Exciplex hosts for blue phosphorescent organic light-emitting diodes

Abstract

The host material of organic light-emitting diodes (OLEDs) has been advanced from a single host to a mixed host for high efficiency and long lifetime. Several types of mixed host have been reported in the literature, but the exciplex host has been popular as the mixed host of OLEDs. The exciplex host has been developed mostly for red and green phosphorescent OLEDs, and has upgraded device performances, but it is difficult to develop the exciplex host for blue phosphorescent OLEDs. Recently, several works demonstrated the potential of the exciplex host for blue phosphorescent OLEDs. In this paper, the exciplex host for blue OLEDs is reviewed, and its prospects are presented.

Pure-organic phosphine oxide luminescent materials

“…D-A based bimolecular exciplex systems with intermolecular CT interaction have already been developed as a main kind of TADF-emitting system, after the pioneering work of Adachi et al [61]. Such systems have superiority in flexible D-A combination and energy level modulation [62,63]. The accurate regulation of D-A interaction, however, is dependent on not only the electronic inductive effects of the D and A molecules but also the D-A distance determined by the steric hindrances of the two components.…”

Mechanism of Ir(ppy)3 Guest Exciton Formation with the Exciplex-Forming TCTA:TPBI Cohost within a Phosphorescent Organic Light-Emitting Diode Environment

“…For constructing high-efficiency organic light-emitting diodes (OLEDs), exciplex materials that combine hole transporting materials (HTMs) and electron transporting materials (ETMs) to form intermolecular charge transfer (CT) complexes have been widely investigated [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 ]. Because exciplexes tend to display undesirable red-shifted and broadened emissions, they were sometimes considered to negatively affect OLED performance.…”

“…Various host materials have been studied for improving the performances and the lifetimes of phosphorescent OLEDs (PhOLEDs) [ 22 , 27 ]. Recently, exciplex-forming cohost materials have been widely developed for efficient PhOLEDs [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 28 , 29 ]. In more conventional PhOLEDs with a single host material, the host itself is responsible for both hole and electron transports in EML, and balancing the two mobilities is essentially nontrivial as tuning one property in the host inevitably touches the other.…”

“…In contrast, with the exciplex-forming cohost, controlling the mobilities of holes and electrons separately becomes a relatively simple matter, and achieving efficient charge recombination in EML becomes more feasible [ 30 , 31 ]. For example, with better balanced mobilities with an exciplex-forming cohost, the recombination zone in EML can be spatially widened, leading to lower exciton density and to subsequent suppression of the triplet–triplet annihilation (TTA) process [ 7 ]. Indeed, light emission from a triplet exciton in PhOLEDs is a rather slow process, and suppressing the leaky TTA is important in maintaining the emission efficiency.…”

Fluorene–Triphenylamine-Based Bipolar Materials: Fluorescent Emitter and Host for Yellow Phosphorescent OLEDs

“…4,4 -bis(N-carbazolyl)-1,1 -biphenyl (CBP), 1,3-bis(N-carbazolyl)benzene (mCP), and 4,4 -cyclohexyli denebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC) are well-known HT type host materials, but they do not show stable thermal properties. Tris(8-hydroxyquinoline)aluminum (Alq 3 ) and Bis [2-(diphenylphosphino)phenyl] ether oxide (DPEPO) are ET type host materials that are used in phosphorescent and TADF OLEDs [37][38][39][40][41][42][43][44][45][46][47][48][49]. Host materials are essential energy suppliers to the dopant material that also prevent energy flow back from the dopant.…”