Guangchao Han

Graphdiyne Oxides as Excellent Substrate for Electroless Deposition of Pd Clusters with High Catalytic Activity

Graphdiyne (GDY), a novel kind of two-dimensional carbon allotrope consisting of sp- and sp(2)-hybridized carbon atoms, is found to be able to serve as the reducing agent and stabilizer for electroless deposition of highly dispersed Pd nanoparticles owing to its low reduction potential and highly conjugated electronic structure. Furthermore, we observe that graphdiyne oxide (GDYO), the oxidation form of GDY, can be used as an even excellent substrate for electroless deposition of ultrafine Pd clusters to form Pd/GDYO nanocomposite that exhibits a high catalytic performance toward the reduction of 4-nitrophenol. The high catalytic performance is considered to benefit from the rational design and electroless deposition of active metal catalysts with GDYO as the support.

Fine-Tuning of Crystal Packing and Charge Transport Properties of BDOPV Derivatives through Fluorine Substitution

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 cm(2) 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.

A Cofacially Stacked Electron-Deficient Small Molecule with a High Electron Mobility of over 10 cm(2) V-1 s(-1) in Air

A strong, electron-deficient small molecule, F4 -BDOPV, has a lowest unoccupied molecular orbital (LUMO) level down to -4.44 eV and exhibits cofacial packing in single crystals. These features provide F4 -BDOPV with good ambient stability and large charge-transfer integrals for electrons, leading to a high electron mobility of up to 12.6 cm(2) V(-1) s(-1) in air.

Deep‐Red to Near‐Infrared Thermally Activated Delayed Fluorescence in Organic Solid Films and Electroluminescent Devices

The 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.

Terminal π–π stacking determines three-dimensional molecular packing and isotropic charge transport in an A–π–A electron acceptor for non-fullerene organic solar cells

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.

Optimized Fibril Network Morphology by Precise Side-Chain Engineering to Achieve High-Performance Bulk-Heterojunction Organic Solar Cells

A polymer fibril assembly can dictate the morphology framework, in forming a network structure, which is highly advantageous in bulk heterojunction (BHJ) organic solar cells (OSCs). A fundamental understanding of how to manipulate such a fibril assembly and its influence on the BHJ morphology and device performance is crucially important. Here, a series of donor-acceptor polymers, PBT1-O, PBT1-S, and PBT1-C, is used to systematically investigate the relationship between molecular structure, morphology, and photovoltaic performance. The subtle atom change in side chains is found to have profound effect on regulating electronic structure and self-assembly of conjugated polymers. Compared with PBT1-O and PBT1-S, PBT1-C-based OSCs show much higher photovoltaic performance with a record fill factor (FF) of 80.5%, due to the formation of optimal interpenetrating network morphology. Such a fibril network strategy is further extended to nonfullerene OSCs using a small-molecular acceptor, which shows a high efficiency of 12.7% and an FF of 78.5%. The results indicate the formation of well-defined fibrillar structure is a promising approach to achieving a favorable morphology in BHJ OSCs.

Insertion of double bond π-bridges of A–D–A acceptors for high performance near-infrared polymer solar cells

Double bond π-bridges were introduced into an A–D–A structured n-type organic semiconductor (n-OS) IDT-IC and a new n-OS acceptor SJ-IC was synthesized. In comparison with IDT-IC, the SJ-IC film shows significantly red-shifted absorption, improved electron mobility and tuned crystallinity, which make its blend film with a polymer donor easier to form appropriate phase separation. The polymer solar cells with polymer J61 as a donor and SJ-IC as an acceptor demonstrated a higher PCE of 9.27% with a higher Jsc of 16.99 mA cm−2, while the device based on J61/IDT-IC only delivered a PCE of 6.95% with a Jsc of 13.70 mA cm−2, which should be ascribed to the red-shifted and broadened absorption of SJ-IC. Therefore, inserting double bond π-bridges into n-OS acceptor molecules is a simple and effective way to broaden and red-shift their absorption to improve their photovoltaic performance. In addition, the near-infrared absorption of the SJ-IC acceptor should be beneficial to its future application in semitransparent and tandem PSCs.

Revealing the influence of the solvent evaporation rate and thermal annealing on the molecular packing and charge transport of DPP(TBFu)2

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.

“H”-like Organic Nanowire Heterojunctions Constructed from Cooperative Molecular Assembly for Photonic Applications

“H”‐like organic nanowire heterojunctions with two parallel 2‐acetyl‐6‐dimethylamino‐naphthalene wires vertically bridged by one 2,4,5‐triphenylimidazole wire are prepared via cooperative molecular assembly in liquid phase. The exciton conversion at the junction interfaces is beneficial for the design of multichannel light‐controlled photoswitches. The results provide better understanding of molecular assembly toward specific structures and open up new prospects for the creation of novel photonic materials.

Reduction of the singlet–triplet energy gap of a thermally activated delayed fluorescence emitter by molecular interaction between the host and the emitter

We reported the comparative effect of 1,3-bis(9H-carbazol-9-yl) benzene (mCP) and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI) hosts on the PL characteristics of 2-phenyl-4′-carbazole-9H-thioxanthen-9-one-10,10-dioxide (TXO-PhCz). Strong interaction between TXO-PhCz and TPBI can be observed, leading to the lower singlet–triplet energy gap of 8.8 meV and non-monotonic increase of ΦTotle, ΦD, and ΦP with temperature. OLEDs based on TXO-PhCz:TPBI films afford a maximum current efficiency of 71.9 cd A−1, a maximum power efficiency of 45.2 lm W−1, and a maximum EQE of 23.2%.