Direct imaging and probing of the p-n junction in a planar polymer light-emitting electrochemical cell

A vast array of semiconductor applications relies on the ability to dope the materials by the controlled introduction of impurities in order to achieve desired charge carrier concentration and conduction type. In this way, various functional metal/semiconductor or semiconductor/semiconductor junctions can be constructed for device applications. Conjugated polymers are organic semiconductors that can be electrochemically doped to form a dynamic p-n junction. The electronic structure and even the existence of such a polymer p-n junction had been the subject of intense scrutiny and debate. In this work, the formation of the world's largest frozen polymer p-n junction and its light-emission are visualized. With a pair of micromanipulated probes, we mapped the potential distribution of the p-n junction under bias across the entire interelectrode gap of over 10 mm. Site-selective current-voltage measurements reveal that the polymer junction is a graded p-n junction, with a much more conductive p region than n region.     

The importance of p-n junction interfaces for efficient small molecule-based organic solar cells

The efficiency of small-molecule solar cells critically depends on the match of the junction of the donor and acceptor semiconductors used in these devices to create charged carriers and on the mobility of individual components to transport holes and electrons. In the present study, a 2% efficient bilayer organic solar cell consisting of a p-type semiconductor, pentacene, and an n-type semiconductor, N,N'-diheptyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C(7)), is fabricated. The morphology of PTCDI-C(7) interestingly follows pentacene due to the matched surface energy of these two active layers and the easily deposited PTCDI-C(7) monomers on an inclined plane of the pentacene grains. This condition results in the low trap states in the PTCDI-C(7) film and at the pentacene/PTCDI-C(7) interface for the enhancement of exciton dissociation and carrier transport compared with the photoactive layer comprised of pentacene and N,N-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C(13)). The detailed exciton and carrier transport mechanisms are investigated using time-resolved photoluminescence and X-ray diffraction spectroscopy.     

Electrical properties of electrochemically doped organic semiconductors using light-emitting electrochemical cells

We present a study of the electrical properties of electrochemically doped conjugated polymers using polymeric light-emitting electrochemical cells (PLECs) and interpreting the results according to a phenomenological model (PM) which assumes that, above the device turn-on voltage, the bulk transport properties of the doped organic semiconductor are responsible for the main contribution to the whole device conductivity. To confirm the predictions of this model, the dependence of the conductivity of PLECs with different parameters is evaluated and compared with the behavior expected for a doped semiconducting polymeric material. The organic semiconductor doping level, the blend concentration of organic semiconducting molecules, the device thickness, the charge carrier mobility, and the temperature are the parameters varied to perform this analysis. We observed that the device conductivity is independent of the active layer thickness, weakly dependent on the temperature, but strongly dependent on the semiconductor doping level, on the semiconductor fraction in the blend, and on the intrinsic charge carrier mobility. These results were well described by the variable range hopping (VRH) model, which has been widely employed to describe the charge transport in doped semiconducting polymeric materials, confirming the prediction of the phenomenological model. The current analysis demonstrates that PLECs are a suitable system for studying, in situ, the electrochemical doping of semiconducting polymers, permitting the evaluation of material properties as, for instance, the density of electronic charge carriers (and, consequently, the ionic charge carrier concentration) necessary to achieve the maximum electrochemical doping level of the organic semiconductor.     

有机发光材料的电化学稳定性及其对器件稳定性的影响

有机发光器件（OLED）在平板显示和固体照明领域有着广阔的应用前景.过去的二十多年来,OLED的效率得到了大幅提升,但是器件的稳定性仍有待提高.在OLED器件中,通常认为载流子的传输涉及分子反复的氧化还原.因此,OLED材料的电化学性质是影响器件稳定性的重要因素.本文总结了近年来有关OLED材料电化学性质的研究进展,并重点探讨了材料的电化学稳定性与器件稳定性之间的关系.总结发现：（1）单极性材料的电化学不稳定性是导致器件衰减的本质原因之一;（2）双极性材料高度的电化学稳定性有助于提高器件的稳定性,但并不一定保证器件具有高稳定性;（3）有关材料分子结构的稳定性对器件稳定性的影响以及器件的本征衰变机制还有待深入研究.相信,对OLED发光材料稳定性和器件衰变机制的深入研究将有助于提高其他有机光电材料和器件的稳定性,从而推动有机电子学和相关产业的发展.     

