Quantitative understanding of the photophysical processes is essential for developing novel thermally activated delayed fluorescence (TADF) materials. Taking as an example 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene, a typical TADF-active molecule, we calculated the interconversion and decay rates of the lowest excited singlet and triplet states at different temperatures as well as the prompt and delayed fluorescence efficiencies at 300 K at the first-principles level. Our results can reproduce well the experimentally available data. It is found that the reverse intersystem crossing rate (kRISC) is sharply increased by 3 orders of magnitude, while the other rates increase slightly or remain unchanged when the temperature rises from 77 to 300 K. Importantly, kRISC reaches up to 1.23 × 106 s–1 and can compete with the radiative and nonradiative decay rates of S1 (1.11 × 107 and 2.37 × 105 s–1) at 300 K, leading to an occurrence of delayed fluorescence. In addition, our calculations indicate that it is the freely rotational motions of the carbazolyl between two cyano groups that are responsible for the interconversion between S1 and T1. The large torsional barriers of other three adjacent carbazolyl groups block the nonradiative decay channels of S1 → S0, leading to strong fluorescence. This work would provide useful insight into the molecular design of high-efficiency TADF emitters.

Electronic couplings of charge-transfer states with the ground state and localized excited states at the donor/acceptor interface are crucial parameters for controlling the dynamics of exciton dissociation and charge recombination processes in organic solar cells. Here we propose a quasi-adiabatic state approach to evaluate electronic couplings through combining maximum occupation method (mom)-ΔSCF and state diabatization schemes. Compared with time-dependent density functional theory (TDDFT) using global hybrid functional, mom-ΔSCF is superior to estimate the excitation energies of charge-transfer states; moreover it can also provide good excited electronic state for property calculation. Our approach is hence reliable to evaluate electronic couplings for excited state electron transfer processes, which is demonstrated by calculations on a typical organic photovoltaic system, oligothiophene/perylenediimide complex.

The charge-transport properties in C8BTBT–FnTCNQ and DMQtT–F4TCNQ mixed-stack crystals have been investigated by means of density functional theory, molecular dynamics and kinetic Monte Carlo simulations. The super-exchange nature of charge transport in these crystals is elucidated by the Larsson partition-based electronic coupling method that was developed recently by us. Compared with hole transport, in addition to the donor HOMO–acceptor LUMO interaction, the interaction between the donor HOMO−1 and the acceptor LUMO will also make an important contribution to electron transport. Moreover, this additional interaction plays an opposite role and results in electron-dominant and hole-dominant transport in the C8BTBT–FnTCNQ and DMQtT–F4TCNQ crystals, respectively. Most importantly, our calculations point out that the nonlocal electron–phonon couplings are very weak and much smaller than the electronic couplings in all the studied crystals. This implies that the nonlocal couplings have little influence on charge transport. In contrast to the experimental measurements, the external reorganization energies are thus expected to play an essential role in determining charge carrier mobilities. These findings pave the way for rational design of high performance organic donor–acceptor mixed-stack semiconductors.

AbstractThe design and synthesis of highly efficient deep red (DR) and near-infrared (NIR) organic emitting materials with characteristic of thermally activated delayed fluorescence (TADF) still remains a great challenge. A strategy was developed to construct TADF organic solid films with strong DR or NIR emission feature. The triphenylamine (TPA) and quinoxaline-6,7-dicarbonitrile (QCN) were employed as electron donor (D) and acceptor (A), respectively, to synthesize a TADF compound, TPA-QCN. The TPA-QCN molecule with orange-red emission in solution was employed as a dopant to prepare DR and NIR luminescent solid thin films. The high doped concentration and neat films exhibited efficient DR and NIR emissions, respectively. The highly efficient DR and NIR organic light-emitting devices (OLEDs) were fabricated by regulating TPA-QCN dopant concentration in the emitting layers.

A new acceptor–donor–acceptor-structured nonfullerene acceptor ITCC (3,9-bis(4-(1,1-dicyanomethylene)-3-methylene-2-oxo-cyclopenta[b]thiophen)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d′:2,3-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene) is designed and synthesized via simple end-group modification. ITCC shows improved electron-transport properties and a high-lying lowest unoccupied molecular orbital level. A power conversion efficiency of 11.4% with an impressive V OC of over 1 V is recorded in photovoltaic devices, suggesting that ITCC has great potential for applications in tandem organic solar cells.

