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Nonfused-ring electron acceptors have attracted much attention in recent years due to their advantages of simple synthetic routes, high yields, low costs, reasonable power conversion efficiencies (PCEs), and so on. Herein, three simple A-π-D-π-A-type acceptors (DTC-BO-4F, DTS-BO-4F, and DTP-BO-4F) comprising a tricyclic fused-ring core, two 2,5-bis(alkyloxy)phenylene spacers, and two difluorinated terminal groups (DF-IC) were developed. Compared with DTS-BO-4F, DTC-BO-4Fand DTP-BO-4Fexhibit higher molar extinction coefficients, stronger crystallinity, and more orderly stacking. The PBDB-T:DTC-BO-4F-based blend film shows suitable phase separation and higher and more balanced charge mobilities. Finally, the photovoltaic devices based on DTC-BO-4Fgive an outstanding PCE of 13.26% with a small nonradiative voltage loss of 0.23 eV.

Pioneering work referring to perylene diimide (PDI) acceptors has been conducted for a long period, and great efforts have been devoted to hinder the excessive self-aggregation. The main strategy adopted is constructing a three-dimensional molecular structure either with a twisted core or by forming a congestion center. In addition, it is especially important to maintain an essential intermolecular stacking affinity to endow the acceptor with a high charge-transport ability. With these considerations in mind, herein, we adopt a saddle-shaped cyclooctatetrathiophene (COTh) during the PDI acceptor design and integrate the flexibility of COTh into the final PDI acceptor molecules. The geometries for these acceptors have been verified by quantum calculation, and the main excitations have been well analyzed, which demonstrates that the linking manners between PDI and COTh would directly impact their entire configurations, conjugations, and electronic wave-function distributions. The three-dimensional skeleton guarantees the isotropic crystalline characters and effectively suppresses the severe aggregation. When the saddle feature and the large π-extended characteristic intimately combine in COTh-FPDI, balanced mobility and favorable morphology can be achieved. Finally, a power conversion efficiency (PCE) of 8.37% can be accomplished by the as-cast polymer solar cells of the COTh-FPDI:PBDB-T blend. All of the results demonstrate that this flexible saddle core could provide a new block for the novel PDI-based acceptor design in photovoltaic applications.

Thiophene-based organic semiconductors used as the active components have received much attention. Their photoelectric properties can be easily tuned with various substitutions at different positions on molecular structures. Here, we synthesized series cyclopentadithiophene (CDT) derivatives with sulfur atoms at ortho- (o-CDT), meta- (m-CDT) and para-position (p-CDT) of the bridge carbon. These CDT derivatives were substituted by carbonyl/dicyanomethylene at the bridge position and/or by phenyl groups at α position, respectively. Due to the different conjugation extent and the variation of donor-acceptor (D-A) interaction originating from the change of sulfur atom position, diverse absorption spectra were observed. Especially for dicyanomethylene substituted o-CDT with phenyl as substitution group (DPCN-o-CDT), its absorption spectrum covers the whole region of visible light. Combining with the electrochemical behaviors and theoretical calculations, it was found that the sulfur atoms mainly contribute to the molecular conjugation in these CDT derivatives, especially for o-CDT derivatives. For phenyl groups, they primarily act as electron donor in m-CDT derivatives, and chiefly contribute to molecular conjugation in p-CDT derivatives, and simultaneously work as electron donor and conjugation component in o-CDT derivatives, respectively.

A new centrosymmetrical dipyran unit (DTDP) is successfully prepared by means of an efficient and universal way, and a series of PDTDPpolymers have been prepared so as to assess their potential application in organic photovoltaic. The function of pyran moiety is not merely limited to tune the electron-donating roles and energy levels but it also contributes to solubility improvement. Interestingly, all pyran-based polymers displayed wide absorption ranging from 350 to 780 nm, but varied aggregation phenomena are observed. Furthermore, the quantum chemistry calculations for dimers, morphology study and grazing-incidence wide-angle X-ray scattering analysis for blend films have been utilized to understand the variations at photovoltaic performances. Finally, PDTDP-4achieved the highest power conversion efficiency of 7.26% (Voc= 0.72 V, FF = 0.66, and Jsc= 15.30 mA/cm2), demonstrating promising usage for high-efficiency polymer donors in polymer solar cells. In all, not only a promising dipyran building block is provided by this study, more dipyran derivatives and polymers could be prepared via this facile synthetic route.