Identifying and alleviating electrochemical side-reactions in light-emitting electrochemical cells

We demonstrate that electrochemical side-reactions involving the electrolyte can be a significant and undesired feature in light-emitting electrochemical cells (LECs). By direct optical probing of planar LECs, comprising Au electrodes and an active material mixture of {poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) + poly(ethylene oxide) (PEO) + KCF3SO3}, we show that two direct consequences of such a side-reaction are the appearance of a "degradation layer" at the negative cathode and the formation of the light-emitting p-n junction in close proximity to the cathode. We further demonstrate that a high initial drive voltage and a high ionic conductivity of the active material strongly alleviate the extent of the side reaction, as evidenced by the formation of a relatively centered p-n junction, and also rationalize our findings in the framework of a general electrochemical model. Finally, we show that the doping concentrations in the doped regions at the time of the p-n junction formation are independent of the applied voltage and relatively balanced at approximately 0.11 dopants/MEH-PPV repeat unit in the p-type region and approximately 0.15 dopants/MEH-PPV repeat unit in the n-type region.     

The efficiency of light-emitting electrochemical cells

We report on the efficiency behavior of light-emitting electrochemical cells (LECs) fabricated from a methyl-substituted ladder-type poly(p-phenylene) (mLPPP) that was blended with a crown ether based solid state electrolyte. Unlike organic light-emitting diodes (oLEDs) utilizing mLPPP as an active layer, the LECs suffer from a loss of efficiency at elevated current densities. From scan rate dependent studies we deduce that this efficiency drop is not only due to device decomposition upon high voltage operation and we also reveal the intrinsic mode of LEC operation. The decreasing width of the intrinsic region between the p- and n-type doped zones upon ongoing pin-junction formation causes distinct (either field or electrode induced) luminance quenching effects.     

Polymer light-emitting electrochemical cells: Recent developments to stabilize the p-i-n junction and explore novel device applications

Polymer light-emitting electrochemical cells (PLECs) employ a thin layer of a luminescent conjugated polymer admixed with an ionic source and an ionic conductor for the in-situ formation of p-i-n junction and subsequent efficient injections of both electrons and holes.The junction formation enables the use of air-stable conductors as the cathode and a relatively thick emissive polymer layer that is more compatible with low-cost solution-based processes.This paper overviews the operation mechanism of the PLECs,the properties and drawbacks of the devices.The employment of crosslinkable ionic conductors to stabilize the p-i-n junction is reviewed.The resulting static junction electroluminesces light at high brightness,high efficiency,and prolonged lifetime.Silver paste and carbon nanotubes can be used as the cathode,thus,PLECs were fabricated by lamination.Using single wall carbon nanotubes coated elastic substrate as both anode and cathode,the PLECs can be made highly stretchable.     

白光有机发光二极管的最新进展

有机发光二极管(Organic Light-Emitting Diodes,OLEDs)以其制备工艺简单、成本低、发光颜色可在可见光区内任意调节以及易于大面积制作和柔韧弯曲等优点,被认为是未来重要的显示技术之一,在未来照明光源领域也显示了诱人的应用前景.一般认为,如果OLED的发光效率超过100 lm/W,就有可能取代一般照明.本文综述了实现白光OLED的方法及其最新进展,并对白光OLED存在的问题及其发展趋势进行了讨论.     

Near-quantitative internal quantum efficiency in a light-emitting electrochemical cell

A green-light-emitting iridium(III) complex was prepared that has a photoluminescence quantum yield in a thin-film configuration of almost unity. When used in a simple solid-state single-layer light-emitting electrochemical cell, it yielded an external quantum efficiency of nearly 15% and a power efficiency of 38 Lm/W. We argue that these high external efficiencies are only possible if near-quantitative internal electron-to-photon conversion occurs. This shows that the limiting factor for the efficiency of these devices is the photoluminescence quantum yield in a solid film configuration. The observed efficiencies show the prospect of these simple electroluminescent devices for lighting and signage applications.     

