(321g) Electric-Field Induced Luminescence Shift for Excited States at Organic Semiconductor Interfaces

The realization of high efficiency in organic light-emitting devices (OLEDs) requires the harvesting of all electrically generated excitons. In conventional fluorescent materials, this is limited by optically dark spin triplet excitons, leaving only spin singlet excitons to undergo radiative recombination. OLEDs exhibiting thermally assisted delayed fluorescence (TADF) allow harvesting of triplets, maximizing exciton utilization. TADF has been observed for both single molecules and interfacial exciplexes, which are excited states formed between electron donating and accepting molecules. OLEDs based on exciplex electroluminescence continue to grow in efficiency and warrant further study due to their wide tunability in emission color, high OLED efficiency, and the emergent behavior of their delocalized excited states that could influence device performance. Prior work has demonstrated the sensitivity of exciplex emission energy to external bias, providing an additional route to tune emission color and perhaps photophysics, although the origin of this effect remains unclear. This talk will expand upon earlier indirect study of electric-field induced shifts of exciplex emission energy to explicitly propose an origin of this effect. To predictably control the interfacial electric field, we apply biased, non-injection-type devices and observe a reversible and linearly field-dependent energy shift of exciplex photoluminescence. Further we relate our findings to the more general phenomenon of the giant Stark effect observed in other excitonic material systems. Finally, we note the relationship between our findings in photoluminescence to observations and opportunities for tunable electroluminescence and exciton management in exciplex OLEDs.