Dynamic Behaviors of Exciplex States in Rubrene/ C60 -Based OLEDs with Sub-Band-Gap Turn-On Electroluminescence

Although the origin of sub-band-gap electroluminescence has been extensively studied for organic light-emitting diodes (OLEDs) with a planar heterojunction of rubrene-fullerene (${\mathrm{C}}_{60}$), the dynamic behaviors of exciplexes formed at the $\mathrm{rubrene}/{\mathrm{C}}_{60}$ interface are still vague. Herein, employing magnetoconductance (MC) and magnetoelectroluminescence (MEL) detection techniques, we find an unreported reverse intersystem crossing (RISC, ${\mathrm{EX}}^{3}$ \ensuremath{\rightarrow} ${\mathrm{EX}}^{1}$) from triplet to singlet exciplexes dominated at the $\mathrm{rubrene}/{\mathrm{C}}_{60}$ interface. Besides this RISC evolution channel, the triplet exciplexes also participate in another two processes: one is the scattering channel of triplet-charge annihilation (TCASC, ${\mathrm{EX}}^{3}$ + $\stackrel{\ensuremath{\leftarrow}}{e}$ \ensuremath{\rightarrow} ${S}_{0}$+ \ensuremath{\downarrow}e) with excessive charge carriers, which is disadvantageous for RISC to occur, and the other is Dexter-energy transfer to produce a triplet exciton (${\mathrm{EX}}^{3}$ \ensuremath{\rightarrow} ${T}_{1}$) in rubrene followed by triplet-triplet annihilation (TTA, ${T}_{1}$+ ${T}_{1}$ \ensuremath{\rightarrow} ${S}_{1}$+ ${S}_{0}$ \ensuremath{\rightarrow} h\ensuremath{\nu}\ensuremath{\uparrow} + 2${S}_{0}$). This TTA is the physical origin of sub-band-gap EL because TTA needs no respective injection of electrons and holes into the LUMO and HOMO energy levels and requires no direct recombination of LUMO electrons with HOMO holes in rubrene. Moreover, both the bias-current dependences of MC and MEL traces reveal that the RISC process is more obvious in relatively balanced devices due to weak TCASC processes, and they show a monotonic decrease and nonmonotonic variation in unbalanced and relatively balanced $\mathrm{rubrene}/{\mathrm{C}}_{60}$-based OLEDs, respectively, because of the combined effects of the applied electrical field and the TCASC process. Theoretically, a stronger RISC process should be observed in devices by inserting a thin bathocuproine (BCP) interlayer to modify the $\mathrm{rubrene}/{\mathrm{C}}_{60}$ interface due to the increased separation distance between electron and hole in exciplex states, but the opposite experimental results are obtained because of strong TCASC channels simultaneously occurred with the insertion of a BCP interlayer. The RISC process decreases monotonically upon lowering the operational temperature due to its endothermic property. This work deepens our comprehensive understanding of the dynamic behaviors of exciplexes and the physical origin of sub-gap EL features in $\mathrm{rubrene}/{\mathrm{C}}_{60}$-based OLEDs.