Operando sum‐frequency generation spectroscopy of high‐efficiency OLEDs for probing the charge carrier behavior

Abstract

Operando analysis of organic devices is crucial for understanding the device operation mechanism and intrinsic degradation mechanism. Buried organic layers in OLEDs during operation have been investigated using electric-field-induced doubly resonant sum-frequency generation (EFI-DR-SFG) and time-resolved EFI-DR-SFG experiments. The time-resolved EFI-SFG technique allows us to observe the carrier transportation directly and non-destructively. DC electric field leads to a significant increase in the SFG intensity produced by the charged carriers. Electric-field-induced SFG results clearly show that charge accumulation occurs at the interface between the light emitting layer and the electron-blocking layer in the blue OLED and that this charge accumulation strongly contributes for the light emission. Particularly in the TTA process, high charge accumulation is effective in generating triplet excitons and improving the luminescence efficiency, but at the same time, it reduces the lifetime of the device.

Charge Transfer Dynamics in Organic–Inorganic Hybrid Heterostructures—Insights by Vibrational‐Sum Frequency Generation Spectroscopy

“…This additional third-order contribution to the signal is often termed as electric-field induced VSFG or EFI-VSFG. [46] This process is comparable to the electric field-induced effects seen in second harmonic generation. [47] Nakai et al exploited such local field enhancement to obtain an increased VSFG response of polyvinyl phenol (PVP) in a pentacene/PVP/ SiO 2 interface, which reflected the spatial distribution of charge density at the interface.…”

Ladungstransferdynamiken in organisch–anorganischen hybriden Heterostrukturen – Einblicke mithilfe von Summenfrequenzspektroskopie

“…Dieser zusätzliche Beitrag dritter Ordnung wird häufig als "electric-field induced" SFS oder EFI-SFS bezeichnet. [46] Dieser Prozess ist vergleichbar mit den durch ein elektrisches Feld induzierten Effekten, die bei der Erzeugung der zweiten Harmonischen beobachtet werden. [47] Nakai et al nutzten eine solche lokale Feldverstärkung, um ein verstärktes SFS-Signal von Polyvinylphenol (PVP) an einer Pentacen/PVP/SiO 2 -Grenzfläche zu erhalten, die die räumliche Verteilung der Ladungsdichte an der Grenzfläche widerspiegelt.…”

Electrostatic charges and their distribution on the charged surfaces probed by sum-frequency generation spectroscopy

“…SFG is a vibrational spectroscopic technique that can also observe the electric-field-induced effects caused by the charges on the surfaces and interfaces because SFG spectroscopy is based on the second-order nonlinear optical process as in the SHG. [14][15][16][17] In contrast to the case of SHG, SFG has the significant advantage of identifying materials by their vibration modes.…”

“…When an electrostatic field E₀ is applied to a sample within the measurement range, the intensity of SFG light is shown by the following Eq. (1) [14][15][16][17]…”

“…Thus, the additional χ (3) E 0 term induces the increase in the intensity of the SFG signal, which is known as the electric-field-induced effects. 16) χ (2) is inherently surface and/or interface sensitive, while χ (3) is third-order contribution and thus bulk sensitive. 19,20) The intensity of the SFG output is given by the following equation:…”