The rapid advance of fused-ring electron acceptors (FREAs) has made them a potential substitute to fullerene-based acceptors and offered new avenues for the construction of organic solar cells (OSCs). Nonfused-ring acceptors (NFRAs) could significantly reduce the synthetic cost while achieving reasonable power conversion efficiencies (PCEs). Widely used fullerene acceptors have been applied as a second acceptor to regulate the morphology, absorption, and electron transport. To take full advantage of both nonfullerene and fullerene acceptors at the same time, we rationally designed and synthesized two novel NFRAs with phenyl-C61-butyric acid methyl ester (PCBM) as the lateral pendent. With the incorporation of fullerene pendent in PCBM-C6and PCBM-C10, varied UV–vis absorption and photoluminescence (PL) quenching behaviors were observed, and isotropic diffraction patterns were obtained via grazing incidence wide-angle X-ray scattering (GIWAXS) measurements. The bulky, spherical, and electronic isotropic fullerene pendent could effectively suppress severe molecular aggregation and form the preferred blend morphology. This strategy significantly improved the efficiencies for exciton separation and charge collection relative to the control acceptor CH3COO-C6. Finally, the Voc, Jsc, and fill factor (FF) of PCBM-C10-based devices were simultaneously improved and an enhanced PCE of 13.55% was accomplished.

Single pure organic molecular white light emitters (SPOMWLEs) are of significance as a new class of material for white lighting applications; however, few of them are able to emit white electroluminescence from organic light-emitting diodes. Herein, donor−π–acceptor conjugated emitters, 2PQ–PTZ and 4PQ–PTZ, were designed and synthesized as SPOMWLEs for white light emission considering the distinct advantages of their conformation isomers. The coexistence of conformational isomers in 2PQ–PTZ, which is the first experimental evidence of the coexisting quasi-axial and quasi-equatorial conformers, provides ideal flexibility to obtain white light emission from their simultaneous and well-separated fluorescence and thermally activated delayed fluorescence. With these remarkable properties, a 2PQ–PTZ-based white light-emitting diode (LED) with a CIE of (0.32, 0.34) and color rendering index (CRI) of 89 is demonstrated. Further, the white organic light-emitting diode (OLED) of 2PQ–PTZ exhibits a high external quantum efficiency (EQE) of 10.1%, which is the reported highest performance among SPOMWLE-based OLEDs.

We precisely design and synthesize two A−π–D−π–A type dipyran-cored nonfullerene acceptors (NFAs) Ph-DTDPo-OTand Ph-DTDPi-OTwith oxygen atoms at the outer and inner positions, respectively. 3-Hexyloxythiophene is used as the π-spacer to expand the effective conjugation length of the acceptors. These two NFAs possess similar optical band gaps and energy levels. However, the position of the oxygen atom at the dipyran core can markedly influence the molecular packing and aggregation behavior of the resulted acceptors. Ph-DTDPo-OTwith a strong intermolecular affinity tends to form larger aggregates blending with PBDB-T, leading to a lower photovoltaic performance; Ph-DTDPi-OTpresents good miscibility with PBDB-Tand the blend films preferentially adopt a face-on orientation. Ph-DTDPi-OT-based devices display high and balanced hole and electron mobilities, leading to an optimal power conversion efficiency of 11.38%, which is much higher than those of Ph-DTDPo-OT-based ones (7.60%). Moreover, Ph-DTDPi-OT-based devices also exhibit a lower nonradiative recombination voltage loss of 0.268 eV. Our work demonstrates that the π-spacer and chemical structure of the core unit can greatly influence the molecular packing and the morphology of blend films, which are critical to the photovoltaic performance of devices.

