Dal Ho Huh

Soluble Self‐Doped Conducting Polymer Compositions with Tunable Work Function as Hole Injection/Extraction Layers in Organic Optoelectronics

Charge injection/extraction layers greatly affect the efficiency and lifetime of organic optoelectronic devices. Therefore, reliable and soluble charge injection/extraction buffer layers, which are widely applicable to various organic optoelectronic devices, should be developed. Solution processable conducting polymers doped with poly(4-styrenesulfonate) are especially good candidates for use as hole injection buffer layers (HILs) and hole extraction buffer layers (HELs) in organic light-emitting diodes (OLEDs) and organic photovoltaic cells (OPVs). However, these polymers have practical limitations for large area electronics because the particles are dispersed in water and thus form poor-quality films with defects caused by aggregation of the particles; they also present a high hole injection/extraction barrier to overlying organic layers, and are hygroscopic. Therefore, to improve the device lifetime and hole injection/extraction capability in organic optoelectronics, a soluble, efficient, and stable buffer layer should be developed. We introduce soluble, self-doped conducting polyaniline graft copolymer compositions based on poly(4-styrenesulfonate)-g-polyaniline (PSS-g-PANI) (Figure 1) that have tunable work functions WF for HILs/ HELs in OLEDs and OPVs. We systematically controlled the self-organized surface-enriched layer of the HIL/HELs after incorporating a perfluorinated ionomer (PFI) into the PSS-gPANI solutions. Then, we investigated the influence of the surface layer on the values of WF of spin cast films and the correlation of WF values with hole injection/extraction capabilities and device lifetimes in OLEDs and OPVs. Water-soluble PSS-g-PANI with a 8:1 weight ratio (Figure 1) was synthesized (see the Supporting Information) by oxidative polymerization of aniline with an oxidant and an aqueous solution of random copolymer composed of stryenesulfonate and p-aminostyrene derivatives. Doping of the polymeric acid dopant PSS is structurally stable because this graft type conducting polymer bonds covalently to the PSS. We used ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) at the 4B1 beam line of Pohang Acceleration Laboratory (PAL) to characterize the surfaces of spin-cast films of the soluble conducting polymer and of the blends with the PFI. The value of WF was affected by the compositions of the solutions (Table 1). In PSS-g-PANI, WF= 5.28 eV, which is higher than that of the Figure 1. Self-organized film of PSS-g-PANI compositions after spincoating, and chemical structures of PSS-g-PANI and PFI.

Hole-injecting conducting-polymer compositions for highly efficient and stable organic light-emitting diodes

This letter introduces conducting polymer compositions which can be used for hole-injection layer in organic light-emitting diodes. The compositions are composed of poly (3,4-ethylenedioxythiophene) (PEDOT), polystyrene sulfonic acid (PSS) and a perfluorinated ionomer. The films based on these compositions showed much higher workfunction (∼5.3–5.7eV) than conventional PEDOT/PSS (∼5.0–5.2eV). When we fabricated blue polymer light-emitting diodes by using these compositions as a hole-injection layer, the luminescent efficiency was improved and the device lifetime was also enhanced relative to the device using the commercially available PEDOT/PSS. These compositions including perfluorinated ionomers can be one of the promising candidates for a hole-injection layer in organic light-emitting devices.

New interfacial materials for rapid hole-extraction in organic photovoltaic cells

Two new hole-extraction materials, TPDI (5,10,15-triphenyl-5H-diindolo[3,2-a:3′,2′-c]carbazole) and TBDI (5,10,15-tribenzyl-5H-diindolo[3,2-a:3′,2′-c]carbazole), were synthesized and explored in planar heterojunction organic solar cells (OSCs). The synthesized materials have good transparency in the visible spectrum, high hole mobility, and compatible highest occupied molecular orbital (HOMO) values with the donor material, which make them excellent hole-extraction layers (HELs) for OSCs. The SubPc (subphthalocyanine chloride)/C60 and SubNc (subnaphthalocyanine chloride)/C60 OSCs with a TBDI HEL show impressive 35.9% and 29.1% improvements in power conversion efficiencies compared to reference devices. Their HOMO values were evaluated as 5.2–5.3 eV and hole mobilities were measured as 5.9–6.1 × 10−3 cm2 V−1 s−1 at 0.3 MV cm−1 by the space-charge limited current method.

An Alternative Host Material for Long‐Lifespan Blue Organic Light‐Emitting Diodes Using Thermally Activated Delayed Fluorescence

It has been challenging to find stable blue organic light emitting diodes (OLEDs) that rely on thermally activated delayed fluorescence (TADF). Lack of stable host materials well‐fitted to the TADF emitters is one of the critical reasons. The most popular host for blue TADF, bis[2‐(diphenylphosphino)phenyl] ether oxide (DPEPO), leads to unrealistically high maximum external quantum efficiency. DPEPO is however an unstable material and has a poor charge transporting ability, which in turn induces an intrinsic short OLED operating lifespan. Here, an alternative host material is introduced which educes the potential efficiency and device lifespan of given TADF emitters with the appropriateness of replacing the most popular host material, DPEPO, in developing blue TADF emitters. It simultaneously provides much longer device lifespan and higher external quantum efficiency at a practical brightness due to its high material stability and electron‐transport‐type character well‐fitted for hole‐transport‐type TADF emitters.

P-207: Novel Buffer Materials with High Work Function for Efficient and Stable Organic Light-Emitting Diodes

We introduce novel water based buffer materials with high work function (5.5 − 5.6 eV), thermal stability, and low acidity (> pH 3). These materials are even stable at both neutral and basic conditions (> pH 7). When PLED devices are fabricated, even though there are high energy barrier between ITO (ca. 4.8 eV) and the buffer material and each buffer layers compared with that between ITO and Baytron-P, J-V characteristics of the device with our buffer layers are more improved than those with Baytron-P series (AI 4083 and CH 8000). Also their performance showed higher luminous efficiency (170% @ 1000 nit) and better device lifetime (>1.7 times) using new buffer materials compared with those of Baytron-P series even though we have yet to optimize the device structures.