Monitoring the Charge Accumulation Process in Polymeric Field-Effect Transistors via in Situ Sum Frequency Generation

Abstract

Vibrational sum frequency generation (VSFG) spectroscopy is used to probe the polymer−silica interface of poly(3-hexylthiophene) (P3HT) organic field-effect transistors (oFETs) in situ during device operation. The VSFG spectra exhibit dramatic changes upon charge accumulation at the buried interface. Proper modeling of the data reveals that the changes in the spectroscopic features are almost exclusively due to changes in the amplitude and relative phase of the nonresonant signal, while the P3HT alkyl CH3 and CH2 vibrations remain unperturbed. We interpret the spectroscopic data in light of vibrationally resonant, electronically resonant, and electric field dependent enhancements that occur upon oxidative doping of the P3HT, as measured by visible to mid-IR spectroelectrochemistry. Notably, we observe electric field enhancement of the VSFG signals at both positive and negative gate biases despite unipolar current−voltage responses, which we attribute to the trapping of electrons at the dielectric interface.

Probing buried organic layers in organic light-emitting diodes under operation by electric-field-induced doubly resonant sum-frequency generation spectroscopy

“…17 In previous EFI-SFG spectroscopy of OFETs, only the background intensity was enhanced. 15 In contrast, the SFG signals derived from Alq 3 were significantly enhanced in our SFG results. This observation must be due to the doubleresonance effect, because the output SFG wavelength (in this work, x SFG ¼ x VIS þ x IR % 430 nm) is at the absorption edge of Alq 3 .…”

“…9-11 IR-visible SFG spectroscopy has also been applied to OFETs to probe the molecular structural changes and charge accumulation processes caused by injected charges. [12][13][14][15][16] In this letter, multilayer OLED devices were investigated by EFI-DR-SFG. Buried layers were probed by applying a bias voltage to multilayer OLED devices during measurements of the SFG under the doubly resonant condition.…”

“…This observation must be due to the doubleresonance effect, because the output SFG wavelength (in this work, x SFG ¼ x VIS þ x IR % 430 nm) is at the absorption edge of Alq 3 . When a static electric field E 0 is applied to the system, the SFG signal intensity (I SFG ) is given by [7][8][9][10][11][12][13][14][15][16] …”

Experimental evidence for an optical interference model for vibrational sum frequency generation on multilayer organic thin film systems. I. Electric dipole approximation

“…69 Briefly, the regeneratively amplified system (regen) outputs 40 fs (FWHM) pulses centered around 800 nm with 30 nm of bandwidth at 1 kHz for 1.7 W of power before compression. About 300 mW of this is spectrally filtered to 3 nm of bandwidth and used to seed a home built multipass amplifier followed by further spectral narrowing in a 4f pulse shaper for use as the visible pulse in the VSFG experiment.…”

The Maxwell-Wagner model for charge transport in ambipolar organic field-effect transistors: The role of zero-potential position

“…Hence, the direct observation of the potential profile is crucial for deep understanding. It was proposed many methods capable to study the potential, electric field, or carrier profiles across the channel, such as Kelvin probe force microcopy, [9][10][11][12] electric-field induced second-harmonic generation 13 (EFISHG), sum frequency generation, 14 chargeinduced elastic modulation interferometry, 15 Stark spectroscopy, 16 and field and charge modulation spectroscopy. 17,18 However, the potential distribution in the channel region of ambipolar OFET has not been sufficiently analyzed.…”