Molecular electronic structure plays a vital role in the photovoltaic performances in polymer solar cells (PSCs) due to their influences on light-harvesting, charge carrier transfer, π–π stacking, etc. Indacenodithiophene as a star unit has been well studied in PSCs; various structural derivation methods have been tried, but they are still not efficient in improvement of power conversion efficiencies (PCE) due to the narrow optical absorptions. In this contribution, a novel planar DMIDT with extended lateral π-electron delocalization is efficiently synthesized via introduction of sp2 hybrid carbons as the bridge atoms. Based on this novel building block, a two-dimensional conjugated polymer PDMIDT-TPD is prepared, and the unique structure improves the conjugation at the lateral direction, enlarges the electron delocalization area, and greatly broadens the absorption spectrum with a full coverage from 350 to 700 nm. Finally, a PCE of 8.26% is achieved when blended with PC71BM, which is the highest result among the IDT-based polymer donors. Meanwhile, PDMIDT-TPD also presents good compatibility with the non-fullerene acceptor, and a preliminary PCE of 6.88% is obtained. In all, this work not only provides an excellent donor material but also offers a general and simple derivation strategy for fused aromatic building blocks.

Three planar nonfullerene acceptors (FTIC-C8C6, FTIC-C6C6 and FTIC-C6C8) comprising a central fluorenedicyclopentathiophene (FT) core and two 2-methylene-(3-(1,1-dicyanomethylene)-indanone) terminal groups are designed and synthesized. The coplanarity of the molecular backbone can be maintained through a locked conformation via intramolecular noncovalent interactions. The solubility of these nonfullerene acceptors is very good because the FT core can bear enough flexible aliphatic side-chain substitutions. Thus, the dilemma of the planarity–solubility tradeoff can be minimized. Through changing the length of the six flexible aliphatic side chains at the central FT core, we can easily adjust the π–π interactions of nonfullerene acceptors and optimize the nanoscale morphology of the photoactive layers. Among these three small molecular acceptors, FTIC-C6C8 based active layers show the best morphology together with the highest electron and hole mobility. These inherent advantages of FTIC-C6C8 guarantee it a high power conversion efficiency of 11.12% when used in non-fullerene polymer solar cells with a wide-bandgap polymer donor PBDB-T. Our results provide an appropriate molecular design strategy for building high-performance nonfullerene acceptors and show that optimizing alkyl-side chains is a very effective way to further improve the photovoltaic performance of devices.

Fluorescent conjugated polymers (CPs) have been successfully used to make conjugated polymer nanoparticles (CPNs) for cellular imaging, bio-orthogonal labeling, and in vivo tumor 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.

We demonstrate facile and efficient construction of conjugated double helical ladder oligomers from the saddle-shaped cyclooctatetrathiophene (COTh) building blocks. The key step involves deprotonation of tetra[3,4]thienylene (β,β-COTh) with n-BuLi which displays remarkably high ipsilateral selectivity. Three racemic double helical ladder oligomers, rac-DH-1, rac-DH-2, and rac-DH-3, containing two, three, and five COTh annelated moieties are efficiently synthesized by diastereoselective coupling of the racemic precursors. The X-ray crystallographic studies of rac-DH-1, rac-DH-2 and rac-DH-3 unambiguously revealed that each double helical scaffold has two single helices intertwined with each other via the C–C single bonds. Following removal of TMS groups, double helical ladder oligomer rac-DH-1-D had sufficient solubility to be resolved via chiral HPLC, thus enabling determination of its chirooptical properties such as CD spectra and optical rotation. (+)-DH-1-D has a large barrier for racemization, with lower limit of ΔG‡ > 48 kcal mol–1, which may be compared to DFT-computed barrier of 51 kcal mol–1. The enantiomers of DH-1-D show 1 order of magnitude stronger chirooptical properties than the carbon–sulfur [7]helicene, as determined by the anisotropy factor g = Δε/ε = −0.039, based on Δεmax = −11 and ε = 2.8 × 102 L mol–1 cm–1 in cyclohexane at 327 nm.

