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Cyano substitution is vital to the molecular design of polymer semiconductors toward highly efficient organic solar cells. However, how regioselectivity impacts relevant optoelectronic properties in cyano-substituted bithiophene systems remain poorly understood. Three regioisomeric cyano-functionalized dialkoxybithiophenes BTHH, BTHT, and BTTT with head-to-head, head-to-tail, and tail-to-tail linkage, respectively, were synthesized and characterized in this work. The resulting polymer semiconductors (PBDTBTs) based on these building blocks were prepared accordingly. The regiochemistry and property relationships of PBDTBTs were investigated in detail. The BTHH moiety has a higher torsional barrier than the analogs BTHT and BTTT, and the regiochemistry of dialkoxybithiophenes leads to fine modulation in the optoelectronic properties of these polymers, such as optical absorption, band gap, and energy levels of frontier molecular orbitals. Organic field-effect transistors based on PBDTBTHH had higher hole mobility (4.4 × 10−3 cm2/(V·s)) than those (ca. 10−4 cm2/(V·s)) of the other two polymer analogs. Significantly different short-circuit current densities and fill factors were obtained in polymer solar cells using PBDTBTs as the electron donors. Such difference was probed in greater detail by performing space-charge-limited current mobility, thin-film morphology, and transient photocurrent/photovoltage characterizations. The findings highlight that the BTHH unit is a promising building block for the construction of polymer donors for high-performance organic photovoltaic cells. Graphical abstract

Summary There is an urgent demand for all-polymer organic solar cells (AP-OSCs) to gain higher efficiency. Here, we successfully improve the performance to 16.09% by introducing a small amount of BN-T, a B←N-type polymer acceptor, into the PM6:PY-IT blend. It has been found that BN-T makes the active layer, based on the PM6:PY-IT:BN-T ternary blend, more crystalline but meanwhile slightly reduces the phase separation, leading to enhancement of both exciton harvesting and charge transport. From a thermodynamic viewpoint, BN-T prefers to reside between PM6 and PY-IT, and the fraction of this fine-tunes the morphology. Besides, a significantly reduced nonradiative energy loss occurs in the ternary blend, along with the coexistence of energy and charge transfer between the two acceptors. The progressive performance facilitated by these improved properties demonstrates that AP-OSCs can possibly comparably efficient with those based on small molecule acceptors, further enhancing the competitiveness of this device type.

The purpose of this study was to synthesize a magnetic material that could be easily separated by a magnetic field and combined the catalytic function of an acid/base ionic liquid with silicon for biodiesel preparation. A kind of magnetic catalyst-immobilized ionic liquid was synthesized by a three-step method. The synthesis conditions in each step were optimized by single-factor analysis. Under the optimum conditions, 206.83 mg of ionic liquid (>43.63%) was immobilized on SiO2 (per gram). Heating under reflux was applied to extract Schisandra chinensis seed oil with an average yield of 10.9%. According to the biodiesel yields, Fe3O4@SiO2@[C4mim]HSO4 was the most efficient catalyst in the methyl esterification reaction. Under the optimum reaction conditions, seed oil (10.0 g) was mixed with methanol (70 mL) under continuous mechanical stirring for 3 h, and the yield of biodiesel was 0.557 g/g (the catalyst efficiency was about 89.2%). Also, the thermal value was increased from 32.14 kJ/g (seed oil) to 38.28 kJ/g (biodiesel). The catalytic efficiency of Fe3O4@SiO2@[C4mim]HSO4 was 87.6% of the first being used after four reuse cycles, and 71.4% of the first being used after six reuse cycles in the methylation reaction. The yields and physical and chemical properties of biodiesel were determined.

Compared with p-type terpolymers, less effort has been devoted to n-type analogs. Herein, we synthesized a series of n-type terpolymers via incorporating three electron-deficient third components including thienopyrroledione (TPD), phthalimide, and benzothiadiazole into an imide-functionalized parent n-type copolymer to tune optoelectronic properties without sacrificing the n-type characteristics. Due to effects of the third components with different electron-accepting ability and solubility, the resulting three polymers feature distinct energy levels and crystallinity. In addition, heteroatoms (S, O, and N) attached on the third components trigger intramolecular noncovalent interactions, which can increase molecule planarity and have a significant effect on the packing structures of the polymer films. As a result, the best power conversion efficiency of 8.28% was achieved from all-polymer solar cells (all-PSCs) based on n-type terpolymer containing TPD. This is contributed by promoted electron mobility and face-on polymer packing, showing the pronounced advantages of the TPD used as a third component for thriving efficient n-type terpolymers. The generality is also successfully validated in a benchmark polymer donor/acceptor system by introducing TPD into the benchmark n-type polymer N2200. The results demonstrate the feasibility of introducing suitable electron-deficient building blocks as the third components for high-performance n-type terpolymers toward efficient all-PSCs.