Dynamic doping and degradation in sandwich-type light-emitting electrochemical cells

Photoluminescence spectroscopy has been performed in situ during device operation and after switch-off on ionic transition metal complex (iTMC)-based sandwich-type light-emitting electrochemical cells (LECs). It is demonstrated that the photoluminescence of the LECs decreases with increasing operating time. For operating times up to three hours the decline in photoluminescence is fully recoverable after switching off the bias. These results imply that doping of the iTMC layer is responsible, not only, for the turn-on of LECs but also for their lifetimes.     

Proteins as functional interlayer in organic field-effect transistor

The paper summarizes and discusses the recent advances of proteins as functional interlayers in organic field-effect transistors (OFETs). Specific focus is given on the proteins integrated into the device structure, either to act as dielectric materials or to perform as the functional interlayer between the dielectric and the organic semiconductor (OSC). The main emphasis is give to the location and the specific effect of protein layers in the structure of OFETs. Besides, the possibility of amyloid serving as useful building blocks for OFET is discussed.     

Performance and defects in phosphorescent organic light-emitting diodes

Phosphorescent heavy metal complexes can utilize both singlet and triplet excitons and thus are interesting for doping polymer to obtain highly efficient organic light-emitting diodes. In this study, we have investigated devices using a new phosphorescent–metal complex containing fluorene and platinum added to a luminescent polymer blend, composed of 2-(4-biphenylyl)-5-(4-tert-butyl-phenyl)-(1,3,4-oxadiazole) (PBD) and poly(9-vinylcarbazole) (PVK). The performance of devices (luminance and yield) is measured in indium tin oxide (ITO)/poly(3-4 ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/(PVK–PBD-complex)/Al diodes. The devices emit an orange light with a brightness of 607 cd/m² and an external quantum efficiency of 0.28 cd/A at 25 V. In order to investigate the structural modifications of the polymer by the incorporation of phosphorescent–metal complex, we have studied the defect states in diodes by charge-based Deep Level Transient Spectroscopy (Q-DLTS). Analysis of Q-DLTS spectra obtained in undoped and doped devices, revealed at least three trap levels distributed in the range 0.2–0.5 eV within the band gap of the hybrid composite with trap density in the range around 10¹⁶ cm^?3. Incorporation of Pt complex into the polymer blend modified the trap states by reducing the density of traps in the blend and by creating new trap levels in the band gap.     

Spatial and temporal electrochemical control of singlet oxygen production and decay in photosensitized experiments

Active spatial and temporal modulation of domains of singlet oxygen activity is demonstrated using electrochemical tools. Using singlet oxygen microscopy in photosensitized experiments, it is shown that singlet oxygen concentrations around an ultramicroelectrode can be controlled by applying a bias voltage to the electrode. Two phenomena that can be exploited separately or collectively are examined: (1) the singlet oxygen concentration can be altered by local oxidation or reduction of the photosensitizer, which is the precursor to singlet oxygen, and (2) the reduction of oxygen to produce the superoxide anion which, among other things, is an effective singlet oxygen quencher, results in a local decrease in the concentration of singlet oxygen around the electrode. Both of these phenomena depend significantly on the diffusion of molecules along concentration gradients established by the biased electrode. The results reported herein demonstrate that one can indeed exert local electrochemical control and readily manipulate the population of singlet oxygen produced in a photosensitized process.     

Organometallic complexes as hole-transporting materials in organic light-emitting diodes

