Polymer Light-emitting Electrochemical Cells Research Articles

Surface modification and characterization of indium–tin oxide for organic light-emitting devices

In this work, we used different treatment methods (ultrasonic degreasing, hydrochloric acid treatment, and oxygen plasma) to modify the surfaces of indium–tin oxide (ITO) substrates for organic light-emitting devices. The surface properties of treated ITO substrates were studied by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), sheet resistance, contact angle, and surface energy measurements. Experimental results show that the ITO surface properties are closely related to the treatment methods, and the oxygen plasma is more efficient than the other treatments since it brings about smoother surfaces, lower sheet resistance, higher work function, and higher surface energy and polarity of the ITO substrate. Moreover, polymer light-emitting electrochemical cells (PLECs) with differently treated ITO substrates as device electrodes were fabricated and characterized. It is found that surface treatments of ITO substrates have a certain degree of influence upon the injection current, brightness, and efficiency, but hardly upon the turn-on voltages of current injection and light emission, which are in agreement with the measured optical energy gap of the electroluminescent polymer. The oxygen plasma treatment on the ITO substrate yields the best performance of PLECs, due to the improvement of interface formation and electrical contact of the ITO substrate with the polymer blend in the PLECs.

Surface modifications of ITO electrodes for polymer light-emitting devices

Surface modifications were performed on the indium tin oxide (ITO) substrates for polymer light-emitting devices, using the different treatment methods including solvent cleaning, hydrochloric acid treatment and oxygen plasma. The influence of modifications on the surface properties of ITO electrodes were investigated by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), contact angle, and four-point probe. The surface energies of the ITO substrates were also calculated from the measured contact angles. Experimental results demonstrate that the surface properties of the ITO substrates strongly depend on the modification methods, and oxygen plasma more effectively improves the ITO surface properties compared with the other treatments. Furthermore, the polymer light-emitting electrochemical cells (LECs) with the differently treated ITO substrates as device electrodes were fabricated and characterized. It is observed that the surface modifications on ITO electrodes have a certain degree of influence upon the injection current, luminance and efficiency, but hardly upon the turn-on voltages of current injection and light emission which are close to the measured energy gap of electroluminescent polymer. Oxygen plasma treatment on the ITO electrode yields the better performance of the LECs, due to the improvement of interface formation and electrical contact of the ITO electrode with the polymer blend in the LECs.

Improving the efficiency of polymer light-emitting electrochemical cells by controlled doping relaxation

We show that a frozen p-n junction in a polymer light-emitting electrochemical cell can relax into a p-i-n junction when heated briefly to above the glass transition temperature of the polymer electrolyte used. This results in a less doped “quasi-intrinsic” emission zone with reduced photoluminescence quenching due to electrochemical doping. A 408%–2678% increase in electroluminescence efficiency has been demonstrated by controlled doping relaxation. The formation of a p-i-n junction has been confirmed by spatially resolved imaging of the doping profile in a planar device structure.

High-resolution imaging of electrochemical doping and dedoping processes in luminescent conjugated polymers

We report fluorescence imaging of extremely large planar polymer light-emitting electrochemical cells (LECs) with high spatial and temporal resolution. The type of cation used has been shown to strongly affect n-doping propagation. When left to relax, p doping undergoes significantly faster relaxation than n doping. Imaging also confirms that a p-n junction, rather than a p-i-n junction, is formed in fully turned on LECs, which suffer from heavy photoluminescence quenching throughout the polymer film. However, a p-i-n junction with a much less quenched “intrinsic” region can be formed by relaxing a p-n junction without bias, suggesting a simple approach for achieving more efficient LECs.

