Enhanced Efficiency of NIR-OLEDs by Deuteration of Both Host and Guest Molecules

The external quantum efficiency (EQE) of organic light-emitting diodes (OLEDs) in the visible area has reached its theoretical limit due to advancements in material development and device physics. Although OLEDs that emit near-infrared (NIR) light have the potential for valuable sensing applications in the future, they currently face the serious problem of low EQE, preventing practical use. This issue stems from the low photoluminescence quantum yield (PLQY) of NIR-emitting molecules resulting from a significant nonradiative decay rate following the energy gap law. Particularly, NIR-OLEDs emitting wavelengths over 900 nm are limited to only a few percent efficiency.<br/>Recently, a strategy involving the deuteration of C-H bonds in NIR emitters has been proposed to suppress nonradiative decay. Deuteration reduces the molecular vibrations of the emitting molecule, potentially suppressing nonradiative decay processes. However, there are only a few reports on the effect of deuteration on the NIR PL properties in host-guest codeposited films.<br/>In this study, we investigated the impact of deuteration of both the host (mCP-<i>d₂₀</i>: 1,3-dicarbazole-benzene-<i>d₂₀</i>) and guest (BBT-TPA-<i>d₂₈</i>: 4,8-bis[4-(N,N-diphenylamino)phenyl]benzo[1,2-c:4,5-c']bis[1,2,5]thiadiazole-<i>d₂₈</i>) on the NIR PL and EL properties in the host-guest codeposited film. The 1wt%-BBT-TPA-<i>d₂₈</i>:mCP-<i>d₂₀</i> codeposited film exhibited a PLQY of 15 ± 2% with an emission peak wavelength at approximately 900 nm that is about three times higher than that of the film composed of the undeuterated molecule (~5%). Furthermore, the fluorescence lifetime of the codeposited film with 1wt% BBT-TPA-<i>d₂₈</i>:mCP-<i>d₂₀</i> is significantly longer (4.7 ns) compared to the undeuterated system (2.0 ns). This indicates suppression of nonradiative decay in the deuterated film.<br/>We observed that the deuteration of only one of the host or guest molecules does not suppress the nonradiative decay process compared to the deuteration of both. This suggests that the host-guest interaction is important for suppressing the nonradiative decay process. Furthermore, NIR-OLEDs incorporating the deuterated codeposited film as an emissive layer demonstrated a maximum external EL quantum efficiency of 2.3 ± 0.2% with an EL peak wavelength of around 900 nm.