Non-monotonic current dependence of intersystem crossing and reverse intersystem crossing processes in exciplex-based organic light-emitting diodes

Intersystem crossing (ISC) and reverse ISC (RISC) between singlet and triplet polaron-pair and exciplex state are important spin-mixing processes in exciplex-based organic light-emitting diodes (EB-OLEDs). These two processes usually show normal current dependence which weakens with the increase of bias-current. This is because the bias-current increases by improving the device bias-voltage. When the bias-voltage rises, the electric field within the device is enhanced, 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 process and RISC process, leading these two processes to weaken. 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 mu A) process to abnormal ISC (25-200 mu A) process, whereas those of the balanced device show conversions from normal ISC (1-5 mu A) into abnormal RISC (10-50 mu A) and then into normal RISC (50-150 mu A) and finally into abnormal ISC (200-300 mu A) process. By fitting and decomposing the current-dependent MEL traces of the unbalanced and balanced devices, we find that the ISC process and RISC process in these two devices first increase then decrease as the bias-current increases. These non-monotonic current-dependent ISC process and RISC process are attributed to the competition between the increased number and the reduced lifetime of polaron-pair state and exciplex state 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 the number of triplet exciplex states to increase. That is to say, more triplet exciplex states can be converted into singlet exciplex states through the RISC process, causing the external quantum efficiency of the balanced device to be higher 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 an insight into the device physics for designing and fabricating high-efficiency EB-OLEDs.