Nonfullerene acceptors (NFAs), which usually possess symmetric skeletons, have drawn great attention in recent years due to their pronounced advantages over the fullerene counterparts. Moreover, breaking the symmetry of NFAs could fine tune the molecular dipole, solubility, energy level, intermolecular interaction, molecular packing, crystallinity, etc., and give rise to improved photovoltaic performance. Currently, there are three main strategies for the design of asymmetric NFAs. This review highlights the recent advances of high-performance asymmetric NFAs and briefly outlooks the materials exploration for the future.

Two A−π–D−π–A-type non-fullerene acceptors (IDT-ToFICand IDT-TiFIC) with 5-hexylthienyl chains substituted at the inner and outer β-positions of the thiophene π-bridge have been designed, respectively. Impacts of varied positional modifications are systematically studied. By utilizing PBDB-T as the donor, polymer solar cells are constructed with these two molecules as acceptors. Power conversion efficiencies of 11.09 and 9.46% are acquired for IDT-ToFIC- and IDT-TiFIC-based devices, respectively. Our studies have demonstrated that the use of thiophene spacers carrying one conjugated side chain at different positions can markedly enhance the photovoltaic properties relative to the corresponding control molecule IDTT2F.

Molecular electronic structure plays a vital role in the photovoltaic performances in nonfullerene polymer solar cells (PSCs) due to their influences on light-harvesting, charge carrier transfer, π-π stacking and morphology tuning etc. Oxygen as an electron donating atom, which has been incorporated into the nonfullerene acceptors (NFAs) system in the alkoxy forms at central, terminal or π bridged moieties, could effectively tune the electron donating ability, absorption spectra, energy levels of the highest occupied molecular orbital (HOMO) & lowest unoccupied molecular orbital (LUMO) etc. In this contribution, a novel centrosymmetric pyran based ladder-typed conjugated building block (DTDP) has been designed and synthesized via a facile route and it was applied as the donor (D) core in acceptor-donor-acceptor (A-D-A) type NFAs design to evaluate its potential usage in polymer solar cells. This incorporated-O in the conjugated backbones effectively elevated the electron donating ability for the D moiety and wide absorption behaviors are observed. Although slight bent appears at the central part of the DTDPblock from the geometry quantum optimization, strong crystallinity are found for both molecules. Photovoltaic performances of the solar cell devices are systematically investigated with poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene)-co-(1,3-bis(5-thiophene-2-yl)−5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c]dithiophene-4,8-dione)] (PBDB-T) as the polymer donor and a high power conversion efficiency (PCE) of 9.39% (Voc= 0.80V, Jsc= 16.82mA/cm2and FF= 69.95%) could be afforded by DTDP-MICafter the preliminary optimization, demonstrating the great promise in PSCs. Meanwhile, the morphologies as well as the carrier mobilities of the blend films are gleaned to assist understanding the structure-property relationships. Overall, the study in this work not only provides a promise ladder-typed dipyran donor unit for NFAs PSCs, it also paves a new way to use oxygens in small molecule NFAs design.

Heat and mass transfer between porous media and fluid is a complex coupling process, which is widely used in various fields of engineering applications, especially for natural and artificial fractures in oil and gas extraction. In this study, a new method is proposed to deal with the flow and heat transfer problem of steady flow in a fracture. The fluid flow in a fracture was described using the same method as Mohais, who considered a fracture as a channel with porous wall, and the perturbation method was used to solve the mathematical model. Unlike previous studies, the shear jump boundary condition proposed by Ochoa-Tapia and Whitaker was used at the interface between the fluid and porous media. The main methods were perturbation analysis and the application of shear jump boundary conditions. The influence of permeability, channel width, shear jump degree and effective dynamic viscosity on the flow and heat transfer in the channel was studied by analysing the analytical solution. The distribution of axial velocity in the channel with the change of the typical parameters and the sensitivity of the heat transfer was obtained.

Fluorescent conjugated polymers (CPs) have been successfully used to make conjugated polymer nanoparticles (CPNs) for cellular imaging, bio-orthogonal labeling, and in vivotumor targeting, since CPNs have superior bright fluorescent emission and photostability. However, the preparation of red fluorescent CPNs with reasonably high quantum yield (QY) is still very challenging. We have synthesized bright red fluorescence CPNs based on a conjugated polymer that contains an annulated pyran ring and branched alkoxy chains, which have helped to reduce close packing in the solid phase. We examined the impact of the molecular weight of CPs on the QYs of CPNs. QY as high as 51.8–36.8% was observed in all CPNs with normally used CPs, with a surfactant mass ratio of 1 : 2. Our CPNs provided intense and stable fluorescence which could be traced through several generations in cell imaging, indicating the considerable photostability and biocompatibility of our CPNs.

