Activated Singlet Fission Dictated by Anti-Kasha Property in a Rylene Imide Dye.

A key challenge in the search of new materials capable of singlet fission (SF) arises from the primary energy conservation criterion, i.e., the energy of the triplet exciton has to be half that of the singlet (E(S1) ≥ 2E(T1)), which excludes most photostable organic materials from consideration and confines the design strategy to materials with low energy triplet states. One potential way to overcome this energy requirement and improve the triplet energy is to enable a SF channel from higher energy ("hot") excitonic states (Sn) in a process called activated SF. Herein, we demonstrate that efficient activated SF is achieved in a rylene imide-based derivative acenaphth[l, 2-a]acenaphthylene diimide (AADI). This process is enabled by an increase in the energy gap to greater than 1.0 eV between the S3 and S1 states due to the incorporation of an antiaromatic pentalene unit, which leads to the emergence of anti-Kasha properties in the isolated molecule. Transient spectroscopy studies show that AADI undergoes ultrafast SF from higher singlet excited states in thin film, with excitation wavelength-dependent SF yields. The SF yield of ∼200% is observed upon higher energy excitation, and long-lived free triplets persist on the µs time scale suggesting that AADI can be used in SF-enhanced devices. Our results suggest that enlarging the Sn-S1 energy gap is an effective way to turn on the activated SF channel and shed light on the development of novel, stable SF materials with high triplet energies.

The impact of molecular orientations on the energy levels of A-D-A acceptors: implications for the charge separation driving force of organic solar cells.

The energetic landscape of charge carriers, namely the ionization potential (IP) and electron affinity (EA), can play a crucial role in the charge separation and migration processes for organic solar cells (OSCs). However, the impact of molecular orientations on the energy levels remains elusive, especially in acceptor-donor-acceptor (A-D-A) type nonfullerene acceptors (NFAs) with intrinsic anisotropy. Using the self-consistent quantum mechanics/embedded charge (sc-QM/EC) approach, we have investigated the energy level shifts from the edge-on or face-on surfaces to the bulk phase for three typical NFA crystals, IDIC-4F, INIC-4F, and Y6. The results point out that the surface-to-bulk changes in IP are limited within 0.2 eV for both the orientations due to the mutual counteraction between the electrostatic and induction effects. In sharp contrast, the EA values are substantially decreased from the bulk to the surfaces; especially, for the face-on orientation, the reduction reaches 0.5-0.8 eV. This indicates that the face-on orientation can provide a significant driving force for electrons moving from the surface or the interface to the bulk phase and thus improve the charge separation efficiency. Our work indicates that enhancing the face-on orientation is an effective method to increase the charge separation driving force for the OSCs based on A-D-A NFAs.

Intermolecular donor-acceptor stacking to suppress triplet exciton diffusion for long-persistent organic room-temperature phosphorescence.

Controlling triplet states is crucial to improve the efficiency and lifetime of organic room temperature phosphorescence (ORTP). Although the intrinsic factors from intramolecular radiative and non-radiative decay have been intensively investigated, the extrinsic factors that affect triplet exciton quenching are rarely reported. Diffusion to the defect sites inside the crystal or at the crystal surface may bring about quenching of triplet exciton. Here, the phosphorescence lifetime is found to have a negative correlation with the triplet exciton diffusion coefficient based on the density functional theory (DFT)/time-dependent density functional theory (TD-DFT) calculations on a series of ORTP materials. For systems with a weak charge transfer (CT) characteristic, close π-π stacking will lead to strong triplet coupling and fast triplet exciton diffusion in most cases, which is detrimental to the phosphorescence lifetime. Notably, for intramolcular donor-acceptor (D-A) type systems with a CT characteristic, intermolecular D-A stacking results in ultra-small triplet coupling, thus contributing to slow triplet diffusion and long phosphorescence lifetime. These findings shed some light on molecular design toward high-efficiency long persistent ORTP.

Nonfullerene Acceptor Featuring Unique Self-Regulation Effect for Organic Solar Cells with 19 % Efficiency.

