Various current-dependent intersystem crossing and reverse intersystem crossing processes induced by different charge balances in exciplex-based OLEDs

Abstract

Intersystem crossing (ISC) and reverse ISC (RISC) between singlet and triplet polaron-pair and exciplex states are important spin-mixing processes in exciplex-based organic light-emitting diodes (EB-OLEDs). These two processes usually show normal current dependencies which weaken with increasing the bias-current. This is because the increase in the bias-current is realized by improving the device bias-voltage. When the bias-voltage rises, the electric field within the device enhances, which facilitates the electric-field-induced dissociation of polaron-pair and exciplex states and then reduces their lifetime. That is, less polaron-pair and exciplex states participate in the ISC and RISC processes leading to the reduction of these two processes. Here, magneto-electroluminescence (MEL) is used as a fingerprint probing tool to observe various current-dependent ISC and RISC processes in EB-OLEDs with different charge balances via modifying the device hole-injection layer. Interestingly, current-dependent MEL traces of the unbalanced device display a conversion from normal ISC (1-25 mA) to abnormal ISC (25-200 mA) processes, whereas those of the balanced device show conversions from normal ISC (1-5 mA) to abnormal RISC (10-50 mA) and then to normal RISC (50-150 mA) and finally to abnormal ISC (200-300 mA) processes. By fitting and decomposing the current-dependent MEL traces of the unbalanced and balanced devices, we find that the ISC and RISC processes in these two devices first enhance but then weaken as the bias-current increases. These non-monotonic current-dependent ISC and RISC processes are attributed to the competition between the increased number and the reduced lifetime of polaron-pair and exciplex states during improving the bias-current. Furthermore, the RISC process in the balanced device is stronger than that in the unbalanced device. This is because the balanced carrier injection can facilitate the formation of triplet exciplex states and weaken the triplet-charge annihilation (TQA) process between triplet exciplex states and excessive charge carriers, which leads to the increased number of triplet exciplex states. That is, more triplet exciplex states can convert into singlet exciplex states through the RISC process, causing a higher external quantum efficiency of the balanced device than that of the unbalanced device. Obviously, this work not only deepens the understandings of current-dependent ISC and RISC processes in EB-OLEDs, but also provides insights of device physics for designing and fabricating high-efficiency EB-OLEDs.