A solution-processed diketopyrrolopyrrole (DPP)-based small molecule, namely BDT-DPP, with broad absorption and suitable energy levels has been synthesized. The widely used solvents of chloroform (CF) and o-dichlorobenzene (o-DCB) were used as the spin-coating solvent, respectively, and 1,8-diiodooctane (DIO) was used as additive to fabricate efficient photovoltaic devices with BDT-DPP as the donor material and PC71BM as the acceptor material. Devices fabricated from CF exhibit poor fill factor (FF) of 43%, low short-circuit current density (Jsc) of 6.86 mA/cm2, and moderate power conversion efficiency (PCE) of 2.4%, due to rapid evaporation of CF, leading to poor morphology of the active layer. When 0.3% DIO was added, the FF and Jsc were improved to 60% and 8.49 mA/cm2, respectively, because of the better film morphology. Active layer spin-coated from the high-boiling-point solvent of o-DCB shows better phase separation than that from CF, because of the slow drying nature of o-DCB, offering sufficient time for the self-organization of active-layer. Finally, using o-DCB as the parent solvent and 0.7% DIO as the cosolvent, we obtained optimized devices with continuous interpenetrating network films, affording a Jsc of 11.86 mA/cm2, an open-circuit voltage (Voc) of 0.72 V, an FF of 62%, and a PCE of 5.29%. This PCE is, to the best of our knowledge, the highest efficiency reported to date for devices prepared from the solution-processed DPP-based small molecules.Keywords: device optimization; diketopyrrolopyrrole; organic solar cells; solution-processed small molecules;

A solution-processed diketopyrrolopyrrole (DPP)-based small molecule, namely BDT-DPP, with broad absorption and suitable energy levels has been synthesized. The widely used solvents of chloroform (CF) and o-dichlorobenzene (o-DCB) were used as the spin-coating solvent, respectively, and 1,8-diiodooctane (DIO) was used as additive to fabricate efficient photovoltaic devices with BDT-DPP as the donor material and PC71BM as the acceptor material. Devices fabricated from CF exhibit poor fill factor (FF) of 43%, low short-circuit current density (Jsc) of 6.86 mA/cm2, and moderate power conversion efficiency (PCE) of 2.4%, due to rapid evaporation of CF, leading to poor morphology of the active layer. When 0.3% DIO was added, the FF and Jsc were improved to 60% and 8.49 mA/cm2, respectively, because of the better film morphology. Active layer spin-coated from the high-boiling-point solvent of o-DCB shows better phase separation than that from CF, because of the slow drying nature of o-DCB, offering sufficient time for the self-organization of active-layer. Finally, using o-DCB as the parent solvent and 0.7% DIO as the cosolvent, we obtained optimized devices with continuous interpenetrating network films, affording a Jsc of 11.86 mA/cm2, an open-circuit voltage (Voc) of 0.72 V, an FF of 62%, and a PCE of 5.29%. This PCE is, to the best of our knowledge, the highest efficiency reported to date for devices prepared from the solution-processed DPP-based small molecules.Keywords: device optimization; diketopyrrolopyrrole; organic solar cells; solution-processed small molecules;

In this report, an atom efficient and facile synthetic strategy for accessing multi-diketopyrrolopyrrole (DPP)-based oligomers used in solution-processed organic field effect transistors (OFETs) and organic solar cells (OSCs) has been developed. The DPP units were successfully installed onto benzene and pyrene cores via palladium-catalyzed dehydrohalogenative coupling of mono-capped DPPs with multi-bromo-benzene or -pyrene (direct arylation), affording four oligomer small molecules (SMs 1–4) containing bis-, tri-, tri-, and tetra-DPP, respectively, in high yields of 78–96%. All the designed linear or branched DPP-based oligomers exhibit broad light absorptions, narrow band-gaps (1.60–1.73 eV), deep highest occupied molecular orbital (HOMO) levels (−5.26 ∼ −5.18 eV), and good thermal stability (Td = 390–401 °C). OFETs based on SMs 1–4 showed hole mobilities of 0.0033, 0.0056, 0.0005, and 0.0026 cm2 V–1 s–1, respectively. OSCs based on SMs 1–4 under one sun achieved power conversion efficiencies of 3.00%, 3.71%, 2.47%, and 1.86% accordingly, along with high open-circuit voltages of 0.86–0.94 V. For OSC devices of SM 1, SM 3, and SM 4, the solvent CHCl3 was solely employed to the formation of active layers; neither high boiling point additives nor annealing post-treatment was needed. Such a simple process benefits the large-scale production of OSCs via roll to roll technology.Keywords: diketopyrrolopyrrole; direct arylation; organic field effect transistors; organic solar cells; pyrene; solution-processed small molecules;