The photophysical properties of three TX-based D–A isomers (TXO-PhCz1, TXO-PhCz3, and TXO-PhCz4) with a PhCz donor at different substitution positions of phenyl group on a TXO unit were investigated. The substitution position of the PhCz unit significantly impacts the photophysical properties of these isomers. TXO-PhCz1 exhibits a very weak emission in both undoped and doped films, while TXO-PhCz3 and TXO-PhCz4 exhibit a strong emission with the requisite properties for TADF emitters, including a small ΔEST and transient PL decay curves with a prompt and delayed fluorescent component. TXO-PhCz4 exhibits a much stronger orbital coupling than TXO-PhCz3 and then the phosphorescent emission causes the inverse temperature dependence of the transient PL decay, which is contrary to that of TXO-PhCz3 and other TADF emitters. TXO-PhCz4 exhibits a small ΔEST of 23 meV and a short decay time of 14 μs at room temperature, which are much smaller and shorter than those of TXO-PhCz3. Multilayer OLEDs based on TXO-PhCz4 exhibit a very low-efficiency roll-off with a maximum current efficiency of 49.2 cd A−1, a maximum power efficiency of 47.7 lm W−1, and a maximum EQE of 16.3%.

In recent years, great progress has been achieved in the field of non-fullerene organic solar cells. In particular, the power conversion efficiencies for the photovoltaic devices based on A–π–A fused-ring electron acceptors, e.g. ITIC, can catch up with or even surpass the fullerene-based ones. However, the detailed molecular packing structures and charge transport properties of these acceptors are rarely studied and still unclear, which has become the major obstacle for rational molecular design to further improve the photovoltaic performance. Here, we have unravelled the intermolecular arrangements in the ITIC film via atomistic molecular dynamics simulations. The simulated results point to that three-dimensional molecular packing is formed in the ITIC film through local intermolecular π–π stacking between the terminal acceptor units. In sharp contrast, the ITIC crystal grown by the slow solvent vapor diffusion approach exhibits a one-dimensional edge-to-face stacking structure. Consequently, excellent isotropic electron mobilities along three dimensions are found for the film and unprecedentedly, the overall mobility is even higher than that of the crystal. Our work suggests that judicious modulation of the terminal acceptor unit to increase local intermolecular π–π interaction would be an effective way to improve the electron mobilities and photovoltaic performance of the A–π–A electron acceptors.

Side-chain engineering is crucial to improve the performance of solution-processed organic solar cells. However, the correlation between side-chain structures and photovoltaic properties is still unclear. Here, we have investigated the local interface morphologies of PC71BM blended with two donors, DR3TBDT and DR3TSBDT with alkyloxy and alkylthio side chains on the BDT core, by means of atomistic molecular dynamics simulations. Compared with alkyloxy, alkylthio exhibits larger steric hindrance after changing the side-chain anchoring atom from oxygen to sulfur, leading to an obvious reduction of the PC71BM–BDT face-on orientations in which charge recombination is demonstrated to be the most severe by quantum-chemical calculations. This suggests that the performance of DR3TBDT/PC71BM solar cells is likely to be more affected by charge recombination than that of DR3TSBDT/PC71BM-based devices. For the first time, our work unravels the important role of side-chain anchoring atoms in tuning the donor/fullerene interfacial arrangements toward high-performance organic solar cells.

By means of atomistic molecular dynamics simulations, we have investigated the effect of the solvent evaporation rate and thermal annealing on the molecular packing morphology of a diketopyrrolopyrrole based organic photovoltaic donor material, DPP(TBFu)2, which displays excellent hole mobility. It is observed that slow evaporation of solvent will lead to a relatively high degree of molecular packing order while leaving many voids in the as-cast sample. Upon thermal annealing, the as-cast samples at both fast and slow evaporation rates become more compact and much more apparently at the slow evaporation rate. Interestingly, the effect of thermal annealing on molecular packing order depends on the solvent evaporation rates of the as-cast samples. Upon thermal annealing, the molecular packing order of the fast evaporated sample is enhanced with increased π–π stacks. In contrast, thermal annealing will decrease the degree of packing order for the slow evaporated sample since the orientations and conformations of the molecules at the aggregate boundaries are substantially modulated to squeeze the voids. Electrical network analyses point to the fact that the mesoscopic electrical connectivities for all the samples are quite effective and insensitive to the modifications of local molecular ordering due to the delocalized HOMO of DPP(TBFu)2 providing efficient intermolecular electronic interactions. The hole mobilities of all the fabricated samples are thus estimated to be similar and quite high. Finally, our simulations point to the fact that the modest enhancement of mobility upon thermal annealing is correlated with the increased density rather than the varied ordering of molecular packing. Our work provides an atomistic insight into the evolution of thin-film morphology of organic photovoltaic molecular materials during solution processing and thermal annealing treatments and sheds light on the correlation between the molecular structure, packing morphology and hole transport capability.

