Ultrafast Intermolecular Energy Transfer in OLED Materials: Excited-State Dynamics of a Blend of Poly(vinylcarbazole) and Oxadiazole Derivative in Solution and Film States

Abstract

Intermolecular energy transfer dynamics between layers of organic materials controls the efficiency of organic lightemitting diode (OLED) devices. Here, to understand the ultrafast intermolecular energy transfer dynamics occurring in widely used UV-OLED materials of poly(vinylcarbazole) (PVK, donor: holetransporting material) and synthesized oxadiazole derivative (OXD, acceptor: electron-transporting material), their excited-state relaxation pathways are investigated in tetrahydrofuran (THF) and film. Upon addition of OXD, the nanosecond fluorescence lifetime studies of PVK exhibited efficient quenching of dynamics, supporting the occurrence of Forster resonance energy transfer from PVK to OXD with an efficiency of ∼90%. The femtosecond time-resolved absorption measurements of the mixture of PVK and OXD revealed the intermolecular resonance energy transfer with a time constant of ∼1.32 ps from all of the intermediates in the excited states of PVK to OXD. Interestingly, in the film state, the formation of exciplex beneficial for devices with a lifetime of ∼32.66 ns was observed at ∼516 nm upon excitation of PVK in a blend of OXD and PVK. Nanosecond transient absorption spectra confirmed the occurrence of energy transfer also from the triplet state of PVK. Understanding of these excited-state dynamics occurring in solution and film states is a prerequisite to design and improve the performance of OLED materials.

Experimental Verification of the Accuracy of Oxygen-Varying Correlated Fluorescence for Determining the Quenching Constant KSV

“…Theoretically, the Stern–Volmer equation , is utilized to express the relationship between phosphorescence quenching and oxygen content, which is as follows I normalp 0 I normalp = τ normalp 0 τ normalp = 1 + K normalS normalV [ O 2 ] where I p0 and I p and τ p0 and τ p represent the phosphorescence intensity and lifetime in the absence and presence of oxygen, respectively …”

Stereoselective Polymerization of 3,6-Disubstituted N-Vinylcarbazoles

“…1 The electron-rich carbazole heterocycle, when doped, provides sufficiently stable radical cations for through-space charge transport, while the nonconjugated vinyl backbone imparts synthetic accessibility, processability, and stability. 1,2 Because of these advantageous properties, PNVC-H has proven to be a valuable hole-transport layer in organic photovoltaics, 3 perovskite interlayers, 4−6 electroluminescent materials, 7 light-emitting diodes, 8 and organic cathode materials. 9 Despite the success of PNVC-H in a variety of applications, the polymer has limited electronic tunability and is prone to decomposition at the 3,6-positions of the carbazole due to the strong withdrawing character of open-shelled species (Figure 1A).…”

“…Poly­( N -vinylcarbazole) (PNVC-H) is a photoconductive nonconjugated vinyl polymer that has found extensive use as a donor material in optoelectronics (Figure A) . The electron-rich carbazole heterocycle, when doped, provides sufficiently stable radical cations for through-space charge transport, while the nonconjugated vinyl backbone imparts synthetic accessibility, processability, and stability. , Because of these advantageous properties, PNVC-H has proven to be a valuable hole-transport layer in organic photovoltaics, perovskite interlayers, − electroluminescent materials, light-emitting diodes, and organic cathode materials . Despite the success of PNVC-H in a variety of applications, the polymer has limited electronic tunability and is prone to decomposition at the 3,6-positions of the carbazole due to the strong withdrawing character of open-shelled species (Figure A). , Derivatization of the carbazole motif at the 3,6-positions with various electronic, steric, and solubilizing functional groups would provide a route to tuning bulk optoelectronic properties and stability of PNVC-H, but the free radical polymerization used to synthesize PNVC-H industrially is not tolerant to many potential derivatives of N -vinylcarbazole .…”