Imaging the doping and electroluminescence in extremely large planar polymer light-emitting electrochemical cells

An extremely large planar polymer light-emitting electrochemical cell with an interelectrode spacing of 11mm has been demonstrated. The large planar device structure allows for the imaging of doping propagation, photoluminescence, and electroluminescence (EL) with high spatial and temporal resolutions. Several unconventional EL modes have been observed based on the direct imaging of these devices with an interelectrode spacing ranging from 0.6to11mm. These include transient EL in a dynamic-junction device, EL from planar polymer/polymer heterojunction devices, and anomalous reverse-bias EL in a frozen-junction device. Transient EL occurs when the polarity of the applied bias is reversed after the device is fully turned on. The reversal of the applied bias causes the initiation and uneven propagation of fresh p doping within previously n-doped regions, and vise versa. This leads to the formation of transient, discrete, light-emitting p-n junctions near the electrodes before a continuous light-emission zone is formed by the complete reversal of the doping profile. In addition, planar cells consisting of a side-by-side polymer/polymer heterojunction have been demonstrated and imaged. The heterojunction in all working devices is found to be electronically conductive, but exhibits different ion-transport properties. Three types of polymer/polymer interface have been identified based on the imaging of doping and EL profiles. Finally, an anomalous reverse-bias EL mode has been observed in a planar frozen-junction device. The device was turned on at elevated temperature and then cooled to 200K. Stress under a constant reverse bias leads to the activation of anomalous EL that originates from the same region as forward-bias EL. Furthermore, both forward- and reverse-bias EL have been found to increase with time under reverse-bias stress.

Electrochemical and optical characterization of p- and n-doped poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene

We study electrochemical p- and n-type doping in the well-known light-emitting polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV). Doping reactions are characterized using cyclic voltammetry. Optical measurements including photoluminescence and UV/Vis/NIR transmission were performed on doped samples. We find that oxidation in MEH-PPV is a highly reversible reaction resulting in stable freestanding doped films, while the reduced form is unstable and the reaction irreversible. We discuss the dependence of doping reactions on scan rate, film thickness, salt type and concentration, and working electrode type. We observe the development of two additional broad absorption bands in both lightly and heavily doped films accompanied by a slight blueshift in the primary optical transition, suggesting bipolaron band formation. Finally we find that both p and n dopings result in extremely sensitive photoluminescence quenching. We propose a physical model for understanding electrochemical doping in MEH-PPV and the implications this has on the development of such technologies as polymer light-emitting electrochemical cells, electrochromic devices, actuators, and sensors.

Polymer light-emitting electrochemical cells: Frozen-junction operation of an “ionic liquid” device

We report frozen-junction operation of a polymer light-emitting electrochemical cell containing a mixture of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and the ionic liquid tetra-n-butylammonium trifluoromethanesulfonate (TBA-TF) as the active material. We find fast turn-on time, unipolar light emission, and significant operational lifetime up to T=200K for planar Au∕(TBA-TF+MEH-PPV)∕Au surface cells, which had been charged (i.e., electrochemically p- and n-type doped in situ) at T=393K and V=4V and then cooled to 80 K at V=4V. We employed differential scanning calorimetry to demonstrate that (TBA-TF+MEH-PPV) exhibits two melting transitions of TBA-TF crystalline phases located at Tm,1≈280K and Tm,2≈380K, respectively. The lower Tm,1 sets the upper limit for frozen-junction operation (with zero-ionic conductivity), while the larger Tm,2 correlates to the lower limit for the charging regime (with high ionic conductivity).

Bringing light to solid-state electrolytes: The polymer light-emitting electrochemical cell

A brief review is given on the operational mechanism and performance of various types of polymer light-emitting electrochemical cells (LECs). We discuss two key characteristics of LECs—the turn-on time and the operational lifetime—in the context of ion mobility, phase separation and electrochemical stability, and we propose that, with the appropriate selection of materials and pre-treatment, ternary {polymer + small molecule + salt} and single-component LECs could offer the desired combination of a fast turn-on time and long lifetime during device operation at room temperature.

