Mi-Ri Choi

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.

Molecularly Controlled Interfacial Layer Strategy Toward Highly Efficient Simple-Structured Organic Light-Emitting Diodes

A highly efficient simplified organic light-emitting diode (OLED) with a molecularly controlled strategy to form near-perfect interfacial layer on top of the anode is demonstrated. A self-organized polymeric hole injection layer (HIL) is exploited increasing hole injection, electron blocking, and reducing exciton quenching near the electrode or conducting polymers; this HIL allows simplified OLED comprised a single small-molecule fluorescent layer to exhibits a high current efficiency (∼20 cd/A).

Polyaniline‐Based Conducting Polymer Compositions with a High Work Function for Hole‐Injection Layers in Organic Light‐Emitting Diodes: Formation of Ohmic Contacts

It is a great challenge to develop solution-processed, polymeric hole-injection layers (HILs) that perform better than small molecular layers for realizing high-performance small-molecule organic light-emitting diodes (SM-OLEDs). We have greatly improved the injection efficiency and the current efficiency of SM-OLEDs by introducing conducting polymer compositions composed of polyaniline doped with polystyrene sulfonate and perfluorinated ionomer (PFI) as the HIL. During single spin-coating of conducting polymer compositions, the PFI layer was self-organized at the surface and greatly increased the film work function. It enhanced hole-injection efficiency and current efficiency by introducing a nearly ohmic contact and improving electron blocking. Our results demonstrate that solution-processed polyaniline HILs with tunable work functions are good candidates for reducing process costs and improving OLED performance.

High-efficiency polymer photovoltaic cells using a solution-processable insulating interfacial nanolayer: the role of the insulating nanolayer

We employed a low-cost solution-processed ultrathin insulating polymeric layer of poly(4-hydroxystyrene) (PHS), with a high glass transition temperature (Tg ∼ 185 °C), as an interfacial layer between the polymer:fullerene photoactive layer and the Al negative electrode for enhancing device power conversion efficiency (PCE) of polymer bulk-heterojunction photovoltaic cells and investigated the roles of the interfacial nanolayer by ultraviolet photoemission spectroscopy and capacitance–voltage measurement. The thin polymeric layer forms a dipole layer and causes the vacuum level of the adjacent negative electrode to shift upward, which resulted in an increase of the built-in potential. As a result, the open-circuit voltage and PCE of the device using a PHS nanolayer were remarkably improved. We finally achieved a very high PCE of 6.5% with the PHS/Al negative electrode which is even much better than that of the device using an Al electrode (5.0%). The solution-processed inexpensive PHS layer with a high Tg can be an attractive alternative to conventional vacuum-deposited low-work-function metal and insulating metal fluoride interfacial layers.

Role of Ultrathin Metal Fluoride Layer in Organic Photovoltaic Cells: Mechanism of Efficiency and Lifetime Enhancement

Although rapid progress has been made recently in bulk heterojunction organic solar cells, systematic studies on an ultrathin interfacial layer at the electron extraction contact have not been conducted in detail, which is important to improve both the device efficiency and the lifetime. We find that an ultrathin BaF2 layer at the electron extraction contact strongly influences the open-circuit voltage (Voc ) as the nanomorphology evolves with increasing BaF2 thickness. A vacuum-deposited ultrathin BaF2 layer grows by island growth, so BaF2 layers with a nominal thickness less than that of single-coverage layer (≈3 nm) partially cover the polymeric photoactive layer. As the nominal thickness of the BaF2 layer increased to that of a single-coverage layer, the Voc and power conversion efficiency (PCE) of the organic photovoltaic cells (OPVs) increased but the short-circuit current remained almost constant. The fill factor and the PCE decreased abruptly as the thickness of the BaF2 layer exceeded that of a single-coverage layer, which was ascribed to the insulating nature of BaF2 . We find the major cause of the increased Voc observed in these devices is the lowered work function of the cathode caused by the reaction and release of Ba from thin BaF2 films upon deposition of Al. The OPV device with the BaF2 layer showed a slightly improved maximum PCE (4.0 %) and a greatly (approximately nine times) increased device half-life under continuous simulated solar irradiation at 100 mW cm(-2) as compared with the OPV without an interfacial layer (PCE=2.1 %). We found that the photodegradation of the photoactive layer was not a major cause of the OPV degradation. The hugely improved lifetime with cathode interface modification suggests a significant role of the cathode interfacial layer that can help to prolong device lifetimes.

Ultrahigh-efficiency solution-processed simplified small-molecule organic light-emitting diodes using universal host materials

Researchers achieved ultrahigh efficiency of solution-processed simplified small-molecule OLEDs that use novel universal host materials. Although solution processing of small-molecule organic light-emitting diodes (OLEDs) has been considered as a promising alternative to standard vacuum deposition requiring high material and processing cost, the devices have suffered from low luminous efficiency and difficulty of multilayer solution processing. Therefore, high efficiency should be achieved in simple-structured small-molecule OLEDs fabricated using a solution process. We report very efficient solution-processed simple-structured small-molecule OLEDs that use novel universal electron-transporting host materials based on tetraphenylsilane with pyridine moieties. These materials have wide band gaps, high triplet energy levels, and good solution processabilities; they provide balanced charge transport in a mixed-host emitting layer. Orange-red (~97.5 cd/A, ~35.5% photons per electron), green (~101.5 cd/A, ~29.0% photons per electron), and white (~74.2 cd/A, ~28.5% photons per electron) phosphorescent OLEDs exhibited the highest recorded electroluminescent efficiencies of solution-processed OLEDs reported to date. We also demonstrate a solution-processed flexible solid-state lighting device as a potential application of our devices.