Woosum Cho

Electrical properties of silica-polyimide composite dielectric thin films prepared via sol-gel reaction and thermal imidization

Abstract Thin dielectric films composed of silica and polyimide were prepared from tetraethoxysilane (TEOS) and precursor of poly(p-phenylene biphenyltetracarboximide)(BPDA-PDA). Silica particles were generated from TEOS via sol-gel process. BPDA-PDA polyimide was prepared from its flexible and soluble precursors, poly(p-phenylene biphenyltetracarboxamic acid)(BPDA-PDA PAA), and poly(p-phenylene biphenyltetracarboxamic diethyl ester)(BPDA-PDA ES), through thermal imidiration process. In the present work, the electrical properties of the silica-polyimide hybrid composite films were examined utilizing a capacitance measurement by a high resolution electrometer/function generator system. The effects of TEOS contents and precursor types for polyimide matrix of the composites on the relative dielectric constant, dielectric dissipation factor, and resistivity of the composite films were investigated.

Morphology of organic-inorganic hybrid composites in thin films as multichip packaging material

Silica-polyimide hybrid composites were prepared via a sol-gel process and thermal imidization. Two different types of soluble precursors, poly(amic acid) (PAA) and poly(amic diethyl ester) (ES), chemically convertible to poly(p-phenylene biphenyltetracarboximide), were used as organic polymer matrix component, and tetraethoxysilane (TEOS), convertible to silica, as the inorganic component. The structure of composites prepared as thin films was investigated by means of small-angle X-ray scattering, scanning electron microscopy and atomic force microscopy. Nanometre-scale composites were successfully obtained for < 30 wt% TEOS-loaded mixtures with ES and PAA. It was considered from the microstructural investigation that the composite films based on ES were not significantly affected by the inorganic particles generated, maintaining the structure of the homopolyimide, while those based on PAA did not preserve the structure due to the nanoparticles grown in situ during the sol-gel process.

A simple structured and efficient triazine-based molecule as an interfacial layer for high performance organic electronics

Achieving the state-of-the-art performance of solution processable and flexible organic electronics requires efficient, stable, and cost-effective interfacial layers (ILs). Here, we report an alcohol soluble phosphine oxide functionalized 1,3,5-triazine derivative (PO-TAZ) as an IL, which remarkably tailors the work function of conductors including metals, transparent metal oxides and organic materials, making it an ideal candidate for an interfacial material in organic electronics. Consequently, PO-TAZ thin films enable the fabrication of organic and organic–inorganic (perovskite) solar cells with power conversion efficiencies of 10.04% and 16.41%, respectively, and n-channel organic field-effect transistors with an electron mobility of 8 cm2 V−1 s−1. Owing to the low-cost processing associated with PO-TAZ and the tremendous improvement in device performances as compared to the devices without PO-TAZ along with ambient stability, PO-TAZ is a good choice for efficient organic electronics in large area printing processes.

Facile synthesis and characterization of iridium(III) complexes containing an N-ethylcarbazole–thiazole main ligand using a tandem reaction for solution processed phosphorescent organic light-emitting diodes

A new series of highly efficient phosphorescent Ir(III) complexes, which have potential applications in solution processable phosphorescent organic light-emitting diodes (PhOLEDs), were synthesized and their photophysical, electrochemical, and electroluminescent (EL) properties were investigated. The Ir(III) complexes, including (Et-Cz–Tz)2Ir(pic), (Et-Cz–Tz)2Ir(pic-N-O), (Et-Cz–Tz)2Ir(EO2–pic), and (Et-Cz–Tz)2Ir(EO2–pic-N-O), are comprised of linked N-ethylcarbazole (Et-Cz) and thiazole (Tz) units as the main ligand (Et-Cz–Tz) and picolinic acid (pic) and picolinic acid N-oxide (pic-N-O) as ancillary ligands. In addition, some of the Ir(III) complexes contain an ethylene oxide solubilizing group attached to the ancillary ligands via a tandem reaction. High performance, solution processable PhOLEDs, fabricated using (Et-Cz–Tz)2Ir(EO2–pic), were observed to have a maximum external quantum efficiency of 6.08% and a luminance efficiency of 10.98 cd A−1. This is the first report on the use of EO2–pic and EO2–pic-N-O ancillary ligands for the synthesis of solution processable Ir(III) complexes via a tandem reaction. The performances of the PhOLEDs based on these Ir(III) complexes correlate well with the theoretical properties predicted by using density functional theory calculations.