The blend nanomorphology of electron-donor (D) and -acceptor (A) materials is of vital importance to achieving highly efficient organic solar cells. Exogenous additives especially aromatic additives are always needed to further optimize the nanomorphology of blend films, which is hardly compatible with industrial manufacture. Herein, we proposed a unique approach to meticulously modulate the aggregation behavior of NFAs in both crystal and thin film nanomorphology via self-regulation effect. Nonfullerene acceptor Z9 was designed and synthesized by tethering phenyl groups on the inner side chains of the Y6 backbone. Compared with Y6, the tethered phenyl groups participated in the molecular aggregation via the π-π stacking of phenyl-phenyl and phenyl-2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC-2F) groups, which induced 3D charge transport with phenyl-mediated super-exchange electron coupling. Moreover, ordered molecular packing with suitable phase separation was observed in Z9-based blend films. High power conversion efficiencies (PCEs) of 19.0 % (certified PCE of 18.6 %) for Z9-based devices were achieved without additives, indicating the great potential of the self-regulation strategy in NFA design.

Dimeric Giant Molecule Acceptors Featuring N-type Linker: Enhancing Intramolecular Coupling for High-Performance Polymer Solar Cells.

Giant molecular acceptors (GMAs) are typically designed through the conjugated linking of individual small molecule acceptors (SMAs). This design imparts an extended molecular size, elevating the glass transition temperature (Tg) relative to their SMA counterparts. Consequently, it effectively suppresses the thermodynamic relaxation of the acceptor component when blended with polymer donors to construct stable polymer solar cells (PSCs). Despite their merits, the optimization of their chemical structure for further enhancing of device performance remains challenge. Different from previous reports utilizing p-type linkers, here, we explore an n-type linker, specifically the benzothiadiazole unit, to dimerize the SMA units via a click-like Knoevenagel condensation, affording BT-DL. In comparison with B-DL with a benzene linkage, BT-DL exhibits significantly stronger intramolecular super-exchange coupling, a desirable property for the acceptor component. Furthermore, BT-DL demonstrates a higher film absorption coefficient, redshifted absorption, larger crystalline coherence, and higher electron mobility. These inherent advantages of BT-DL translate into a higher power conversion efficiency of 18.49 % in PSCs, a substantial improvement over the 9.17 % efficiency observed in corresponding devices with B-DL as the acceptor. Notably, the BT-DL based device exhibits exceptional stability, retaining over 90 % of its initial efficiency even after enduring 1000 hours of thermal stress at 90 °C. This work provides a cost-effective approach to the synthesis of n-type linker-dimerized GMAs, and highlight their potential advantage in enhancing intramolecular coupling for more efficient and durable photovoltaic technologies.

Tethered Trimeric Small-molecular Acceptors through Aromatic-core Engineering for Highly Efficient and Thermally Stable Polymer Solar Cells.

Polymer solar cells (PSCs) rely on a blend of small molecular acceptors (SMAs) with polymer donors, where thermodynamic relaxation of SMAs poses critical concerns on operational stability. To tackle this issue, tethered SMAs, wherein multiple SMA-subunits are connected to the aromatic-core via flexible chains, are proposed. This design aims to an elevated glass transition temperature (Tg) for a dynamical control. However, attaining an elevated Tg value with additional SMA subunits introduces complexity to the molecular packing, posing a significant challenge in realizing both high stability and power conversion efficiency (PCE). In this study, we initiate isomer engineering on the benzene-carboxylate core and find that meta-positioned dimeric BDY-ß exhibits more favorable molecular packing compared to its para-positioned counterpart, BDY-α. With this encouraging result, we expand our approach by introducing an additional SMA unit onto the aromatic core of BDY-ß, maintaining a meta-position relative to each SMA unit location in the tethered acceptor. This systematic aromatic-core engineering results in a star-shaped C3h-positioned molecular geometry. The supramolecular interactions of SMA units in the trimer contribute to enhancements in Tg value, crystallinity, and a red-shifted absorption compared to dimers. These characteristics result in a noteworthy increase in PCE to 18.24 %, coupled with a remarkable short-circuit current density of 27.06 mA cm-2. More significantly, the trimer-based devices delivered an excellent thermal stability with over 95 % of their initial efficiency after 1200 h thermal degradation. Our findings underscore the promise and feasibility of tethered trimeric structures in achieving highly ordered aggregation behavior and increased Tg value in PSCs, simultaneously improving in device efficiency and thermal stability.

