Unveiling the Double-Edged Roles of DPEPO Doping in Exciplex-Based OLEDs: The Nonmonotonic Device Efficiency Due to the Competition between RISC and TQA

This study employs organic magneto-electroluminescence (MEL) and magneto-conductance (MC) as a core probe to unveil a critical, yet previously unconfirmed, physical mechanism governing the device performance of m-MTDATA:Bphen exciplex OLEDs doped with the phosphorescent host DPEPO. We present the first experimental confirmation that DPEPO acts not only as a molecular spacer to modulate the energy gap between singlet and triplet exciplexes but, more importantly, facilitates the triplet-charge annihilation (TQA) between the triplet exciplex and free charge. This technical solution of doping molecular spacer proves to be a double-edged sword. On one hand, it reduces the energy barrier that needs to be overcome for the occurrence of reverse intersystem crossing (RISC), which is beneficial for the device performance; on the other hand, it significantly intensifies the competing and detrimental pathway of TQA. The TQA process consumes valuable triplet excitons and hinders the carrier mobility. This competition between enhanced RISC (beneficial) and amplified TQA (detrimental) perfectly explains the nonmonotonic trend of device efficiency with varying DPEPO doping concentration. Experimental results show that the device efficiency peaks at a 20% concentration. Below this threshold, the enhancement in RISC dominates, improving efficiency; above 20% concentration, the dramatically intensified TQA becomes the prevailing mechanism, causing a sharp efficiency roll-off. This work offers a new physical framework and design strategy for optimizing molecule distance between donor and acceptor in exciplex-based OLEDs, highlighting the necessity of precisely balancing the gains from RISC against the losses from parasitic quenching pathways.