High-efficiency deep blue organometallic phosphors are imperative to organic luminescence devices. While green iridium complexes commonly exhibit high luminescence efficiencies, the luminescence quantum efficiency always drops sharply when emission becomes deep blue. In this work, the microscopic mechanism of such a drastic decrease is elucidated from detailed computational investigation. Both radiative (kr) and non-radiative (knr) decay rates of the lowest triplet state (T1) are calculated for five representative cyclometalated iridium(III) complexes with emission color ranging from green to deep blue, based on phenylpyridyl, phenylpyrazolyl, bipyridinato, pyrimidinpyridyl, and pyrimidinprazolyl ligands. For all compounds, the T1 states are characteristic of mixed intraligand (π → π*) transition and iridium-to-ligand charge transfer (d → π*), and the increased π → π* and decreased d → π* portions lead to the blue-shifted emission of 1 < 2 < 4 < 5 < 3. Strikingly, it is found that the drastic increase of knr arising from severe intra-ligand vibration relaxations induced by the enhanced π → π* transition is mainly responsible for the droop of the phosphorescence quantum efficiency, which provides a different deactivation mechanism from the thermally-activated transformation into a dark metal-centred ligand field excited state reported in many previous studies. Compared with the well-studied compounds 1–3, the newly designed compounds 4 and 5 achieve a good balance between high efficiency and a large energy gap and are very promising as deep blue phosphors. These findings are expected to be helpful for the rational design of high-efficiency blue organometallic phosphors, especially in terms of ligands.

Electronic delocalization at donor/acceptor (D/A) interfaces can play an important role in photocurrent generation for organic solar cells. Here, we have investigated the nature of local excited and interfacial charge transfer (CT) states in model complexes including one to four anti-parallel stacking dipolar donor (DTDCTB) molecules and one fullerene (C60) molecule by means of density functional theory (DFT) and time-dependent DFT (TDDFT). For all the donor-to-acceptor CT states, despite the number of DTDCTB molecules in the complexes, the hole is mainly localized on a single DTDCTB, and moves farther away from C60 for the energy higher level. However, the highest occupied molecular orbitals (HOMOs) and the excitonic states (EX) including the bright and dark EX are delocalized over the whole donor stacks in the complexes. This implies that the formation of ordered DTDCTB arrangements can substantially shorten the exciton diffusion process and facilitate ultrafast charge generation. Interestingly, owing to strong intermolecular Coulomb attraction, the donor-to-donor CT states are situated below the local excited states, but can approach the donor-to-acceptor CT states, indicating a weak role as charge traps. Our work would be helpful for understanding the electronic delocalization effects in organic solar cells.

•Two twistacene functionalized anthracenes were synthesized.•The obtained molecules show high thermal stability and strong blue fluorescence.•The fabricated devices exhibit good electroluminescent performance.Two novel 9,10-diarylanthracene derivatives containing small twistacenes units (9,14-diphenylbenzo[f]tetraphene and 2,7-di-tert-butyl-9,14-diphenyldibenzo[de,qr]tetracene) have been successfully synthesized and characterized. The resulting molecules show high thermal decomposition temperatures (5% weight) of 443 °C–516 °C, and emit strong fluorescence in dichloromethane and thin film with full widths at half maximum of ca. 55 nm, falling into blue region. The organic light-emitting diodes doped with them into 3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl were prepared and showed the maximum luminescence in the range of 1784–4122 cd/m2 respectively, which indicates these new diarylanthracene derivatives are appealing fluorescent dyes for OLEDs.Two novel anthracene derivatives containing the twistacenes were synthesized for use as active layers in blue organic light emitting diodes.

