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The bulk heterojunction (BHJ) morphology of photovoltaic materials is crucial to the fundamental optoelectronic properties of organic solar cells (OSCs). However, in the photoactive layer, the intrinsic crystalline packing structure of Y6, currently the hallmark molecule among Y-series non-fullerene acceptors (NFAs), has not been unambiguously determined. Here, employing grazing-incidence wide-angle X-ray scattering (GIWAXS), we managed to uncover the intrinsic crystalline packing structure of Y6 in the BHJ active layer of OSCs, which is found to be different from its single-crystal structure reported previously. Moreover, we find that solvent additive 1-chloronaphthalene (CN) can induce highly ordered packing of Y6 in BHJ thin films. With the help of atomistic molecular dynamics simulations, it is revealed that π–π interactions generally exist between naphthalene derivatives and IC terminals of Y6 analogues, which would essentially improve their long-range ordering. Our work reveals the intrinsic crystalline packing structure of Y6 in the BHJ active layer as well as its crystallization mechanism in thin films, thus providing direct correlations between this crystalline packing and the device characteristics and photophysical properties. [ABSTRACT FROM AUTHOR]

The device performance, including efficiency and stability, of polymer solar cells (PSCs) is mainly correlated with the bulk microstructure of specific active layer systems. Generally, developing a single-component (SC) active layer is an effective approach to solving the inherent shortcomings of bulk heterojunctions (BHJs). Herein, we designed and synthesized a conjugated-block copolymer (CBC) PBDB-YTCl-2 and further compared the photovoltaic performance of the CBC PBDB-YTCl-2 and BHJ PBDB-T:PYCl-2 systems. Although both PSC systems show high device efficiencies of over 13%, the contributions of relevant photovoltaic parameters are quite different. We systematically evaluated multiple target parameters, including morphological characteristics, physical kinetics, and active layer stability issues, and compared the correlations and differences between the photovoltaic systems, blend morphology, and device performance of the corresponding CBC and BHJ systems. Our work demonstrates that CBC materials are promising active layer systems, which are conducive to the reduced efficiency-stability gap of PSCs in comparison to the corresponding BHJ all-polymer systems. [ABSTRACT FROM AUTHOR]

Despite the significant progress in all-polymer solar cells (all-PSCs) in recent years, obtaining both high open-circuit voltage (VOC) and short-circuit current density (JSC) simultaneously has been a challenging issue. Herein, a novel polymer acceptor PY-DF was developed by polymerizing small molecule acceptor (SMA) monomers with difluorothiophene linkers. Compared to non-fluorinated PYT, PY-DF exhibits a more coplanar and rigid molecular conformation, which leads to better intra-molecular conjugation and enhanced interchain packing, resulting in improved electron mobility and reduced energetic disorder. Furthermore, PY-DF exhibits a relatively up-shifted lowest unoccupied molecular orbital (LUMO) energy level (−3.76 eV) than PYT (−3.80 eV), which is favorable for improving VOC. In addition, the polymer acceptor demonstrates good miscibility with polymer donor, thus leading to optimized phase segregation for superior exciton dissociation and charge transport. As a result, the PY-DF-based all-PSCs achieved a higher PCE of 15.7% with simultaneously enhanced JSC (23.1 mA cm−2) and VOC (0.97 V) in comparison with PYT-based all-PSCs (PCE = 13.2%, JSC = 21.7 mA cm−2, and VOC = 0.93 V). This work provides a promising polymer acceptor for all-PSCs and shows that fluorination of linkers is a potential strategy to build high-performance polymer acceptors. [ABSTRACT FROM AUTHOR]

Designing new high-performance non-fullerene acceptors is the key driving force for the development of organic solar cells (OSCs). In this work, a new acceptor, BOEH-4Cl, was designed based on the end-group chlorination of L8-BO. Theoretical calculations successfully predicted the expected experimental results based on the optoelectronic properties of BOEH-4Cl and L8-BO and intermolecular interaction of PM6/BOEH-4Cl or L8-BO. A high-boiling-point solvent (HBPS, chlorobenzene) was also introduced as a calculation factor, which is beneficial to future industrialization. In agreement with the calculated results, the optimized HBPS-processed BOEH-4Cl film exhibited tighter molecular packing, a more efficient interfacial hole transfer process and lower non-radiative energy loss, demonstrating necessary properties as a promising acceptor. The efficiency of optimized HBPS-processed PM6/BOEH-4Cl OSCs reached 17.4%, much higher than that of PM6/L8-BO (14.5%). Hence, this work demonstrates the great potential of utilizing theoretical chemical calculations to assist in the design of acceptor molecules to reduce time and cost. [ABSTRACT FROM AUTHOR]

