ConspectusThe active layer in a solution processed organic photovoltaic device comprises a light absorbing electron donor semiconductor, typically a polymer, and an electron accepting fullerene acceptor. Although there has been huge effort targeted to optimize the absorbing, energetic, and transport properties of the donor material, fullerenes remain as the exclusive electron acceptor in all high performance devices. Very recently, some new non-fullerene acceptors have been demonstrated to outperform fullerenes in comparative devices. This Account describes this progress, discussing molecular design considerations and the structure–property relationships that are emerging.The motivation to replace fullerene acceptors stems from their synthetic inflexibility, leading to constraints in manipulating frontier energy levels, as well as poor absorption in the solar spectrum range, and an inherent tendency to undergo postfabrication crystallization, resulting in device instability. New acceptors have to address these limitations, providing tunable absorption with high extinction coefficients, thus contributing to device photocurrent. The ability to vary and optimize the lowest unoccupied molecular orbital (LUMO) energy level for a specific donor polymer is also an important requirement, ensuring minimal energy loss on electron transfer and as high an internal voltage as possible. Initially perylene diimide acceptors were evaluated as promising acceptor materials. These electron deficient aromatic molecules can exhibit good electron transport, facilitated by close packed herringbone crystal motifs, and their energy levels can be synthetically tuned. The principal drawback of this class of materials, their tendency to crystallize on too large a length scale for an optimal heterojunction nanostructure, has been shown to be overcome through introduction of conformation twisting through steric effects. This has been primarily achieved by coupling two units together, forming dimers with a large intramolecular twist, which suppresses both nucleation and crystal growth. The generic design concept of rotationally symmetrical aromatic small molecules with extended π orbital delocalization, including polyaromatic hydrocarbons, phthalocyanines, etc., has also provided some excellent small molecule acceptors. In most cases, additional electron withdrawing functionality, such as imide or ester groups, can be incorporated to stabilize the LUMO and improve properties. New calamitic acceptors have been developed, where molecular orbital hybridization of electron rich and poor segments can be judiciously employed to precisely control energy levels. Conformation and intermolecular associations can be controlled by peripheral functionalization leading to optimization of crystallization length scales. In particular, the use of rhodanine end groups, coupled electronically through short bridged aromatic chains, has been a successful strategy, with promising device efficiencies attributed to high lying LUMO energy levels and subsequently large...

Conventional semiconducting polymer synthesis typically involves transition metal-mediated coupling reactions that link aromatic units with single bonds along the backbone. Rotation around these bonds contributes to conformational and energetic disorder and therefore potentially limits charge delocalisation, whereas the use of transition metals presents difficulties for sustainability and application in biological environments. Here we show that a simple aldol condensation reaction can prepare polymers where double bonds lock-in a rigid backbone conformation, thus eliminating free rotation along the conjugated backbone. This polymerisation route requires neither organometallic monomers nor transition metal catalysts and offers a reliable design strategy to facilitate delocalisation of frontier molecular orbitals, elimination of energetic disorder arising from rotational torsion and allowing closer interchain electronic coupling. These characteristics are desirable for high charge carrier mobilities. Our polymers with a high electron affinity display long wavelength NIR absorption with air stable electron transport in solution processed organic thin film transistors.

A novel photoactive polymer with two different molecular weights is reported, based on a new building block: thieno[3,2‐b][1]benzothiophene isoindigo. Due to the improved crystallinity, optimal blend morphology, and higher charge mobility, solar‐cell devices of the high‐molecular‐weight polymer exhibit a superior performance, affording efficiencies of 9.1% without the need for additives, annealing, or additional extraction layers during device fabrication.

We report a detailed comparison of absorption spectroscopy, electrochemistry, DFT calculations, fieldeffect charge mobility, as well as organic photovoltaic characteristics between thiophene-and selenophene-bridged donor-acceptor low-band-gap copolymers. In these copolymers, a significant reduction of the band-gap energy was observed for selenophene-bridged copolymers by UV-visible absorption spectroscopy and cyclic voltammetry. Field-effect charge mobility studies reveal that the enhanced hole mobility of the selenophene-bridged copolymers hinges on the solubilising alkyl side chain of the copolymers. Both cyclic voltammetry experiments and theoretical calculations showed that the decreased band-gap energy is mainly due to the lowering of the LUMO energy level, and the raising of the HOMO energy level is just a secondary cause. These results are reflected in a significant increase of the short circuit current density (J SC ) but a slight decrease of the open circuit voltage (V OC ) of their bulk-heterojunction organic photovoltaics (BHJ OPVs), of which the electron donor materials are a selenophene-bridged donor-acceptor copolymer: poly{9-dodecyl-9H-carbazole-alt-5,6bis(dodecyloxy[1,2,5]-thiadiazole} (pBDTSe), or a thiophene-bridged donor-acceptor copolymer: poly{9-dodecyl-9Hcarbazole-alt-5,6-bis(dodecyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole} (pCzS) or poly {4,8-bis(2-ethylhexyloxy)benzo[1,2-b;4,5-b 0 ]dithiophene-alt-5,6-bis(dodecyloxy)-4,7-di(thiophen-2-yl) benzo[c][1,2,5]-thiadiazole} (pBDTS); the electron acceptor material is [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Judging from our device data, the potential Se-Se interactions of the selenophene-bridged donor-acceptor copolymers, which is presumably beneficial for the fill factor (FF) of BHJ OPVs, is rather susceptible to the device fabrication conditions.