This paper summarizes our recent works on theoretical modelling of molecular packing and electronic processes in small-molecule organic solar cells. Firstly, we used quantum-chemical calculations to illustrate exciton-dissociation and charge-recombination processes at the DTDCTB/C60 interface and particularly emphasized the major role of hot charge-transfer states in the exciton-dissociation processes. Then, we systematically analyzed the influence of DTDCTB surfaces with different features on the vacuum vapor deposition growth and packing morphologies of C60via atomistic molecular dynamics simulations, and found that the formation of crystalline fullerene is the result of an integrated impact of stability, landscape, and molecular orientation of the substrate surfaces. Also, we investigated the impact of different film-processing conditions, such as solvent evaporation rates and thermal annealing, on molecular packing configurations in a neat small-molecule donor material, DPP(TBFu)2, and discussed the correlation between charge mobility and molecular packing via atomistic simulations in combination with electronic-structure calculations and kinetic Monte Carlo simulations.Electronic processes and molecular packing morphologies at the donor/acceptor interface or in the pure phase for organic small-molecule photovoltaic materials are revealed by a combination of atomistic simulations and quantum-chemical calculations.

•Two novel colorimetric and fluorescent off–on enantiomers (R/S-RBOL) have been developed and characterized.•The fluorescence intensity of R-RBOL is linearly proportional with increasing the concentration of Fe3+ ranging from 0 to 20 equiv.•R/S-RBOL could be successfully applied to monitor Fe3+ in living cells without cytotoxicity under physiological conditions.Two novel colorimetric and fluorescent off–on enantiomers (R/S-RBOL) were developed to detect Fe3+ in aqueous and biological samples through probing the integrated changes in fluorescence, circular dichroism (CD) and circularly polarized luminescence (CPL). The fluorescence intensity of R/S-RBOL is linearly proportional to the Fe3+ concentration in the range of 0–20 equiv., bringing out a detection limit of 1.83 × 10−7 M. The Job's plot indicates a 1:1 binding stoichiometry of RBOL:Fe3+, which is further confirmed by ESI-MS analysis. Moreover, the RBOLs exhibit a dual-readout response in colour and fluorescence, rendering them suitable to sense Fe3+ in living cells with low cytotoxicity. In order to shed some light on the detection mechanism, the coordination between R/S-RBOL and Fe3+ is unravelled by DFT calculations. From the free RBOL to the RBOL–Fe3+ complex, the spiro-lactam ring is opened to interact with Fe3+ via the lactam N and O, imine N and hydroxyl O atoms in the form of a ferric six-coordinate structure.Graphical abstract

The surface patterns of fused thiophene indacene (FTI) derivatives with different alkyl chains are investigated with scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The FTI with ethyl chains (FTI 2) arranges into linear and fishbone structures. Both of them are defective, and FTI 2 is easily disturbed during experiments. FTI 6, the molecule carrying hexyl chains, is fixed on the surface much better than FTI 2, and defects in its assembly are reduced greatly. However, the essential characteristics of FTI 6 assembly are similar to those of the FTI 2 linear one. For the compound with dodecyl chains, FTI 12 organizes into the lamellar structure showing critical alterations from that of FTI 6 or FTI 2. The DFT-optimized models disclose that hydrogen bonds exist in all the rows of three studied molecules although the stacking of the respective compound is different. STM and DFT results indicate that hydrogen bonds are responsible for the formation of FTI 2 assembly, whereas the interactions related to alkyl chains lead to the formation of the lamellar structure of FTI 12. Thus, we may estimate the balance point of hydrogen bonds and the interactions related to alkyl chains.

Molecular packing in organic single crystals greatly influences their charge transport properties but can hardly be predicted and designed because of the complex intermolecular interactions. In this work, we have realized systematic fine-tuning of the single-crystal molecular packing of five benzodifurandione-based oligo(p-phenylenevinylene) (BDOPV)-based small molecules through incorporation of electronegative fluorine atoms on the BDOPV backbone. While these molecules all exhibit similar column stacking configurations in their single crystals, the intermolecular displacements and distances can be substantially modified by tuning of the amounts and/or the positions of the substituent fluorine atoms. Density functional theory calculations showed that the subtle differences in charge distribution or electrostatic potential induced by different fluorine substitutions play an important role in regulating the molecular packing of the BDOPV compounds. Consequently, the electronic couplings for electron transfer can vary from 71 meV in a slipped stack to 201 meV in a nearly cofacial antiparallel stack, leading to an increase in the electron mobility of the BDOPV derivatives from 2.6 to 12.6 cm2 V–1 s–1. The electron mobility of the five molecules did not show a good correlation with the LUMO levels, indicating that the distinct difference in charge transport properties is a result of the molecular packing. Our work not only provides a series of high-electron-mobility organic semiconductors but also demonstrates that fluorination is an effective approach for fine-tuning of single-crystal packing modes beyond simply lowering the molecular energy levels.

