by the well-demonstrated guest-dependent characteristics. Accordingly, the NKU-111⊃guest reveals highly tunable donor-acceptor properties such as charge-transfer-based emissions and electrical conductivity. This work indicates the potential of crystalline host-guest systems as an ideal platform for systematic investigations of donor-acceptor materials.