With the efficient synthesis of the crucial dibenzopyran building block, a series of PDBPTBT polymers 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-4 tend 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-5 displayed 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-5 with 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.Keywords: dibenzopyran; electron-transporting layer; polymer solar cells; quantum chemistry calculations; supramolecular interaction; wide band gap

•Different dithienosilole building blocks are prepared with different alkyl side chains.•A series of dithienosilole based conjugated copolymers are prepared.•Side chain plays an important role in morphology and photovoltaic performance.A series of dithienosilole copolymers are designed and prepared from dithienosilole monomers with different acyl groups and benzodithiophene as the comonomer. Microwave assisted Stille coupling method is employed for the polymerization. The photophysical, thermal, electrochemical and photovoltaic properties of the new polymers were characterized. The series of the polymers presented similar absorption behavior, but slight differences could be observed. Finally, the polymers were evaluated as the donor materials for polymer solar cells, which all presented high open-circuit voltage around 1.0 V, and the branched 3,7-dimethyloctane modified polymer displayed the best performance with a PCE of 2.66% (Voc = 1.01 V, Jsc = 6.71 mA/cm2 and FF = 0.39). This photovoltaic variation might be attributed to the morphology difference raised by the substituted alkyl chains.

Two novel helicene-like molecules based on naphthotetrathiophene are successfully synthesized. All target molecules and intermediates are characterized by 1H NMR, 13C NMR, IR, and HRMS. Their electrochemical and photophysical properties are studied. The configurations of the molecules are optimized by DFT quantum calculations and UV–vis behaviors are also predicted to further understand the origin of different absorption bands. We believe the current work illustrated an efficient way for the design and synthesis of sophisticated structures with naphthotetrathiophene as building blocks.

The unsymmetric dithieno[3,2-b:3′,4′-d]thiophene (ts-DTT) was efficiently synthesized, and two novel heptathienoacenes with linear and bull’s horn shapes were designed and prepared via different ring cyclization connection manners. All intermediates and aimed heptathienoacenes were fully characterized by 1H NMR, 13C NMR, and HRMS. Their UV–vis absorption behavior, fluorescence, and electrochemical properties are characterized. In addition, DFT quantum calculation was employed to further understand the electron distribution and the origin of the absorption bands.

Four cross-conjugated butterfly-shaped molecules were designed and synthesized with the branched tetrakis(thiophene-2-yl)ethene or the planar naphthotetrathiophene as the donors and dicyanovinylene as the acceptor. Owing to the donor–acceptor conjugated structures, these molecules exhibited broad and strong absorbance in the UV–vis region. Their electrochemical and photophysical properties were studied; in addition, the UV–vis behaviors were also described by virtue of DFT calculations to further understand the origins of different absorption bands and efficient charge transfer was observed for given optical transitions from the ground states to the excited states in natural transition orbitals. Finally, these butterfly-shaped molecules were applied to fabricate organic photovoltaic devices and we believe the current work illustrated an efficient way for the design and synthesis of sophisticated structures with the tetrathiophene building blocks.

Based on the selectivity of deprotonation of 5,5′-bistrimethylsilyl-2,3′-bithiophene (4) in the presence of n-BuLi, three new cyclooctatetrathiophenes (COThs), COTh-1, COTh-2, and COTh-3 have been efficiently developed via intermolecular or intramolecular cyclizations. Their crystal structures clearly show that the different connectivity sequence of the thiophene rings in the molecules. The CV data and UV–vis absorbance spectra of COThs are also described. In addition, the time-dependent density functional theory (TDDFT) calculations accurately reproduce experimental observations and afford band assignment.

Three planar nonfullerene acceptors (FTIC-C8C6, FTIC-C6C6 and FTIC-C6C8) comprising a central fluorenedicyclopentathiophene (FT) core and two 2-methylene-(3-(1,1-dicyanomethylene)-indanone) terminal groups are designed and synthesized. The coplanarity of the molecular backbone can be maintained through a locked conformation via intramolecular noncovalent interactions. The solubility of these nonfullerene acceptors is very good because the FT core can bear enough flexible aliphatic side-chain substitutions. Thus, the dilemma of the planarity–solubility tradeoff can be minimized. Through changing the length of the six flexible aliphatic side chains at the central FT core, we can easily adjust the π–π interactions of nonfullerene acceptors and optimize the nanoscale morphology of the photoactive layers. Among these three small molecular acceptors, FTIC-C6C8 based active layers show the best morphology together with the highest electron and hole mobility. These inherent advantages of FTIC-C6C8 guarantee it a high power conversion efficiency of 11.12% when used in non-fullerene polymer solar cells with a wide-bandgap polymer donor PBDB-T. Our results provide an appropriate molecular design strategy for building high-performance nonfullerene acceptors and show that optimizing alkyl-side chains is a very effective way to further improve the photovoltaic performance of devices.