Crystal-Liquid-Glass Transition and Near-Unity Photoluminescence Quantum Yield in Low Melting Point Hybrid Metal Halides.

Hybrid metal halides (HMHs) are a class of materials that combine extraordinary photophysical properties and excellent processability. Their chemical variability allows for the solid-liquid transition toward melt-processable HMHs. Herein, we report the design and synthesis of zero-dimensional HMHs [M(DMSO)6][SbCl6], where the isolated octahedra of [M(DMSO)6]3+ and [SbCl6]3- are alternatively aligned in the crystal structure. The luminescent center of [SbCl6]3- enables the photogeneration of self-trapped excitons, resulting in broadband photoluminescence with a large Stokes shift and a nearly 100% quantum yield. Meanwhile, the release of DMSO ligands from [M(DMSO)6]3+ is controlled by the M-O coordination and thus a low melting point of ∼90 °C is achieved for HMHs. Interestingly, the glass phase is obtained by melt quenching, with a sharp change in photoluminescence colors compared to the crystal phase of melt-processable HMHs. The robust crystal-liquid-glass transition opens a new avenue to tailoring structural disorder and optoelectronic performance in organic-inorganic materials.

Humidity-Controlled Molecular Assembly and Photoisomerization Behavior with a Bubble-Assisted Patterning Approach.

Precise control of molecular assembly is of great significance in the application of functional molecules. This work has systematically investigated the humidity effect in bubble-assisted molecular assembly. This work finds humidity is critical in the evolution of the soft confined space, leading to the formation of microscale liquid confined space under high humidity, and nanoscale liquid confined space under low humidity. It is also revealed that the differences in surface wettability and adhesion play the key role. Consequently, a flat pattern with thermodynamically favorable ordered structure and a sharp pattern with dynamically favorable disordered structure are achieved, which show different solid-state photoisomerization behaviors and photoresponsiveness. Interestingly, conductivity of sharp pattern with disordered structure is higher than that of flat pattern with layered ordered structure due to electronic transport mechanism of different spatial dimensions. This work opens a new way for manipulating the molecular self-assembly to control the morphology and function of molecular patterns.

Molecular Insight into Efficient Charge Generation in Low-Driving-Force Nonfullerene Organic Solar Cells.

ConspectusFor organic solar cells (OSCs), charge generation at the donor/acceptor interfaces is regarded as a two-step process: driven by the interfacial energy offsets, the excitons produced by light absorption are first dissociated into the charge-transfer (CT) states, and then the CT states are further separated into free charge carriers of holes and electrons by overcoming their Coulomb attraction. Meanwhile, the CT states can recombine through radiative and nonradiative decay. Owing to the emergence of narrow-band-gap A-D-A small-molecule acceptors, nonfullerene (NF) OSCs have developed rapidly in recent years and the power conversion efficiencies (PCEs) surpass 18% now. The great achievement can be attributed to the high-yield charge generation under low exciton dissociation (ED) driving forces, which ensures both high photocurrent and small voltage loss. However, it is traditionally believed that a considerable driving force (e.g., at least 0.3 eV in fullerene-based OSCs) is essential to provide excess energy for the CT states to achieve efficient charge separation (CS). Therefore, a fundamental question open to the community is how the excitons split into free charge carriers so efficiently under low driving forces in the state-of-the-art NF OSCs.In this Account, we summarize our recent theoretical advances on the charge generation mechanisms in the low-driving-force NF OSCs. First, the A-D-A acceptors are found to dock with the D-A copolymer or A-D-A small-molecule donors mainly via local π-π interaction between their electron-withdrawing units, and such interfacial geometries can provide sufficient electronic couplings, thus ensuring fast ED. Second, the polarization energies of holes and electrons are enhanced during CS, which is beneficial to reduce the CS energy barrier and even leads to barrierless CS in the OSCs based on fluorinated A-D-A acceptors. Moreover, the exciton binding energies (Eb) are substantially decreased by the strong polarization of charge carriers for the A-D-A acceptors; especially for the Y6 system with three-dimensional molecular packing structures, the remarkable small Eb can enable direct photogeneration of free charge carriers. Accordingly, the excess energy becomes unnecessary for CS in the state-of-the-art NF OSCs. Third, to simultaneously decrease the driving force and suppress charge recombination via the triplet channel, it is imperative to reduce the singlet-triplet energy difference (ΔEST) of the narrow-band-gap A-D-A acceptors. Importantly, the intermolecular end-group π-π stacking is demonstrated to effectively decrease the ΔEST while keeping strong light absorption. Finally, hybridization of the CT states with local excitation can be induced by small interfacial energy offset. Such hybridization will result in direct population of thermalized CT states upon light absorption and a significant increase of luminescence quantum efficiency, which is beneficial to concurrently promote CS and reduce nonradiative voltage loss. We hope this Account contributes to the molecular understanding of the mechanisms of efficient charge generation with low driving forces and would be helpful for further improving the performance of organic photovoltaics in the future.

