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

Abstract

A high‐level reverse intersystem crossing (HL‐RISC, T2 → S1 → S0 + hν) process has recently been discovered as a promising route for achieving highly efficient organic light‐emitting diodes (OLEDs), but the prerequisites for the occurrence of HL‐RISC in rubrene is still vague and the reported external quantum efficiencies (EQEs) of rubrene‐doped OLEDs are typically limited to several percent. Herein, using the fingerprint magneto‐electroluminescence tools, it is found that the energy confinement of high‐lying triplet states (T2, rub) is of great importance for the achievement of the HL‐RISC process. Namely, when the triplet energies of hosts satisfy the criterion of E(T1, host) ≥ E(T2, rub), the high‐level Dexter energy transfer channel (T1, host → T2, rub) can facilitate the occurrence of HL‐RISC (T2, rub → S1, rub) in rubrene. Most importantly, through selecting an exciplex with a high triplet energy as the co‐host for rubrene dopant so as to simultaneously utilize the HL‐RISC of the dopant and the RISC of the host, a record high EQE up to 16.1% is achieved and no obvious efficiency roll‐off is observed at high luminance due to the absence of triplet‐charge annihilation. Accordingly, this work not only deepens the physical understanding of this amazing HL‐RISC channel, but also provides a new direction for designing a series of highly efficient OLEDs.

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

“…Therefore, the I -dependent MC, Mη, or MEL traces are determined by the combined effects of the B -mediated PP-ISC and EX-RISC processes in the exciplex-based OLEDs. Moreover, the occurrence probability of the EX-RISC process is dependent on the energy confinement of EX 3 states, which is affected by the difference in triplet energy levels between the EX 3 states and the triplet excitons of donor or acceptor. , Specifically, as shown in Figure e for Device 1, because the triplet exciton (T 1 ) energy (2.56 eV) of Bphen or that (2.70 eV) of m -MTDATA is just a little higher than the EX 3 energy (2.47 eV) of the m -MTDATA:Bphen exciplex, the DET process may occur from EX 3 back to T 1 of Bphen and m -MTDATA via the thermally assisted reverse energy transfer, resulting in the partial energy loss of EX 3 so that the EX-RISC process of EX 3 (EX 1 ← EX 3 ) only occurs partly. The partial EX-RISC channel could be proven by the delayed PL emission of m -MTDATA:Bphen in Device 1, as depicted in Figure S7 (Supporting Information).…”

Critical Role of High‐Lying Triplet States for Efficient Excitons Utilization in High‐Performance Non‐Doped Deep‐Blue Fluorescent and Hybrid White Organic Light‐Emitting Diodes

“…The decrease in exciton utilization of doped devices may be because the energy of T 2, guest > T 1, host (2.56 eV) > T 1, guest , resulting in the loss of triplet excitons through efficient highlevel Dexter energy transfer channels. [12] The exciton utilization of these devices exceeded the spin limit of 25% from the results of the above devices, which indicates that the triplet exciton is effectively utilized. As shown in Figure 3e, because PT-1 and PT-2 possess a large energy gap between S 1 and T 1 , and the short single-exponential fluorescence decay lifetimes in the time-resolved PL decay spectrum, the possibility of the TADF mechanism is excluded.…”

Overcoming energy loss of thermally activated delayed fluorescence sensitized-OLEDs by developing a fluorescent dopant with a small singlet–triplet energy splitting

“…The FRET between the co-host of TCTA:PO-T2T and the FDs was investigated first. Here, the bulk exciplex of TCTA and PO-T2T was selected as the co-host because it can form a TADF-type exciplex 23 with green emission. In our experiments, the optimized ratio of TCTA and PO-T2T as co-hosts is 80 : 20, and therefore, we mainly focused on the Fo ¨rster energy transfer between the co-hosts TCTA : PO-T2T (80 : 20) and 67dTPA-FQ/ 267tTPA-FQ.…”

“…For common FDs, the T 1 energy is usually over 0.6 eV lower than its S 1 energy. [21][22][23] Thus, the energy loss caused by the T 1 states of FDs can be mainly attributed to two aspects in TSF-OLEDs. One is that the T 1 exciton of the FDs cannot go back to the ground state via radiative transition, and the other is that its T 1 energy is significantly lower than that of the host or TADF sensitizer, meaning that the T 1 excitons residing on the FDs cannot return to the host or TADF sensitizer.…”