Efficient Light Emitting Diodes from Ternary Blends of PPV-Based Copolymers Ali Cirpan, 1 Z. Ayse Aroguz, 2 Frank E. Karasz 1 1 Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003 2 Department of Chemistry, Engineering Faculty, University of Istanbul, 34850 Avcilar, Istanbul, Turkey Received 2 January 2006; accepted 4 April 2006 DOI 10.1002/app.24650 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Light emitting diodes (LEDs) were prepared using ternary blends of block copolymers emitting in the blue (I and II) and green (III) regime. These blends were studied in terms of their absorbance, photoluminescence (PL), and electroluminescence (EL) spectra. The PL spectra in dilute so- lution and in the solid state were compared. Fo ¨ rster energy transfer from I and II to III was studied in the solid state. The EL device made using a ternary blend with the ratio (1 : 1 : 1) showed the highest EL quantum efficiency. Differential scan- ning calorimetry (DSC) measurements have been made to investigate the phase separation in these copolymer blends. Ó 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2509– 2511, 2006 Key words: light emitting diode; photoluminescence; elec- troluminescence; polymer blends INTRODUCTION Polymer blends have been used as chromophores and it has been demonstrated that emission colors can be varied as a function of voltage 1 while combinations of polymers with blue, red, and green emissions can be used in a blend to obtain a white light-emitting diode. 2 Polymer blends also provide a platform with which to combine two polymers with different properties, such as hole transport or emission efficiencies with each other as opposed to preparing a multilayer device. 3 Polymer blending can be used as a tool to increase the efficiency of devices 4,5 by improving charge injection 6 and localization. Energy transfer between the compo- nents in polymer blends leads to an enhancement in the PL efficiency of films and EL efficiency in resulting devices. 7,8 DSC measurements have also been studied on a series of electro-optically active alternating block copolymers and their blends. 9 We report the utilization of a ternary polymer blend system consisting of three conjugated polymers, which are blue emitting (I, II) and green emitting (III). Fo ¨ rster energy transfer in the photoluminescence and electro- luminescence of the blends was studied. The devices based on the 1 : 1 : 1 weight ratio blend showed opti- mally enhanced external quantum efficiency and higher brightness compared to pure copolymers and other blends studied. Glass transition temperatures of these copolymers and copolymer blends were also measured to assess the nature of phase separation in these copolymers. EXPERIMENTAL Chloroform (Aldrich) and poly(3,4-ethylenedioxythio- phene)/poly(styrene sulfonate) (PEDOT : PSS) (Bayer) were used as purchased. The PPV-based copolymers as shown in Figure 1 were synthesized as described previ- ously. 10–12 Solutions of polymer blends were prepared (10 mg/mL in chloroform) in selected proportions of copolymers (I : II : III) (1 : 1 : 1), (1 : 1 : 2), (1 : 2 : 2), (2 : 1 : 2), (2 : 2 : 1) filtered through 0.2 mm millex FGS Filters (Milipore). Thin films for optical absorption and photo- luminescence measurements were spin-cast onto glass slides. All the films were dried in vacuum at room tem- perature for 1 h to remove residual solvent. Optical absorption spectra were taken on a Hitachi U-3010 UV– visible spectrophotometer. PL spectra were recorded on a PerkinElmer LS 50B luminescence spectrometer using a xenon discharge lamp for excitation. Double-layer PLEDs with the configuration ITO/ PEDOT : PSS/polymer/Ca/Al were fabricated on ITO- coated glass slides cleaned in ultrasonic baths of water and acetone. A hole injection layer of PEDOT:PSS was spin-coated on top of the ITO and dried at 1008C for 1 h under vacuum. A 100 nm layer of the copolymers and of their blends were spin-coated from the respective chloroform solutions onto the PEDOT : PSS layer under nitrogen. A 400 nm thick calcium electron-injecting cathode was deposited on the polymer film through a 6 mm 2 mask by vacuum evaporation at a pressure less than 2 Â 10 À6 Torr, followed by a protective coating of aluminum. The devices were characterized using a sys- Correspondence to: Z. A. Aroguz (aroguz@istanbul.edu.tr). Journal of Applied Polymer Science, Vol. 102, 2509–2511 (2006) V V C 2006 Wiley Periodicals, Inc.