Knowledge of the critical factors that determine compatibility, blend morphology, and performance of bulk heterojunction (BHJ) solar cells composed of an electron-accepting polymer and an electron-donating polymer remains limited. To test the idea that bulk crystallinity is such a critical factor, we have designed a series of new semiconducting naphthalene diimide (NDI)-selenophene/perylene diimide (PDI)-selenophene random copolymers, xPDI (10PDI, 30PDI, 50PDI), whose crystallinity varies with composition, and investigated them as electron acceptors in BHJ solar cells. Pairing of the reference crystalline (crystalline domain size Lc = 10.22 nm) NDI-selenophene copolymer (PNDIS-HD) with crystalline (Lc = 9.15 nm) benzodithiophene-thieno[3,4-b]thiophene copolymer (PBDTTT-CT) donor yields incompatible blends, whose BHJ solar cells have a power conversion efficiency (PCE) of 1.4%. However, pairing of the new 30PDI with optimal crystallinity (Lc = 5.11 nm) as acceptor with the same PBDTTT-CT donor yields compatible blends and all-polymer solar cells with enhanced performance (PCE = 6.3%, Jsc = 18.6 mA/cm(2), external quantum efficiency = 91%). These photovoltaic parameters observed in 30PDI:PBDTTT-CT devices are the best so far for all-polymer solar cells, while the short-circuit current (Jsc) and external quantum efficiency are even higher than reported values for [70]-fullerene:PBDTTT-CT solar cells. The morphology and bulk carrier mobilities of the polymer/polymer blends varied substantially with crystallinity of the acceptor polymer component and thus with the NDI/PDI copolymer composition. These results demonstrate that the crystallinity of a polymer component and thus compatibility, blend morphology, and efficiency of polymer/polymer blend solar cells can be controlled by molecular design.

The lack of suitable acceptor (n-type) polymers has limited the photocurrent and efficiency of polymer/polymer bulk heterojunction (BHJ) solar cells. Here, we report an evaluation of three naphthalene diimide (NDI) copolymers as electron acceptors in BHJ solar cells which finds that all-polymer solar cells based on an NDI-selenophene copolymer (PNDIS-HD) acceptor and a thiazolothiazole copolymer (PSEHTT) donor exhibit a record 3.3% power conversion efficiency. The observed short circuit current density of 7.78 mA/cm(2) and external quantum efficiency of 47% are also the best such photovoltaic parameters seen in all-polymer solar cells so far. This efficiency is comparable to the performance of similarly evaluated [6,6]-Phenyl-C61-butyric acid methyl ester (PC60BM)/PSEHTT devices. The lamellar crystalline morphology of PNDIS-HD, leading to balanced electron and hole transport in the polymer/polymer blend solar cells accounts for its good photovoltaic properties.

New electron-acceptor materials are long sought to overcome the small photovoltage, high-cost, poor photochemical stability, and other limitations of fullerene-based organic photovoltaics. However, all known nonfullerene acceptors have so far shown inferior photovoltaic properties compared to fullerene benchmark [6,6]-phenyl-C60-butyric acid methyl ester (PC60BM), and there are as yet no established design principles for realizing improved materials. Herein we report a design strategy that has produced a novel multichromophoric, large size, nonplanar three-dimensional (3D) organic molecule, DBFI-T, whose π-conjugated framework occupies space comparable to an aggregate of 9 [C60]-fullerene molecules. Comparative studies of DBFI-T with its planar monomeric analogue (BFI-P2) and PC60BM in bulk heterojunction (BHJ) solar cells, by using a common thiazolothiazole-dithienosilole copolymer donor (PSEHTT), showed that DBFI-T has superior charge photogeneration and photovoltaic properties; PSEHTT:DBFI-T solar cells combined a high short-circuit current (10.14 mA/cm(2)) with a high open-circuit voltage (0.86 V) to give a power conversion efficiency of 5.0%. The external quantum efficiency spectrum of PSEHTT:DBFI-T devices had peaks of 60-65% in the 380-620 nm range, demonstrating that both hole transfer from photoexcited DBFI-T to PSEHTT and electron transfer from photoexcited PSEHTT to DBFI-T contribute substantially to charge photogeneration. The superior charge photogeneration and electron-accepting properties of DBFI-T were further confirmed by independent Xenon-flash time-resolved microwave conductivity measurements, which correctly predict the relative magnitudes of the conversion efficiencies of the BHJ solar cells: PSEHTT:DBFI-T > PSEHTT:PC60BM > PSEHTT:BFI-P2. The results demonstrate that the large size, multichromophoric, nonplanar 3D molecular design is a promising approach to more efficient organic photovoltaic materials.

All-polymer solar cells with 4.8% power conversion efficiency are achieved via solution processing from a co-solvent. The observed short-circuit current density of 10.5 mA cm(-2) and external quantum efficiency of 61.3% are also the best reported in all-polymer solar cells so far. The results demonstrate that processing the active layer from a co-solvent is an important strategy in achieving highly efficient all-polymer solar cells.

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Several new solution-processable organic semiconductors based on dendritic oligoquinolines were synthesized and were used as electron-transport and hole-blocking materials to realize highly effi cient blue phosphorescent organic light-emitting diodes (PhOLEDs). Various substitutions on the quinoline rings while keeping the central meta -linked tris(quinolin-2-yl)benzene gave electron transport materials that combined wide energy gap ( > 3.3 eV), moderate electron affi nity (2.55-2.8 eV), and deep HOMO energy level ( < -6.08 eV) with electron mobility as high as 3.3 × 10 − 3 cm 2 V − 1 s − 1 . Polymer-based PhOLEDs with iridium (III) bis(4,6-(di-fl uorophenyl)pyridinato-N ,C 2 ′ )picolinate (FIrpic) blue triplet emitter and solution-processed oligoquinolines as the electrontransport layers (ETLs) gave luminous effi ciency of 30.5 cd A − 1 at a brightness of 4130 cd m − 2 with an external quantum effi ciency (EQE) of 16.0%. Blue PhOLEDs incorporating solution-deposited ETLs were over two-fold more effi cient than those containing vacuum-deposited ETLs. Atomic force microscopy imaging shows that the solution-deposited oligoquinoline ETLs formed vertically oriented nanopillars and rough surfaces that enable good ETL/ cathode contacts, eliminating the need for cathode interfacial materials (LiF, CsF). These solution-processed blue PhOLEDs have the highest performance observed to date in polymer-based blue PhOLEDs.

The rapid emergence of organic (opto)electronics as a promising alternative to conventional (opto)electronics has been achieved through the design and development of novel π‐conjugated systems. Among various semiconducting structural platforms, 4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY) π‐systems have recently attracted attention for use in organic thin‐films transistors (OTFTs) and organic photovoltaics (OPVs). This Review article provides an overview of the developments in the past 10 years on the structural design and synthesis of BODIPY‐based organic semiconductors and their application in OTFT/OPV devices. The findings summarized and discussed here include the most recent breakthroughs in BODIPYs with record‐high charge carrier mobilities and power conversion efficiencies (PCEs). The most up‐to‐date design rationales and discussions providing a strong understanding of structure‐property‐function relationships in BODIPY‐based semiconductors are presented. Thus, this review is expected to inspire new research for future materials developments/applications in this family of molecules.

Side chain engineering of an n-type polymer provides a means of maintaining solubility while increasing crystallinity and electron mobility, leading to enhanced photocurrent. Bulk heterojunction solar cells composed of a side chain engineered copolymer (PNDIS-30BO) as acceptor and PSEHTT as donor give 10.4 mA cm(-2) photocurrent and 4.4% efficiency.