Direct observation of a frozen junction in polymer light-emitting electrochemical cells

Doping propagation in polymer light-emitting electrochemical cells (LECs) has been studied based on the direct imaging of extremely large planar LECs at various temperatures and driving voltages. In situ electrochemical p doping, manifested as strong photoluminescence quenching in the polymer film, propagates at a nearly constant rate from anode toward cathode until it encounters an n-doped region to form a p-n junction. The propagation rate is found to be proportional to the applied bias at a fixed temperature. At a fixed bias, the propagation rate has a superexponential dependence on temperature. Below the glass transition temperature (Tg), the propagation rate becomes negligible, and a frozen junction has been confirmed by direct imaging of the doping profiles.

Studies on Polymer Light-emitting Electrochemical Cells ——Introduction of the Studies on the Electrochemistry of Conducting Polymers in CAS Key Laboratory of Organic Solids (Ⅱ)

Studies on Polymer Light-emitting Electrochemical Cells ——Introduction of the Studies on the Electrochemistry of Conducting Polymers in CAS Key Laboratory of Organic Solids (Ⅱ)

Polymer light-emitting electrochemical cells: Doping, luminescence, and mobility

We utilize the planar ``surface cell'' device configuration with Au contacts and a mixture of a soluble phenyl-substituted poly(para-phenylene vinylene) copolymer (``superyellow''), a dicyclohexano-18-crown-6 crown ether, and a ${\mathrm{LiCF}}_{3}{\mathrm{SO}}_{3}$ salt as the active material. Because the lowest thermal transition occurs well above room temperature (RT), we can study the charging process at an elevated temperature and probe the exact location of the electroluminescence (EL) and doping-induced quenching of photoluminescence in charged devices at RT. We also employ the same active material in thin-film field-effect transistor structures to study the influence of electrochemical doping on transistor performance. Our results demonstrate that reversible bipolar electrochemical doping indeed takes place at applied voltages above the band gap of the semiconducting polymer $(V\ensuremath{\geqslant}{E}_{g}∕e)$, but also that limited unipolar electrochemical doping can take place at $V<{E}_{g}∕e$ if the barrier heights for hole and electron injection are asymmetric. The EL originates in, or in close proximity to, the thin $p\text{\ensuremath{-}}i\text{\ensuremath{-}}n$ junction, which is located close to the cathode.

Planar polymer light-emitting device with fast kinetics at a low voltage

Polymer light-emitting electrochemical cells containing a ternary mixture of a soluble phenyl-substituted poly(para-phenylene vinylene) copolymer (“superyellow”), a dicyclohexano-18-crown-6 (DCH18C6) crown ether and a LiCF3SO3 salt as the active material have been assembled. Planar Au/{superyellow+DCH18C6+LiCF3SO3}/Au devices, with an interelectrode gap of 50 μm, were initially charged (i.e., electrochemically p- and n-doped in situ) at T=85 °C and then cooled to room temperature under applied voltage. When operated at T=23 °C charged devices show electroluminescence with fast response (< 1 s) at a low applied voltage (V⩾6 V). Charged devices could be stored under open-circuit conditions at room temperature for a prolonged time without detectable changes in device performance, and they can be completely (reversibly) discharged by raising the temperature to 85 °C. The active material mixtures were studied by atomic force microscopy and differential scanning calorimetry. The results demonstrate that superyellow phase separates from a crystalline DCH18C6–LiCF3SO3 complex on a ∼25 nm scale. The superyellow phase exhibits a glass transition at Tg∼180 °C, while the crystalline DCH18C6–LiCF3SO3 phase melts at Tm≈56 °C. Thus, we attribute the stabilization of charged Au/{superyellow+DCH18C6+LiCF3SO3}/Au devices in going from 85 to 23 °C as being directly related to the passage of Tm of the DCH18C6–LiCF3SO3 phase. The ionic distribution related to the p- and n-doped regions is “frozen-in” by this crystallization allowing for the observed fast kinetics at low voltages at room temperature.