Despite the dominant position of aromatic fluorophores, we report herein the design and synthesis of quinoidal fluorophores based on rarely emissive quinoidal bithiophene. Quinoidal bitheno[3,4-b]thiophene, QBTT-C6, consisting of cruciform-fused (E)-1,2-bis(5-hexylthiophen-2-yl)ethene and quinoidal bithiophene, shows a fluorescence quantum yield of 8.5%, 25-fold higher than that of the parent quinoidal QBT chromophore, but its maximum emission is at similar wavelengths. QBTT-Ar’s featuring intramolecular charge transfer can further shift the maximum emission into the near-infrared region. The intramolecular charge transfer is programmably enhanced by tuning the substituents on the aryl groups from the electron-withdrawing trifluoromethyl to the electron-donating methoxy groups. Unexpectedly, a positive relationship between intramolecular charge transfer and fluorescence quantum yield is observed; as a result, QBTT-FL gives an unprecedentedly high fluorescence quantum yield of up to 53.1% for quinoidal oligothiophenes. With detailed photophysical and theoretical investigations, we demonstrate that the nonradiative intersystem crossing (S1 → T2) is significantly restrained in QBTT-Ar’s, which can be attributed to the faster reverse intersystem crossing (T2 → S1) characteristic of a small activation energy. This work reveals the possibility for developing red/near-infrared fluorophores from the less explored quinoidal molecules because of their intrinsically narrow bandgaps.

The charge-transport parameters in three 4,10-dihalogenated anthanthrones (AAOs) are investigated by means of density functional theory (DFT) and molecular dynamics (MD) calculations. Our calculations point to similar hole and electron reorganization energies for each molecule. Significant electronic couplings and bandwidths (particularly for electron transport) are found along the parallel π–π stacking directions in all the dihalogenated AAO crystals. The calculated effective masses are small or moderate for both holes and electrons. Especially for the iodinated AAO crystals, remarkable ambipolar charge transport can be anticipated due to the smallest and similar effective masses for holes and electrons (both are around 1.0 m0). In addition, due to the presence of two small effective masses, two-dimensional charge transport would take place for electrons in the chlorinated AAO crystal and for both holes and electrons in the iodinated AAO crystal. Also, our calculations reveal large nonlocal electron–phonon couplings along the π-stacks in the brominated AAO and, in particular, the chlorinated AAO crystals, which can further improve the balance in transport of holes and electrons.

A phenylbenzoxazole-based organic compound, N-(3-(benzo[d]oxazol-2-yl)phenyl)-4-tert-butylbenzamide (3OTB), has been synthesized, and the mechanism of its condensed-state emission enhancement has been studied. Experimental and theoretical investigations indicate that prohibition of transition from the local excited state to the nonemissive twisted intramolecular charge transfer (TICT) excited state, but halfway to the intermediate emissive quasi-TICT excited state that results from partial restriction of free intramolecular rotations in condensed states, is responsible for the emission enhancement. Furthermore, it is easy to grow 3OTB nanosheets from THF/H2O mixed solvents. In addition, when the molecular arrangement is more ordered, restriction of molecular rotation becomes severer, and consequently, stronger emission can be observed, so that the emission quantum efficiency is in the order of crystalline > powder > nanosheet > amorphous film.

A heterogeneous catalytic system, Pd(OAc)2/n-Bu4NOAc, for the efficient synthesis of diaceno[a,e]pentalenes via a tandem Pd catalytic cycle is reported. The catalytic partner n-Bu4NOAc played indispensable and versatile roles, acting as both the media for recovering active Pd(0) species and their stabilizer. A series of new diaceno[a,e]pentalenes were obtained in moderate to high yields, among which the octacyclic dianthracenopentalene was found to be highly emissive.

