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

Abstract

Intersystem crossing (ISC) and reverse ISC (RISC) between singlet and triplet polaron-pairs (PP) and exciplex (EX) states, triplet-charge annihilation (TCA), and triplet–triplet annihilation (TTA) are well-known key spin-mixing processes in EX-based organic light-emitting diodes (EX-OLEDs) for improving the device optoelectronic performances. These microscopic processes usually show normal current dependencies; i.e., ISC and RISC weaken while TCA and TTA enhance with increasing bias current, respectively. Here, planar- and bulk-heterojunction EX-OLEDs (PHJ- and BHJ-EX-OLEDs) with good and poor hole-injection abilities were fabricated by modifying the device hole-injection layer. Amazingly, electroluminescence (EL) spectra are used to find that electron–hole coupling distances (r) of PP and EX states in PHJ-EX-OLEDs with good and poor hole-injection abilities are short and long, respectively. This unreported phenomenon only exists in PHJ-EX-OLEDs because holes and electrons separately accumulate on different sides of the donor/acceptor interface, and the distance from holes to the interface increases with decreasing hole-injection ability. However, r of PP and EX states in BHJ-EX-OLEDs are always short whether the devices have good or poor hole-injection abilities since holes and electrons are mixed together within the donor:acceptor blend layer. Furthermore, magneto-EL (MEL) traces of these devices are used as fingerprint detection techniques to distinguish various current-dependent ISC, RISC, TCA, and TTA processes of PP and EX states with short and long r. Specifically, current-dependent low-field effects (LFEs, B ≤ 10 mT) of MEL traces from PP and EX states with short r show the abnormal ISC process as the bias current increases (100 → 2000 μA), whereas those from PP and EX states with long r present rich conversions from normal ISC (100 → 400 μA) to abnormal RISC (600 → 1000 μA) and then to normal RISC (1000 → 1400 μA) and finally to abnormal ISC (1600 → 2000 μA) processes. These different current-dependent LFEs occur because PP and EX states with short and long r undergo monotonically and nonmonotonically current-dependent ISC and RISC processes, respectively. Moreover, current-dependent high-field effects (HFEs, 10 < B ≤ 300 mT) of MEL traces from EX states with a short r show the normal TCA process with increasing the bias current (100 → 2000 μA), but those from EX states with long r display an interesting conversion from abnormal TCA (100 → 1000 μA) to normal TTA (1000 → 2000 μA) processes. These different current-dependent HFEs happen because EX states with short and long r separately participate in the monotonically enhanced TCA process and the competition between the TCA and TTA processes. Obviously, this work enriches the physical understanding of the current-dependent ISC, RISC, TCA, and TTA processes of PP and EX states with short and long r in EX-OLEDs.