Molecular Design of A-D-A Electron Acceptors Towards Low Energy Loss for Organic Solar Cells.

Low energy loss is a prerequisite for organic solar cells to achieve high photovoltaic efficiency. Electron-vibration coupling (i. e., intramolecular reorganization energy) plays a crucial role in the photoelectrical conversion and energy loss processes. In this Concept article, we summarize our recent theoretical advances on revealing the energy loss mechanisms at the molecular level of A-D-A electron acceptors. We underline the importance of electron-vibration couplings on reducing the energy loss and describe the effective molecular design strategies towards low energy loss through decreasing the electron-vibration couplings.

Uniform Colloidal Polymer Rods by Stabilizer-Assisted Liquid-Crystallization-Driven Self-Assembly.

The synthesis of anisotropic colloidal building blocks is essential for their self-assembly into hierarchical materials. Here, a highly efficient stabilizer-assisted liquid-crystallization-driven self-assembly (SA-LCDSA) strategy was developed to achieve monodisperse colloidal polymer rods. This strategy does not require the use of block copolymers, but only homopolymers or random copolymers. The resulting rods have tunable size and aspect ratios, as well as well-defined columnar liquid crystal structures. The integrated triphenylene units enable the rods to exhibit unusual photo-induced fluorescence enhancement and accompanying irradiation memory effect, which, as demonstrated, are attractive for information encryption/decryption of paper documents. In particular, unwanted document decryption during delivery can be examined by fluorescence kinetics. This SA-LCDSA-based approach can be extended to synthesize other functional particles with desired π-molecular units.

Diazulenorubicene as a Non-benzenoid Isomer of peri-Tetracene with Two Sets of 5/7/5 Membered Rings Showing Good Semiconducting Properties.

Non-benzenoid polycyclic aromatic hydrocarbons (PAHs) have received a lot of attention because of their unique optical, electronic, and magnetic properties, but their synthesis remains challenging. Herein, we report a non-benzenoid isomer of peri-tetracene, diazulenorubicene (DAR), with two sets of 5/7/5 membered rings synthesized by a (3+2) annulation reaction. Compared with the precursor containing only 5/7 membered rings, the newly formed five membered rings switch the aromaticity of the original heptagon/pentagon from antiaromatic/aromatic to non-aromatic/antiaromatic respectively, modify the intermolecular packing modes, and lower the LUMO levels. Notably, compound 2 b (DAR-TMS) shows p-type semiconducting properties with a hole mobility up to 1.27 cm2  V-1 s-1 . Moreover, further extension to larger non-benzenoid PAHs with 19 rings was achieved through on-surface chemistry from the DAR derivative with one alkynyl group.

Singlet-Triplet Energy Gap as a Critical Molecular Descriptor for Predicting Organic Photovoltaic Efficiency.