•Two A-π-D-π-A type indacenodithiophene-based small molecules have been designed and synthesized.•The indacenodithiophene-based small molecules exhibit good performance of OFETs and OPVs simultaneously.•Side chain positions of the π-bridges in the molecules influence the performance of the OFETs and OPVs.•The power conversion efficiency of the OSCs based on the molecules as donor reached ca. 3%.Solution-processed indacenodithiophene (IDT)-based small molecules with 1,3-indanedione (ID) as terminal acceptor units and 3,3′-hexyl-terthiophene (IDT-3Th-ID(I)) or 4,4′-hexyl-terthiophene (IDT-3Th-ID(II)) as π-bridges, have been designed and synthesized for the application in organic field-effect transistors (OFETs) and organic solar cells (OSCs). These molecules exhibited excellent solubility in common organic solvents, good film-forming ability, reasonable thermal stability, and low HOMO energy levels. For the OFETs devices, high hole motilities of 0.52 cm2 V−1 s−1 for IDT-3Th-ID(I) and 0.61 cm2 V−1 s−1 for IDT-3Th-ID(II) were achieved, with corresponding high ION/IOFF of ca. 107 and ∼109 respectively. The OSCs based on IDT-3Th-ID(I)/PC70BM (2:1, w/w) and IDT-3Th-ID(II)/PC70BM (2:1, w/w) without using any treatment of solvent additive or thermal annealing, showed power conversion efficiencies (PCEs) of 3.07% for IDT-3Th-ID(I) and 2.83% for IDT-3Th-ID(II), under the illumination of AM 1.5G, 100 mW/cm2. The results demonstrate that the small molecules constructed with the highly π-conjugated IDT as donor unit, 3Th as π-bridges and ID as acceptor units, could be promising organic semiconductors for high-performance OFETs and OSCs applications.Graphical abstract

The assembling structure of square and triangular macrocycle molecules constructed with diethynylcarbazole units was investigated by scanning tunneling microscopy (STM) on a graphite surface. STM observation revealed that the square macrocycle molecule (M1) forms a multilayer on the graphite surface. In the first layer, M1 assembles into medium-sized domains with a few defects and dislocations, whereas, for the second layer, most of M1 are dispersed on the first layer separately. A tentative stacking mode of the bilayer structure is provided in this paper on the basis of information given by STM experiments. Considering the interlayer distance given by the crystal data on the similar molecules and the length of the M1 alkyl chains, we think that it is possible that part of the top M1 side chains adsorbs on the cavity area of the bottom M1 and probably plays a dominant role in stabilizing the second layer. This postulation is verified by a control experiment in which coronene is filled in the cavity of M1 and no bilayer structure of M1 is found. The triangular molecule (M2) organizes into a single layer with larger and less defect domains. Two M2 are paired together in parallel, but opposite-oriented style, and are responsible for the serrate edge of the molecular row. The alkyl chains of M2 adopt rather diverse arrangements without disturbing the assembly of M2 core parts. When the solution contains both coronene and M2, no M2–coronene complex is observed and the adlayer characteristics of M2 are essentially the same as those of only M2 in solution. The results may help us to learn the stacking behavior of macrocycle molecules with different shapes, understand surface self-assembling principles, and develop high-performance devices based on related materials.

Two solution-processable acceptor–donor–acceptor (A-D-A) structured organic molecules with bithienyl-substituted benzodithiophene (BDTT) as central and donor unit, indenedione (ID) as acceptor unit and end groups, and thiophene (T) or bithiophene (bT) as π-bridges, D1 and D2, are designed and synthesized for the application as donor materials in organic solar cells (OSCs). Two corresponding molecules with alkoxy side chains on BDT, DO1, and DO2 are also synthesized for comparison. The four compounds possess broad absorption covering the wavelength range 450–740 nm and relatively lower HOMO energy levels from −5.16 to about −5.19 eV. D2 and DO2 with bithiophene π-bridges demonstrate stronger absorbance and higher hole mobilities than the compounds with thiophene π-bridges. The power conversion efficiency (PCE) values of the OSCs based on the organic compounds/PC70BM (1.5:1, w/w) are 6.75% for D2, 5.67% for D1, 5.11% for DO2, and 4.15% for DO1. The results indicate that the molecules with thienyl conjugated side chains and bithiophene π-bridges show better photovoltaic performance. The PCE of the D2-based OSC are among the highest values in the OSCs based on the solution-processed organic small molecules.Keywords: D-A-D structured molecules with conjugated side chains; organic solar cells; solution-processable organic photovoltaic materials;

•The incorporation of alkyl-thiophene spacers were beneficial for 2D BDT-RT and TPD combination polymer at absorption coefficient.•The polymer with spacers exhibited better PSC performance up to 6.08%.•The result was distinct from alkyl-thiophene spacer effect on other BDT-TPD structured polymers reported.Two novel donor–acceptor (D–A) type conjugated polymers using thiophene and hexyl-thiophene spacers between two-dimensional alkyl-thiophene substituted benzo[1,2-b:4,5-b′]dithiophene (BDT-RT) and alkylthieno[3,4-c]pyrrole-4,6-dione (TPD) units are synthesized and characterized. The effects of incorporation of alkyl-thiophene spacers in the polymer backbone on the optical, electrochemical, charge transport and photovoltaic properties are studied. The bulk-heterojunction (BHJ) polymer solar cells (PSCs) based on the polymer with hexyl-thiophene spacers show much better performance (with power conversion efficiency up to 6.08%) than that of polymer without spacers, which is very distinct from alkyl-thiophene spacer effect of other BDT-TPD structured polymers reported previously. The experimental results and theoretical calculations show that a subtle tuning of chemical structure can significantly influence the absorption coefficient by inserting alkyl-thiophene spacers in the polymer backbone. This provides an important route for designing new materials to obtain higher current density (Jsc) and fill factors (FF).Graphical abstract