Pd particles coated with ZIF-67 (Pd@ZIF-67) was prepared from the self-reduction of palladium acetate. With isopropanol as the solvent, the furfural hydrogenation reaction performances of Pd@ZIF-67 and ZIF-67 heated at different temperatures were tested in H2 and N2 reaction atmosphere, respectively. The pyrolytic carbonization of Pd@ZIF-67 and ZIF-67 produced Pd–Co@C and Co@C in carbon coating structures, respectively. It was found that furfural hydrogenation reaction systems with different catalytic active sites included direct hydrogenation with H2 as the hydrogen source, and transfer hydrogenation with isopropanol as the hydrogen donor. The Pd–Co@C and Co@C with metal active sites both experienced direct hydrogenation and transfer hydrogenation simultaneously, and the direct hydrogenation activity was higher. The ZIF-67 with Lewis acidic sites experienced transfer hydrogenation, and the catalytic activity was higher than the transfer hydrogenation activity of Pd–Co@C and Co@C. Moreover, reaction routes in which furfural direct hydrogenation and transfer hydrogenation occurred in metal active sites and Lewis acidic sites were identified. [ABSTRACT FROM AUTHOR]

All-small-molecule OSCs (ASM-OSCs) are more suitable for commercial-scale manufacturing owing to the merits of small molecules, such as well-defined chemical molecular structures, easy synthesis, and less batch-to-batch variation. With the rapid development of non-fullerene acceptors, the design of small molecule donors and the optimization of bulk heterojunction (BHJ) morphology will play a greater role in improving the power conversion efficiencies (PCE) of ASM-OSCs. Herein, a novel small molecule donor, BTR-SCl, with alkylthio and chlorine substituents on the side-chains was designed and synthesized. BTR-SCl exhibits strong absorption in the wavelength range of 400–700 nm with a wide optical bandgap of 1.77 eV, a low-lying highest occupied molecular orbital (HOMO) energy level of −5.51 eV, and strong crystallization properties. Consequently, a PCE of 14.6% was obtained from BTR-SCl:Y6 solar cells with a Voc of 0.88 V, a Jsc of 23.4 mA cm−2, and an FF of 70.8%. Notably, with the incorporation of polymer PM7 as a morphology modulator, the BTR-SCl:Y6 matrix achieved well-formed bicontinuous interpenetrating networks and ordered molecular packing. As a result, PM7-optimized devices achieved a significantly enhanced PCE of 15.8% with a higher Jsc of 24.5 mA cm−2 and FF of 73.1%. [ABSTRACT FROM AUTHOR]

Morphology optimization is key for the high-performance of organic solar cells (OSCs). Herein, we develop a solid additive (i.e. uv-9) showing immiscibility with active layers for morphology control, and comprehensively study its effect on the morphology evolution and device performance of PM6:Y6 OSCs. The addition of uv-9 leads to more refined phase separation and stronger molecular packing in PM6:Y6 blend films, and improves the charge generation, exciton dissociation, charge transport and collection, which contribute to higher photocurrent and fill factor for PM6:Y6 OSCs. Consequently, the OSC device with the uv-9 solid additive exhibits an improvement in the power conversion efficiency from 16.00% to 17.18%, compared to the control device without an additive. Moreover, the generality of uv-9 as an effective solid additive has been verified by applying it to diverse OSCs. This work suggests a new class of solid additives to optimize the morphology of OSCs for high performance. [ABSTRACT FROM AUTHOR]

