Finding effective molecular design strategies to optimize the active layer blend morphology is among the long-standing challenges in developing efficient allpolymer solar cells (all-PSCs). Here we show that new biselenophene/selenophenelinked naphthalene diimide random copolymer acceptors BSSx (x = 10, 20, 50) facilitate the achievement of high-performance all-PSCs without the use of any solution processing additive. Blends of BSS10 with donor polymer PBDB-T combined 10.1% power conversion efficiency with 97% internal quantum efficiency and 0.59 eV optical band gap energy loss (E loss ). BSS10-and BSS20-based devices have the best combination of high external quantum efficiency (>85%) and small E loss (<0.6 eV) among all-PSCs yet reported. The results demonstrate that the blend morphology, charge carrier mobilities, and photovoltaic properties of all-PSCs could be rationally optimized by means of a synthetic variablethe random copolymer composition.

Understanding the influence of polymer molecular weight on the morphology, photophysics, and photovoltaic properties of polymer solar cells is central to further advances in the design, processing, performance and optimization...

We study the organic electrochemical transistor (OECT) performance of the ladder polymer poly-(benzimidazobenzophenanthroline) (BBL) in an attempt to better understand how an apparently hydrophobic side-chain-free polymer is able to operate as an OECT with favorable redox kinetics in an aqueous environment. We examine two BBLs of different molecular masses from different sources. Regardless of molecular mass, both BBLs show significant film swelling during the initial reduction step. By combining electrochemical quartz crystal microbalance gravimetry, in-operando atomic force microscopy, and both ex-situ and in-operando grazing incidence wideangle X-ray scattering (GIWAXS), we provide a detailed structural picture of the electrochemical charge injection process in BBL in the absence of any hydrophilic side-chains. Compared with ex-situ measurements, in-operando GIWAXS shows both more swelling upon electrochemical doping than has previously been recognized and less contraction upon dedoping. The data show that BBL films undergo an irreversible hydration driven by the initial electrochemical doping cycle with significant water retention and lamellar expansion that persists across subsequent oxidation/ reduction cycles. This swelling creates a hydrophilic environment that facilitates the subsequent fast hydrated ion transport in the absence of the hydrophilic side-chains used in many other polymer systems. Due to its rigid ladder backbone and absence of hydrophilic side-chains, the primary BBL water uptake does not significantly degrade the crystalline order, and the original dehydrated, unswelled state can be recovered after drying. The combination of doping induced hydrophilicity and robust crystalline order leads to efficient ionic transport and good stability.

Electron transport is critical to the use of n-type semiconducting polymers in diverse electronic and optoelectronic devices. Herein, we combine measurements of field-effect electron mobility and bulk electron mobility with thin-film microstructure characterization to elucidate the polymer chain length dependence of electron transport in n-type semiconducting polymers, exemplified by a naphthalene diimide-biselenophene copolymer, PNDIBS. Both bulk electron mobility measured by the space–charge limited current method and field-effect electron mobility of PNDIBS and other n-type semiconducting copolymers exhibit a peak at a critical degree of polymerization (DPc) of 45–60 repeat units. The decreased electron mobility below DPc is shown to originate from reduced intercrystallite connectivity while above DPc, intrachain twisting/folding, interchain entanglements, and intracrystallite limitations dominate electron transport. These findings provide a unified picture of the effects of polymer molecular weight on electron transport in naphthalene diimide-based polymers and offer a more quantitative design rule for high-mobility n-type polymers with donor–acceptor architecture.

Unlike naphthalene diimides, perylene diimides, and other classes of n-type conjugated polymers with numerous derivatives that enable understanding of structure–property relationships, the electronic structure and properties have not been reported for any derivative of ladder poly­(benzimidazobenzophenanthroline) (BBL). Herein, we report the synthesis and properties of BBL-P, a phenazine derivative of BBL. In acid solution, BBL-P has a broad absorption spectrum with a lowest energy absorption peak at 840 nm due to protonation-enhanced intramolecular charge transfer. Compared to BBL, BBL-P thin films have decreased crystallinity with face-on molecular orientations on substrates, resulting in a substantially decreased field-effect electron mobility of 1.2 × 10–4 cm2/V s. BBL-P films have excellent mechanical properties exemplified by a Young modulus of 11 GPa. The results demonstrate that BBL-P is a promising n-type semiconducting polymer and provide new insights into the effects of backbone structure on electronic structure, thin film microstructure, and charge transport properties of conjugated ladder polymers.

