Room-Temperature Observation for Reverse Intersystem Crossing in Exciplex-Based OLEDs with Balanced Charge Injection

Abstract

Room-temperature observation for reverse intersystem crossing (RISC) from triplet to singlet charge-transfer states (CT 3 → CT 1 ) and clarification of its physical mechanisms are the key requirements for designing highly efficient exciplex-based organic light-emitting diodes (OLEDs). Herein, balanced and unbalanced exciplex-based OLEDs were fabricated by employing different holeinjection layers, and RISC of CT states was directly observed via analyzing magnetoconductance (MC) and magneto-electroluminescence (MEL) traces of the balanced device at room temperature. Specifically, current-dependent MC traces of the balanced device always present B-mediated RISC features, whereas those from the unbalanced one depict the superposition of B-mediated intersystem crossing (ISC) and the dissociation of CT 3 by excessive charge carriers. Simultaneously, MEL curves of the balanced device display the conversion from ISC to RISC with lowering bias current, but those from the unbalanced one always show ISC under all of bias currents. Moreover, although all of current-dependent magneto-efficiency (Mη) traces exhibit ISC, Mη values are ∼2 times lower in the balanced device than the unbalanced one. These rich changes of magnetic-field responses demonstrate that balanced carrier injection can facilitate the occurrence of RISC by reducing the dissociation of CT 3 . Expectedly, the current efficiency of electroluminescence from the balanced device is increased by ∼2.2 times, which originates from the improvement of delayed luminescence because of the enhanced RISC. Accordingly, this work not only clarifies the prerequisite for observing RISC of CT states but also provides strategies for designing high-efficiency exciplex-based OLEDs.

Conversions from Normal to Abnormal Current‐Dependent ISC and from Abnormal to Normal Current‐Dependent RISC Processes in Exciplex‐Based OLEDs

“…Moreover, saturated HFE of Dev. 1 at 25 μA indicates negligible TQA process, [ 21 ] that is, the balanced carrier injection. Thus, Dev.…”

“…Moreover, HFE of Dev. 2 saturates rapidly at all bias I , which indicates negligible TQA process, [ 21 ] that is, the balanced carrier injection is obtained for almost all bias currents. This is because the hole‐injection barrier (0.2 eV) from the LUMO level of HAT‐CN to the HOMO level of TAPC is nearly equal to the electron‐injection barrier (0.1 eV) from the LiF/Al cathode to the LUMO level of PO‐T2T in Dev.…”

“…2 is negligible, that is, the carrier injection is almost balanced. [ 16,17,21 ] When the carrier injection in Dev. 2 is nearly balanced, the quantity of CT 3 is far more than that of PP 1 because CT 3 is evolved from PP 3 and the number of PP 3 is three times that of PP 1 according to the spin statistics rule.…”

“…[ 20 ] In our previous works, we used MC to directly observe ISC and RISC processes in EB‐OLEDs. [ 21–24 ] And these two processes often present normal current ( I ) dependencies which weaken with increasing I because of the enhanced electric‐field‐induced dissociation of exciplex states. [ 4,21,24 ] Although some understandings of the physical mechanisms related to ISC and RISC processes in EB‐OLEDs have been obtained, more in‐depth explorations are still needed because the behaviors of ISC and RISC processes depend significantly on the device architecture parameters [ 19–24 ] and the film morphology properties.…”

A reverse intersystem crossing managing assistant dopant for high external quantum efficiency red organic light-emitting diodes

“…Based on the measured τ p , τ d , and PLQY, PL-related rate constants were calculated and are summarized in Table 1. 36–38 The rate constant for RISC ( k RISC ) increased in the order of 4CzTPN (8.03 × 10 5 s −1 ) < BTCz3CzTPN (9.08 × 10 5 s −1 ) < 2BTCz2CzTPN (10.22 × 10 5 s −1 ) due to the shortened τ d . However, the red-shifted emission spectra of BTCz3CzTPN and 2BTCz2CzTPN resulted in increased non-radiative rate constants ( k nr ) of 2.18 × 10 5 s −1 and 3.27 × 10 5 s −1 , respectively, by the energy gap law.…”