In recent years, great progress has been achieved in the field of non-fullerene organic solar cells. In particular, the power conversion efficiencies for the photovoltaic devices based on A–π–A fused-ring electron acceptors, e.g. ITIC, can catch up with or even surpass the fullerene-based ones. However, the detailed molecular packing structures and charge transport properties of these acceptors are rarely studied and still unclear, which has become the major obstacle for rational molecular design to further improve the photovoltaic performance. Here, we have unravelled the intermolecular arrangements in the ITIC film via atomistic molecular dynamics simulations. The simulated results point to that three-dimensional molecular packing is formed in the ITIC film through local intermolecular π–π stacking between the terminal acceptor units. In sharp contrast, the ITIC crystal grown by the slow solvent vapor diffusion approach exhibits a one-dimensional edge-to-face stacking structure. Consequently, excellent isotropic electron mobilities along three dimensions are found for the film and unprecedentedly, the overall mobility is even higher than that of the crystal. Our work suggests that judicious modulation of the terminal acceptor unit to increase local intermolecular π–π interaction would be an effective way to improve the electron mobilities and photovoltaic performance of the A–π–A electron acceptors.

Side-chain engineering is crucial to improve the performance of solution-processed organic solar cells. However, the correlation between side-chain structures and photovoltaic properties is still unclear. Here, we have investigated the local interface morphologies of PC71BM blended with two donors, DR3TBDT and DR3TSBDT with alkyloxy and alkylthio side chains on the BDT core, by means of atomistic molecular dynamics simulations. Compared with alkyloxy, alkylthio exhibits larger steric hindrance after changing the side-chain anchoring atom from oxygen to sulfur, leading to an obvious reduction of the PC71BM–BDT face-on orientations in which charge recombination is demonstrated to be the most severe by quantum-chemical calculations. This suggests that the performance of DR3TBDT/PC71BM solar cells is likely to be more affected by charge recombination than that of DR3TSBDT/PC71BM-based devices. For the first time, our work unravels the important role of side-chain anchoring atoms in tuning the donor/fullerene interfacial arrangements toward high-performance organic solar cells.

High-efficiency deep blue organometallic phosphors are imperative to organic luminescence devices. While green iridium complexes commonly exhibit high luminescence efficiencies, the luminescence quantum efficiency always drops sharply when emission becomes deep blue. In this work, the microscopic mechanism of such a drastic decrease is elucidated from detailed computational investigation. Both radiative (kr) and non-radiative (knr) decay rates of the lowest triplet state (T1) are calculated for five representative cyclometalated iridium(III) complexes with emission color ranging from green to deep blue, based on phenylpyridyl, phenylpyrazolyl, bipyridinato, pyrimidinpyridyl, and pyrimidinprazolyl ligands. For all compounds, the T1 states are characteristic of mixed intraligand (π → π*) transition and iridium-to-ligand charge transfer (d → π*), and the increased π → π* and decreased d → π* portions lead to the blue-shifted emission of 1 < 2 < 4 < 5 < 3. Strikingly, it is found that the drastic increase of knr arising from severe intra-ligand vibration relaxations induced by the enhanced π → π* transition is mainly responsible for the droop of the phosphorescence quantum efficiency, which provides a different deactivation mechanism from the thermally-activated transformation into a dark metal-centred ligand field excited state reported in many previous studies. Compared with the well-studied compounds 1–3, the newly designed compounds 4 and 5 achieve a good balance between high efficiency and a large energy gap and are very promising as deep blue phosphors. These findings are expected to be helpful for the rational design of high-efficiency blue organometallic phosphors, especially in terms of ligands.

