High Efficiency and High Stability Exciplex-Based OLEDs Monima Sarma 1 , Ken-Tsung Wong 1,2 *, Shun-Wei Liu 3 , Wen-Yi Hung 4 1 Department of Chemistry National Taiwan University, Taipei 10617, Taiwan 2 Institute of Atomic and Molecular Science Academia Sinica, Taipei 10617, Taiwan 3 Department of Electronic Engineering, Organic Electronics Research Center, Ming Chi University of Technology, New Taipei City 24301, Taiwan 4 Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 202, Taiwan Abstract Organic materials exhibiting thermally activated delayed fluorescence (TADF) are an outstanding class of functional materials that have witnessed a significant development in recent years. Besides the pure TADF emitters, a new class of efficient TADF-based OLEDs have also been fabricated which encompasses the intermolecular charge transfer between physically blended electron donor and acceptor moieties. In such systems, the electron and hole are positioned on two different molecules, thereby giving small exchange energy and thus, the ΔE ST values are observed to be very small in the range 0–0.05 eV, even smaller than that of pure TADF materials. Consequently, exciplex-based OLEDs have the potential to maximize the contribution from TADF and realize theoretical 100% internal quantum efficiency. As a result, the challenging task of achieving small ΔE ST in organic systems has been resolved. In this paper, we demonstrated the molecular design strategy of using ” remote steric effect” on the donor molecules for improving the efficiency of exciplex-based OLEDs and the importance of acceptor structure on the stability of exciplex- hosted phosphorescent OLEDs. Author Keywords OLED, TADF, exciplex, host/co-host, remote steric effect 1. Introduction In the last three decades, organic light-emitting devices (OLEDs) have attracted enormous attention for flat-panel displays and lighting applications. In OLEDs, the recombination of holes and electrons in the emitting layer (EML) forms excitons. The electrically generated singlet and triplet excitons are in a ratio of 1:3 due to the spin statistics. 1 Light can be extracted from singlet excitons by a traditional organic fluorescent emitter with non-radiative triplet excitons left behind, thereby limiting its efficacy. An effective way to harvest all the triplet excitons produced is to utilize transition metal phosphorescent complexes as emitters. The strong heavy-atom- induced spin–orbit coupling (SOC) in transition metal complexes (especially iridium and platinum complexes) expedites intersystem crossing (ISC) and so, singlet excitons can be down-converted to the emissive triplet state. As both the singlet and triplet excitons can be harvested in such system, the maximum internal quantum efficiency (IQE) can reach as high as 100 %. 2 However, these transition metals are expensive, thereby leading to high cost of commercially available products based on phosphorescent OLEDs and thus, researchers are in constant search of alternative approaches with the aim of achieving higher external electroluminescence quantum efficiencies (η ext ). Among the several methods adopted for harvesting singlet excitons from triplet excitons, such as, triplet– triplet annihilation and other up-conversion techniques, 3 thermally activated delayed fluorescence (TADF) has received great attention in recent years. OLEDs adopting TADF emitters have the potential to attain 100% IQE by up-converting the triplet excitons to the radiative singlet state via reverse intersystem crossing (RISC). 4 But, the RISC from triplet to singlet excited state is feasible only when the S 1 −T 1 energy gap (ΔE ST ) is adequately small. Therefore, TADF can be practically achieved by tailor-made molecules with weakly coupled electron donor and acceptor components since they can induce a subtle intramolecular charge transfer (ICT) leading to a low electron exchange energy which eventually gives a small ΔE ST value. In addition to this, TADF can also be attained via exciplex exciton generated by two oppositely charged molecules with weak Coulomb interactions. 5 The exciplex states are generally interfacial charge transfer states whose emission occurs owing to interaction between an injected electron in the conduction band of the acceptor and a hole in the valence band of the donor. Analogous to pure TADF emitters, the exciplex emitters display an innately small ΔEST value (∼ 0–50 meV) and can feasibly undergo TADF emission via RISC from non- radiative T 1 to radiative S 1 . Therefore, exciplex-based fluorescent OLEDs have theoretical 100% IQE, corresponding to a maximum EQE of 20% if we consider the device out- coupling efficiency to be 20%, rendering them promising candidates for practical applications in full-color displays and lighting. The basic mechanism and working principle of exciplex systems is shown in Figure 1. In addition to serve as emitting layer, exciplex can also play important role as host materials for fluorescent, phosphorescent and pure TADF emitters. 6 For fluorescent dopant, only the Forest energy transfer between excipelx host and guest is effective, therefore, low doping concentration is necessary for suppressing the non- emissive Dexter energy transfer pathway. For triplet-based dopants, namely, phosphorescent and TADF emitters, Forest and Dexter processes are both effective (Figure 1). Therefore, the potential of using exciplex forming system as co-host for triplet- based emitters is obvious since the exciplex excitons can be efficiently transferred to the dopant, leading to high efficiency and possibly high stability of OLEDs. Figure 1. Basic mechanism and working principle of exciplex as emitters or hosts. 26-1 / M. Sarma Invited Paper 328 • SID 2018 DIGEST ISSN 0097-996X/18/4701-0328-$1.00 © 2018 SID