Charge injection in an LED with a hybrid composite as the emissive layer G. Gozzi a , D.L. Chinaglia b , T.F. Schmidt a , O.N. Oliveira Jr. a, ⁎ a Instituto de Física de São Carlos, USP, CP 369, São Carlos, SP, Brazil b Departamento de Física, IGCE, UNESP, Av. 24A 1515, CP 178, Rio Claro, SP, Brazil abstract article info Article history: Received 6 September 2010 Received in revised form 14 December 2010 Accepted 17 February 2011 Available online 26 February 2011 Keywords: Hybrid composite Electroluminescent device Charge injection Hopping mechanisms Impedance spectroscopy Understanding and controlling charge transport are crucial to achieve optimized organic devices, including light emitting diodes. In this study, we investigate the charge injection in devices made with a hybrid composite (HC) containing Zn 2 SiO 4 :Mn (ZSP:Mn) in a polymeric blend consisting of poly(o-methoxyaniline) (POMA) and poly(vinylidene co-trifluorethylene) P(VDFTrFE), with the architecture ITO/HC/metallic electrode (ME). Charge injection was found to depend mainly on the POMA semiconducting phase. For ITO/HC/Au, an Ohmic junction was observed because the work function of ITO is close to that of Au, which also matches the energy levels of HC. Holes are injected through the HC/Au junction, as the highest occupied molecular orbital (HOMO) level of POMA matches the Fermi level of Au. The impedance spectroscopy data for the ITO/HC/ME devices were analyzed with a theoretical model where charge injection was assumed to occur via hopping with a distribution of potential energy barriers. The average hopping distance was estimated as 5.5 Å and only the device with the Al electrode had the current limited by the interface mechanism (charge injection). For ITO/HC/Cu and ITO/HC/Au devices the limiting factor for the charge transport was the bulk resistance of the samples, in spite of the existence of a small interface energy barrier. The disorder parameter was 0.18 and 0.19 for the HC/Cu and HC/Al interfaces, respectively, which arises from the disordered nature of the hybrid material. The combination of the Cole–Cole model and the Miller–Abrahams function are a good approach to describe charge a.c. injection processes in disordered materials. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The advent of semiconducting polymers [1] has motivated a variety of fundamental studies in physics and chemistry, in addition to new developments in device engineering with the promise of unprecedented versatility in the choice of materials [2–4]. Of special relevance are the low mass density and flexibility, which make the polymers strong candidates for large-area luminescent displays [5]. There are nevertheless major challenges for achieving materials properties that are suitable for real-world applications, including improved film processability and control of molecular architectures in devices. A possible enhancement in device properties may be obtained with hybrid materials [6], as in light-emitting diodes (LEDs) that emitted green light with high purity [7]. The limitation in the latter device was the excessively high operation voltage (ca. 80 V), which calls for further work in blend composition and molecular engineering to optimize the device characteristics. The properties of LEDs made with composites depend on the three phases of the materials: the inorganic phase enhances the luminescence; the organic insulating phase increases mechanical stability; and the organic conducting phase enhances the charge transport. An efficient charge transport is perhaps one of the main require- ments for lowering the operation voltage. For the hybrid composite containing zinc silicate phosphor manganese doped (Zn 2 SiO 4 :Mn (ZSP: Mn)) and a polymer blend [8], the low conductivity was identified as the limiting factor. Two mechanisms were determined as responsible for the charge transport, namely the charge transport at the interface between ZSP:Mn and the polymeric matrix, and the charge transport across the polymer matrix. Charge transport was found to occur mainly in the conducting phase of doped polymer [8]. Also relevant for charge transport is the injection into the polymer materials from the metallic electrodes. Two processes have been invoked to explain injection: hopping [9] and tunneling [10], where the first was extended by Arkhipov [11] for a symmetric distribution of potential barriers and taking into account the probability of charge recombination at the interface. The hopping mechanism may be considered as a tunneling process assisted by phonons, being predominant at high temperatures and for low interface potential barriers. In contrast, direct tunneling (especially the Fowler–Nordhein tunneling [12]) is either independent of the temperature or displays a weak dependence [13], thus prevailing at low temperatures and for high electrical field and interface potential barriers. In this study, we determine the mechanisms of charge injection in a hybrid LED, in addition to assessing whether charge injection occurs via the conducting phase. The devices have been made with a hybrid composite containing a polymer blend and a zinc silicate phosphor Materials Science and Engineering C 31 (2011) 969–974 ⁎ Corresponding author. Tel.: +55 16 33739825; fax: +55 16 33715365. E-mail address: chu@ifsc.usp.br (O.N. Oliveira). 0928-4931/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2011.02.022 Contents lists available at ScienceDirect Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec