Modification of Conductive Polymer for Polymeric Anodes of Flexible Organic Light-Emitting Diodes
© to the authors 2009
Received: 24 January 2009
Accepted: 5 March 2009
Published: 17 March 2009
A conductive polymer, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), was modified with dimethyl sulfoxide (DMSO) in solution state, together with sub-sequential thermal treatment of its spin-coated film. The electrical conductivity increased by more than three orders of magnitude improvement was achieved. The mechanism for the conductivity improvement was studied at nanoscale by particle size analysis, field emission scanning electron microscopy (FESEM), and X-ray photoelectron spectroscopy (XPS). Smaller particle size was observed, resulting in larger contact area and better electrical conductive connections. Connection of conductive PEDOT increased on the surface of the PEDOT:PSS particles, which promoted high conductivity. Flexible anodes based on the modified PEDOT:PSS were fabricated. Flexible organic light-emitting diodes (FOLED) based the polymeric anodes have a comparable performance to those on indium–tin–oxide (ITO) anodes.
Conjugated conductive polymers have seen prosperous applications, e.g., printed sensors or timers, electronic newspaper, wearable electronics, and displays. Poly(3,4-ethylenedioxythiophene) (PEDOT), a derivative of polythiophene, firstly developed by Bayer AG research laboratories, shows excellent transparency in the visible region, good electrical conductivity, and environmental stability . However, PEDOT prepared by using oxidative chemical or electrochemical polymerization methods is found to be insoluble and therefore difficult to process in thin-film form or other shapes . Water soluble PEDOT:PSS was achieved by using a water-soluble poly(styrene sulfonic acid) (PSS) as the charge-balancing dopant during polymerization , which is a system with good film-forming properties, high visible light transmittance and excellent environmental stability. It has been successfully used in anti-static coating and organic optoelectronic deices etc. [4–7]. PEDOT:PSS electrodes are good candidates to replace brittle indium tin oxide (ITO) electrodes for the fully flexible electronic devices, except their lower conductivity [8, 9], as the high Joule heat generated by the current flowing through the organic electrodes impairs the performance as well as the lifetime of the devices. Grain-like structure was reported on spin-coated PEDOT:PSS films, in which the PEDOT core was surrounded by a thin layer of PSS shell with a non-homogenous distribution of the PEDOT and PSS species in a single grain . This core-shell structure is likely to limit the conductivity, as commercially available products have a conductivity of 1 S/cm (Baytron P, Bayer Corporation), which limits its application in optoelectronic devices.
Recently, efforts have been focused on improvement of electrical conductivity of PEDOT:PSS films. Electrical conductivity of PEDOT:PSS can be enhanced by acidic and thermal treatments. Improvement was achieved by adding small amount of solvents with high boiling temperature such as dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), glycerol, and sorbitol, etc. [11–14]. Conformational modification, morphology modifications, and reduction of the Coulombic interaction by a screening effect were proposed to explain the conductivity improvements [13, 15–18].
In this letter, PEDOT:PSS was modified with dimethyl sulfoxide (DMSO) in its solution state, followed by thermal treatment on the spin-coated films to improve its electrical conductivity. The mechanism of the improvement was investigated by particle size analysis, field emission scanning electron microscopy (FESEM), and X-ray photoelectron spectroscopy (XPS). Flexible PEDOT:PSS anodes were fabricated to replace ITO in flexible organic light-emitting diodes (FOLED). FOLEDs based on the modified PEDOT:PSS anodes have a comparable performance to the those on the ITO anodes.
Pre-Treatment of Substrates
Poly(ethylene terephthalate) (PET) films with a thickness of 175 μm (DuPont Teijin Films/Melinex®ST506) were adopted as the flexible substrate. The PET substrates were sequentially rinsed in four ultrasonic baths: non-ionic detergent, de-ionized (DI) water, acetone, and isopropyl alcohol, each for 10 min, respectively. The substrates were then dried in a vacuum oven for 12 h at 60 °C. At last, the substrates were treated by mild O2plasma for 8 min.
