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.     

Solid-state white light-emitting electrochemical cells using iridium-based cationic transition metal complexes

White electroluminescent (EL) emission from single-layered solid-state light-emitting electrochemical cells (LECs) based on host-guest cationic iridium complexes has been successfully demonstrated. The devices show white EL spectra (Commission Internationale de l'Eclairage coordinates ranging from (x, y) = (0.45, 0.40) to (0.35, 0.39) at 2.9-3.3 V with high color rendering indices up to 80. Peak external quantum efficiency and peak power efficiency of the white LEC reach 4% and 7.8 lm/W, respectively. These results suggest that white LECs based on host-guest cationic transition metal complexes may be a promising alternative for solid-state lighting technologies.     

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.     

Electronic properties of silole-based organic semiconductors

We report on a detailed quantum-chemical study of the geometric structure and electronic properties of 2,5-bis(6(')-(2('),2(")-bipyridyl))-1,1-dimethyl-3,4-diphenylsilole (PyPySPyPy) and 2,5-di- (3-biphenyl)-1,1-dimethyl-3,4-diphenylsilole (PPSPP). These molecular systems are attractive candidates for application as electron-transport materials in organic light-emitting devices. Density Functional Theory (DFT), time-dependent DFT, and correlated semiempirical (ZINDO/CIS) calculations are carried out in order to evaluate parameters determining electron-transport and optical characteristics. Experimental data show that PyPySPyPy possesses an electron-transport mobility that is significantly greater than PPSPP, while PPSPP has a significantly larger photoluminescence quantum yield; however, the theoretical results indicate that the two systems undergo similar geometric transformations upon reduction and have comparable molecular orbital structures and energies. This suggests that intermolecular interactions (solid-state packing, electronic coupling) play significant roles in the contrasting performance of these two molecular systems.     

Electrical transport properties of an organic semiconductor on polyaniline doped by boric acid

The electrical conductivity, thermoelectric power, and dielectric properties of polyaniline doped by boric acid (PANI‐B) have been investigated. The room temperature electrical conductivity of PANI‐B was found to be 1.02 × 10^?4 S cm^?1. The thermoelectric power factor for the polymer was found to be 0.64 µW m^?1 K^?2. The optical band gap of the PANI‐B was determined by optical absorption method, and the PANI‐B has a direct optical band gap of 3.71 eV. The alternating charge transport mechanism of the polymer is based on the correlated barrier hopping (CBH) model. The imaginary part of the dielectric modulus for the PANI‐B suggests a temperature dependent dielectric relaxation mechanism. Electrical conductivity and thermoelectric power results indicate that the PANI‐B is an organic semiconductor with thermally activated conduction mechanism. Copyright © 2008 John Wiley & Sons, Ltd.     

Electrical properties of MOS and MIS diodes using an electrochemically prepared poly(N-methylpyrrole)

Organic semiconductor devices using electrochemically prepared poly(N-methylpyrrole) (PNP) were characterized by capacitance-voltage (C-V) measurements. A device with aluminum evaporated on the PNP behaved as a MOS (metal-oxide semiconductor) diode in vacuum. Assuming the oxide layer between the metal and the PNP to be pure Al₂O₃, the MOS parameters were calculated. The values obtained for depletion width W = 166 Å and acceptor density N_A = 10¹⁸ cm^?3 in the PNP were reasonable by comparison with the previously reported values obtained by Schottky analysis. An indium/poly(p-phenylene)-1,3,4-oxadiazole (POD)/PNP diode was prepared, and C-V measurements were carried out in air. The diode behaved like an MIS (metal-insulator semiconductor) device, but the characteristics showed large variation with frequency. We have proposed a schematic model for this mechanism on the basis of the results obtained by dielectric measurements of POD and the temperature dependence of the MIS characteristics.     