Three star-shaped D–A small molecules, (P-DPP)3TPA, (4-FP-DPP)3TPA, and (4-BuP-DPP)3TPA were designed and synthesized with triphenylamine (TPA) as the core, diketopyrrolopyrrole (DPP) as the arm, and unsubstituted or substituted benzene rings (phenyl, P; 4-fluoro-phenyl, 4-FP; 4-n-butyl-phenyl, 4-BuP) as the end-group. All the three small molecules show relatively narrow optical band gaps (1.68–1.72 eV) and low-lying highest occupied molecular orbital (HOMO) energy levels (−5.09∼−5.13 eV), implying that they are potentially good electron donors for organic solar cells (OSCs). Then, photovoltaic properties of the small molecules blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as electron acceptor were investigated. Among three small molecules, the OSC based on (P-DPP)3TPA:PCBM blend exhibits a best power conversion efficiency (PCE) of 2.98% with an open-circuit voltage (Voc) of 0.72 V, a short-circuit current density (Jsc) of 7.94 mA/cm2, and a fill factor (FF) of 52.2%, which may be ascribed to the highest hole mobility of (P-DPP)3TPA.Keywords: diketopyrrolopyrrole; organic solar cells; small molecule; star-shaped; triphenylamine;

Two novel solution-processable acceptor–donor–acceptor (A–D–A)-structured organic small molecules with diketopyrrolopyrrole (DPP) as terminal acceptor units and pentathiophene (PTA) or pyrrole-modified pentathiophene (NPTA) as the central donor unit, namely, DPP2(PTA) and DPP2(NPTA), were designed and synthesized. We examined the effects of changing the central bridging heteroatoms of the five-ring-fused thienoacene core identity from sulfur [DPP2(PTA)] to nitrogen [DPP2(NPTA)] in the small-molecule donor material. Replacement of the bridging atom with a different electronic structure has a visible effect on both the optical and electrical properties: DPP2(NPTA), which contains much more electron-rich pyrrole in the central thienoacene unit, possesses red-shifted absorption and a higher HOMO level relative to DPP2(PTA) with the less electron-rich thiophene in the same position. More importantly, substitution of the bridging atoms results in a change of the substituting alkyl chains due to the nature of the heteroatoms, which significantly tailored the crystallization behavior and the ability to form an interpenetrating network in thin-film blends with an electron acceptor. Compared to DPP2(PTA) with no alkyl chain substituting on the central sulfur atom of the PTA unit, DPP2(NPTA) exhibits improved crystallinity and better miscibility with PC71BM probably because of a dodecyl chain on the central nitrogen atom of the NPTA unit. These features endow the DPP2(NPTA)/PC71BM blend film higher hole mobility and better donor/acceptor interpenetrating network morphology. Optimized photovoltaic device fabrication based on DPP2(NPTA)/PC71BM (1.5:1, w/w) has resulted in an average power conversion efficiency (PCE) as high as 3.69% (the maximum PCE was 3.83%). This study demonstrates that subtle changes and tailoring of the molecular structure, such as simply changing the bridging heteroatom in the thienoacene unit in D/A-type small molecules, can strongly affect the physical properties that govern their photovoltaic performances.Keywords: cycle extension; diketopyrrolopyrrole; heterocycle-modified pentathiophene; organic solar cell; solution-processable small molecule; structure−property relationship;