Summary High-performance n-type (electron-transporting or n-channel) polymer semiconductors are critical components for the realization of various organic optoelectronic devices and complementary circuits, and recent progress has greatly advanced the performance of organic thin-film transistors, all-polymer solar cells, and organic thermoelectrics, to cite just a few. This Perspective focuses on the recent development of high-performance n-type polymer structures, particularly those based on the most investigated and novel electron-deficient building blocks, as well as summarizes the performance achieved in the above devices. In addition, this Perspective offers our insights into developing new electron-accepting building blocks and polymer design strategies, as well as discusses the challenges and opportunities in advancing n-type device performance.

Summary Generally, it is important to fine-tune the energy levels of donor and acceptor materials in the field of polymer solar cell (PSCs) to achieve a minimal highest occupied molecular orbital (HOMO) energy offset, which yet is still sufficient for charge separation. Based on the high-performance small-molecule acceptor (SMA) of BTP-4F, we modified the end groups of BTP-4F from IC-2F to CPTCN-Cl. It was found that when both end groups were substituted by CPTCN-Cl, the energy level upshift was too large that caused unfavorable energetic alignment, thus poor device performance. By using the strategy of asymmetric end groups, we were able to achieve near optimal energy level match, resulting in higher open-circuit voltage (VOC) and power conversion efficiency (PCE) compared with those given by the PM6:BTP-4F system. Our strategy can be useful and potentially applied to other material systems for maximizing efficiency of non-fullerene PSCs.

Chemical doping is a key process for investigating charge transport in organic semiconductors and improving certain (opto)electronic devices

Significant progress has been made in nonfullerene small molecule acceptors (NF‐SMAs) that leads to a consistent increase of power conversion efficiency (PCE) of nonfullerene organic solar cells (NF‐OSCs). To achieve better compatibility with high‐performance NF‐SMAs, the direction of molecular design for donor polymers is toward wide bandgap (WBG), tailored properties, and preferentially ecofriendly processability for device fabrication. Here, a weak acceptor unit, methyl 2,5‐dibromo‐4‐fluorothiophene‐3‐carboxylate (FE‐T), is synthesized and copolymerized with benzo[1,2‐b:4,5‐b′]dithiophene (BDT) to afford a series of nonhalogenated solvent processable WBG polymers P1‐P3 with a distinct side chain on FE‐T. The incorporation of FE‐T leads to polymers with a deep highest occupied molecular orbital (HOMO) level of −5.60−5.70 eV, a complementary absorption to NF‐SMAs, and a planar molecular conformation. When combined with the narrow bandgap acceptor ITIC‐Th, the solar cell based on P1 with the shortest methyl chain on FE‐T achieves a PCE of 11.39% with a large V oc of 1.01 V and a J sc of 17.89 mA cm−2. Moreover, a PCE of 12.11% is attained for ternary cells based on WBG P1, narrow bandgap PTB7‐Th, and acceptor IEICO‐4F. These results demonstrate that the new FE‐T is a highly promising acceptor unit to construct WBG polymers for efficient NF‐OSCs., A series of wide bandgap donor polymers are designed and synthesized by incorporating a monothiophene functionalized with both a fluorine atom and an ester group. Fabricated from nonhalogenated solvent, power conversion efficiencies of 11.39% and 12.11% are achieved for binary and ternary nonfullerene solar cells, respectively.

In article number 1801743, Xugang Guo and co‐workers develop two phthalimide‐based polymers featuring a D‐A1‐D‐A2 backbone motif. Eliminating benzodithiophene leads to polymers with substantial mobility. Nonfullerene polymer solar cells utilizing these high‐mobility polymers achieve a remarkable power conversion efficiency >13%. The results demonstrate that phthalimides are excellent building blocks for enabling polymer semiconductors with outstanding solar cell performances and benzodithiophenes are not necessary for constructing such polymers.