In contrast to the inorganic and perovskite solar cells, organic photovoltaics (OPV) depend on a series of charge generation and recombination processes, which complicates molecular design to improve the power conversion efficiencies (PCEs). Herein, we first propose the singlet-triplet energy gap (ΔEST ) as a critical molecular descriptor for predicting the PCE considering that minimizing ΔEST is beneficial to simultaneously reduce voltage loss and triplet recombination. Remarkably, the results from data-driven machine learning verify that the prediction accuracy of the ΔEST (Pearson's correlation coefficient r=0.72) is apparently superior to that of two commonly used molecular descriptors in OPV, i.e., the optical gap (r=0.65) and the driving force (r=0.53). Moreover, an impressive prediction accuracy of r=0.81 is achieved just by combining the three descriptors. This work paves the way toward rapid and precise screening of efficient OPV materials.

The Intrinsic Role of the Fusion Mode and Electron-Deficient Core in Fused-Ring Electron Acceptors for Organic Photovoltaics.

The A-DA'D-A fused-ring electron acceptors with an angular fusion mode and electron-deficient core has significantly boosted organic photovoltaic efficiency. Here, the intrinsic role of the peculiar structure is revealed by comparing representative A-DA'D-A acceptor Y6 with its A-D-A counterparts having different fusion modes. Owing to the more delocalized HOMO and deeper LUMO level, Y6 exhibits stronger and red-shifted absorption relative to the linear and angular fused A-D-A acceptors, respectively. Moreover, the change from linear to angular fusion substantially reduces the electron-vibration couplings, which is responsible for the faster exciton diffusion, exciton dissociation, and electron transport for Y6 than the linear fused A-D-A acceptor. Notably, the electron-vibration coupling for exciton dissociation is further decreased by introducing the electron-deficient core, thus contributing to the efficient charge generation under low driving forces in the Y6-based devices.

Non-Equal Ratio Cocrystal Engineering to Improve Charge Transport Characteristics of Organic Semiconductors: A Case Study on Indolo[2,3-a]carbazole.

Rare studies of cocrystal engineering have focused on improving carrier mobility of organic semiconductors mainly because of the generation of ambipolarity, the alteration of the charge carrier polarity or the reduction of electronic couplings. Herein, we utilize indolo[2,3-a]carbazole (IC) as the model compound and 2,6-diphenylanthraquinone (DPAO) and 9-fluorenone (FO) as the coformers to construct IC2-DPAO and IC-FO cocrystals with 2 : 1 or 1 : 1 ratios, respectively, through hydrogen bonds and donor-acceptor interactions. Interestingly, the more appropriate packing structure, possessing not only enhanced electronic couplings but also increased intermolecular distances, is achieved in IC2-DPAO, which shows an improved carrier mobility of 0.11 cm2  V-1 s-1 by four orders of magnitude relative to the IC crystal. These results suggest that non-equal ratio cocrystal engineering opens up the possibility to develop organic semiconductors with enhanced charge transport behaviors.

Hydrogen-Rich 2D Halide Perovskite Scintillators for Fast Neutron Radiography.

A fast neutron has strong penetration ability through dense and bulky objects, which makes it an ideal nondestructive technology for detecting voids, cracks, or other defects inside large equipment. However, the lack of effective fast neutron detection materials limits its application. Perovskites have shown excellent optical properties in many areas, but they are absent from fast neutron detection imaging because they cannot directly absorb fast neutrons and emit luminescence. Here, we demonstrate a hydrogen-rich long-chain organic amine modified two-dimensional (2D) perovskite fast neutron scintillator, Mn-(C18H37NH3)2PbBr4(Mn-STA2PbBr4). Its hydrogen density can reach 9.51 × 1028 m-3, and the photoluminescence quantum yield can reach 58.58%, so it is possible to integrate fast neutron absorption and luminescence into a single compound. More importantly, Mn-STA2PbBr4 can be made into a large-area self-supporting fast neutron scintillator plate with satisfactory spatial resolution (0.5 lp/mm (lp: line pairs)). This strategy provides a simple and promising choice for fast neutron scintillator nondestructive testing.

Visualization of Crystallographic Orientation and Twist Angles in Two-Dimensional Crystals with an Optical Microscope.

