Shanfeng Xue

Electrochemical route to fabricate film-like conjugated microporous polymers and application for organic electronics.

Film-like conjugated microporous polymers (CMPs) are fabricated by the novel strategy of carbazole-based electropolymerization. The CMP film storing a mass of counterions acting as an anode interlayer provides a significant power-conversion efficiency of 7.56% in polymer solar cells and 20.7 cd A(-1) in polymer light-emitting diodes, demonstrating its universality and potential as an electrode interlayer in organic electronics.

Color‐stable White Electroluminescence Based on a Cross‐linked Network Film Prepared by Electrochemical Copolymerization

2010 WILEY-VCH Verlag Gmb In recent years, white organic light-emitting diodes (WOLEDs) have received great attention because of their potential application in full-color flat-panel displays and solid-state lighting. A variety of common approaches to obtain white emission from OLEDs is the use of blends as emissive layers and multi-layer device structures. Nevertheless, both of them have some disadvantages, for instance, the intrinsic phase separation for the former and the interfacial mixing of different layers for the latter, which make WOLEDs suffer from bias-dependent electroluminescence (EL) spectra and limit their applications. Another approach to WOLEDs is the use of a single white-emitting polymer, but phase stability is still a potential issue especially for a device working at a temperature close to the glass-state transition temperature (Tg) of the polymers. To achieve colorstable white EL, a cross-linked network that fixes the luminescent chromophores is an ideal structure. However, the poor solubility of cross-linked polymer networks inhibits their processability, which is a basic factor for the fabrication of OLEDs. Here we present the first white emissive network films formed by in situ electrochemical copolymerization (ECP), and their use as emissive layers for highly efficient and color-stable WOLEDs. ECP with concurrent polymer film deposition has proven to be an especially useful method for the preparation of electroactive and conducting copolymer films. In this method, two or several kinds of precursors are mixed in an electrolyte solution. The precursors are electrochemically oxidized and the coupling reaction between the monomers occurs at the electrode surface with deposition of the copolymer film onto the electrode as is shown in Scheme 1, thus it is an in situ method to process cross-linked polymer network films. Films obtained by this method exhibit excellent structural and phase stability because the cross-linked structures in ECP films fix the position of the molecules and avoid phase separation in ECP films. The monomers dispersed in ECP films are uniform because the oxidation of monomers is random. The above advantages of ECP films are the essence of the present studies of highly efficient and color-stable WOLEDs. By utilizing the ECP films as emitters in WOLEDs, the devices emit illumination-quality white light with high brightness, high luminous efficiency, a high color rendering index (CRI), and extremely stable Commission Internationale d’Eclairage (CIE) coordinates. To achieve a full coverage of the whole visible spectroscopic range and high efficiency in white emissive ECP films, we used three electroactive and highly blue, green, and red fluorescent compounds as ECP precursors. The blue-, green-, and red-emissive molecules TCPC, TCBzC, and TCNzC, are fluorene-based compounds with an emission-adjustive unit and peripheral carbazole groups (see Scheme 1). Wide-bandgap TCPC (photoluminescence (PL) quantum efficiency in the solid state of FPL1⁄4 0.99) was used as the host and the blue-light-emitting species, and TCBzC (FPL1⁄4 0.84) and TCNzC (FPL1⁄4 0.63) were respectively selected as the greenand red-light-emitting moieties in terms of energy levels and spectroscopic overlap. The white-light emission was achieved by blending a small amount of the greenand red-emissive components with the blue emissive host in electrolyte solutions and electrochemically copolymerizing these three materials into an ECP film. The resulting copolymer can be considered as a system with two individual chromophores (green dopant and red dopant) molecularly dispersed in a blue-emissive polymeric host. Therefore, white-light emission was obtained from three simultaneously individual emissive species. The synthetic route and structural characterization of the three materials are included within the Supporting Information. The existing research for electrochemical behaviors of carbazole and its derivatives have shown the formation of dimeric carbazyl from the reaction of a carbazyl radical cation by electro-oxidation. The four carbazoles in the three materials, which increase the reaction points, help to form the cross-linked ECP films. The resulting ECP films exhibit a very smooth surface morphology and insolubility in commonly good solvents such as CH2Cl2 (see Fig. S1c and d), and demonstrate the formation of cross-linked films in contrast with the high solubility of the three precursors in CH2Cl2 (> 20mgmL ). X-ray photoelectron spectroscopy (XPS) analysis for N 1s shows a 0.5 eV shift in the TCPC electropolymerized (EP) film (see Fig. S2), which indicates the increase of the valent state of N 1s and the formation of dimeric carbazyl in the EP films, which is the essence of the cross-linked films. The absorption spectra of the three materials are presented in Figure 1a. The emissions of TCPC (439 nm) and TCBzC (509 nm) overlap the absorption of the 2,1,3-benzothiadiazole (BTz) unit in

