Identifying the Exciplex-to-Exciplex Energy Transfer in Tricomponent Exciplex-Based OLEDs through Magnetic Field Effect Measurements

Abstract

Tricomponent exciplex-based organic light-emitting diodes (TE-OLEDs) with high- and low-energy exciplexes have recently gained much attentions because of their already-reported high external quantum efficiency (EQE) and their exciplex-to-exciplex energy transfer (EE-ET) is considered as an important factor to influence the device performance. However, few works provide evidence to prove this EE-ET process due to the lack of the absorption band of exciplexes. Herein, the EE-ET channel from a high-energy exciplex donor (EXED) to another low-energy exciplex acceptor (EXEA) in a TE-OLED is demonstrated by probing magnetic field effects (MFEs) on the electrical and optical properties of devices including magneto-conductance (MC), magneto-efficiency (Mη), and magneto-electroluminescence (MEL), because this EE-ET can influence the evolution channels of spin-pair states in the TE-OLED which could be visualized by these featured MFE traces. Specifically, all the MC, Mη, and MEL curves of the single EXED-based OLED depict the normal bias-current (I)-dependent intersystem crossing (ISC) from singlet to triplet polaron pairs, which weakens with increasing I. Moreover, the Mη and MEL traces of the single EXEA-based OLED, respectively, present the abnormal and normal I-dependent ISC, while its MC curves show the conversion from reverse ISC (RISC) of exciplexes to ISC of polaron pairs with increasing I. However, both MEL and Mη traces of the TE-OLED with simultaneous EXED and EXEA show the abnormal I-dependent RISC from triplet to singlet exciplexes, which enhances with increasing I, while its MC traces display normal I-dependent RISC. These RISC behaviors have seldom been observed previously in the literature, which are induced by the EE-ET process from EXED to EXEA that facilitate the RISC channel of EXEA via increasing the quantity of triplet exciplex states. Moreover, higher EQE is obtained in the TE-OLED with this EE-ET channel from EXED to EXEA than the single EXEA-based OLED. Thus, these MFE measurements provide new strategies for recognizing the EE-ET process in OLEDs based on multiple exciplex emitter systems and pave the way for designing superior performance exciplex-based OLEDs.

Abundant Evolution Processes of Polaron-Pairs and Exciplex States with Various Electron–Hole Coupling Distances in Exciplex-Based OLEDs by Modifying Device Hole-Injection Abilities

“…1 using eq . According to the characteristic MEL traces reported in the literature (also shown in Figure S1), − LFEs and HFEs of Dev. 1 are separately attributed to the B -mediated ISC process from PP 1 to PP 3 (PP 1 → PP 3 ) and the B -mediated TCA process between EX 3 and excessive charge carriers.…”

“…23,24 Similar to PP 1 and PP 3 , EX 1 and EX 3 also can interconvert via the ISC and RISC processes under the HFI because EX 1 and EX 3 are intrinsically intermolecular excited states and almost degenerate in energy. 10,11,27 As well reported in the literature, 10−12 the interconversion of EX states is often governed by the RISC process from EX 3 to EX 1 (EX-RISC, EX 1 ← EX 3 ), i.e., EX-RISC is stronger than the ISC process from EX 1 to EX 3 (EX 1 → EX 3 ). This is because the quantity of EX 3 is 3 times that of EX 1 and the lifetime of EX 3 (∼10 −6 s) is 3 orders of magnitude longer than that of EX 1 (∼10 −9 s).…”

“…2 converts from a sharp rise to a quick reduction as the bias current increases from 100 to 1000 μA. According to the characteristic MEL traces in Figure S1, − quickly reduced LFEs of Dev. 2 are attributed to the B -mediated RISC process from EX 3 to EX 1 (EX 3 → EX 1 ).…”

“…1–4, the formation and evolution mechanisms of PP and EX states with short and long r at different bias currents are exhibited in Figure a,b, respectively. As can be seen, holes and electrons injected from their own electrodes first recombine under the Coulomb attraction and then form weakly bound PP 1 and PP 3 on separate molecules (i.e., intermolecular electron–hole pairs). − PP 1 and PP 3 can interconvert via the ISC and RISC processes because PP 1 and PP 3 are nearly degenerate in energy and the spin flipping can be achieved by the hyperfine interaction (HFI) with the scale of several milli-Tesla. − Next, PP 1 and PP 3 will further evolve into weakly bound EX 1 and EX 3 with their rate constants of k S and k T . − Note that, although ISC and RISC processes of PP states occur simultaneously, the interconversion of PP states is often dominated by the ISC process from PP 1 to PP 3 (PP-ISC, PP 1 → PP 3 ) since k T is usually larger than k S . , Similar to PP 1 and PP 3 , EX 1 and EX 3 also can interconvert via the ISC and RISC processes under the HFI because EX 1 and EX 3 are intrinsically intermolecular excited states and almost degenerate in energy. ,, As well reported in the literature, − the interconversion of EX states is often governed by the RISC process from EX 3 to EX 1 (EX-RISC, EX 1 ← EX 3 ), i.e., EX-RISC is stronger than the ISC process from EX 1 to EX 3 (EX 1 → EX 3 ). This is because the quantity of EX 3 is 3 times that of EX 1 and the lifetime of EX 3 (∼10 –6 s) is 3 orders of magnitude longer than that of EX 1 (∼10 –9 s). , Although a previous paper reported that the EX-RISC process is induced by the Δ g mechanism, Δ g mechanism is not suitable for our work, which has been explained in detail in Text S2 and Figure S6.…”

Mediation of energy transfer on magnetic field effects in Px-CNP-based OLEDs

“…The persistent blue shift of the device luminescence peak with increasing mCP and Alq 3 concentrations is due to the luminescence of the host material is superimposed on the luminescence of the guest material, causing a blue shift in the EL peak. 36 As the concentration of the host material increases, the emission of mCP and Alq 3 increases, resulting in a blue-shift. A small peak around 410 nm and 520 nm in the device with mCP and Alq 3 as the host material is close to the mCP and Alq 3 EL peak, indicating that the energy transfer from the host mCP and Alq 3 to the guest Px-CNP is incomplete, and the singlet exciton (S 1 ) in mCP and Alq 3 , leaps to the ground state through the excitation radiation and produces prompt fluorescence (PF).…”