Field-Induced Spectral Shift in Organic Charge-Transfer State Luminescence

Control of exciton energy by externally applied electric fields could lead to dynamically tunable color (allochroism) and photophysics in organic semiconductor devices, such as organic light-emitting devices. Unfortunately, few reports have shown dramatic electric-field induced modulation of exciton energy in organic semiconductors, largely confined to electro-absorption. Here we employ excited states formed across electron donor-acceptor heterojunctions, called charge-transfer (CT) states, as a field-responsive electro-optical medium. CT states exhibiting thermally activated delayed fluorescence provide highly luminescent states that enable optical measurement of excited state energy. We employ biased, non-injection-type devices to show a reversible and linearly field-dependent energy shift of CT state photoluminescence. Further we relate our findings to the more general phenomenon of the Stark effect observed in electro-absorption and electro-emission across other excitonic material systems. Finally, we note the relationship between our findings in photoluminescence to observations and opportunities in organic light-emitting devices exhibiting electroluminescence.