The use of metal complexes fac-tris(1-phenylpyrazolato-N,C(2)('))cobalt(III) [fac-Co(ppz)(3)], fac-tris(2-phenylpyridinato-N,C(2)(') cobalt(III) [fac-Co(ppy)(3)], and [tris[2-((pyrrole-2-ylmethylidene)amino)ethyl]amine]gallium(III) [Ga(pma)] as materials for hole-transporting layers (HTL) in organic light-emitting diodes (OLEDs) is reported. Co(ppz)(3) and Co(ppy)(3) were prepared by following literature procedures and isolated as mixtures of facial (fac) and meridional (mer) isomers. The more stable fac isomers were separated from the unstable mer forms via column chromatography and thermal gradient sublimation. Crystals of fac-Co(ppz)(3) are monoclinic, space group P2(1)/c, with a = 13.6121(12) A, b = 15.5600(12) A, c = 22.9603(17) A, beta = 100.5 degrees, V = 4781.3(7) A(3), and Z = 8. [Tris[2-((pyrrol-2-ylmethylidene)amino)ethyl]amine]gallium [Ga(pma)] was prepared by the reaction of gallium(III) nitrate with the pmaH(3) ligand precursor in methanol. Ga(pma) crystallizes in the cubic space group I3d with cell parameters a = 20.2377(4) A, b = 20.2377(4) A, c = 20.2377(4) A, beta = 90.0 degrees, V = 8288.6(3) A(3), and Z = 16. These cobalt and gallium complexes are pale colored to colorless solids, with optical energy gaps ranging 2.6-3.36 eV. A two-layer HTL/ETL (ETL = electron-transporting layer) device structure using fac-Co(ppz)(3) and fac-Co(ppy)(3) as the HTL does not give efficient electroluminescence. However, the introduction of a thin layer of a hole-transporting material (N,N'-bis(1-naphthyl)-N,N'-diphenylbenzidine, NPD) as an energy "stair-step" and electron/exciton-blocker dramatically improves the device performance. Both fac-Co(ppz)(3) and fac-Co(ppy)(3) devices give external quantum efficiencies higher than 1.0%, with brightness 5000 and 7000 Cd/m(2) at 10 V, respectively. Ga(pma) also functions as an efficient interface layer, giving device performances very similar to those of analogous devices using NPD as the interface layer. Stability tests have been carried out for Co(ppz)(3)/NPD/Alq(3) and Co(ppy)(3)/NPD/Alq(3) devices. While fac-Co(ppy)(3) gave stable OLEDs, the fac-Co(ppz)(3)-based devices had very short lifetimes. On the basis of the experimental results of chemical oxidation of fac-Co(ppz)(3), the major cause for the fast decay of the fac-Co(ppz)(3) device is proposed to be the decomposition of fac-Co(ppz)(3)(+) in the HTL layer during the device operation.     

Progress in small-molecule luminescent materials for organic light-emitting diodes

Organic light-emitting diodes(OLEDs) have been extensively studied since the first efficient device based on small molecular luminescent materials was reported by Tang. Organic electroluminescent material, one of the centerpieces of OLEDs, has been the focus of studies by many material scientists. To obtain high luminosity and to keep material costs low, a few remarkable design concepts have been developed. Aggregation-induced emission(AIE) materials were invented to overcome the common fluorescence-quenching problem, and cross-dipole stacking of fluorescent molecules was shown to be an effective method to get high solid-state luminescence. To exceed the limit of internal quantum efficiency of conventional fluorescent materials, phosphorescent materials were successfully applied in highly efficient electroluminescent devices. Most recently, delayed fluorescent materials via reverse-intersystem crossing(RISC) from triplet to singlet and the "hot exciton" materials based on hybridized local and charge-transfer(HLCT) states were developed to be a new generation of low-cost luminescent materials as efficient as phosphorescent materials. In terms of the device-fabrication process, solution-processible small molecular luminescent materials possess the advantages of high purity(vs. polymers) and low procession cost(vs. vacuum deposition), which are garnering them increasing attention. Herein, we review the progress of the development of small-molecule luminescent materials with different design concepts and features, and also briefly examine future development tendencies of luminescent materials.     

