Polymorphic Phase-Dependent Optical and Electrical Properties of a Diketopyrrolopyrrole-Based Small Molecule.

Four different polymorphic conformations of diethyl 5,5'-[5,5'-[2,5-bis(2-ethylhexyl)-3,6-dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrole-1,4-diyl]bis(thiophene-5,2-diyl)]difuran-2-carboxylate (DPP-(CF)2), namely, DPP-(CF)2-α, DPP-(CF)2-ß, DPP-(CF)2-γ, and DPP-(CF)2-ω, were identified from X-ray diffraction analysis conducted on their thin films and single crystals. Highly crystalline and well-textured thin films of these four polymorphs were successfully prepared via postgrowth solvent vapor and thermal annealing treatments to investigate the polymorphic phase-dependent optical and electrical properties of DPP-(CF)2. Interestingly, during the phase transition from DPP-(CF)2-α to DPP-(CF)2-ω, the optical band gap decreases from 1.75 to 1.5 eV because of the enhanced π-π interaction between the neighboring molecules. Except for DPP-(CF)2-γ, the other three phases show ambipolar charge transport. Although DPP-(CF)2-ß and DPP-(CF)2-γ exhibit a similar way of packing, a small increment in the π-π-stacking distance (0.006 Å) and twist conformation of the grafted electron-donating moieties of DPP-(CF)2-γ are found to reduce its hole mobility.

Interfacing Solution-Grown C60 and (3-Pyrrolinium)(CdCl3 ) Single Crystals for High-Mobility Transistor-Based Memory Devices.

Aligned ferroelectric single crystals of (3-pyrrolinium)(CdCl3 ) can be prepared from solution on top of aligned semiconducting C60 single crystals using an orthogonal solvent. Memory devices based on these ferroelectric/semiconductor bilayered heterojunctions exhibit much larger hysteresis compared with that of only C60 single crystals. More importantly, the introduction of the ferroelectric layer induces the memory window without dramatically reducing the charge mobility.

Solution-grown organic single-crystalline donor-acceptor heterojunctions for photovoltaics.

Organic single crystals are ideal candidates for high-performance photovoltaics due to their high charge mobility and long exciton diffusion length; however, they have not been largely considered for photovoltaics due to the practical difficulty in making a heterojunction between donor and acceptor single crystals. Here, we demonstrate that extended single-crystalline heterojunctions with a consistent donor-top and acceptor-bottom structure throughout the substrate can be simply obtained from a mixed solution of C60 (acceptor) and 3,6-bis(5-(4-n-butylphenyl)thiophene-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (donor). 46 photovoltaic devices were studied with the power conversion efficiency of (0.255±0.095)% under 1 sun, which is significantly higher than the previously reported value for a vapor-grown organic single-crystalline donor-acceptor heterojunction (0.007%). As such, this work opens a practical avenue for the study of organic photovoltaics based on single crystals.

Pyrene and diketopyrrolopyrrole-based oligomers synthesized via direct arylation for OSC applications.

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.

A direct arylation-derived DPP-based small molecule for solution-processed organic solar cells.

A diketo-pyrrolo-pyrrole (DPP) oligomer containing three DPP cores (Ph4Th4(DPP)3) was synthesized via direct arylation of C-H bonds (DACH). Ph4Th4(DPP)3 has good solubility in many organic solvents, and shows a broad absorption band from the visible to near-infrared region as well as a field-effect hole mobility as high as 0.006 cm(2) V(-1) s(-1). Solution-processed bulk heterojunction organic solar cells based on blends of Ph4Th4(DPP)3 as electron donor and fullerene derivative as electron acceptor were fabricated. An optimized power conversion efficiency of 3.76% with a high open-circuit voltage of 0.85 V was achieved after finely tuning the morphology by changing the blend ratio and by adding additives. These results indicate that DACH is an effective way to produce π-conjugated oligomers for organic solar cells.

Solution-grown organic single-crystalline p-n junctions with ambipolar charge transport.

Organic single-crystalline p-n junctions are grown from mixed solutions. First, C60 crystals (n-type) form and, subsequently, C8-BTBT crystals (p-type) nucleate heterogeneously on the C60 crystals. Both crystals continue to grow simultaneously into single-crystalline p-n junctions that exhibit ambipolar charge transport characteristics. This work provides a platform to study organic single-crystalline p-n junctions.