Seok-Heon Jung

Charge-Transport Anisotropy in a Uniaxially Aligned Diketopyrrolopyrrole-Based Copolymer.

Aligned films of a semiconducting DPP‐based copolymer exhibit highly anisotropic charge transport with a band‐like temperature dependence along the alignment direction and hole mobilities of up to 6.7 cm2 V−1 s−1. X‐ray diffraction measurements reveal an exceptional degree of in‐plane alignment, high crystallinity, and a dominant face‐on orientation of the polymer backbones. The surprising charge‐transport properties are interpreted in a tie‐chain model consistent with anisotropic activation energies.

Semiconductor nanocrystals in fluorous liquids for the construction of light-emitting diodes

This communication reports a materials handling strategy based on fluorous materials chemistry using CdSe/CdS/CdZnS core–shell type semiconductor nanocrystals. When the crystals were treated with the semi-perfluoroalkanethiol ligands in the presence of iPr2EtN, the surface-modified nanocrystals (RF1-NC and RF2-NC) became soluble in the fluorous liquids. Solutions of RF1-NC and RF2-NC in HFE-7500 enabled the solution-casting of nanocrystalline films on top of a small-molecular hole-transporting layer and provided layered structures suitable for light-emitting diode fabrication.

Effect of Donor Building Blocks on the Charge-Transfer Characteristics of Diketopyrrolopyrrole-Based Donor–Acceptor-Type Semiconducting Copolymers

We investigate the effect of donor (D) building blocks on the charge transportation characteristics of donor (D)-acceptor (A)-type semiconducting copolymers with alternating electron-donating and electron-accepting units to provide a basis for the rational design of high-performance semiconducting polymers. For this purpose, we studied three different diketopyrrolopyrrole (DPP)-based semiconducting copolymers comprising a common dithienyl-DPP [3,6-dithienyl-2,5-diketopyrrolo(3,4-c)pyrrole] and variable donor moieties: phenylene (P)-PDPPTPT, thiophene (T)-PDPP3T, and thienothiophene (TT)-PDPP2T-TT. Structural analysis using grazing incidence X-ray diffraction indicates that all three DPP-based copolymer films have edge-on phases but poor crystallinity of the films, except the PDPP2T-TT copolymer with branched alkyl side chains that are relatively long. The electrical measurements show that the DPP-based copolymer with a TT donor unit has the highest field-effect mobility value of 0.30 cm2/V s. To understand the role of the donor units in DPP-based D-A copolymers, further insight into the charge transportation behavior is realized by analyzing the temperature-dependent transfer curves of the DPP semiconducting copolymer-based field-effect transistors using the Gaussian disorder model. Compared to the DPP-based D-A-type semiconducting copolymer with a P-moiety and shorter-branched alkyl side chains that exhibit a broad distribution in the density of localized states (DOS) and a higher thermal-activated energy for charge hopping, the DPP copolymers with a TT-moiety and longer branched side chains have the narrowest DOS, the lowest activation energy, and thus the highest hole mobility. These results suggest that the higher mobilities obtained from PDPP2T-TT with a TT donor unit can be attributed to the suppressed DOS distribution near the transport level, which mainly originates from the narrowest energy band gap tuned with the orbital couplings of the DPP acceptor and TT donor units.

Analysis of charge injection and contact resistance as a function of electrode surface treatment in ambipolar polymer transistors

Ambipolar organic field-effect transistors (OFETs) have both of hole and electron enhancements in charge transport. The characteristics of conjugated diketopyrrolopyrrole ambipolar OFETs depend on the metal-contact surface treatment for charge injection. To investigate the charge-injection characteristics of ambipolar transistors, these devices are processed via various types of self-assembled monolayer treatments and annealing. We conclude that treatment by the self-assembled monolayer 1-decanethiol gives the best enhancement of electron charge injection at both 100 and 300 °C annealing temperature. In addition, the contact resistance is calculated by using two methods: One is the gated four-point probe (gFPP) method that gives the voltage drop between channels, and the other is the simultaneous contact resistance extraction method, which extracts the contact resistance from the general transfer curve. We confirm that the gFPP method and the simultaneous extraction method give similar contact resistance, which means that we can extract contact resistance from the general transfer curve without any special contact pattern. Based on these characteristics of ambipolar p- and n-type transistors, we fabricate inverter devices with only one active layer.

Electroluminescence from Solution-Processed Pinhole-Free Nanometer-Thickness Layers of Conjugated Polymers

We report the formation of robust, reproducible, pinhole-free, thin layers of fluorinated polyfluorene conjugated copolymers on a range of polymeric underlayers via a simple solution processing method. This is driven by the different characters of the fluorinated and nonfluorinated sections of these polymers. Photothermal deflection spectroscopy is used to determine that these layers are 1-2 nm thick, corresponding to a molecularly thin layer. Evidence that these layers are continuous and pinhole-free is provided by electroluminescence data from polymer LED devices that incorporate these layers within the stacked LED structure. These reveal, remarkably, light emission solely from these molecularly thin layers.