Organic Electroluminescent Devices Using Fluorine-Containing Polyimides as a Hole Transporting Layer

In this work, organic electroluminescent devices (OELDs) were fabricated using fluorine-containing polyimide as a matrix polymer to hole transporting layer. The device consists of indium tin oxide (ITO) coated glass/triphenyl diamine derivative (TPD)-dispersed polyimide/Alq3/Al structure. The effect of the molecular structure of both the polyimides containing fluorine atoms or having no fluorine atoms on the device performances was investigated.

Influential effects of π-spacers, alkyl side chains, and various processing conditions on the photovoltaic properties of alkylselenyl substituted benzodithiophene based polymers

π-spacers and alkyl side chains play a key role in the optical, electrochemical, and photovoltaic properties of π-conjugated polymers. To investigate the collective effects of π-spacers, alkyl side chains, and various processing conditions on the photovoltaic properties, an array of four new low bandgap (LBG) π-conjugated polymers (P1–P4) was designed and synthesized for their application as donor materials in bulk heterojunction polymer solar cells (BHJ PSCs). These π-conjugated polymers contain a benzodithiophene (BDT) donor unit substituted with 2-ethylhexylselenyl or 2-hexyldecylselenyl as π-conjugated side chains and a dialkoxybenzothiadiazole (dialkoxyBT) electron deficient unit connected with thiophene or selenophene as π-spacers. Among the four polymers, the absorption spectra of P3 with the thiophene π-spacer showed a well enhanced vibronic shoulder peak between 620 and 650 nm, indicating that P3 possesses a strong interchain aggregation tendency and attains a planar backbone structure due to the non-covalent interactions arising between the sulfur atom in thiophene and the oxygen atom in dialkoxyBT. Under suitable device processing conditions optimized pristine PSCs of P3 showed a maximum power conversion efficiency (PCE) of 4.09%. After employing 1,8-diiodooctane as an additive, one of the PSC devices based on P2 displayed a PCE of 5.34%. The active layers of P1–P4 showed a positive response towards methanol treatment, especially the P3-based devices delivered an improved PCE of 5.63%, which was further assessed by electrical impedance spectroscopy. These findings in the current article provide a good specimen for efficiently fine tuning the optical and photovoltaic properties of π-conjugated polymers via varying the size of alkyl chains, π-spacer groups and device processing conditions for the imminent growth of LBG π-conjugated polymers.

Substituent position engineering of diphenylquinoline-based Ir(III) complexes for efficient orange and white PhOLEDs with high color stability/low efficiency roll-off using a solution-processed emission layer

Three new heteroleptic Ir(III) complexes o-LIrpic, m-LIrpic, and p-LIrpic (L = CF3DPQ) consisting of 2,4-diphenylquinoline (DPQ) with a –CF3 group at ortho (o)/meta (m)/para (p) positions of the metalated phenyl ring, respectively, as the main ligands were synthesized and used as emitters in phosphorescent organic light-emitting diodes (PhOLEDs). We realized that –CF3 position extremely affects the crucial photophysical and electronic properties such as emission color, photoluminescence quantum yield (PLQY) and energy levels of these Ir(III) complexes resulting in –CF3 position-dependent performance of their PhOLEDs. To verify the effect of –CF3 group position on device performance, three other Ir(III) complexes o-LIrtmd, m-LIrtmd, and p-LIrtmd were synthesized using the same main ligands but a different ancillary ligand. In the two series of Ir(III) complexes, the devices with m-CF3 based complexes are outstanding in performance compared to o- or p-CF3 based ones due to the enhanced PLQY and well suppressed non-radiative deactivations by m-substitution. Finally, the single emission layer solution-processed orange and two-component white PhOLEDs fabricated using m-LIrpic as orange emitter achieved the maximum external quantum efficiency of 17.1% (43.9 cd A−1) and 21.1% (48.8 cd A−1), respectively, with highly stable color coordinates and low efficiency roll-off. This is the highest efficiency reported to date for solution-processed orange PhOLEDs using a small molecular host with easily accessible emitter.

