Hyun Ah Um

High mobility isoindigo-based π-extended conjugated polymers bearing di(thienyl)ethylene in thin-film transistors

New isoindigo and di(thienyl)ethylene-containing π-extended conjugated polymers with different branched side-chains and molecular weights (MWs) have been synthesized. Octyldodecyl (OD) and decyltetradecyl (DT) groups were employed as side chain moieties tethered to isoindigo units. In addition, the MW of the polymer was varied by changing the polymerization time. The polymers bearing DT side chains showed much better charge transport behavior, compared to that of polymers with OD side chains. The hole mobility of high-MW PIIDDTE-III with DT side-chains was determined to be higher than that of the low-MW polymer, PIIDDTE-II with identical side-chains. The thermally annealed film of PIIDDTE-III exhibited outstanding hole mobilities of ∼2.20 cm2 V−1 s−1 measured under ambient conditions. This work unambiguously demonstrates the effect of the branched side-chain bulkiness and MW on the thin film transistor (TFT) performances.

Bis(thienothiophenyl) Diketopyrrolopyrrole-Based Conjugated Polymers with Various Branched Alkyl Side Chains and Their Applications in Thin-Film Transistors and Polymer Solar Cells

New thienothiophene-flanked diketopyrrolopyrrole and thiophene-containing π-extended conjugated polymers with various branched alkyl side-chains were successfully synthesized. 2-Octyldodecyl, 2-decyltetradecyl, 2-tetradecylhexadecyl, 2-hexadecyloctadecyl, and 2-octadecyldocosyl groups were selected as the side-chain moieties and were anchored to the N-positions of the thienothiophene-flanked diketopyrrolopyrrole unit. All five polymers were found to be soluble owing to the bulkiness of the side chains. The thin-film transistor based on the 2-tetradecylhexadecyl-substituted polymer showed the highest hole mobility of 1.92 cm2 V(-1) s(-1) due to it having the smallest π-π stacking distance between the polymer chains, which was determined by grazing incidence X-ray diffraction. Bulk heterojunction polymer solar cells incorporating [6,6]-phenyl-C71-butyric acid methyl ester as the n-type molecule and the additive 1,8-diiodooctane (1 vol %) were also constructed from the synthesized polymers without thermal annealing; the device containing the 2-octyldodecyl-substituted polymer exhibited the highest power conversion efficiency of 5.8%. Although all the polymers showed similar physical properties, their device performance was clearly influenced by the sizes of the branched alkyl side-chain groups.

High Aspect Ratio Conjugated Polymer Nanowires for High Performance Field-Effect Transistors and Phototransistors

We synthesized a highly crystalline DPP-based polymer, DPPBTSPE, which contained 1,2-bis(5-(thiophen-2-yl)selenophen-2-yl)ethene as a planar and rigid electron donating group. High- and low-molecular weight (MW) DPPBTSPE fractions were collected by Soxhlet extraction and were employed to investigate their unique charge transport properties in macroscopic films and single crystalline polymer nanowire (SC-PNW), respectively. The low-MW polymer could provide well-isolated and high aspect ratio SC-PNWs, in which the direction of π-π stacking was perpendicular to the wire growing axis. The field effect transistors made of SC-PNWs exhibited remarkably high carrier mobility of 24 cm(2) V(-1) s(-1). In addition, phototransistors (PTs) made of SC-PNW showed very high performance in terms of photoresponsivity (R) and photoswitching ratio (P). The average R of the SC PNW-based PTs were in the range of 160-170 A W(-1) and the maximum R was measured at 1920 A W(-1), which is almost three orders higher than that of thin film-based PT device.

New Bipolar Host Materials for Realizing Blue Phosphorescent Organic Light-Emitting Diodes with High Efficiency at 1000 cd/m2

New host molecules such as 9-(6-(9H-carbazol-9-yl)pyridin-3-yl)-6-(9H-carbazol-9-yl)-9H-pyrido[2,3-b]indole (pPCB2CZ) and 9-(6-(9H-carbazol-9-yl)pyridin-2-yl)-6-(9H-carbazol-9-yl)-9H-pyrido[2,3-b]indole (mPCB2CZ) were designed and synthesized for blue phosphorescent organic light-emitting diodes (PhOLEDs). The glass transition temperatures of two host molecules were measured higher than 120 °C, and the identical triplet energies were determined to be 2.92 eV for both molecules. The bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(III) (FIrpic)-doped mPCB2CZ-based PhOLED exhibited practically useful driving voltage of 4.8 V in a simple organic three layer device configuration which has a smaller number of interfaces in conventional multilayer PhOLEDs. Also, the high quantum efficiency of 23.7% is reported at the practically useful brightness value of 1000 cd/m2.