Modification of PEDOT:PSS
DMSO, with a weight concentration of 3, 5, 7, and 9%, respectively, was added into the aqueous PEDOT:PSS (BAYTRON®PH 500) and mixed uniformly under ultrasonic condition for 10 min. Films of DMSO modified PEDOT:PSS were fabricated by the spin-coating method. Then the spin-coated films were baked at 100, 130, 160, and 190 °C, respectively.
Characterization of PEDOT:PSS
The sheet electrical resistance of the films on PET substrates was determined with the four point resistivity measurement. The particle size of PEDOT:PSS in solution state was examined by a particle size analyzer (LS 13 320, Universal Liquid Module, Beckman Coulter Inc.). Single particles of PEDOT:PSS were examined by FESEM (JEOL, JSM-6335F). XPS analysis was performed using a Physical Electronics 5600 multi-technique system, with a base pressure of 1 × 10−10 Torr (ultra-high vacuum) using monochromatic AlKαX-ray source, at hv = 1486.6 eV, with a resolution of 200 × 200 μm. The take-off angle was varied from 45° to 25° to obtain the chemical information in different depth nearer the surface region.
Fabrication and Characterization of FOLEDs
Double layer FOLEDs with a hole transport layer of TPD and emitting/electron transport layer of Alq3film sandwiched between DMSO modified PEDOT:PSS anode and a LiF/Al cathode. FOLEDs based on a PET coated with ITO anodes (CPFilms Inc., a sheet resistance of 120 ohm/□) were fabricated for reference. The TPD, Alq3, LiF, and Al films were deposited under a vacuum of 2 × 10−6 Torr with a deposition rate of 0.5, 0.2, 0.3, and 0.5 nm/s, respectively. The effective size of the devices was 10 mm2. The voltage–luminescence (V–L) characteristics of the FOLEDs were measured using 814 photomultiplier detection system (Photon Technology International) under ambient environment at steady-state.
Results and Discussion
Particles Size Analysis
The bigger the take-off angle, the deeper the chemical information can be obtained. In PEDOT and PSS chain, both have one sulfur atom in one repeat unit. The sulfur atom from PEDOT has a different chemical condition from the sulfur atom from PSS, resulting in different S(2p) binding energy so that the composition of the PEDOT:PSS from various depth can be analyzed . The S(2p) core level spectra of pristine PEDOT:PSS film and DMSO modified PEDOT:PSS film are shown in Fig. 4b, c, with a take-off angle of 45° and 25°, respectively. The S(2p) core level spectra of the pristine PEDOT:PSS and modified PEDOT:PSS film at the two take-off angles have identical peaks, around 168.5 eV and 164.0 eV, corresponding to sulfur from PSS and PEDOT. At a take-off angle of 45°, the pristine and the modified PEDOT:PSS films exhibit no difference. On the contrast at a take-off angle of 25°, the relative intensity S(2p) of sulfur from PEDOT increases, which indicates that conductive PEDOT concentration increases at the surface of the PEDOT:PSS particle, which is preferred for high conductivity. Particles with higher conductivity and better contacts are formed, resulting in the three orders of degree conductivity improvement by the DMSO modification and thermal treatment.
FOLEDs Based on Flexible Electrodes
In this letter, PEDOT:PSS was modified with DMSO and sequential thermal treatment. The mechanism for the conductivity improvement was studied with particle size analysis, FESEM, and XPS. Concentration of PEDOT and PSS was found to redistribute in the modification process. PEDOT concentration increases on the surface, which enhances electrical conductivity. In addition, electrical conductive particles with smaller size and more uniform, better packing and bigger contact area facilitate in better electrical connection. Three orders of magnitude enhancement in conductivity was achieved with the modification process. Based on the modified polymeric anodes, FOLEDs were fabricated and compared with the FOLEDs of ITO anodes. A lower turn voltage and higher luminous intensity were achieved. These FOLEDs based on the modified PEDOT:PSS anodes have a comparable performance to those of ITO anodes.
The authors thank Research Grant Council for funding support (No. Polyu5286/03E). Dr. Wang acknowledges a postgraduate scholarship from the same source.
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