Recently, twisted two-dimensional (2D) bilayers have attracted intense interest due to the emergence of exotic physics in moiré superlattices arising from strong interactions between electrons in the two misaligned lattices. Accurate determination and control of crystallographic orientation is vital to construct twisted 2D bilayers. Here, we demonstrate a very simple method to visualize crystallographic orientation in various 2D crystals by directly imaging n-tritriacontane crystallites epitaxially grown on 2D crystals using an ordinary optical microscope. The specific orientation of the molecular assembly allows the lattice orientation of the underlying 2D crystals to be determined with a high precision using an optical microscope. When tested on a 2D bilayer comprising staggered stacked MoS2 crystals, the twist angle can also be determined accurately. Our findings offer a nondestructive method for determining the lattice structure of 2D crystals and their bilayers and can also be a metrology tool for establishing large-area surface crystalline order.

Small Exciton Binding Energies Enabling Direct Charge Photogeneration Towards Low-Driving-Force Organic Solar Cells.

Organic solar cells (OSCs) with nonfullerene acceptors (NFAs) exhibit efficient charge generation under small interfacial energy offsets, leading to over 18 % efficiency for the single-junction devices based on the state-of-the-art NFA of Y6. Herein, to reveal the underlying charge generation mechanisms, we have investigated the exciton binding energy (Eb ) in Y6 by a joint theoretical and experimental study. The results show that owing to strong charge polarization effects, Y6 has remarkable small Eb of -0.11-0.15 eV, which is even lower than perovskites in many cases. Moreover, it is peculiar that the photoluminescence is enhanced with temperature, and the energy barrier for separating excitons into charges is evidently lower than the thermal energy according to the temperature dependence of photoluminescence, manifesting direct photogeneration of charge carriers enabled by weak Eb in Y6. Thus, charge generation in NFA-based OSCs shows little dependence on interfacial driving forces.

New Synthetic Approaches to N-Aryl and π-Expanded Diketopyrrolopyrroles as New Building Blocks for Organic Optoelectronic Materials.

Diketopyrrolopyrrole (DPP) as a building block has been intensively investigated for organic semiconductors and light emitting materials. The synthesis of N-aryl DPPs remains challenging. Herein, we firstly report a new easily handled synthetic method toward N-aryl DPPs through H-DPP with diaryliodonium salt in the presence of CuI, which shows broad reaction scope. Sixteen N-aryl DPPs, including phenyl, furan and thiophene as flanking aromatic groups, were synthesized with yields up to 78 %. These N-aryl DPPs are fluorescent in both solutions and solid states, with quantum yields up to 96 % and 40 %, respectively. Moreover, we show that the reaction between H-DPP and diaryliodonium salt can lead to π-expanded DPPs by using Pd(OAc)2 as catalyst. Nine π-expanded DPPs were obtained in 27-61 % yields. These π-expanded DPPs exhibit good semiconducting properties with hole mobility of 0.71 cm2 V-1 s-1 , demonstrating that they are useful building blocks for high performance organic semiconductors.

Electrical Loss Management by Molecularly Manipulating Dopant-free Poly(3-hexylthiophene) towards 16.93 % CsPbI2 Br Solar Cells.

Inorganic cesium lead halide perovskites offer a pathway towards thermally stable photovoltaics. However, moisture-induced phase degradation restricts the application of hole transport layers (HTLs) with hygroscopic dopants. Dopant-free HTLs fail to realize efficient photovoltaics due to severe electrical loss. Herein, we developed an electrical loss management strategy by manipulating poly(3-hexylthiophene) with a small molecule, i.e., SMe-TATPyr. The developed P3HT/SMe-TATPyr HTL shows a three-time increase of carrier mobility owing to breaking the long-range ordering of "edge-on" P3HT and inducing the formation of "face-on" clusters, over 50 % decrease of the perovskite surface defect density, and a reduced voltage loss at the perovskite/HTL interface because of favorable energy level alignment. The CsPbI2 Br perovskite solar cell demonstrates a record-high efficiency of 16.93 % for dopant-free HTL, and superior moisture and thermal stability by maintaining 96 % efficiency at low-humidity condition (10-25 % R. H.) for 1500 hours and over 95 % efficiency after annealing at 85 °C for 1000 hours.