Diketopyrrolopyrrole-based copolymers bearing highly π-extended donating units and their thin-film transistors and photovoltaic cells

New diketopyrrolopyrrole (DPP)-based π-extended conjugated polymers containing relatively long conjugated donor monomers were successfully synthesized in order to investigate their physical properties and device performance in thin-film transistors and photovoltaic cells. The solubility of the polymers was improved via side-chain engineering, and relatively high molecular weights were achieved by modifying the synthetic procedures. The highest occupied molecular orbital and the lowest unoccupied molecular orbital and optical bandgaps of the polymers were easily tuned by incorporating different donor monomers with different donating abilities. All polymers were highly crystalline with a predominant edge-on orientation on the substrate. The polymers displayed different hole mobilities in TFTs depending on the donor monomer. The highest mobility was 4.17 cm2 V−1 s−1, which was shown by the TFT fabricated with a terselenophene-containing DPP polymer after thermal annealing. Furthermore, bulk heterojunction organic photovoltaic cells made from a blend film of the polymers and (6,6)-phenyl C71-butyric acid methyl ester demonstrated promising device performance with power conversion efficiencies in the range of 3.48–5.05%.

Tunable light harvesting properties of a highly crystalline alternating terpolymer for high-performing solar cells

To achieve power conversion efficiency in organic photovoltaic devices, we designed and synthesized a new DPP-based alternating terpolymer, PDPPPyT, containing DPP as an electron accepting unit and pyrene and thiophene as electron donating units. The donor–acceptor alternating copolymers PDPPPy and PDPPT were also prepared as control materials. The optical properties and the highest occupied molecular orbital of PDPPPyT were found to be between those of the PDPPPy and PDPPT copolymers. GI-XRD analysis indicated that the PDPPPy and PDPPT copolymers had a typical edge-on orientation on the substrate, whereas the PDPPPyT terpolymer showed face-on orientation in pristine and annealed films. Although PDPPPyT has two different donating units, pyrene and thiophene, in the repeating unit, the corresponding TFT exhibited a high hole mobility of ∼0.8 cm2 V−1 s−1. A PSC made of PDPPPyT and PC71BM displayed power conversion efficiency values of around 4.0%, which is higher than those of PDPPPy- and PDPPT-based PSCs. This improved performance was attributed to the more efficient light-harvesting properties of PDPPPyT in the solar spectrum. These results unambiguously demonstrated that this new synthetic strategy for regular conjugated terpolymers could be used to tune the molecular energy levels and optical properties to obtain high-performance PSCs.

Side-chain engineering of diketopyrrolopyrrole-based copolymer using alkyl ester group for efficient polymer solar cell

The new diketopyrrolopyrrole-based copolymer, referred to as PDPPBDTE, bearing linear alkyl ester groups, was synthesized with the objective of improving the miscibility between the donor polymer and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM). PDPPBDTA bearing ethylhexyl side-chains was also synthesized as a control polymer. The PDPPBDTE:PC71BM blend film showed a relatively small crystalline domain size compared to the PDPPBDTA:PC71BM blend film. A polymer solar cell fabricated with PDPPBDTE and PC71BM exhibited twofold higher power conversion efficiency than that employing PDPPBDTA.

High-Performing Thin-Film Transistors in Large Spherulites of Conjugated Polymer Formed by Epitaxial Growth on Removable Organic Crystalline Templates

Diketopyrrolopyrrole (DPP)-based conjugated polymer PDTDPPQT was synthesized and was used to perform epitaxial polymer crystal growth on removable 1,3,5-trichlorobenzene crystallite templates. A thin-film transistor (TFT) was successfully fabricated in well-grown large spherulites of PDTDPPQT. The charge carrier mobility along the radial direction of the spherulites was measured to be 5.46-12.04 cm(2) V(-1) s(-1), which is significantly higher than that in the direction perpendicular to the radial direction. The dynamic response of charge transport was also investigated by applying a pulsed bias to TFTs loaded with a resistor (∼20 MΩ). The charge-transport behaviors along the radial direction and perpendicular to the radial direction were investigated by static and dynamic experiments through a resistor-loaded (RL) inverter. The RL inverter made of PDTDPPQT-based TFT operates well, maintaining a fairly high switching voltage ratio (Vout(ON)/Vout(OFF)) at a relatively high frequency when the source-drain electrodes are aligned parallel to the radial direction.

New fluorene-based chiral copolymers with unusually high optical activity in pristine and annealed thin films

Polyfluorene (PF)-based chiral alternating copolymers, i.e., PFPh and PFTh, were successfully synthesized to obtain high circular dichroism (CD) in pristine thin films. By tethering chiral side-chains to the fluorene moiety, the resultant PFPh and PFTh films intriguingly exhibited high CD even in pristine films, which was attributed to the relatively low steric hindrance between the repeating units and intermolecular interactions between the polymer chains. Compared to PFTh bearing a five-membered heteroaromatic thiophene monomer, PFPh containing a six-membered aromatic benzene monomer, showed significantly enhanced CD in an annealed film. Of particular importance, results of optical microscopy and atomic force microscopy can be used to assess the CD of a chiral polymer thin film after thermal annealing.

High-performance n-type field-effect transistors based on a highly crystalline tricyanovinyldihydrofuran derivative

We propose a novel tricyanovinyldihydrofuran (TCF)-based molecule called DBOB-DTCF that is designed and synthesized for application in n-type field-effect transistors (FETs). It can be operated in a stable manner under ambient conditions. DBOB-DTCF is successfully fabricated as crystalline microplates (CMs) because of its capability of self-assembly. A high electron mobility of ∼1.9 cm2 V-1 s-1 is observed for a CM-based FET, measured under ambient conditions. This suggests that TCF is an excellent acceptor unit that organizes air stable n-type organic semiconductors.