In-depth understanding of the structure–property relationship in organic solar cells (OSCs) is of great importance in the development of the OSC field. In this study, we designed and synthesized two new chlorinated small molecule donor materials, named SM1-α-Cl (with α-chlorinated thiophene as a conjugated side chain) and SM1-β-Cl (with β-chlorinated thiophene as a conjugated side chain), by a side chain isomerization strategy. The morphology results indicate that the two small molecules present different aggregation characteristics, and the SM1-β-Cl:Y6 blend film displays enhanced molecular stacking properties, benefiting the charge transport. Besides, the ultrafast spectroscopic study indicates a higher charge transfer (CT) state yield in the SM1-β-Cl based blend. Therefore, the devices based on SM1-β-Cl:Y6 demonstrate efficient exciton dissociation, balanced electron/hole mobility and a higher power conversion efficiency (PCE) of 12.14%, while SM1-α-Cl:Y6 based devices give an inferior PCE of 8.93%. These results imply that side-chain isomerization of small molecule donors could be an effective method to fine-tune the morphological features and improve the photovoltaic performance. [ABSTRACT FROM AUTHOR]

In spite of the great success of all-polymer solar cells (all-PSCs) in terms of device efficiency mainly owing to the vigorous development of polymer donors (PDs) and polymer acceptors (PAs), the synergistic effects of the molecular structure and molecular weight (Mw) of PD and PA materials as well as PD–PA pair miscibility on device performance are still unclear and rarely reported. Herein, we introduced PBDB-T and its congener materials (PM6 and PM7) as PDs with comparable Mws and two PYT batches as PAs with different Mws (PYT-M and PYT-H) to deeply investigate the effects of molecular mechanisms on the device efficiency and stability in these six systems. Benefiting from proper PD–PA miscibility owing to the matched molecular structure and Mws, both PBDB-T:PYT-H and PM6:PYT-M systems with suitable phase separation show comparable device efficiencies, which are much better than those of the other four all-polymer systems. Impressively, further investigation demonstrates that the PBDB-T:PYT-H active layer is more stable than the PM6:PYT-M one, resulting from the trade-offs between molecular miscibility and Mw. This work not only employs the synergetic effect of the molecular structure and molecular weight on device efficiency and stability but also provides a promising strategy to simultaneously improve the device performance of all-PSCs. [ABSTRACT FROM AUTHOR]

With the state-of-the-art impressive power conversion efficiency (PCE) of all-polymer solar cells (all-PSCs) surpassing 17%, it has become increasingly urgent to achieve highly stable devices under long-term operational conditions. Herein, the photostability of all-PSCs based on three isomeric polymer acceptors (PAs, PYTT-x (x = 1–3)), as well as their underlying degradation mechanisms, were investigated. Impressively, the relatively amorphous PA PYTT-1 and crystalline PA PYTT-3 provided completely different degradation mechanisms and morphological evolutionary processes, deriving from domain shrinkage-related increased energetic bulk traps in the former, and increased surface-assisted recombination together with domain growth and vertical phase rearrangement in the latter. In contrast, the decent longevity of PYTT-2 with moderate crystallinity almost preserves the initial nanostructure with a slight loss of molecular ordering. The analysis further revealed that three types of degradation behaviors along with related blend microstructure evolution are correlated with the synergistic effects of intermolecular interactions and crystallization characteristics. Not only do these findings underscore the key role of photovoltaic material design regarding the regioisomerized end-capping groups in the determination of film formation and its blend microstructure evolution under light-soaking for long exposure times, but more importantly, also denote the correlation between the molecular structure and device photostability directly. [ABSTRACT FROM AUTHOR]

Researchers are endeavoring to decode the fundamental reasons for the non-fullerene acceptor, Y6, to deliver high-performance in organic solar cells. In this work, we tackle this problem from the molecular packing point of view. By gradually increasing the thermal annealing temperature of Y6, its backbone order in thin films is significantly enhanced and eventually reoriented to the "backbone-on" orientation with the backbone plane standing on the substrate. It displays well-defined Bragg peaks in two-dimensional grazing-incidence wide-angle X-ray scattering patterns for a clear crystal indexing. The 250 °C annealed pure Y6 and binary PM6:Y6 films exhibit square and rectangular diffraction patterns, respectively. It is suggested that both are polymorphs of two-dimensional Y6 packing in its backbone plane, which resulted from its "L" shaped core-group and the biaxial backbone order through the end-group π–π stacking. Interestingly, the square lattice can be restored in the blend PM6:Y6 film by adding a small amount of IDIC. However, on the other hand, a larger portion of IDIC in the ternary blend hinders the long-range crystalline packing of Y6 and correlatedly deteriorates the device performance. This work provides an in-depth understanding of the molecular packing mechanism of Y6 in thin films for the future rational design of high-performance organic photovoltaic molecules with advantageous crystalline packing motifs. [ABSTRACT FROM AUTHOR]