Spatial control of p-n junction in an organic light-emitting electrochemical transistor

Low-voltage-operating organic electrochemical light-emitting cells (LECs) and transistors (OECTs) can be realized in robust device architectures, thus enabling easy manufacturing of light sources using printing tools. In an LEC, the p-n junction, located within the organic semiconductor channel, constitutes the active light-emitting element. It is established and fixated through electrochemical p- and n-doping, which are governed by charge injection from the anode and cathode, respectively. In an OECT, the electrochemical doping level along the organic semiconducting channel is controlled via the gate electrode. Here we report the merger of these two devices: the light-emitting electrochemical transistor, in which the location of the emitting p-n junction and the current level between the anode and cathode are modulated via a gate electrode. Light emission occurs at 4 V, and the emission zone can be repeatedly moved back and forth within an interelectrode gap of 500 μm by application of a 4 V gate bias. In transistor operation, the estimated on/off ratio ranges from 10 to 100 with a gate threshold voltage of -2.3 V and transconductance value between 1.4 and 3 μS. This device structure opens for new experiments tunable light sources and LECs with added electronic functionality.     

Phosphorescent sensitized fluorescent solid-state near-infrared light-emitting electrochemical cells

We report phosphorescent sensitized fluorescent near-infrared (NIR) light-emitting electrochemical cells (LECs) utilizing a phosphorescent cationic transition metal complex [Ir(ppy)(2)(dasb)](+)(PF(6)(-)) (where ppy is 2-phenylpyridine and dasb is 4,5-diaza-9,9'-spirobifluorene) as the host and two fluorescent ionic NIR emitting dyes 3,3'-diethyl-2,2'-oxathiacarbocyanine iodide (DOTCI) and 3,3'-diethylthiatricarbocyanine iodide (DTTCI) as the guests. Photoluminescence measurements show that the host-guest films containing low guest concentrations effectively quench host emission due to efficient host-guest energy transfer. Electroluminescence (EL) measurements reveal that the EL spectra of the NIR LECs doped with DOTCI and DTTCI center at ca. 730 and 810 nm, respectively. Moreover, the DOTCI and DTTCI doped NIR LECs achieve peak EQE (power efficiency) up to 0.80% (5.65 mW W(-1)) and 1.24% (7.84 mW W(-1)), respectively. The device efficiencies achieved are among the highest reported for NIR LECs and thus confirm that phosphorescent sensitized fluorescence is useful for achieving efficient NIR LECs.     

Electrical transport properties of Zn doped ZnO

Electrical resistivity and Hall effect measurements at 77–373°K are presented for Zn doped ZnO crystals. The crystals have been doped systematically at 600–1100°C in controlled pressures of Zn. The concentration of electrons at room temperature is in the range n_RT = 2.5 × 10¹⁶, to 3.6 × 10¹⁸, cm^?3. The donor level E_D and the concentrations of donors N^D and acceptors N_A have been calculated from a best fit to the experimental relationships log n versus $\text{1}\text{T}$ and log μ_H versus log T. At dilute concentrations of donors, two donor levels have been observed, E_D^I = 0.043–0.045 eV and a deeper level E^II_D greater than 0.165 eV. The ZnO was found to behave as a metal at N_D ～ 6 × 10¹⁸, cm^?3.At least two different donors have to be assumed in order to explain the experimental results. It is suggested that interstitial Zn is the electrical active donor at higher doping levels. The nature of the other donor is not clear. Neither 1s¹ H-type nor 1s² He-type donors seem to explain all the observations consistently.     

Efficient green-blue-light-emitting cationic iridium complex for light-emitting electrochemical cells

A highly luminescent novel cationic iridium complex [iridium bis(2-phenylpyridine)(4,4'-(dimethylamino)-2,2'-bipyridine)]PF6 was synthesized and characterized using NMR, UV-visible absorption, and emission spectroscopy and electrochemical methods. This complex displays intense photoluminescence maxima in the green-blue region of the visible spectrum and exhibits unprecedented phosphorescence quantum yields, 80 +/- 10% with an excited-state lifetime of 2.2 mus in a dichloromethane solution at 298 K. Single-layer light-emitting electrochemical cells with the charged complex as conducting and electroluminescent material sandwiched between indium-tin oxide and Ag electrodes were fabricated, which emit green-blue light with an onset voltage as low as 2.5 V. Density functional theory calculations were performed to provide insight into the electronic structure of the [iridium bis(2-phenylpyridine)(4,4'-(dimethylamino)-2,2'-bipyridine)]PF6 complex, comparing these results with those obtained for [iridium bis(2-phenylpyridine)(4,4'-tert-butyl-2,2'-bipyridine)]PF6.     

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.     