A novel fused heterocycle-flanked diketopyrrolopyrrole (DPP) monomer, thieno[2,3-b]pyridine diketopyrrolopyrrole (TPDPP), was designed and synthesized. When copolymerized with 3,4-difluorothiophene using Stille coupling polymerization, the new polymer pTPDPP-TF possesses a highly planar conjugated polymer backbone due to the fused thieno[2,3b]pyridine flanking unit that effectively alleviates the steric hindrance both with the central DPP core as well as the 3,4-difluorothiophene repeat unit. This new polymer exhibits a high electron affinity (EA) of-4.1 eV and was successfully utilized as an n-type polymer semiconductor for applications in organic field-effect transistors (OFETs) and all polymer solar cells. A promising ntype charge carrier mobility of 0.1 cm 2 V-1 s-1 was obtained in bottom-contact, top-gate OFETs and power conversion efficiency (PCE) of 2.72 % with a high open-circuit voltage (V OC) of 1.04 V was achieved for all polymer solar cells using PTB7-Th as the polymer donor. INTRODUCTION Solution-processable polymer semiconductors are promising candidates to realize next-generation lightweight and flexible electronics, such as organic field-effect transistors (OFETs) and organic solar cells (OSCs), via low cost and large area printing. The last decade has witnessed a tremendous advance in p-type polymer semiconductors with hole mobilities surpassing 10 cm 2 V-1 s-1 in OFETs 1,2 and power conversion efficiencies (PCEs) above 11% in OSCs. 3,4 To achieve broad applications of organic electronics such as organic complementary circuits and all polymer solar cells, congruent performances for both p-and n-type polymer semiconductors are desired. However, the progress in n-type polymer semiconductor development

An all carbon non-fullerene electron acceptor material based on diindeno[1,2-g:1′,2′-s]rubicene (DIR) was readily synthesized and processed for bulk-heterojunction organic solar cells.

A series of new 9,10-diphenylanthracene-based, 2,6-linked blue-light-emitting copolymers bearing hole-or electron-transporter as well as bulky substituent were successfully synthesized. Photophysical, thermal, electrochemical, and electroluminescence (EL) properties of these copolymers were studied and characterized. Bright and efficient blue fluorescence in the solid state was achieved by incorporating bulky substituent into the copolymer backbone. Both hole-and electron-transport-substituted copolymers apparently enhanced the electroluminescent performance of their polymeric light-emitting diodes (PLEDs). A diphenylvinyl-bearing copolymer (pDPV) PLED exhibited sky-blue EL (λ max EL =473 nm, CIE x,y =0.16, 0.28) with peak luminous efficiency of 2.21 cd/A; a N-carbazole bearing copolymer (pCBZ) PLED displayed a blue EL (λ max EL =469 nm, CIE x,y =0.15, 0.22) with peak luminous efficiency of 2.15 cd/A. OXD-7 (1,3-bis(2-(4-tertbutylphenyl)-1,3,4-oxadiazol-5-yl)benzene) as an electron-transporting dopant was found to improve the performance of PLED significantly. A better balanced hole/electron charge carrier was ascribed to electrontransporting, 1,3,4-oxadiazole-bearing copolymer (pOXD) PLED. It showed a very mild efficiency rolls off: only 0.13 cd/A luminous efficiency drops from current densities of 10-100 mA/cm 2 , corresponding to EL brightness of 169-1558 cd/m 2 .

The development, optimization, and assessment of new methods for the preparation of conjugated materials is key to the continued progress of organic electronics. Direct C–H activation methods have emerged and developed over the last 10 years to become an invaluable synthetic tool for the preparation of conjugated polymers for both redox-active and solid-state applications. Here, we evaluate direct (hetero)­arylation polymerization (DHAP) methods for the synthesis of indaceno­[1,2-b:5,6-b′]­dithiophene-based polymers. We demonstrate, using a range of techniques, including direct visualization of individual polymer chains via high-resolution scanning tunneling microscopy, that DHAP can produce polymers with a high degree of regularity and purity that subsequently perform in organic thin-film transistors comparably to those made by other cross-coupling polymerizations that require increased synthetic complexity. Ultimately, this work results in an improved atom economy by reducing the number of synthetic steps to access high-performance molecular and polymeric materials.