Fabrication and characterization of OLEDs using PEDOT:PSS and MWCNT nanocomposites

doi:10.1016/j.compscitech.2007.11.004

Composites Science and Technology

Volume 68, Issue 14, November 2008, Pages 2837-2841

https://doi.org/10.1016/j.compscitech.2007.11.004 Get rights and content

Abstract

Nanocomposites of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and multi-wall carbon nanotubes (MWCNTs) were investigated as hole injection layers in organic light-emitting diodes (OLEDs) consisting of a hole transport layer of N,N′-diphenyl-N,N′-di(m-tdyl)benzidine (TPD) and a light-emitting/electron transport layer of tris(8-hydroxyquinolinato)aluminium (Alq3) sandwiched between a transparent ITO anode based on polyethylene terephthalate (PET) substrate and a metal cathode. The OLEDs were fabricated by physical vapor deposition method and the current–voltage–luminescence (I–V–L) characteristics of the devices were examined. It was found that high light intensity can be obtained by adopting nanocomposites of PEDOT:PSS and MWCNTs as hole injection layer of OLEDs. Reasons for the improved performance of the devices were discussed. The improved electric properties of the nanocomposites might be the reason for the improvement of OLED performance achieved by incorporating MWCNTs into hole injection layer of PEDPT:PSS.

Introduction

Since efficient organic light-emitting device (OLED) was reported by Tang and Van Slyke [1], great progress has been made to utilize OLED technology in the field of displays because of its unique properties, such as lightweight, low drive voltage, wide view angle and high resolution [2]. The OLEDs and PLEDs have a multi-layered structure with light-emitting layer and buffer layer sandwiched between two electrodes. ITO is most widely used as a transparent anode for OLEDs or PLEDs due to its high optical transparency and electrical conductivity. The performance of OLEDs or PLEDS is highly affected by the hole injection ability of the ITO anode. Mechanical polishing, chemical, solvent, or dry cleaning, and surface functionalization were adopted on ITO to control hole injection ability and improve the performance of OLEDs [3], [4], [5], [6].

Hole injection layers, such as poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) [7], polyaniline [8], 4,4′,4′′-tris(N,N-diphenyl-amino)triphenylamine (TDATA), 4,4′,4″-tris(N-dibenzo[a,g]carbazolyl)triphenylamine (TDCTA) [9], Ag₂O [10], SiO₂ [11], and nickel oxide [12], etc., were inserted between the hole transport layer and the ITO anode to improve the performance of OLEDs, by decreasing the energy level between the highest occupied molecular orbital (HOMO) energy of the hole transport layer and the ITO anode. Among them, PEDOT:PSS was a very good hole injection materials for OLEDs or PLEDS. In addition, PEDOT:PSS is a system with good film-forming properties, high conductivity, high visible light transmittance and excellent environmental stability. It has been successfully used in a number of applications, such as hole injection layer of OLEDs, anti-static coating and sensors [13].

Nanocomposites of PEDOT:PSS with inorganic fillings exhibits better electrical properties and are found more applications in optoelectronic devices. Devices with a better performance were achieved by inclusion of nanocomposites in the hole injection layer [14] or in the hole transport layer [15]. Carbon nanotubes (CNTs), first observed by IIJIMA [16], were one of a good filling for in polymer based nanocomposites, which make possible large variety of applications, such as devices in nano-electronics, field emitters or reinforcing materials, as they exhibit high mechanical properties, both metal and p-type semiconductor properties, and have a relative high work function. In addition, they also have high aspect ratio, high conductivity, low percolation threshold and low absorption in visible region [17], [18], [19].

They can improve the electrical properties of the host polymers with a low concentration and have a low side-effect on optical properties. CNTs were successfully introduced as a structure element in nanocomposites to reinforce thermo-set polymers and thermoplastic polymers or as a conductive filling in nanocomposites to improve their conductivity.

In this research, nanocomposites of PEDOT:PSS and MWCNTs were fabricated and used as hole injection layer for OLEDs based on flexible PET substrates. MWCNTs network formulated in PEDOT:PSS system might can facility the hole injection and improve the performance the OLEDs. Since the good electrical properties of MWCNTs, it is expected that the hole injection and hole transport ability of PEDOT:PSS might be improved and improvement might be obtained in the OLEDs.

Section snippets

Experimental

The MWCNTs, supplied from Shenzhen Nanotech Port Co. Ltd., have a diameter of 40 nm and purity over 98%. The CNTs were further purified with strong acid to remove the impurities. The acid-modification of CNTs was based on a mixture of concentrated nitric and sulfuric acids in a molar ratio of 1:3, respectively. The suspended MWCNTs were added into the mixture of the acids and refluxed at 140 °C for 20 min. After washing with de-ionized water until the supernatant attained a pH around 7, the

Results and discussion

Fig. 2 shows the surface morphology of the MWCNTs before acid-treated (Fig. 2a) and after acid-treated (Fig. 2b). It was found that particle-like impurity of MWCNTs was removed and the morphology, as in terms of the surface smoothness, was improved after the acid-treatment. The MWCNTs after acid-treated can disperse well in aqueous PEDOT:PSS solution. No precipitation was found in the solution kept for several months. Very uniform solution was obtained by adding acid-treated MWCNTs in the

Conclusions

In conclusion, nanocomposites of PEDOT:PSS and MWCNTs were fabricated by inclusion of acid-treated MWCNTs into aqueous PEDOT:PSS solution. Improved luminance intensity and decrease of turn-on voltage were obtained in the OLEDs with the nanocomposites as hole injection layer. The metal or semimetal properties of the MWCNTs might contribute to the improvement of the nanocomposites of PEDOT:PSS and MWCNTs, which leads to the improvement of the OLEDs.

Acknowledgements

The authors thanks Research Grant Council for funding support (No. PolyU5286/03E). Mr. Wang acknowledges a postgraduate scholarship from the same source.

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Fabrication and characterization of OLEDs using PEDOT:PSS and MWCNT nanocomposites

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Eng Fract Mech

Syn Met

Thin Sol Films

Surf Coat Tech

Thin Sol Films

Chem Phys Lett

Thin Sol Films

Infrared Phys Techn

Thin Sol Films