Color tunable organic light-emitting diodes using coumarin dopants

A series of 4-hydroxy coumarin derivatives, C-1, C-2 and C-3, were synthesized, and their photoluminescence and electroluminescent properties were investigated. The peak emission wavelengths of the dopants can be tuned between 574 and 618 nm, depending upon the electronic properties of the substituents. Multilayer electroluminescent devices were fabricated using C-1, C-2 and C-3 as the dopants. Among these devices, an external quantum efficiency of 3.2% and a maximum brightness of 1.6×10⁴ cd/m² were achieved.     

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.     

Electrical transport properties of pristine and iodine doped polyetherimides

Electrical conduction behavior of pristine and iodine doped polyetherimides(PEI) has been investigated under both transient and steady state conditions in the operating temperature range 50-200℃at various electric fields of 12-60 kV/cm.The transient currents show the hyperbolic decay character,and the decay exponent p(a measure of current decay rate) decreases with temperature(7) and doping concentration.The origin of transient currents has been attributed to the dipolar nature of carbonyl((?)C=O) groups...     

Synthesis of polymeric organic semiconductors using semifluorinated polymer precursors

The transesterification of 1,1,1,3,3,3-hexafluoroisopropyl acrylate (HFIPA) homopolymers and block copolymers with methyl acrylate (MA) or n-butyl acrylate (n-BuA) were investigated for the efficient synthesis of p-type and n-type organic semiconductor acrylates. HFIPA, MA, and n-BuA are readily prepared in a one-pot synthesis by Cu(0) reversible deactivation radical polymerization in high yield with low dispersity to give poly(HFIPA)₁₀₀, PMA₁₀₀-b-poly(HFIPA)_x, and PnBuA₁₀₀-b-poly(HFIPA)_x (x = 100, 50,25). The transesterification of poly(HFIPA)₁₀₀ was also studied using ¹⁹F NMR spectroscopy. Based on this method, a series of eight homopolymers and three copolymers based on semiconductor motifs commonly found in organic light-emitting diodes, organic thin-film transistors, and organic photovoltaics were synthesized with narrow dispersity and conversions up to 99%. These experiments demonstrate that poly(HFIPA) can provide a useful building block in the synthesis of organic electronic materials, providing a simpler route to complex materials which may be challenging to access directly via controlled radical polymerization. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2183–2191     

Improving the balance of carrier mobilities of host-guest solid-state light-emitting electrochemical cells

We report efficient host-guest solid-state light-emitting electrochemical cells (LECs) utilizing a cationic terfluorene derivative as the host and a red-emitting cationic transition metal complex as the guest. Carrier trapping induced by the energy offset in the lowest unoccupied molecular orbital (LUMO) levels between the host and the guest impedes electron transport in the host-guest films and thus improves the balance of carrier mobilities of the host films intrinsically exhibiting electron preferred transporting characteristics. Photoluminescence measurements show efficient energy transfer in this host-guest system and thus ensure predominant guest emission at low guest concentrations, rendering significantly reduced self-quenching of guest molecules. EL measurements show that the peak EQE (power efficiency) of the host-guest LECs reaches 3.62% (7.36 lm W(-1)), which approaches the upper limit that one would expect from the photoluminescence quantum yield of the emissive layer (～0.2) and an optical out-coupling efficiency of ～20% and consequently indicates superior balance of carrier mobilities in such a host-guest emissive layer. These results are among the highest reported for red-emitting LECs and thus confirm that in addition to reducing self-quenching of guest molecules, the strategy of utilizing a carrier transporting host doped with a proper carrier trapping guest would improve balance of carrier mobilities in the host-guest emissive layer, offering an effective approach for optimizing device efficiencies of LECs.     

Electrical conductivity and electrochemical properties of γ-irradiated TiO2

Electrical conductivity and Seebeck voltage developed in -irradiated TiO₂ was studied as a function of temperature. The conversion of n-type TiO₂ into p-type observed after irradiation was explained on the basis of formation of inversion layer of p-type TiO₂ due to the chemisorption of oxygen on the oxide surface during irradiation. Sign of the EMF of the electrochemical concentration cell of Ag/Ag⁺ with the addition of irradiated TiO₂ was found to be exactly opposite to that due to the addition of nonirradiated oxide.