Efficient p-phenylene based OLEDs with mixed interfacial exciplex emission P. Data a,b,c, * ,1 , R. Motyka b , M. Lapkowski b,c , J. Suwinski c , S. Jursenas d , G. Kreiza d , A. Miasojedovas d , A.P. Monkman a a Physics Department, Durham University, South Road, Durham DH1 3LE, United Kingdom b Faculty of Chemistry, Silesian University of Technology, M. Strzody 9, 44-100 Gliwice, Poland c Center of Polymer and Carbon Materials, Polish Academy of Science, M. Curie-Sklodowskiej 34, 41-819 Zabrze, Poland d Institute of Applied Research, Vilnius University, Sauletekio 9-III, LT-10222 Vilnius, Lithuania A R T I C L E I N F O Article history: Received 5 August 2015 Received in revised form 19 September 2015 Accepted 19 September 2015 Available online 25 September 2015 Keywords: exciplex OLED TADF selenophene thiophene A B S T R A C T Organic electronics, mainly due to the advancement of OLED (Organic Light Emitting Diode) technology, is a fast developing research area, and has already revolutionized the displays market. This direction presents the use of exciplex emitters and thermally activated delayed fluorescence (TADF) in OLEDs. This is shown through electrochemical characterisation of six p-phenylene derivatives for application in optoelectronic devices and presents the possibility the compounds’ use as OLED emitters. In these OLED devices, it is established that selenophene based compounds with a “heavy-atom effect” can be used as potential emitters when exciplex phenomena are involved. ã 2015 Elsevier Ltd. All rights reserved. 1. Introduction Some of the first polymeric and small molecules developed as OLED emitters were synthesized from of p-phenylenevinylenes [1–4] and thiophenes [5–8] as they are both electrochemically stable and highly fluorescent materials. The most difficult to obtain are blue emission OLED devices, usually their efficiency is couple times lower than for green and red based emitters. Blue (high energy) emitters are necessary as they are part of every display in a form of small micrometer pixel. One method used to decrease the emission wavelength is to insert molecules containing heavy atoms (Se, Te, Ir etc.) into the molecular structure. This however, unfortunately also leads to a decrease in photoluminescence quantum yield due to “heavy-atom” effects [9–11]. Iridium compounds have previously been used as OLED emitters due to their efficiently intermixed singlet and triplet states, producing effective heavy atom spin orbit coupling, yielding nearly 100% emission output via phosphorescence [12–15]. The quantum efficiency of a normal fluorescent emitting device is limited to 25%, upon electrical excitation, meaning that 75% of triplet excitons are be wasted [16,17]. In order to increase the efficiency of devices, it is possible to link the processes of fluorescence and phosphorescence and get 100% efficiency, employing a TADF process [18,19]. In this process the excited singlet and triplet state must have very similar energies so the molecule at triplet excited state may move back to the singlet excited state with thermal activation. The Molecule at excited singlet state then relaxes by emitting a photon. The maximum efficiency of this process is 100% [18,19]. The recent idea of devices generating photons such as in light emitting diodes, is used in exciplex - bimolecular systems (BMs), which consist of two different low molecular weight compounds, one of which is the electron donor (D) and the second is the electron-acceptor (A) [20]. The application of the BMs in optoelectronic systems allows for much greater freedom of the choice of the active compounds. The length of the emitted wave in a BM system is independent of the energy gap in a single compound but is determined by the energy gap for the exciplex, where the HOMO (Highest Occupied Molecular Orbital) energy level is the same as the electron-donor compound and the LUMO (Lowest Unoccupied Molecular Orbital) energy level is the same as for electron-acceptor compound. In this case the emitted wavelength is adjusted by appropriate selection of the D and A compounds, which allows for the use of soluble, small molecular compounds [21–23]. * Corresponding author. Tel.: +48606782634. E-mail address: Przemyslaw.Data@dur.ac.uk (P. Data). 1 ISE member. http://dx.doi.org/10.1016/j.electacta.2015.09.110 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved. Electrochimica Acta 182 (2015) 524–528 Contents lists available at ScienceDirect Electrochimica Acta journal homepa ge: www.elsev ier.com/locate/electacta