Solid state polymer light-emitting electrochemical cells: Recent developments (original) (raw)
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IEEE/OSA Journal of Display Technology, 2007
Organic light-emitting devices exhibiting high power conversion efficiency and long operating lifetime may potentially be achieved with the polymer light-emitting electrochemical cell (LEC) configuration. An LEC device typically uses a thin layer of conjugated polymer sandwiched between two contact electrodes. The polymer layer contains an ionically conductive species that are essential in the formation of a light-emitting p-i-n junction. LEC devices are characterized with balanced electron and hole injections, high current density at relatively low bias voltages (2-4 V), and high electroluminescent power efficiency. We will describe the working mechanism of the LECs and review the recent developments in LEC materials, device fabrication and performance. Among the important developments are planar (surface-typed) LECs, bilayer LECs that emit different colors at forward and reverse biases, frozen p-i-n junction LECs that functions like diodes, and phosphorescent LECs. Extensive efforts have been made to improve the LEC performance by controlling the blend morphology, including the use of bipolar surfactant additives and new electrolytes, the synthesis of conjugated polymers with ion-transporting main chain segments or side groups and polyelectrolyte. Degradation mechanisms that limit the lifetime of the LECs will also be discussed
Polymer light-emitting electrochemical cells with frozen p-i-n junction
Applied Physics Letters, 1997
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 cdyA 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. ᮊ
Advanced Functional Materials, 2007
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 cdyA 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. ᮊ
Polymer Light-Emitting Electrochemical Cells with Frozen p-i-n Junction at Room Temperature
Advanced Materials, 1998
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 cdyA 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. ᮊ
Polymer light-emitting electrochemical cells: Doping, luminescence, and mobility
Physical Review B, 2004
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 LiCF 3 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 ജ E g / e͒, but also that limited unipolar electrochemical doping can take place at V Ͻ 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-i-n junction, which is located close to the cathode.
Electrochemical properties of luminescent polymers and polymer light-emitting electrochemical cells
Synthetic Metals, 1999
Ž . Ž . Ž . The electrochemical p-doping potentials f and n-doping potentials f of 10 soluble poly 1,4-phenylene vinylene PPV p n Ž . derivatives and of the conjugated polymer blend in a light-emitting electrochemical cell LEC were measured by cyclic voltammetry. The Ž . Ž . energy levels corresponding to the highest occupied molecular orbital HOMO and the lowest unoccupied molecular orbital LUMO of Ž X . Ž X . the polymers were determined from the onset potentials for n-doping f and p-doping f , respectively. The electrochemical energy Ž .
Electrochemical doping during light emission in polymer light-emitting electrochemical cells
Physical Review B, 2008
Polymer light-emitting electrochemical cells ͑LECs͒, the electrochemical analog of light-emitting diodes, are relatively simple to manufacture yet difficult to understand. The combination of ionic and electronic charge carriers make for a richly complex electrochemical device. This paper addresses two curious observations from wide-gap planar LEC experiments: ͑1͒ Both the current and light intensity continue to increase with time long after the p-n junction has formed. ͑2͒ The light-emitting p-n junction often moves, both "straightening out" and migrating toward the cathode, with time. We propose that these phenomena are explained by the continuation of electrochemical doping even after the p-n junction has formed. We hope that this understanding will help to solve issues such as the limited lifetime of LECs and will help to make them a more practical device in commercial and scientific applications.
Applied Physics Letters, 2007
The authors report on enhanced efficiency of polymer light-emitting electrochemical cells ͑LECs͒ by means of forming a n-doping self-assembled monolayer ͑SAM͒ at the cathode-polymer interface. The addition of the SAM, a silane-based salt with structural similarity to the commonly used LEC n-dopant tetra-n-butylammonium, caused a twofold increase in quantum efficiency. Photovoltaic analysis indicates that the SAM increases both the open-circuit voltage and short-circuit current. Current versus voltage data are presented which indicate that the SAM does not simply introduce an interfacial dipole layer, but rather provides a fixed doping region, and thus a more stable p-i-n structure.
Electrical and luminescent properties of double-layer oligomeric/ polymeric light-emitting diodes
Synthetic Metals, 1996
We report the use of α-sexithiophene (T6) thin films sublimed onto glass substrates coated with indium-tin oxide in light-emitting diodes. Absolute photoluminescence quantum efficiencies were found to be in the range 10−2–10−3%, and indicate that T6 should be used as a hole injector into an emissive layer, rather than as a luminescent layer. We have fabricated double-layer organic light-emitting diodes where a cyano-substituted derivative of poly(p-phenylenevinylene) (PPV), MEH-CN-PPV, was spun on top of the T6 layer prior to evaporation of Ca-Al cathodes. Turn-on voltages for electroluminescence of about 3 V were found in structures with total thickness between 160 and 230 nm, while internal quantum efficiencies were up to 0.4%, i.e. at least ten times less than those measured in comparative devices where a PPV hole-injecting layer was used in place of T6. The substantial difference is interpreted on the basis of a significant lowering of the barrier to electron ejection from the luminescent layer into the hole-injecting layer when passing from PPV to T6. This allows inefficient recombination to take place in T6, as confirmed by the electroluminescence spectra.
Efficient, Low Operating Voltage Polymer Light-Emitting Diodes with Aluminum as the Cathode Material
Advanced Materials, 1998
The architecture of polymer light-emitting diodes (LEDs) consists of a thin film of luminescent conjugated polymer sandwiched between two contacts. Electrons and holes are injected through the contacts into the light-emitting semiconductor layer; when they recombine, light is emitted. Dissimilar metals are used for the contacts to facilitate the injection of electrons into the p*-band at one contact and the injection of holes into the p-band at the other. In order to achieve high electroluminescence (EL) efficiency, the electron and hole injection must be balanced.