Improving carrier mobility, redox stability, blend morphology, and photovoltaic performance while elucidating structure–property relationships remains an important design goal for nonfullerene electron acceptors (NFAs) for organic solar cells. Although numerous NFAs have been created from rylene diimide electron-deficient building blocks, they have shown far inferior photovoltaic properties compared to benchmark fused-ring electron acceptors (FREAs) such as ITIC. Herein we show that new bis­(naphthalene-imide)­arylenelidenes (BNIAs), incorporating rylene-imide end-capping groups via methine bridges in donor–acceptor architectures, are endowed with enhanced electrochemical redox stability, high carrier mobilities, and high photovoltaic performance. Pairing of those BNIAs that are also FREAs, NIDT and NIBT, respectively, with donor polymer PBDB-T produced 10.0–10.8% efficient photovoltaic devices, which are comparable to benchmark ITIC devices. Blends of FREAs NIDT and NIBT and those of non-FREA NITV were found to have similar electron mobilities, demonstrating that the much higher photovoltaic efficiency of NIDT and NIBT devices does not originate from enhanced charge transport but from differences in blend morphology and blend photophysics. The results demonstrate that incorporating rylene imides into molecular architectures through the methine-bridged donor–acceptor coupling motif is a promising design strategy toward more efficient and electrochemically rugged materials for organic solar cells.

Naphthalene diimide (NDI)−biselenophene copolymer (PNDIBS), NDI−selenophene copolymer (PNDIS), and the fluorinated donor polymer PM6 were used to investigate how a fluorinated polymer component affects the morphology and performance of all-polymer solar cells (all-PSCs). Although the PM6:PNDIBS blend system exhibits a high open-circuit voltage (V oc = 0.925 V) and a desired low optical bandgap energy loss (E loss = 0.475 eV), the overall power conversion efficiency (PCE) was 3.1%. In contrast, PM6:PNDIS blends combine a high V oc (0.967 V) with a high fill factor (FF = 0.70) to produce efficient all-PSCs with 9.1% PCE. Furthermore, the high-performance PM6:PNDIS all-PSCs could be fabricated by various solution processing approaches and at active layer thickness as high as 300 nm without compromising photovoltaic efficiency. The divergent photovoltaic properties of PNDIS and PNDIBS when paired respectively with PM6 are shown to originate from the starkly different blend morphologies and blend photophysics. Efficient PM6:PNDIS blend films were found to exhibit a vertical phase stratification along with lateral phase separation, while the molecular packing had a predominant face-on orientation. Bulk lateral phase separation with both face-on and edge-on molecular orientations featured in the poor-performing PM6:PNDIBS blend films. Enhanced charge photogeneration and suppressed geminate and bimolecular recombinations with 99% charge collection probability found in PM6:PNDIS blends strongly differ from the poor charge collection probability (66%) and high electron−hole pair recombination seen in PM6:PNDIBS. Our findings demonstrate that beyond the generally expected enhancement of V oc , a fluorinated polymer component in all-PSCs can also exert a positive or negative influence on photovoltaic performance via the blend morphology and blend photophysics.

Effects of the donor moiety substitution on the intrinsic and photovoltaic blend properties of n-type semiconducting naphthalene diimide-arylene copolymers with donor−acceptor structure were investigated. The alternating naphthalene diimide-thiophene copolymer, PNDIT-hd, and naphthalene diimide-selenophene copolymer, PNDIS-hd, were found to have identical electrochemically derived electronic structures and similar bulk electron mobility; however, PNDIShd has an optical bandgap of 1.70 eV, which is 0.07 eV narrower relative to that of PNDIT-hd. All-polymer solar cells incorporating the donor polymer, PBDB-T, and PNDIS-hd were found to combine a high power conversion efficiency of 8.4% with high external quantum efficiency (86%) and a high fill factor of 0.71, which are significantly enhanced compared to the corresponding PBDB-T:PNDIT-hd devices with 6.7% power conversion efficiency and 73% external quantum efficiency. The improved photovoltaic properties of the selenophene-containing acceptor copolymer relative to the thiophene counterpart originate from enhanced light harvesting, more favorable molecular packing in blends, and reduced charge recombination losses in devices. These findings demonstrate that selenophene substitution for thiophene in donor− acceptor copolymers is an effective strategy that enhances the intrinsic polymer properties as well as the performance of all-polymer solar cells incorporating them.