Extremely Stable Perylene Bisimide-Bridged Regioisomeric Diradicals and Their Redox Properties.

Excellent stability is an essential premise for organic diradicals to be used in organic electronic and spintronic devices. We have attached two tris(2,4,6-trichlorophenyl)methyl (TTM) radical building blocks to the two sides of perylene bisimide (PBI) bridges and obtained two regioisomeric diradicals (1,6-TTM-PBI and 1,7-TTM-PBI). Both of the isomers show super stability rather than the monomeric TTM under ambient conditions, due to the increased conjugation and the electron-withdrawing effects of the PBI bridges. The diradicals show distinct and reversible multistep redox processes, and a spectro-electrochemistry investigation revealed the generation of organic mixed-valence (MV) species during reduction processes. The two diradicals have singlet ground states, very small singlet-triplet energy gaps (ΔES-T ) and a pure open-shell character (with diradical character y0 =0.966 for 1,6-TTM-PBI and 0.967 for 1,7-TTM-PBI). This work opens a window to developing very stable diradicals and offers the opportunity of their further application in optical, electronic and magnetic devices.

3D-Heterojunction Based on Embedded Perovskite Micro-Sized Single Crystals for Fast Photomultiplier Photodetectors with Broad/Narrowband Dual-Mode.

A fast photomultiplier photodetector with a broad/narrowband dual mode is implemented using a new 3D heterostructure based on embedded perovskite micro-sized single crystals. Because the single-crystal size is smaller than the electrode size, the active layer can be divided into a perovskite microcrystalline part for charge transport and a polymer-embedded part for charge storage. This induces an additional radial interface in the 3D heterojunction structure, and allows a photogenerated built-in electric field in the radial direction, especially when the energy levels between the perovskite and embedding polymer are similar. This type of heterojunction has a small radial capacitance that can effectively reduce carrier quenching and accelerate the carrier response. By controlling the applied bias direction, up to 300-1000% external quantum efficiency (EQE) and microsecond response can be achieved not only in the wide range of ultraviolet to visible light from 320 to 550 nm, but also in the narrow-band response with a full width at half minimum (FWHM) of 20 nm. This shows great potential for applications in integrated multifunctional photodetectors.

Ultralong Carrier Lifetime Exceeding 20 µs in Lead Halide Perovskite Film Enable Efficient Solar Cells.

The carrier lifetime is one of the key parameters for perovskite solar cells (PSCs). However, it is still a great challenge to achieve long carrier lifetimes in perovskite films that are comparable with perovskite crystals owning to the large trap density resulting from the unavoidable defects in grain boundaries and surfaces. Here, by regulating the electronic structure with the developed 2-thiopheneformamidinium bromide (ThFABr) combined with the unique film structure of 2D perovskite layer caped 2D/3D polycrystalline perovskite film, an ultralong carrier lifetime exceeding 20 µs and carrier diffusion lengths longer than 6.5 µm are achieved. These excellent properties enable the ThFA-based devices to yield a champion efficiency of 24.69% with a minimum VOC loss of 0.33 V. The unencapsulated device retains ≈95% of its initial efficiency after 1180 h by max power point (MPP) tracking under continuous light illumination. This work provides important implications for structured 2D/(2D/3D) perovskite films combined with unique FA-based spacers to achieve ultralong carrier lifetime for high-performance PSCs and other optoelectronic applications.

Unraveling the Role of Non-Fullerene Acceptor with High Dielectric Constant in Organic Solar Cells.

Increasing the relative dielectric constant is a constant pursuit of organic semiconductors, but it often leads to multiple changes in device characteristics, hindering the establishment of a reliable relationship between dielectric constant and photovoltaic performance. Herein, a new non-fullerene acceptor named BTP-OE is reported by replacing the branched alkyl chains on Y6-BO with branched oligoethylene oxide chains. This replacement successfully increases the relative dielectric constant from 3.28 to 4.62. To surprise, BTP-OE offers consistently lower device performance relative to Y6-BO in organic solar cells (16.27% vs 17.44%) due to the losses in open-circuit voltage and fill factor. Further investigations unravel that BTP-OE has resulted in reduced electron mobility, increased trap density, enhanced first order recombination, and enlarged energetic disorder. These results demonstrate the complex relationship between dielectric constant and device performance, which provide valuable implications for the development of organic semiconductors with high dielectric constant for photovoltaic application.