Ionic Liquid Doped Polymer Light-Emitting Electrochemical Cells

The polymer light-emitting electrochemical cells (LECs) were fabricated with MEH−PPV as the luminescent polymer and the ionic liquid of imidazolium salts as the supporting electrolyte. The imidazolium salts utilized include various 1-methyl-3-alkylimidazolium salts with the alkyl substituents of butyl (bmim), dodecyl (dmim), tetradecyl (tmim), or hexadecyl (hmim) and the anions of PF6- or BF4-, which possess different melting points from room temperature for [bmim+][PF6-] to 83 °C for [hmim+][PF6-]. The electroluminescent (EL) properties and the electronic structure of the LECs were characterized by current−voltage (I−V), light intensity−voltage (L−V), and ac impedance measurements. It was found that the phase compatibility between the conjugated polymer and the ionic liquid determines the performance of the light-emitting devices, and the concentration of the ionic liquid and the ionic conductivity of the polymer blend films also play an important role. The imidazolium salts investigated in this work are...

Planar polymer light-emitting electrochemical cells with extremely large interelectrode spacing

Planar polymer light-emitting electrochemical cells have been demonstrated with interelectrode spacings nearly 100 times larger than previously reported. Striking details of the p–i–n junction structure have been imaged directly using a digital camera. With an interelectrode spacing of 1.5 mm, a well-defined emission zone over 70 μm wide is observed near the cathode. Imaging under UV excitation reveals that the p-doped region migrates towards the cathode during operation, causing the emission zone at its frontier to shift. The width of the p-doped region is found to have a large variance, giving rise to an irregularly shaped emission zone.

Polymer light-emitting electrochemical cell based on a novel block copolymer containing tri(ethylene oxide) spacers

We reported the optical properties of a novel block copolymer composed of chromophores, 1,5-di(3,5-dimethloxystyrylene)naphthylene, and spacers, tri(ethylene oxide). Intra- and interchain excitations due to aggregation are responsible for the changes in photoluminescence (PL) spectra when going from the solution to the film. Blue-green light-emitting electrochemical cells (LECs) based on this copolymer were demonstrated with the turn-on voltage of 2.5 V, the maximum electroluminescence (EL) efficiency of 4.2 cd/A, and the d.c. response time of less than 5 s, indicating strongly that an improved morphology of the active layer is formed because of the affinity of TEO with PEO. A.c. impedance measurements suggest these devices operate with an electrochemical doping model.

Synthesis of block copolymers with well-defined alternating chromophore and flexible spacer for electroluminescence application

Two novel light-emitting block copolymers, poly[1,4,7,10-tetraoxadecane-1,10-diyl-1,4-naphthalene-1,2-ethenediyl-1,4-phenylene-1,2-ethenediyl-1,4-naphthalene] (TEO-DNVB) and poly[1,4,7,10-tetraoxadecane-1,10-diyl-1,4-naphthalene-1,2-ethenediyl-(2,5-dimethoxy-1,4-phenylene)-1,2-ethenediyl-1,4-naphthalene] (TEO-MDNVB), were synthesized by using the Wittig reaction. The block copolymers are composed of the fluorescent segments, 1,4-di[2-(1-naphthyl) vinyl] benzene (DNVB) or 2,5-dimethyloxy-1,4-di[2-(1-naphthyl) vinyl] benzene (MDNVB) and the flexible segments, tri(ethylene oxide) (TEO). The copolymers were characterized by Fourier transform infrared (FT-IR), 1H-nuclear magnetic resonance ( 1H-NMR), ultraviolet-visible (UV-Vis), gel permeation chromatography (GPC) and cyclic voltammograms (CV). Thermal properties were investigated with differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) under nitrogen atmosphere. Cyclic voltammetric measurement reveals a reversible p-doping process. Efficient blue–green polymer light-emitting diodes (PLEDs) and polymer light-emitting electrochemical cells (PLECs) were successfully fabricated. The synthesis, characterization and the electroluminescent properties of the copolymers are reported in this paper.