A novel imide-functionalized arene, di(fluorothienyl)thienothiophene diimide (f-FBTI2), featuring a fused backbone functionalized with electron-withdrawing F atoms, is designed, and the synthetic challenges associated with highly electron-deficient fluorinated imide are overcome. The incorporation of f-FBTI2 into polymer affords a high-performance n-type semiconductor f-FBTI2-T, which shows a reduced bandgap and lower-lying lowest unoccupied molecular orbital (LUMO) energy level than the polymer analog without F or with F-functionalization on the donor moiety. These optoelectronic properties reflect the distinctive advantages of fluorination of electron-deficient acceptors, yielding "stronger acceptors," which are desirable for n-type polymers. When used as a polymer acceptor in all-polymer solar cells, an excellent power conversion efficiency of 8.1% is achieved without any solvent additive or thermal treatment, which is the highest value reported for all-polymer solar cells except well-studied naphthalene diimide and perylene diimide-based n-type polymers. In addition, the solar cells show an energy loss of 0.53 eV, the smallest value reported to date for all-polymer solar cells with efficiency > 8%. These results demonstrate that fluorination of imide-functionalized arenes offers an effective approach for developing new electron-deficient building blocks with improved optoelectronic properties, and the emergence of f-FBTI2 will change the scenario in terms of developing n-type polymers for high-performance all-polymer solar cells.

Highly efficient nonfullerene polymer solar cells (PSCs) are developed based on two new phthalimide‐based polymers phthalimide‐difluorobenzothiadiazole (PhI‐ffBT) and fluorinated phthalimide‐ffBT (ffPhI‐ffBT). Compared to all high‐performance polymers reported, which are exclusively based on benzo[1,2‐b:4,5‐b′]dithiophene (BDT), both PhI‐ffBT and ffPhI‐ffBT are BDT‐free and feature a D‐A1‐D‐A2 type backbone. Incorporating a second acceptor unit difluorobenzothiadiazole leads to polymers with low‐lying highest occupied molecular orbital levels (≈−5.6 eV) and a complementary absorption with the narrow bandgap nonfullerene acceptor IT‐4F. Moreover, these BDT‐free polymers show substantially higher hole mobilities than BDT‐based polymers, which are beneficial to charge transport and extraction in solar cells. The PSCs containing difluorinated phthalimide‐based polymer ffPhI‐ffBT achieve a substantial PCE of 12.74% and a large V oc of 0.94 V, and the PSCs containing phthalimide‐based polymer PhI‐ffBT show a further increased PCE of 13.31% with a higher J sc of 19.41 mA cm−2 and a larger fill factor of 0.76. The 13.31% PCE is the highest value except the widely studied BDT‐based polymers and is also the highest among all benzothiadiazole‐based polymers. The results demonstrate that phthalimides are excellent building blocks for enabling donor polymers with the state‐of‐the‐art performance in nonfullerene PSCs and the BDT is not necessary for constructing such donor polymers.

A novel bull’s horn-shaped oligothienoacene with seven fused thiophene rings (1) based on dithieno[2,3-b:2′,3′-d]thiophene (2) was efficiently synthesized. X-ray diffraction data indicate that 1 possesses an extraordinary compressed sandwich-herringbone arrangement and shows strong intermolecular S···C and S···S interactions. In addition, the UV/vis properties, theoretical calculations, and cyclic voltammetry behaviors of 1 are also described.

Electron doping of organic semiconductors is typically inefficient, but here a precursor molecular dopant is used to deliver higher n-doping efficiency in a much shorter doping time. Chemical doping is a key process for investigating charge transport in organic semiconductors and improving certain (opto)electronic devices(1-9). N(electron)-doping is fundamentally more challenging than p(hole)-doping and typically achieves a very low doping efficiency (eta) of less than 10%(1,10). An efficient molecular n-dopant should simultaneously exhibit a high reducing power and air stability for broad applicability(1,5,6,9,11), which is very challenging. Here we show a general concept of catalysed n-doping of organic semiconductors using air-stable precursor-type molecular dopants. Incorporation of a transition metal (for example, Pt, Au, Pd) as vapour-deposited nanoparticles or solution-processable organometallic complexes (for example, Pd-2(dba)(3)) catalyses the reaction, as assessed by experimental and theoretical evidence, enabling greatly increased eta in a much shorter doping time and high electrical conductivities (above 100 S cm(-1); ref. (12)). This methodology has technological implications for realizing improved semiconductor devices and offers a broad exploration space of ternary systems comprising catalysts, molecular dopants and semiconductors, thus opening new opportunities in n-doping research and applications(12, 13).