By means of atomistic molecular dynamics simulations, we have investigated the effect of the solvent evaporation rate and thermal annealing on the molecular packing morphology of a diketopyrrolopyrrole based organic photovoltaic donor material, DPP(TBFu)2, which displays excellent hole mobility. It is observed that slow evaporation of solvent will lead to a relatively high degree of molecular packing order while leaving many voids in the as-cast sample. Upon thermal annealing, the as-cast samples at both fast and slow evaporation rates become more compact and much more apparently at the slow evaporation rate. Interestingly, the effect of thermal annealing on molecular packing order depends on the solvent evaporation rates of the as-cast samples. Upon thermal annealing, the molecular packing order of the fast evaporated sample is enhanced with increased π–π stacks. In contrast, thermal annealing will decrease the degree of packing order for the slow evaporated sample since the orientations and conformations of the molecules at the aggregate boundaries are substantially modulated to squeeze the voids. Electrical network analyses point to the fact that the mesoscopic electrical connectivities for all the samples are quite effective and insensitive to the modifications of local molecular ordering due to the delocalized HOMO of DPP(TBFu)2 providing efficient intermolecular electronic interactions. The hole mobilities of all the fabricated samples are thus estimated to be similar and quite high. Finally, our simulations point to the fact that the modest enhancement of mobility upon thermal annealing is correlated with the increased density rather than the varied ordering of molecular packing. Our work provides an atomistic insight into the evolution of thin-film morphology of organic photovoltaic molecular materials during solution processing and thermal annealing treatments and sheds light on the correlation between the molecular structure, packing morphology and hole transport capability.

The charge-transport properties in C8BTBT–FnTCNQ and DMQtT–F4TCNQ mixed-stack crystals have been investigated by means of density functional theory, molecular dynamics and kinetic Monte Carlo simulations. The super-exchange nature of charge transport in these crystals is elucidated by the Larsson partition-based electronic coupling method that was developed recently by us. Compared with hole transport, in addition to the donor HOMO–acceptor LUMO interaction, the interaction between the donor HOMO−1 and the acceptor LUMO will also make an important contribution to electron transport. Moreover, this additional interaction plays an opposite role and results in electron-dominant and hole-dominant transport in the C8BTBT–FnTCNQ and DMQtT–F4TCNQ crystals, respectively. Most importantly, our calculations point out that the nonlocal electron–phonon couplings are very weak and much smaller than the electronic couplings in all the studied crystals. This implies that the nonlocal couplings have little influence on charge transport. In contrast to the experimental measurements, the external reorganization energies are thus expected to play an essential role in determining charge carrier mobilities. These findings pave the way for rational design of high performance organic donor–acceptor mixed-stack semiconductors.

The charge-transport parameters in three 4,10-dihalogenated anthanthrones (AAOs) are investigated by means of density functional theory (DFT) and molecular dynamics (MD) calculations. Our calculations point to similar hole and electron reorganization energies for each molecule. Significant electronic couplings and bandwidths (particularly for electron transport) are found along the parallel π–π stacking directions in all the dihalogenated AAO crystals. The calculated effective masses are small or moderate for both holes and electrons. Especially for the iodinated AAO crystals, remarkable ambipolar charge transport can be anticipated due to the smallest and similar effective masses for holes and electrons (both are around 1.0 m0). In addition, due to the presence of two small effective masses, two-dimensional charge transport would take place for electrons in the chlorinated AAO crystal and for both holes and electrons in the iodinated AAO crystal. Also, our calculations reveal large nonlocal electron–phonon couplings along the π-stacks in the brominated AAO and, in particular, the chlorinated AAO crystals, which can further improve the balance in transport of holes and electrons.

Electronic delocalization at donor/acceptor (D/A) interfaces can play an important role in photocurrent generation for organic solar cells. Here, we have investigated the nature of local excited and interfacial charge transfer (CT) states in model complexes including one to four anti-parallel stacking dipolar donor (DTDCTB) molecules and one fullerene (C60) molecule by means of density functional theory (DFT) and time-dependent DFT (TDDFT). For all the donor-to-acceptor CT states, despite the number of DTDCTB molecules in the complexes, the hole is mainly localized on a single DTDCTB, and moves farther away from C60 for the energy higher level. However, the highest occupied molecular orbitals (HOMOs) and the excitonic states (EX) including the bright and dark EX are delocalized over the whole donor stacks in the complexes. This implies that the formation of ordered DTDCTB arrangements can substantially shorten the exciton diffusion process and facilitate ultrafast charge generation. Interestingly, owing to strong intermolecular Coulomb attraction, the donor-to-donor CT states are situated below the local excited states, but can approach the donor-to-acceptor CT states, indicating a weak role as charge traps. Our work would be helpful for understanding the electronic delocalization effects in organic solar cells.