Effects of electrode modifications on the performance of polymer light-emitting electrochemical cells

Effects of electrode modifications on the performance of polymer light-emitting electrochemical cells (LECs) were studied by measuring and comparing the current–luminance–voltage characteristics and the electroluminescence (EL) external quantum efficiency of the corresponding devices. A block copolymer of 2,5-dimethoxy-1,4-di(α-(2-naphthalene vinylene)benzene) and tri(ethylene oxide) (MDNVB–TEO) was used as the luminescent polymer in the LECs. Insertion of an ultrathin layer of barium at the polymer/cathode interface yields higher injection current and light intensity, lower threshold for light output, but lower device EL efficiency due to the doping of barium ions into the copolymer. With a conducting polymer layer of poly(ethylenedioxythiophene)-polystyrene sulfonate (PEDOT-PSS) on the indium-tin-oxide (ITO) anode, the operation stability and the lifetime of the LEC devices are significantly improved. However, the larger sheet resistance of the PEDOT-PSS layer results in lower injection current and light intensity, and higher threshold for light output. Oxygen-plasma treatment on the ITO anode increases the injection current and light intensity of the LECs. The threshold for the current injection consistently keeps unchanged for the LEC devices with and without the electrode modifications, which confirms the electrochemical doping mechanism of the LECs.

Polymer light-emitting electrochemical cells with the block copolymers containing PEO segments

Polymer light-emitting electrochemical cells (LECs) were fabricated and characterized with three block copolymers containing PEO segments as the luminescent polymers. The block copolymers are composed of PPV segments with three phenylene vinylene units and PEO segments with three ethylene oxide units. Blue–green light-emission of the LECs was demonstrated with the onset voltage lower than 3 V and EL efficiency of 0.74 cd/A at 2.8 V. The response time of the LECs is approximately 5 s. The transient response and a.c. impedance data indicate that the operating mechanism of the LECs is an electrochemical doping model.

Polymer light‐emitting electrochemical cell based on a block copolymer containing tri(ethyleneoxide) spacers

AbstractWe presented the optical properties of a novel block copolymer containing chromophores, 2,5‐dimethoxy‐1,4‐di(α‐(2‐naphthalene vinylene)benzene (MDNVB) and spacers, tri(ethylene oxide) (TEO). The intrachain and interchain excitations due to the molecular aggregation are responsible for the significant bathochromic shift in the photoluminescence (PL) spectrum when going from the solution to the thin film. With a certain content of the polymer electrolyte, blue–green light‐emitting electrochemical cell based on this block copolymer was demonstrated with the turn‐on voltage of 2.5 V, the maximum electroluminescence (EL) efficiency of 1.2 cd/A, and the d.c. response time of less than 5 sec, indicating strongly that an improved morphology of the active layer is formed because of the effective TEO‐PEO interaction. Lowering the content of the polymer electrolyte results in a higher turn‐on voltage and smaller injection current; increasing the content leads to a considerable microscopic leaking current and lower EL efficiency of the device. Copyright © 2002 John Wiley & Sons, Ltd.

Blue-green light-emission LECs based on block copolymers containing di(α-naphthalene vinylene)benzene chromophores and tri(ethylene oxide) spacers

The polymer light-emitting electrochemical cells (LECs) were fabricated and characterized with two novel block copolymers as the luminescent polymers. The block copolymers are composed of the rigid segments (chromophores), 1,4-di(α-2-naphthalene vinylene)benzene (DNVB) or 2,5-dimethyloxy-1,4-di(α-2-naphthalene vinylene)benzene (MDNVB) and the flexible segments (spacers), tri(ethylene oxide) (TEO). Efficient blue-green light emission of the LECs was demonstrated with onset voltage lower than 3 V and electroluminescence (EL) efficiency of ∼0.2 cd/A. The a.c. impedance data indicate that the operation mechanism of the LECs agrees with the electrochemical doping model.