Charge transfer through amino groups-small molecules interface improving the performance of electroluminescent devices Ali Kemal Havare a,⇑ , Mustafa Can b,⇑ , Cem Tozlu c , Mahmut Kus d , Salih Okur e ,S ßerafettin Demic e , Kadir Demirak e , Mustafa Kurt f , Sıddık Icli g a Electrical and Electronics Engineering, Toros University, Mersin, Turkey b Department of Engineering Sciences, Izmir Katip Celebi University, Izmir, Turkey c Department of Energy Systems Engineering, KMU, Karaman, Turkey d Department of Chemical Engineering, Selcuk University, Konya, Turkey e Department of Materials Science and Engineering, Izmir Kâtip Celebi University, Izmir, Turkey f Department of Physics, Ahi Evran University, Kırs ßehir, Turkey g Energy Department, Ege University, Izmir, Turkey article info Article history: Received 3 February 2016 Received in revised form 10 March 2016 Accepted 14 March 2016 Available online 21 March 2016 Keywords: Charge transfer Electroluminescent device HOMO-LUMO level Self-assembled monolayers Amino groups abstract A carboxylic group functioned charge transporting was synthesized and self-assembled on an indium tin oxide (ITO) anode. A typical electroluminescent device [modified ITO/TPD (50 nm)/Alq 3 (60 nm)/LiF (2 nm)/(120 nm)] was fabricated to investigate the effect of the amino groups-small molecules interface on the characteristics of the device. The increase in the surface work function of ITO is expected to facilitate the hole injection from the ITO anode to the Hole Transport Layer (HTL) in electroluminescence. The modified electroluminescent device could endure a higher current and showed a much higher luminance than the nonmodified one. For the produced electroluminescent devices, the I-V characteris- tics, optical characterization and quantum yields were performed. The external quantum efficiency of the modified electroluminescent device is improved as the result of the presence of the amino groups-small molecules interface. Ó 2016 Elsevier B.V. All rights reserved. 1. Introduction Historical background of the electroluminescent device extends to 1960s, to the observation of electroluminance from organic materials. W. Helfrich and his friends have demonstrated the phenomenon of electroluminance through the process of charge injection, using the electroluminescence of anthracene crystal and a hole and electron injecting electrode [1–4]. These initial studies are considered to be the fundamental studies of the field of organic electronic devices. Possible modifications of inorganic electrodes (anode or cathode) have also attracted much attention in the studies of organic semiconductor devices in the last decade [5–10]. These applications of organic electronic devices are based on different properties of organic/inorganic hetero-junctions with strong chemical interactions [11–13]. In electroluminescent devices, the efficient hole transporting from electrode to the electroluminescence organic material is one of the most critical parameters [14–16]. The electrodes having a high work function, which prevents the movement of charge carriers between the organic materials and the electrode interface, is one of the main current issues in the design of organic devices [17–19]. The work function of ITO alloy, used in optoelectronic applications and also known as transparent conductive glass, was measured using Kelvin Probe and ultraviolet photon spectroscopy (UPS) and was found to vary between 4.2 and 4.4 eV [20,21]. These values are suitable for enhancing via surface chemical processes, so that the work func- tion of ITO can be further tuned between 3.9 and 5.1 eV. Many studies were performed in recent years focusing on the modifica- tions of ITO surface using differevnt methods, thus achieving the desired levels of the work function [22–25]. Modifying the surface of ITO with self-assembled monolayer (SAM) techniques had been shown to increase the charge transfer at the organic/metal inter- faces [26–29]. The work function of ITO can be thus tuned by using SAM between 4.8 and 5.2 eV [30–33]. Particularly to facilitate the transfer of charge carriers with SAM technique, molecular struc- tures with high oxidation potential need to be synthesized. SAM molecules make chemical bound with the groups on the surface of the substrate and thus change their physical and chemical http://dx.doi.org/10.1016/j.optmat.2016.03.020 0925-3467/Ó 2016 Elsevier B.V. All rights reserved. ⇑ Corresponding authors. E-mail addresses: alikemal.havare@toros.edu.tr (A.K. Havare), mustafacan80@ yahoo.com (M. Can). Optical Materials 55 (2016) 94–101 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat