Electrical and electroluminescence properties of ITO/PEDOT:PSS/TPD:Alq 3 :C 60 /Al organic light emitting diodes Mina Neghabi a , Abbas Behjat a, b, * a Atomic and Molecular Group, Physics Department, Yazd University, Yazd, Iran b Photonics Group, Engineering Research Center, Yazd University, Yazd, Iran article info Article history: Received 3 July 2011 Received in revised form 10 September 2011 Accepted 15 September 2011 Available online 28 September 2011 Keywords: Organic light emitting diodes Polymer blends Mobility Fullerene abstract Organic light emitting diodes (OLEDs) of ITO/PEDOT:PSS/TPD:Alq 3 :C 60 /Al with different C 60 concentra- tions (0e6.0 wt.%) have been fabricated. The physical parameters including electrical and optical prop- erties of the samples have been measured by Luminanceecurrentevoltage (LeIeV) characteristics and optical absorbance. The currentevoltage characteristics indicate that field-emission tunneling injection dominates in the diodes at high applied voltages. It is found that with increasing the concentration of C 60 , the injection barrier for holes slightly reduces and the hole’s mobility increases over two orders of magnitude. Also, electroluminescence enhances with the presence of C 60 in the blend; optimum current efficiency occurs at 3 wt% C 60 . The method provides a simple way of increasing the efficiency of OLEDs. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction In recent years, research on organic light emitting diodes (OLEDs) based on the blend of different electroactive materials have been significantly increased. This is because of their possible application in large scale lighting, flexible panel and multi color displays with low power consumption and low cost [1e4]. Charge injection, transport and recombination processes are important factors that can influence efficiency and electroluminescence properties of the devices [5e8]. Also, it has been well demonstrated that increase in the efficiency and lifetime of the device, strongly depends on decreasing of the charge’s injection barriers at the interfaces and the charge balance in a controlled electronehole recombination zone [9e11]. One way to obtain devices with high performance is utilizing the blend structure of solution-processible materials. In the blending technique one enables to incorporate the efficient charge transport organic materials into the device and also increase the device performance, by control of the concentration of blend materials [12e17]. In this paper, we study the optical and electrical characteristics of ITO/PEDOT:PSS/Blend (TPD:Alq 3 :C 60 )/Al heterostructure devices with different C 60 concentrations. Here, fullerene (C 60 ) has been used to increase the hole transport process in the TPD:Alq 3 blend. Also, a comprehensive study is carried out to understand mecha- nisms of charge carrier injection and transport for the devices by currentevoltage and luminanceevoltage characterization. 2. Experimental N, N 0 -bis(3-methylphenyl)-N, N 0 -diphenylbenzidine (TPD), tris(8-hydroxyquinoline) aluminum (Alq 3 ), poly-3,4-ethylenedio- xythiophene/polystyrenesulphonate (PEDOT:PSS) and fullerene (C 60 ) were purchased from Sigma-Aldrich and used as received. The ITO glasses having a sheet resistance of 15 U/sq were also purch- ased from Sigma-Aldrich. A 5-mm wide ITO anode strip line was made by selective etching; using mixed solutions of hydrochloric acid (HCl) and nitric acid (HNO 3 ) at a volume ratio of 3:1 for 10e20 min at room temperature. Then, the patterned anode glass was cleaned sequentially by ultra-sonication in propanol, acetone, deionized water for 10 min, respectively. Then the substrates were dried in a high purity N 2 gas stream. PEDOT:PSS as a hole injection layer was spin-coated onto the anodes and then dried in the oven at 110  C for 1 h. The TPD and Alq 3 were dissolved in chloroform solution with a concentration of 15 mg/ml with 10:2 weight ratios. The C 60 was dissolved in toluene and blended with TPD and Alq 3 with different concentrations (i.e. 0, 0.7, 1.5, 3, and 6% by weight). Then the blend was spin-coated onto the prepared substrate; anode * Corresponding author. Photonics Group, Engineering Research Center, Yazd University, Pajouhesh Street, Yazd 89195-741, Iran. Tel.: þ98 351 8122773; fax: þ98 351 8200132. E-mail address: abehjat@yazduni.ac.ir (A. Behjat). Contents lists available at SciVerse ScienceDirect Current Applied Physics journal homepage: www.elsevier.com/locate/cap 1567-1739/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2011.09.010 Current Applied Physics 12 (2012) 597e601