Anomalous deep-red luminescence of perylene black analogues with strong π-π interactions.

Perylene bisimide (PBI) dyes are known as red, maroon and black pigments, whose colors depend on the close π-π stacking arrangement. However, contrary to the luminescent monomers, deep-red and black PBI pigments are commonly non- or only weakly fluorescent due to (multiple) quenching pathways. Here, we introduce N-alkoxybenzyl substituted PBIs that contain close π stacking arrangement (exhibiting dπ-π ≈ 3.5 Å, and longitudinal and transversal displacements of 3.1 Å and 1.3 Å); however, they afford deep-red emitters with solid-state fluorescence quantum yields (ΦF) of up to 60%. Systematic photophysical and computational studies in solution and in the solid state reveal a sensitive interconversion of the PBI-centred locally excited state and a charge transfer state, which depends on the dihedral angle (θ) between the benzyl and alkoxy groups. This effectively controls the emission process, and enables high ΦF by circumventing the common quenching pathways commonly observed for perylene black analogues.

The Multiplicity of π-π Interactions of Fused-Ring Electron Acceptor Polymorphs on the Exciton Migration and Charge Transport.

Efficient long-range exciton migration and charge transport are the key parameters for organic photovoltaic materials, which strongly depend on the molecular stacking modes. Herein, we extracted the stacked structures of the archetype fused-ring electron acceptor molecule, ITIC, based on the information on four polymorphic crystals and investigated the relationship between molecular stacking modes and exciton migration/charge transport properties through the intermolecular Coulomb coupling and charge transfer integral calculation. Experimentally, the thin film texture is crystallized through a post-annealing treatment through grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements, which lead to the enhanced exciton migration through exciton-exciton annihilation in the femtosecond transient absorption (fs-TA) measurements. This work demonstrates the relationship between the molecular arrangement and the exciton migration and electron transport and highlights the significance of optimizing molecular stacking for the development of high-performance electron acceptor materials.

Orbital Interactions between the Organic Semiconductor Spacer and the Inorganic Layer in Dion-Jacobson Perovskites Enable Efficient Solar Cells.

2D Dion-Jacobson (DJ) perovskites have become emerging photovoltaic materials owing to their intrinsic structure stability. However, as insulating aliphatic cations are widely used as spacers, the interactions between the spacers and inorganic layers in DJ perovskites have rarely been studied. Here, an organic semiconductor spacer with two covalently connected thiophene rings, namely bithiophene dimethylammonium (BThDMA), is successfully developed for 2D DJ perovskite solar cells (PSCs). An important finding is that there are strong orbital interactions between the conjugated organic spacer and adjacent inorganic layers, whereas no such interactions exist in DJ perovskite using an aliphatic octane-1,8-diaminium (ODA) spacer with similar length. The BThDMA spacer with multiple conjugated aromatic rings can also induce crystal growth with large grain size and preferred vertical orientation, resulting in reduced trap density and improved charge-carrier mobility. As a result, the optimized device based on (BThDMA)MAn -1 Pbn I3 n +1 (nominal n = 5) shows an excellent PCE of 18.1% with negligible hysteresis, which is a record efficiency for 2D DJ PSCs using a spacer with two or more covalently linked aromatic rings. These findings provide a novel and important insight on achieving efficient and stable 2D DJ perovskite solar cells by developing organic semiconductor spacers.

Toward Quantifying the Relation between Exciton Binding Energies and Molecular Packing.

Reducing the exciton binding energy Eb of organic photoactive materials is critical to minimize the energy loss and improve the photovoltaic efficiency of organic solar cells. However, the relation between the Eb and molecular packing is not well understood. Herein, the Eb in the crystals of a series of A-D-A type nonfullerene acceptors with different lengths of alkyl side chains has been examined by self-consistent quantum mechanics/embedded charge calculations. The variation of molecular packing induced by the different alkyl chains can have an important impact on the polarization effect of charge carriers and thereby the Eb. More interestingly, the Eb values are found to be linearly increased with the ratio of the void fraction vs the packing coefficient of molecular backbones in the solid crystals. Owing to the smallest ratio, a remarkable low Eb of several tens of meV is achieved for the acceptor with an optimal length of alkyl chains.

