Influence of thickness of functional layer on performance of organic salt-doped OLED with ITO/PVK:PBD:TBAPF 6 /Al structure C.C. Yap a, * , M. Yahaya a , M.M. Salleh b a School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia b Institute of Microengineering and Nanoelectronics (IMEN), Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia Received 20 September 2007; accepted 7 November 2007 Available online 17 November 2007 Abstract The effect of thickness of functional layer on the electrical and electroluminescence (EL) properties of single-layer OLED with ITO/ PVK:PBD:TBAPF 6 /Al structure were investigated where indium tin oxide (ITO) was used as anode, poly(9-vinylcarbazole) (PVK) as polymeric host, 2-(4-biphenylyl)-5-phenyl-1,3,4-oxadiazole (PBD) as electron-transporting molecule, tetrabutylammonium hexafluoro- phosphate (TBAPF 6 ) as organic salt dopant and aluminium (Al) as cathode. A unique transition phenomenon at high bias voltage in the devices was observed and the transition was reversible. The transition voltage and turn on voltage decreased with the decrease of functional layer thickness. The turn on voltage was approximately 5.5 V and 6.5 V for 55-nm-thick and 95-nm-thick devices, respectively. However, the current efficiency of 95-nm-thick device was higher than the 55-nm-thick device. More interestingly, the Commission Inter- nationale d’Eclairage (C.I.E.) coordinates of EL spectra of 95-nm-thick device at bias voltage ranging from 7 V to 13 V located in the white light region even without any dye doping. The PL and EL spectra were found completely different. PBD electromer was proposed to dominate the EL spectrum, but the contribution from PVK–PBD electroplex cannot be completely ruled out. Ó 2007 Elsevier B.V. All rights reserved. PACS: 73.40.Gk; 78.60.Fi; 78.66.Qn; 85.60.Jb Keywords: Organic light emitting diodes; Organic salt; Doping; Functional layer thickness; Tetrabutylammonium hexafluorophosphate 1. Introduction Organic light emitting diodes (OLEDs) using either small molecules or polymers, have attracted considerable interest due to their easy processability, flexibility, low cost, low operating voltages, wide viewing angles, tunability of the color emission, fast response time, and ease of forming large area [1,2]. Generally, OLED is a thin film device in which the emitting organic material is sandwiched between two electrodes. It will emit light when electricity is passed through it. Commercial applications of OLEDs require maximum brightness, high power efficiency, and extended lifetimes. To meet these requirements, light emission at low turn on voltage is necessary since high operating voltage will shorten the device lifetime due to thermal aging and impu- rity diffusion under high-electric field [3,4]. One of the approaches to reduce the turn on voltage is by insertion of a buffer layer at the interface between electrode layer (cathode and anode) and organic layer [5–7]. However, this method may increase the production cost due to additional step in manufacturing process. It has also been reported that the incorporation of ionic species either inorganic or organic salt in OLEDs greatly enhances the charge injec- tion [8–14]. Even so, the turn on voltage for these devices stays quite large (>10 V). While there have been several studies on the effect of doping concentration, heat treat- ment and prebiasing conditions on the performance of organic salt-doped OLEDs, few systematic studies have 1567-1739/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2007.11.006 * Corresponding author. Tel.: +60 3 89215933; fax: +60 3 89213777. E-mail address: chichin83@yahoo.co.uk (C.C. Yap). www.elsevier.com/locate/cap www.kps.or.kr Available online at www.sciencedirect.com Current Applied Physics 8 (2008) 637–644