RGB Small Molecules Based on a Bipolar Molecular Design for Highly Efficient Solution-Processed Single-layer OLEDs

In this paper, we describe a bipolar molecular design for small molecule solution-processed organic light emitting diodes (OLEDs). Combining the rigidity of the conjugated emissive cores and the flexibility of the peripheral alkyl-linked carbazole groups, two series of highly efficient bipolar RGB (red, green, blue) emitters have been synthesized and characterized. The emissive cores are composed of electron-withdrawing groups; the carbazole groups endow the materials electron-donating units. Such bipolar structures are advantageous for the carrier injection and balance. Four peripheral carbazole groups are introduced in T-series materials (TCDqC, TCSoC, TCBzC, TCNzC), and another four in O-series materials (OCDqC, OCSoC, OCBzC, OCNzC). With the single-layer device configuration of ITO/PEDOT:PSS/emitting layer/CsF/Al, two green devices exhibited excellent performance with a maximum luminescence efficiency of over 6.4 cd A(-1), and a high maximum luminance of more than 6700 cd m(-2). In addition, compared with the T-series, the luminescence efficiency of blue and red devices based on O-series materials increased from 1.6 to 2.8 cd A(-1) and 0.2 to 1.3 cd A(-1), respectively. To our knowledge, the performance of the blue device based on OCSoC is among the best of the blue small-molecule solution-processed single-layer devices reported so far.

Synthesis and characterization of new polyfluorene derivatives: using phenanthro[9,10-d]imidazole group as a building block for deep blue light-emitting polymer

A series of novel polyfluorene derivatives P1/4, P2/4, and P3/4, containing phenanthro[9,10-d]imidazole group on backbone are designed, synthesized, and well characterized. They all show high-molecular weights, good solubilities, and excellent thermal stabilities. The CV results of all three compounds show the lower LUMO levels and higher HOMO levels than PF. Among them, P3/4 exhibits deep blue emission both in solution and in solid state. The PLED based on P3/4 shows higher device performance and locates in the deep blue region with a CIE coordinate of (0.17, 0.08).

Simultaneous enhancement of the carrier mobility and luminous efficiency through thermal annealing a molecular glass material and device

The molecule 2,5,2′,5′-tetrakis(2,2-diphenylvinyl)biphenyl (TDPVBi) shows excellent solubility in organic solvents, a fully amorphous (glass state) property and strong blue emission in the solid state, endowing it with potential to fabricate solution-processed small-molecular devices. To further improve the quality of TDPVBi films, thermal annealing of method was performed for the pristine films, as prepared from solution, and proved to be an efficient way to repair the defects (e.g. pinhole) that exist in the pristine films. It is found that the films tend to become compact with level surface morphology after thermal annealing at a temperature around their glass transition temperature (Tg). The device characteristics show that thermal annealing induces a simultaneous enhancement of the hole mobility and luminous efficiency. The luminous efficiency reaches 4.60 cd A−1 (corresponding external quantum efficiency of ∼3.0%) after thermal annealing at a temperature of 120 °C, which is double that of the device without annealing.

Low-temperature annealing to enhance efficiency in organic small-molecule solution-processable OLEDs

Single-layer solution-processable organic light-emitting diodes (OLEDs) based on a green emissive molecule of 4,7-bis(9,9-bis(6-9H-carbazol-9-yl)hexyl)-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazole (TCBzC) are fabricated. The luminous efficiency reaches 6.9 cd A−1 after annealing at low temperature of 40 °C, which is double that of the un-annealing device, and this performance is among the highest efficiency of single-layer solution-processable OLEDs reported so far. This enhancement may be due to rearrangement of the peripheral alkyl-linked carbazole units in TCBzC after annealing, which helps the hole and electron transport to be more balanced.