Phosphorus-modified Zr-metal organic framework (P/Zr-MOF) catalysts were synthesized by a (NH4)2HPO4 pyrolysis approach starting from Zr-MOFs. The furfural catalytic transfer hydrogenation performance of P/Zr-MOFs was studied with i-propanol as a hydrogen donor. Experimental results show that the furfural conversion of P/Zr-MOFs rose from 59.1% to 96.1% compared with that of Zr-MOFs. In P/Zr-MOFs compared with Zr-MOFs, a part of O–Zr–O in the MOFs was phosphated to form O–Zr–P and P–Zr–P, in which Zr–P possessed stronger Lewis acidity and basicity than Zr–O, which is favorable for hexatomic ring-catalytic reactions. Hence, P/Zr-MOFs exhibited higher Meerwein–Ponndorf–Verley reactivity. Based on this result, a plausible mechanism of catalytic modification was proposed. [ABSTRACT FROM AUTHOR]

As a member of the refractory metal carbide family of materials, TaC is a promising candidate for ultra-high temperature ceramics (UHTC) with desirable mechanical strength. TaC sample quality and therefore mechanical properties are strongly dependent on synthesis method, and atomistic origins of mechanical failure are difficult to assign. Here, we have successfully synthesized high quality densified TaC samples at 5.5 GPa and 1400 °C using the high pressure and high temperature (HPHT) sintering method, with Vickers hardness determined to be 20.9 GPa. First-principles calculations based on the recently developed strain–stress method show that the ideal indentation strength of TaC is about 23.3 GPa in the (11¯0)[001] direction, in excellent agreement with experimental results. The detailed indentation shear deformation analysis and structural snapshots from the calculations indicate that the slip dislocations of TaC layers are the main structural deformation mode during the Vickers indentation process, and that the strong directional Ta–C bonds are responsible for the high mechanical strength of TaC. HPHT synthesis is shown to produce TaC samples with superior strength, and together with accurate first-principles calculations offers crucial insights for rational design and synthesis of novel and advanced UHTC materials. [ABSTRACT FROM AUTHOR]

Modification of properties of boron nanoparticles by doping with transition metals presents a challenging problem because the number of isomers of both doped and un-doped nanoparticles rapidly increases with the nanoparticle size. Here, we perform a study of neutral and anionic Ru-doped boron clusters RuBn (n = 9–20) using the unbiased CALYPSO structural search method in combination with density functional theory calculations. Our results show that the neutral RuB9 cluster possesses a perfect planar wheel-like geometrical structure, whereas the RuBn clusters prefer structures of the half-sandwich type in the range of 10 ≤n≤ 14, drum-like type in the range of 15 ≤n≤ 18 and cage-like structures for larger n values. The geometrical structures of the lowest total energy states of the RuBn− anions are similar to those of the corresponding neutrals, except for RuB10−, RuB11−, RuB14−, RuB15− and RuB20−. The neutral RuB12 and RuB14 clusters are found to exhibit enhanced stability with respect to the rest of the RuBn clusters due to the delocalized bonding between the Ru atom and the boron host. [ABSTRACT FROM AUTHOR]

As an extension of boron based materials, transition-metal doped boron clusters deserve interest in controlling size-dependent structural and electronic properties. Herein, using the Crystal structure AnaLYsis by Particle Swarm Optimization (CALYPSO) method and density functional theory (DFT) calculations, we have performed a global search for the lowest-energy structures of ZrBQn (Q = 0, −1) clusters with n = 10–20. The results show that the ground-state structures of the obtained clusters feature a distinctive structural evolution pattern, from half-sandwich bowl to distorted drum-like and then to Zr-centered distorted tubular motifs. For the sake of validating the current ground-state structures, photoelectron spectra are predicted from time-dependent DFT calculations. More interestingly, the neutral and anionic ZrB12 clusters are found to possess enhanced stability in the size regime studied here. The stability of the closed shell half-sandwich ZrB12 cluster is analyzed by intrinsic bond orbital (IBO) and Adaptive Natural Density Partitioning (AdNDP) methods, which indicates that the stability mechanism is caused by the dopant Zr atom breaking the boron bowl's triangle B3 unit to form a quasi-linear B3 unit in B12 and strengthen both the interaction of the B–B σ-bonds and the Zr–B π-bonds. [ABSTRACT FROM AUTHOR]