With the efficient synthesis of the crucial dibenzopyran building block, a series of PDBPTBTpolymers containing different alkyl side chains and/or fluorine substitution were designed and synthesized via the microwave-assisted Suzuki polycondensation. Quantum chemistry calculations based on density functional theory indicated that different substitutions have significant impacts on the planarity and rigidity of the polymer backbones. Interestingly, the alkyloxy chains of PDBPTBT-4tend to stay in the same plane with the benzothiadiazole unit, but the others appear to be out of plane. With the S···O and F···H/F···S supramolecular interactions, the conformations of the four polymers will be locked in different ways as predicted by the quantum chemistry calculation. Such structural variation resulted in varied solid stacking and photophysical properties as well as the final photovoltaic performances. Conventional devices based on these four polymers were fabricated, and PDBPTBT-5displayed the best PCE of 5.32%. After optimization of the additive types, ratios, and the interlayers at the cathode, a high PCE of 7.06% (Voc= 0.96 V, Jsc= 11.09 mA/cm2, and FF = 0.67) is obtained for PDBPTBT-5with 2.0% DIO as the additive and PFN-OX as the electron-transporting layer. These results indicated DBP-based conjugated polymers are promising wide band gap polymer donors for high-efficiency polymer solar cells.

Thiophene-based organic semiconductors used as the active components have received much attention. Their photoelectric properties can be easily tuned with various substitutions at different positions on molecular structures. Here, we synthesized series cyclopentadithiophene (CDT) derivatives with sulfur atoms at ortho- (o-CDT), meta- (m-CDT), and para-positions (p-CDT) of the bridge carbon. These CDT derivatives were substituted by carbonyl/dicyanomethylene at the bridge position and/or by phenyl groups at the α position, respectively. Due to the different conjugation extent and the variation of donor–acceptor (D–A) interaction originating from the change of sulfur atom position, diverse absorption spectra were observed. Especially for dicyanomethylene substituted o-CDT with phenyl as substitution group (DPCN-o-CDT), its absorption spectrum covers the whole region of visible light. Combining with the electrochemical behaviors and theoretical calculations, it was found that the sulfur atoms mainly contribute to the molecular conjugation in these CDT derivatives, especially for o-CDT derivatives. For phenyl groups, they primarily act as electron donor in m-CDT derivatives, and chiefly contribute to molecular conjugation in p-CDT derivatives, and simultaneously work as electron donor and conjugation component in o-CDT derivatives, respectively.

Four metal-free organic dyes (Dye-1, Dye-2, Dye-3, and Dye-4) comprising a triphenylamine moiety as the electron donor, a cyanoacrylic acid or carboxylic acid moiety as the electron acceptor and anchoring groups, and phenylethyne as the bridge were designed at the molecular level and synthesized for use in dye-sensitized solar cells (DSSCs). Their absorption spectra and electrochemical and photovoltaic properties were fully characterized, and the electron distribution with the different acceptors was calculated using density functional theory at the B3LYP/6-31G level. All of these dyes performed as sensitizers for the DSSC test, and the cyanoacrylic acid Dye-4 showed an overall conversion efficiency of 3.61% (Jsc= 9.30 mA/cm2, Voc= 0.73 V, ff = 0.49) under AM 1.5 irradiation (100 mW/cm2), which reached 79% with respect to that of an N719-based device fabricated under similar conditions. Meanwhile, the photovoltaic performance data of these phenylethyne-bridged dyes showed a higher open circuit voltage (0.70−0.79 V).

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, CH3–2Fcan form ordered molecular stacking and a face-on orientation in the donor/acceptor blend film. A single-crystal analysis demonstrates that CH3–2Fcan 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.

The reaction proceeds under mild conditions to give synthetically important functionalized hydrazides in good yield.