Reasons for “disappearance” phenomenon of both intersystem crossing of polaron-pair states and reverse intersystem crossing of high-lying triplet excitons in pure Rubrene-based OLEDs

With unique advantages of high sensitivity, no-contact, and non-destructiveness, magneto-electroluminescence (MEL) is usually employed as an effective detection tool to visualize the microscopic mechanisms of excited states existing in organic light-emitting diodes (OLEDs) because their evolution channels of many spin-pair states in OLEDs have the fingerprint MEL line-shapes even with opposite signs. The recently-published MEL results (Tang X T, Pan R H, Zhao X, Jia W Y, Wang Y, Ma C H 2020 <i>Adv. Funct. Mater.</i> <b>5</b> 765) have demonstrated the existence of high-level reverse intersystem crossing process (HL-RISC, S_1,Rub ← T_2,Rub) of high-lying triplet excitons (T_2,Rub) in Rubrene when Rubrene with a typical value of several percent in content is doped into a host with high triplet exciton energy and there are no energy loss channels of triplet excitons from charge-carrier transporting layers either. Furthermore, this HL-RISC process can considerably increase the efficiency and brightness of OLEDs operated at room temperature, for example, high external quantum efficiency up to 16.1% and ten thousands of brightness have been achieved in Rubrene-doped OLEDs with a co-host of exciplex. Herein, surprisingly, in the pure Rubrene-based OLEDs (i.e. the pure Rubrene film is used as an emissive layer) with no energy loss channels of triplet excitons from charge-carrier transporting layers, only strong singlet fission (S_1,Rub+S_0,Rub → T_1,Rub+T_1,Rub) processes are detected at room temperature, but this HL-RISC process is not observed. Moreover, even the most usual evolution process of intersystem crossing of polaron-pair (ISC, PP₁ → PP₃) cannot be observed in this pure Rubrene-based OLEDs, where the polaron-pair is generated through the recombination of the injected electrons and holes in the pure Rubrene emissive layer. To determine the cause of the underlying physical mechanism behind this abnormal and fascinating experimental phenomena, two kinds of devices with pure Rubrene and 5% Rubrene-dopant as emissive layers are fabricated and their current- and temperature- dependent MEL responses are systematically investigated. By comparing and analyzing these tremendously different MEL curves of these two types of devices, we find that the positive Lorentzian MEL curves induced from <i>B</i>-mediated ISC of polaron-pair just completely cancel out the negative Lorentzian MEL curves induced from <i>B</i>-mediated HL-RISC process of T_2,Rub excitons. Note that such an abnormal and coincidental experimental phenomenon is the physical reason why the ISC process and HL-RISC process cannot be observed simultaneously in the pure Rubrene-based OLEDs, and this phenomenon has not been found in the literature. Clearly, this work further deepeneds our understanding of some unique microscopic processes and physical phenomena in organic semiconductor “star” material of Rubrene (such as the energy resonance between 2T₁ and S₁ and the energy approach between T₂ and S₁).