All-Organic Sulfonium Salts Acting as Efficient Solution Processed Electron Injection Layer for PLEDs Dimitra G. Georgiadou,* ,†,‡ Maria Vasilopoulou, † Leonidas C. Palilis, § Ioannis D. Petsalakis, ∥ Giannoula Theodorakopoulos, ∥ Vassilios Constantoudis, † Stella Kennou, ⊥ Antonis Karantonis, ‡ Dimitra Dimotikali, ‡ and Panagiotis Argitis* ,† † Institute of Microelectronics, NCSR “Demokritos”, 15310 Athens, Greece ‡ School of Chemical Engineering, National Technical University of Athens, 15780 Athens, Greece § Department of Physics, University of Patras, 26500 Patras, Greece ∥ Theoretical and Physical Chemistry Institute, The National Hellenic Research Foundation, 11635 Athens, Greece ⊥ Department of Chemical Engineering, University of Patras, 26500 Patras, Greece ABSTRACT: Herein we introduce the all-organic triphenylsulfonium (TPS) salts cathode interfacial layers (CILs), deposited from their methanolic solution, as a new simple strategy for circumventing the use of unstable low work function metals and obtaining charge balance and high electroluminescence efficiency in polymer light-emitting diodes (PLEDs). In particular, we show that the incorporation of TPS-triflate or TPS-nonaflate at the polymer/Al interface improved substantially the luminous efficiency of the device (from 2.4 to 7.9 cd/A) and reduced the turn-on and operating voltage, whereas an up to 4-fold increase in brightness (∼11 250 cd/m 2 for TPS-triflate and ∼14 682 cd/m 2 for TPS-nonaflate compared to ∼3221 cd/m 2 for the reference device) was observed in poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-2,1′,3-thiadiazole)] (F8BT)-based PLEDs. This was mainly attributed to the favorable decrease of the electron injection barrier, as derived from the open-circuit voltage (V oc ) measurements, which was also assisted by the conduction of electrons through the triphenylsulfonium salt sites. Density functional theory calculations indicated that the total energy of the anionic (reduced) form of the salt, that is, upon placing an electron to its lowest unoccupied molecular orbital, is lower than its neutral state, rendering the TPS-salts stable upon electron transfer in the solid state. Finally, the morphology optimization of the TPS-salt interlayer through controlling the processing parameters was found to be critical for achieving efficient electron injection and transport at the respective interfaces. KEYWORDS: OLEDs, triflate, nonaflate, counterions, cathode interfacial layer, electron transport 1. INTRODUCTION The interest in polymer light emitting diodes (PLEDs) has increased in the last years, since the possibilities for facile fabrication renders them attractive candidates for large area lighting applications as well as solution-processed small size disposable displays. 1 For efficient low-power consuming devices, low work function metals are needed as cathodes (e.g., Ca, Ba, Mg). However, these are unstable in ambient environment due to oxidation of the metal and result in fast deterioration of the device performance, which is manifested via dark spot formation during operation and low lifetimes. On the other hand, the environmentally stable metals (e.g., Al, Ag) form large injection barriers with the polymer’s lowest unoccupied molecular orbital (LUMO) level, which hampers the injection of electrons. Therefore, the need for devices with low operating voltage remains an open issue, and this has triggered the research on efficient electron injecting/trans- porting layers (EILs/ETLs). 2 The most prominent material approaches presented so far as efficient EILs include insulating inorganic salts, such as alkaline Received: July 23, 2013 Accepted: November 7, 2013 Published: November 7, 2013 Research Article www.acsami.org © 2013 American Chemical Society 12346 dx.doi.org/10.1021/am402991b | ACS Appl. Mater. Interfaces 2013, 5, 12346−12354