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 currentdependent ISC, RISC, TCA, and TTA processes of PP and EX states with short and long r in EX-OLEDs.