Yellow Light-Emitting Highly Soluble Perylene Bisimide Dyes by Acetalization of Bay-Hydroxy Groups.

A new class of perylene bisimide (PBI) derivatives is introduced by bridging 1,12- and 1,6,7,12-hydroxy functionalized bay positions with oxygen-carbon-oxygen linker(s). This functionalization rigidifies the inherently twisted bay-substituted perylene core to afford dyes of high stability and solubility that are characterized by vibronically well-resolved absorption and fluorescence spectra and intense yellow emission with quantum yields close to 100%.

Spatial Identification of "Zeroth Defect" Formation in Organic Light-Emitting Diodes by Multispectral Mappings.

In recently developed organic semiconductors, the continuously improving sample purity makes the stability of the chemical bonds of organic materials themselves become a key factor in device stability, which provides greater uncertainty for the generation of "zeroth defect", and the spatial resolution of performance at different positions becomes particularly important. In this work, complete maps of electroluminescent, photoluminescent, and Raman scattering in the same area on an organic light-emitting diode during its operation have been achieved with a confocal spectrometer with multiple laser sources. The different spectral characters help to establish different regions and suggest the mechanism of degradation. In particular, Raman scattering has been shown to be very sensitive in a multilayer device to a change in thickness of several nanometers. In amorphous films with few defects, the very weak film uniformity, including the thickness and degree of aggregation, would induce dramatic degradation. The relatively thin and/or loosely textured region easily locally overheats and has the highest probability of "zeroth defect" generation. This method has high spatial resolution, a low level of damage to samples, good reproducibility, and multiple interconnected pieces of information, which is significant for online quality prediction and mechanistic analysis.

Three Isomeric Non-Fullerene Acceptors Comprising a Mono-Brominated End-Group for Efficient Organic Solar Cells.

Non-fullerene acceptors (NFAs) carrying a 1,1-dicyanomethylene-3-indanone (IC) end-group are the most powerful ones to boost the power conversion efficiency of organic solar cells (OSCs). However, the well-known Knoevenagel condensation of the mono-halogenated IC end-group will result in an NFA isomeric effect, a chemical issue that needs to be addressed. Herein, facile preparations and separations of three well-defined mono-brominated isomers BTzIC-2Br-δ, BTzIC-2Br-γ, and BTzIC-2Br-δγ via column chromatography with a well-chosen mixing solvent were demonstrated for Knoevenagel condensation, and their structures were verified by NMR spectra and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF) mass spectra. It is the first time that an asymmetric isomer BTzIC-2Br-δγ is reported, and the regioisomeric effect on optoelectronic properties can be investigated based on all three isomers. Moreover, the single-crystal structure was successfully achieved for the symmetric molecule BTzIC-2Br-γ. With benzodithiophene (BDT)-free PFBT4T-T20 as an easily accessible and low-cost polymer donor, the three isomers could show differentiated device performances, with a power conversion efficiency order of BTzIC-2Br-γ (16.00%) > BTzIC-2Br-δγ (15.81%) > BTzIC-2Br-δ (15.29%). The best efficiency of 16.00% achieved with BTzIC-2Br-γ is among the highest ones for binary OSCs based on the low-cost BDT-free donors. The facile and complete synthesis of isomeric NFAs with mono-halogenated IC end-groups would promote the elucidation of the structure-property relationship.

Asymmetric electron acceptor enables highly luminescent organic solar cells with certified efficiency over 18.

Enhancing the luminescence property without sacrificing the charge collection is one key to high-performance organic solar cells (OSCs), while limited by the severe non-radiative charge recombination. Here, we demonstrate efficient OSCs with high luminescence via the design and synthesis of an asymmetric non-fullerene acceptor, BO-5Cl. Blending BO-5Cl with the PM6 donor leads to a record-high electroluminescence external quantum efficiency of 0.1%, which results in a low non-radiative voltage loss of 0.178 eV and a power conversion efficiency (PCE) over 15%. Importantly, incorporating BO-5Cl as the third component into a widely-studied donor:acceptor (D:A) blend, PM6:BO-4Cl, allows device displaying a high certified PCE of 18.2%. Our joint experimental and theoretical studies unveil that more diverse D:A interfacial conformations formed by asymmetric acceptor induce optimized blend interfacial energetics, which contributes to the improved device performance via balancing charge generation and recombination.

