Novel magnetic field effects in TBRb/C<sub>60</sub>-based OLEDs with sub-bandgap turn-on electroluminescence

<p indent="0mm">In order to study the microscopic processes in devices with sub-bandgap turn-on voltages, a rubrene (Rb) derivative named 2,8-di-tert-butyl 5,11-bis (4-tert-butylphenyl)-6,12-diphenyltetracene (TBRb) was selected as the hole-transporting and emission layer, and fullerene (C₆₀) was used as the electron transport layer to fabricate the devices with the planar heterojunction of the TBRb/C₆₀ interface. The TBRb/C₆₀ device possesses the property of sub-bandgap turn-on electroluminescence. In order to clearly analyze the microscopic mechanisms in TBRb/C₆₀ devices, we also prepared traditional Rb/C₆₀ devices as reference devices. Simultaneously, we also measured the temperature and current dependences of magneto-electroluminescence (MEL) and magneto-conductance (MC) response curves for TBRb/C₆₀ and Rb/C₆₀ devices. The experimental results show that the MEL curves of traditional Rb/C₆₀ devices decide by the intersystem crossing (ISC, PP₁→PP₃) from singlet to triplet polaron pairs (PP₁ and PP₃) and triplet-triplet annihilation (TTA, T₁+T₁→S₁+S₀) processes. However, the MEL curves of the TBRb/C₆₀ device show a novel magnetic field effect with a three-stage variation: With the increase of the applied magnetic field, MEL firstly increases rapidly (0 mT&lt;|<italic>B</italic>|≤27 mT), then decreases (27 mT&lt;|<italic>B</italic>|≤100 mT), and then increases slowly (100 mT&lt;|<italic>B</italic>|≤300 mT). According to the analysis of current-dependent MC curves, this novel phenomenon is mainly attributed to the combined effect of ISC, TTA and triplet exciplex charge annihilation (TECA). Notably, since the hole carrier mobility of TBRb is smaller than that of the Rb material, the existence of excess electron carriers at the TBRb/C₆₀ heterojunction interface can interact with triplet exciplex (EX₃) via TECA process (EX₃+e→e’+S₀). At large currents, although carrier mobility remains constant, the increased number of injected carriers can increase the amount of EX₃, leading to the enhancement of the TECA process. At the same time, since the essentially unchanged Dexter energy transfer process does not result in an increase in the number of T₁ as the injection current increases, the TTA process does not vary significantly with the injection current. When the temperature drops from <sc>300 K</sc> to <sc>100 K,</sc> the decreased carrier mobility can weaken the TECA process, while the endothermic characteristic of the TTA process causes it to decrease. This anomalous endothermic property is due to the singlet state energy <sc>(2.19 eV)</sc> of the TBRb material being higher than twice the triplet exciton energy <sc>(0.98 eV).</sc> When the temperature decreases from <sc>100 K</sc> to 20 K, the prolonged lifetime of EX₃ and T₁ sates can lead to the increase of TECA and TTA processes, respectively, with decreasing temperature. This study is helpful in understanding the microscopic evolution of excited states in OLEDs with sub-bandgap electroluminescence, and provides ideas for the design of high-performance devices.