A systematic identification of efficiency enrichment between thiazole and benzothiazole based yellow iridium(III) complexes

Four cyclometalated heteroleptic Ir(III) complexes [(Et-Cz-BTz)2Ir(pic)], [(Et-Cz-BTz)2Ir(pic-N-O)], [(Et-Cz-BTz)2Ir(EO2-pic)], and [(Et-Cz-BTz)2Ir(EO2-pic-N-O)] containing benzothiazole linked ethylcarbazole (Et-Cz-BTz) as a main ligand have been successfully synthesized for solution-processed phosphorescent organic light-emitting diodes (PhOLEDs). All the Ir(III) complexes emit bright yellow (541–582 nm) phosphorescence at room temperature. Among the four Ir(III) complexes, the [(Et-Cz-BTz)2Ir(EO2-pic)] showed the best luminous efficiency of 60 cd A−1 and external quantum efficiency (EQE) of 19% with CIE coordinates (0.467, 0.524), which is one of the best efficiencies for solution-processed yellow PhOLEDs using heteroleptic Ir(III) complexes as an emitting layer. Interestingly, upon replacing the thiazole linking group in the main ligand by benzothiazole, the EQE increased from 6.08 to 19%.

Electrochemical properties of poly(N-substituted pyrrole)s obtained in TBADS/ACN electrolyte system

Abstract The electrochemical properties of polu(N-substituted pyrrole)s were studied. Polypyrrole(PPy) and poly(N-substituted pyrrole)s were prepared via electrochemical methods using tetrabutylammonium dodecyl sulfate(TBADS)/Acetonitrile(ACN) electrolyte system. The electropolymerized films were characterized by potentiostat, SEM and 4-probe conductivity meter. Structural characteristics of the films were discussed using conductivity and morphology as well as spectroscopy. It was found out that the N-substituted pyrroles having larger substituted groups are more hampered when oxidation, resulting in lower conductivity.

The effect of with/without resonance-mediated interactions on the organic solar cell performance of new 2D π-conjugated polymers

With the goal of discovering a new and plausible approach to utilizing the conjugated side chains (CSCs), other than for the previously reported purpose of two-dimensional (2D) π-conjugation extension in π-conjugated polymers, we report, for the first time, the impact of with/without interactions induced via resonance in CSCs on the molecular weight (Mw) and photovoltaic performance of polymers. For this, we designed two donor (D)–acceptor (A) polymers, represented as PBDTBPA(H)-DPP and PBDTBPA(F)-DPP, containing alkoxy-BPA(H) and alkoxy-BPA(F) [BPA = biphenylethynyl] on the benzodithiophene (BDT) unit as novel CSCs, respectively. The introduction of these CSCs generated bis-tolane as an integrated part of the BDT unit, which allowed us to address the difference in the polymerization, photophysical, and photovoltaic properties of PBDTBPA(H)-DPP and PBDTBPA(F)-DPP, as a function of the structural variation of CSCs, which has never been investigated in organic solar cells (OSCs). In contrast to the weak electron-donating nature of BPA(H), BPA(F) exhibited a strong electron-donating ability due to the interaction of the lone pair electrons of the fluorine atom with the triple bond via resonance, which decreased the rigidity of the triple bond, whereas in PBDTBPA(H)-DPP the rigidity of the triple bond was retained with no such interaction. The striking differences in the rigidity and different electron-withdrawing tendencies of the CSCs were well correlated with Mw and with the highest occupied molecular orbital (HOMO) energy levels of the polymers. As a result, the inverted OSCs based on PBDTBPA(H)-DPP achieved an open-circuit voltage (Voc) of 0.74 V, and power conversion efficiency (PCE) of 5.58%, which was 38% higher than that of PBDTBPA(F)-DPP-based inverted OSCs. More significantly, the inverted OSC devices were highly stable, retaining 80% of their original PCE after 60-day storage in air, even without encapsulation. To the best of our knowledge, this 5.58% is the highest PCE reported to date for the arylethynyl-substituted BDT donor-based OSCs. These results reveal that bis-tolane [BDTBPA(H)] as an integrated part of the new BDT unit is a promising donor building block for high Mw donor polymers in addition to 2D extended π-conjugation for high performance bulk heterojunction (BHJ) OSCs.