Achievement of High-Level Reverse Intersystem Crossing in Rubrene-Doped Organic Light-Emitting Diodes

Abstract

Using the fingerprint magneto-electroluminescence trace, we observe a fascinating high-level reverse intersystem crossing (HL-RISC) in rubrene-doped organic light-emitting diodes (OLEDs). This HL-RISC is achieved from high-lying triplet states (T2,rub) transferred from host triplet states by the Dexter energy transfer to the lowest singlet states (S1,rub) in rubrene. Although HL-RISC decreases with bias current, it increases with lowering temperature. This is contrary to the temperature-dependent RISC from conventional thermally activated delayed fluorescence, because HL-RISC is an exothermic process instead. Moreover, owing to the competition of exciton energy transfer with direct charge trap, HL-RISC changes nonmonotonically with the dopant concentration and increases luminous efficiency to a maximum at 10% of rubrene, which is about ten times greater than that from the pure-rubrene device. Additionally, the HL-RISC process is not observed in bare rubrene-doped films because of the absence of T2,rub. Our findings pave the way for designing highly efficient orange fluorescent OLEDs.

Full Confinement of High‐Lying Triplet States to Achieve High‐Level Reverse Intersystem Crossing in Rubrene: A Strategy for Obtaining the Record‐High EQE of 16.1% with Low Efficiency Roll‐Off

“…Namely, under the characteristic saturation magnetic field (i.e., B = 9 mT), the MEL amplitude (|MEL LFE |) at LFEs (MEL LFE = MEL B = 9 mT − MEL B = 0 mT ) decreases with increasing the bias current, as shown by the olive and red lines of Figure 2f. According to the literature reports, [18] this normal current dependence of HL-RISC process can be attributed to the increased dissociation effect of TQA (T 1, host + charge → e + h + charge) with increasing bias current, resulting in the decreased number of T 1, host excitons and the weakened HL-DET process. Consequently, the populations of T 2, rub excitons reduce and the HL-RISC processes weaken at high bias currents.…”

“…LFEs mean that MELs rapidly decrease (or increase) with B in the range of |B| < 9 mT, whereas the HFEs refer that MELs slowly increase (or decrease) within the range of 9 mT < |B| < 300 mT. According to the fingerprint MEL traces reported in the literatures, [3,18,28,29] these sharply-decreased (or increased) LFEs in rubrene-based devices, governed by the hyperfine strength (typically on the order of several mT), are originated from B-mediated HL-RISC (or ISC) processes. HFEs with a full width at half maximum (FWHM) on the order of 100 mT showing positive (or negative) correlations with B can be attributed to B-mediated SF (or TF) processes.…”

“…Therefore, we can conclude that Figure 2a,b shows the HL-RISC-dominated negative LFEs accompanied by SF-induced HFEs, whereas the ISC-governed positive LFEs and TF-induced HFEs are exhibited in Figure 2c,d. Factually, SF/TF-determined HFEs in these four devices were well reported in the literatures, [10,18] so they will not be detailedly discussed in this article. More importantly, the HL-RISC/ISC-dominated LFE behaviors presented in devices 1-4 with different host materials play significant roles in judging whether the HL-RISC process can exist or not.…”

“…This experimental phenomenon can be ascribed to two factors. On one hand, according to the literatures, [18,34] the HL-RISC rate constant k HL-RISC can be expressed as k HL-RISC ∝ exp(−ΔE S-T /k B T), where ΔE S-T , k B , and T are the singlet-triplet energy difference, Boltzmann's constant, and temperature, respectively. Since the energy of S 1, rub state is about 0.1 eV lower than that of the T 2, rub state (i.e., ΔE S-T < 0 eV) in rubrene molecules, k HL-RISC elevates with lowering temperature, indicating the exothermic property of HL-RISC process.…”

“…[13] Obviously, the large energy gap of rubrene ( 1 1 E S T ∆ = 1.15 eV) [17] excludes the existence of the so-called TADF-RISC process from rubrene. Fortunately, in our recent work, [18] we have discovered an exothermic high-level RISC (HL-RISC) process from high-lying triplet (T 2, rub ) to lowest singlet states (S 1, rub ) in rubrene molecules, in which the energy level difference between S 1, rub and T 2, rub is relatively small [ 1 2 E S T ∆ = E(S 1 )−E(T 2 ) = -0.1 eV]. [19] However, the prerequisite for this amazing HL-RISC channel is still vague and the reported external quantum efficiencies of rubrene-based OLEDs are relatively low.…”