Understanding the structural evolution of materials is a challenging problem of condensed matter physics. Solving this problem would open new ways for understanding the behaviors of materials. In this context, we here report unbiased structure searches for a prototypical perovskite oxide, PbTiO3, based on the CALYPSO (Crystal structure AnaLYsis by Particle Swarm Optimization) method in conjunction with first-principles calculations. For the first time, we decipher the structure evolution of PbTiO3 from a zero dimensional (0D) cluster to a two dimensional (2D) layered structure and in the end to a three dimensional (3D) bulk solid. Our unbiased structure search is successful in reproducing the cubic Pm3̅m and tetragonal P4mm phases of PbTiO3 at ambient pressure. We also predict a new quasi-planar kite shape structure of the PbTiO3 cluster, with Cs symmetry and a surprisingly large HOMO–LUMO gap. Furthermore, by using this method, we predict that the 2D planar PbTiO3 monolayer is unstable in the perpendicular direction and the 2D PbTiO3 double layer is dynamically stable, with a hope that it can provide guidance to future synthesis of low dimensional perovskite oxides. [ABSTRACT FROM AUTHOR]

The effect of Mg doping on the growth behavior and the electronic properties of aluminum clusters has been investigated theoretically using the CALYPSO (Crystal structure AnaLYsis by Particle Swarm Optimization) method in combination with density functional theory calculations. Compared to pure aluminum clusters, the structure of Mg-doped clusters shows the charming transformation with increasing atomic number. The photoelectron spectra (PES) of the global minima of anionic Aln and AlnMg (n = 3–20) clusters have been calculated based on the time-dependent density functional theory (TD-DFT) method. The reliability of our theoretical methodology is easily corroborated by the good agreement between the experimental PES and the simulated spectra. Our findings bring forth an ionic bonding with enhanced stability for the Al6Mg cluster, paired with a surprisingly large HOMO–LUMO gap, as would be expected from the magic number of 20 valence electrons. [ABSTRACT FROM AUTHOR]

As an extension of boron based materials, transition-metal doped boron clusters deserve interest in controlling size-dependent structural and electronic properties. Herein, using the Crystal structure AnaLYsis by Particle Swarm Optimization (CALYPSO) method and density functional theory (DFT) calculations, we have performed a global search for the lowest-energy structures of ZrBQn (Q = 0, -1) clusters with n = 10-20. The results show that the ground-state structures of the obtained clusters feature a distinctive structural evolution pattern, from half-sandwich bowl to distorted drum-like and then to Zr-centered distorted tubular motifs. For the sake of validating the current ground-state structures, photoelectron spectra are predicted from time-dependent DFT calculations. More interestingly, the neutral and anionic ZrB12 clusters are found to possess enhanced stability in the size regime studied here. The stability of the closed shell half-sandwich ZrB12 cluster is analyzed by intrinsic bond orbital (IBO) and Adaptive Natural Density Partitioning (AdNDP) methods, which indicates that the stability mechanism is caused by the dopant Zr atom breaking the boron bowl's triangle B3 unit to form a quasi-linear B3 unit in B12 and strengthen both the interaction of the B-B σ-bonds and the Zr-B π-bonds.

Understanding the structural evolution of materials is a challenging problem of condensed matter physics. Solving this problem would open new ways for understanding the behaviors of materials. In this context, we here report unbiased structure searches for a prototypical perovskite oxide, PbTiO 3 , based on the CALYPSO (Crystal structure AnaLYsis by Particle Swarm Optimization) method in conjunction with first-principles calculations. For the first time, we decipher the structure evolution of PbTiO 3 from a zero dimensional (0D) cluster to a two dimensional (2D) layered structure and in the end to a three dimensional (3D) bulk solid. Our unbiased structure search is successful in reproducing the cubic Pm3[combining macron]m and tetragonal P4mm phases of PbTiO 3 at ambient pressure. We also predict a new quasi-planar kite shape structure of the PbTiO 3 cluster, with C s symmetry and a surprisingly large HOMO-LUMO gap. Furthermore, by using this method, we predict that the 2D planar PbTiO 3 monolayer is unstable in the perpendicular direction and the 2D PbTiO 3 double layer is dynamically stable, with a hope that it can provide guidance to future synthesis of low dimensional perovskite oxides.