Boron-Locked Starazine - A Soluble and Fluorescent Analogue of Starphene.

A starlike heterocyclic molecule containing an electron-deficient nonaaza-core structure and three peripheral isoquinolines locked by three tetracoordinate borons, namely isoquinoline-nona-starazine (QNSA), is synthesized by using readily available reactants through a rather straightforward approach. This new heteroatom-rich QNSA possesses a quasi-planar π-backbone structure, and bears phenyl substituents on borons which protrude on both sides of the π-backbones endowing it with good solubility in common organic solvents. Contrasting to its starphene analogue, QNSA shows intense fluorescence with a quantum yield (PLQY) of up to 62 % in dilute solution.

Photoinduced Cascading Charge Transfer in Perylene Bisimide-Based Triads.

We synthesize three perylene bisimide-based triads with donor-acceptor-acceptor (D∼A1-A2) architectures, in which the distance between D and A1 is varied to study its influence on the excited state electron processes. Very different intramolecular charge transfer (D+∼A1-A2-) lifetimes in dichloromethane (DCM) for these three triads are revealed by steady-state and transient spectroscopies. Free-energy changes of charge transfer (CT) are calculated based on the single-crystal X-ray diffraction data and electrochemical measurements. The results show that photoinduced cascading CT comprises two competing processes in DCM (CTs in D∼A1 units and in A1-A2 units) by pumping of the A1 unit, and then the long-distance CT state is formed. The charge recombination (CR) process is restrained effectively by the increased distance between the anion and cation. This research reveals the importance of multistep cascading CTs on tuning the CT lifetime in multichromophoric systems.

High-Performance Simple Nonfused Ring Electron Acceptors with Diphenylamino Flanking Groups.

Four simple nonfused ring electron acceptors (H-2F, CH3-2F, OCH3-2F, and SCH3-2F) were designed and synthesized. The use of diphenylamine derivatives as the flanking group for the construction of nonfused ring electron acceptors can improve solubility, avoid the formation of oversized aggregates, and enhance the intramolecular charge-transfer effect to extend absorption spectra. The substituent group at the diphenylamine unit has a great impact on the absorption and energy level of acceptors, electron mobility and morphology of blend films. Unlike the other three acceptors, CH3-2F can form ordered molecular stacking and a face-on orientation in the donor/acceptor blend film. A single-crystal analysis demonstrates that CH3-2F can form a two-dimensional electron transport network. Among these four acceptors, CH3-2F-based organic solar cells provide the highest PCE of 12.28%. Our work has demonstrated that triarylamine is a helpful construction unit for low-cost and high efficiency nonfused ring electron acceptors.

Molecular insights of exceptionally photostable electron acceptors for organic photovoltaics.

Photo-degradation of organic semiconductors remains as an obstacle preventing their durable practice in optoelectronics. Herein, we disclose that volume-conserving photoisomerization of a unique series of acceptor-donor-acceptor (A-D-A) non-fullerene acceptors (NFAs) acts as a surrogate towards their subsequent photochemical reaction. Among A-D-A NFAs with fused, semi-fused and non-fused backbones, fully non-fused PTIC, representing one of rare existing samples, exhibits not only excellent photochemical tolerance in aerobic condition, but also efficient performance in solar cells. Along with a series of in-depth investigations, we identify that the structural confinement to inhibit photoisomerization of these unique A-D-A NFAs from molecular level to macroscopic condensed solid helps enhancing the photochemical stabilities of molecules, as well as the corresponding OSCs. Although other reasons associating with the photostabilities of molecules and devices should not excluded, we believe this work provides helpful structure-property information toward new design of stable and efficient photovoltaic molecules and solar cells.

Simple Non-Fused Electron Acceptors Leading to Efficient Organic Photovoltaics.