Fabrication of electrochemical transistor based on pi-conjugate polymer Langmuir-Blodgett film

We fabricated an efficient organic electrochemical transistor (OECT) composed of polymer Langmuir-Blodgett (LB) film. The pi-conjugated polymer LB film, which was constructed from a poly(N-dodecylacrylamide) (pDDA) and poly(3-hexylthiophene) (PHT) mixture, was used as a conduction channel layer to connect source and drain electrodes. The mixed-polymer LB film was characterized using UV-vis spectroscopy, X-ray diffraction (XRD), atomic force microscopy (AFM), and cyclic voltammetry. Subsequent UV spectra measurements, XRD measurements, and AFM measurements show that PHT forms a crystalline lamellar domain in the layered structure of pDDA. The OECT included 10 layers of the mixed-polymer LB film as the conduction channel layer. The OECT showed an on/off ratio of 1.1x10(4) and mobility of 7.5x10(-2) cm2 V(-1) s(-1) at low gate (VG=-1.2 V) and source-drain voltages (VDS=-0.5 V). Moreover, the necessary charge to operate the OECT was 1.1x10(-9) mol of e(-1) cm(-2), which was 2 orders smaller than the value reported using a similar device structure. The relatively high on/off ratio and low charge consumption suggest that this OECT, which is fabricated from pi-conjugated polymer LB films, is applicable to macroelectronic devices.     

聚集诱导发光分子的光电功能与器件应用

光电功能分子通常以薄膜和聚集体的形式显示功能, 聚集诱导发光（AIE）分子体系的发现为解决固态下聚集诱导荧光猝灭（ACQ）难题提供了新的思路. 本文总结了近年来本课题组发展的一系列AIE 分子, 侧重介绍这些AIE 分子的光电功能与器件应用, 特别是在有机电致发光器件和有机激光方面的应用. AIE 材料显示非常高的电致发光效率, 在显示与白光器件方面潜力巨大. 在发展电泵有机激光方面, AIE 材料特点突出, 是最有前景的一类材料.     

Enhancement of electrochemical transfer junction for cation extraction

Electrochemical transfer junctions (ETJs) were synthesized via a physical or chemical covering method onto a porous ceramic substrate (mullite and alumina). The Mo₆S₈ layer thickness ranged from 10 to 100 μm. The ETJ composites placed between two tanks lead to a cobalt transfer by applying a current density between electrodes placed in both tanks. The thickness decrease compared to a hot pressed junction allows imposing current densities equal to 7 or 9 mA cm^?2 inducing 4–6 fold faster Co²⁺ fluxes (1.3.10^?4 and 1.7.10^?4 mol h^?1 cm^?2 for physical and chemical covering respectively, versus 2.9.10^?5 mol h^?1 cm^?2 for hot pressed junctions).     

A hybrid living/organic electrochemical transistor based on the Physarum polycephalum cell endowed with both sensing and memristive properties

A hybrid bio-organic electrochemical transistor was developed by interfacing an organic semiconductor, poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate), with the Physarum polycephalum cell. The system shows unprecedented performances since it could be operated both as a transistor, in a three-terminal configuration, and as a memristive device in a two terminal configuration mode. This is quite a remarkable achievement since, in the transistor mode, it can be used as a very sensitive bio-sensor directly monitoring biochemical processes occurring in the cell, while, as a memristive device, it represents one of the very first examples of a bio-hybrid system demonstrating such a property. Our system combines memory and sensing in the same system, possibly interfacing unconventional computing. The system was studied by a full electrical characterization using a series of different gate electrodes, namely made of Ag, Au and Pt, which typically show different operation modes in organic electrochemical transistors. Our experiment demonstrates that a remarkable sensing capability could potentially be implemented. We envisage that this system could be classified as a Bio-Organic Sensing/Memristive Device (BOSMD), where the dual functionality allows merging of the sensing and memory properties, paving the way to new and unexplored opportunities in bioelectronics.     

A novel ambipolar spirobifluorene derivative that behaves as an efficient blue-light emitter in organic light-emitting diodes

A novel ambipolar spiro-configured D-A blue-light emitter bearing hole-transporting diphenylamino groups and electron-transporting phenylbenzimidazole groups was synthesized, characterized, and incorporated into an efficient single-layer organic light-emitting diode (OLED) device exhibiting blue-emission Commission International d'Eclairage (CIE) coordinates of 0.15 and 0.14, a turn-on potential of 4 V, a maximum brightness of 2800 cd/m2 at 830 mA/cm2 (19 V), and a maximum quantum efficiency of 0.53% (0.61 cd/A).