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Adlayers are often placed at metal-on-organic interfaces as a common strategy to alleviate damage during metal deposition by thermal evaporation. Methods of chemically installing adlayers have been recently demonstrated on organic semiconductors that address these interfacial issues while providing many secondary benefits. Chemical installation has yet to be attempted at the cathode-electron transport layer (ETL) interface within organic light-emitting devices (OLEDs), offering a powerful option to optimize electron injection, improve surface wetting, and reduce metal penetration. Here, a reaction between TPBi (2,2′,2′’-(1,2,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) and propylene oxide results in a controllable 1–3 nm thick layer of propylene oxide as shown by high-resolution X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX). The reactive addition of the adlayer at temperatures below 40℃ does not affect the morphology of the thin film and reaches a high degree of coverage within 3 h. Integration of this layer into a phosphorescent OLED does not introduce any significant negative impact on device function. This result opens up the possibility of introducing further specific functionality into the adlayer to engineer OLED performance. 

Abstract Exciton‐polaritons in organic microcavities are applied in devices including lasers, light‐emitting devices, and photodetectors, as well as in structures capable of tuning exciton kinetics and energy transfer. To enable a broader tailoring of polariton properties, it is important to develop means to better control molecular orientation and tune the intensity of the exciton–photon interaction. Vapor‐processed, glassy organic thin films are previously shown to have tunable molecular orientation as evidenced by phenomena including birefringence and transition dipole moment (TDM) alignment. Here, this tunability in TDM orientation with thin film processing conditions is exploited to continuously vary the interaction between the exciton and confined cavity photon mode. By embedding a thin film of 4,4′‐bis[(N‐carbazole)styryl]biphenyl (BSB‐Cz) in a metal‐reflector microcavity, ultrastrong coupling and hybridization of multiple electronic transitions of BSB‐Cz are demonstrated with a common cavity mode. Increasing the temperature during BSB‐Cz deposition tunes the TDM orientation from predominantly in‐plane to random to slightly vertical. This leads to a corresponding ≈30% variation in the associated Rabi splitting, consistent with theoretical predictions. This work demonstrates a means to continuously tune coupling strength from a materials perspective while also providing a handle to tune orientation disorder in thin film.