Despite the remarkable progress achieved in recent years, organic photovoltaics (OPVs) still need work to approach the delicate balance between efficiency, stability, and cost. Herein, two fully non-fused electron acceptors, PTB4F and PTB4Cl, are developed via a two-step synthesis from single aromatic units. The introduction of a two-dimensional chain and halogenated terminals for these non-fused acceptors plays a synergistic role in optimizing their solid stacking and orientation, thus promoting an elongated exciton lifetime and fast charge-transfer rate in bulk heterojunction blends. As a result, PTB4Cl, upon blending with PBDB-TF polymer, has enabled single-junction OPVs with power conversion efficiencies of 12.76 %, representing the highest values among the reported fully unfused electron acceptors so far.

Spacer Engineering Using Aromatic Formamidinium in 2D/3D Hybrid Perovskites for Highly Efficient Solar Cells.

Organic spacers play an important role in 2D/3D hybrid perovskites, which could combine the advantages of high stability of 2D perovskites and high efficiency of 3D perovskites. Here, a class of aromatic formamidiniums (ArFA) was developed as spacers for 2D/3D perovskites. It is found that the bulky aromatic spacer ArFA in 2D/3D perovskites could induce better crystalline growth and orientation, reduce the defect states, and enlarge spatially resolved carrier lifetime thanks to the multiple NH···I hydrogen-bonding interactions between ArFA and inorganic [PbI6]4- layers. As a result, compared to the control device with efficiency of 19.02%, the 2D/3D perovskite device based on such an optimized organic salt, namely benzamidine hydrochloride (PhFACl), exhibits a dramatically improved efficiency of 22.39% along with improved long-term thermal stability under 80 °C over 1400 h. Importantly, a champion efficiency of 23.36% was further demonstrated through device engineering for PhFACl-based 2D/3D perovskite solar cells. These results indicate the great potential of this class of ArFA spacers in highly efficient 2D/3D perovskite solar cells.

Coexistence of Parallel and Rotary Stackings in the Lamellar Crystals of a Perylene Bisimide Dyad for Temperature-Sensitive Bicomponent Emission.

Coexistence of rotationally π-π stacked columns and discrete slip-stacked dimers of perylene bisimide (PBI) chromophores is revealed by single crystal X-ray diffraction in the lamellar crystal of a head-to-tail linked PBI dyad. The rotary π-π stacked columnar moieties show H-type spectral character with relatively higher excitation energy, while the discrete slip-stacked π-π dimers have J-type spectral behavior with lower excitation energy. The lamellar crystals show relatively low photoluminescence efficiency of 12% at room temperature, while this dramatically increases to ∼90% at low temperature (80 K). Both of the rotary and slip-stacked moieties are emissive, and the nonradiative energy transfer processes between them are suppressed at low temperature, ensuring the highly efficient excimer-like long-lived fluorescence.

Chlorinated Benzo[1,2-b:4,5-c']dithiophene-4,8-dione Polymer Donor: A Small Atom Makes a Big Difference.

The position of a chlorine atom in a charge carrier of polymer solar cells (PSCs) is important to boost their photovoltaic performance. Herein, two chlorinated D-A conjugated polymers PBBD-Cl-α and PBBD-Cl-ß are synthesized based on two new building blocks (TTO-Cl-α and TTO-Cl-ß) respectively by introducing the chlorine atom into α or ß position of the upper thiophene of the highly electron-deficient benzo[1,2-b:4,5-c']dithiophene-4,8-dione moiety. Single-crystal analysis demonstrates that the chlorine-free TTO shows a π-π stacking distance (d π-π) of 3.55 Å. When H atom at the α position of thiophene of TTO is replaced by Cl, both π-π stacking distance (d π-π = 3.48 Å) and Cl···S distance (d Cl-S = 4.4 Å) are simultaneously reduced for TTO-Cl-α compared with TTO. TTO-Cl-ß then showed the Cl···S non-covalent interaction can further shorten the intermolecular π-π stacking separation to 3.23 Å, much smaller than that of TTO-Cl-α and TTO. After blending with BTP-eC9, PBBD-Cl-ß:BTP-eC9-based PSCs achieved an outstanding power conversion efficiency (PCE) of 16.20%, much higher than PBBD:BTP-eC9 (10.06%) and PBBD-Cl-α:BTP-eC9 (13.35%) based devices. These results provide an effective strategy for design and synthesis of highly efficient donor polymers by precise positioning of the chlorine substitution.