Optical absorption spectrum of pentacene cation radicals measured by charge-modulation spectroscopy

Abstract

Charge-modulation spectroscopy was applied to vacuum-sublimed films of pentacene in the metal-insulator-semiconductor configuration with Parylene-C or cross-linked polymer as insulator. The spectrum exhibited several peaks in the wavelength range of 300-800 nm. The signal in the visible was ascribed to the field-modulation effect of a neutral pentacene film, while the three broad peaks in the UV exhibited field and frequency dependences different from those in the visible and were ascribed to the optical absorption due to a monocation radical of pentacene. This assignment was supported by the calculation reported earlier on the basis of the time-dependent density functional theory.

Direct probing of selective electron and hole accumulation processes along the channel of an ambipolar double-layer field-effect transistor by optical modulation spectroscopy

“…The modulations at those energies exhibited a linear dependence on V GS (not shown). 20,21 Nevertheless, under the electron injection condition (V GS > 0 V), no significant modulations could be observed in the measurement optical region, which was quite different from Haas' results. In other words, under the hole injection condition, similar modulated spectra and voltage dependence were observed in both experiments; while under the electron injection condition, different results were shown.…”

“…In other words, the transmittance changes of the active layers were measured, which was supported by the similarities between our results and the results of other groups. [20][21][22][23] Figure 2(a) shows the modulation spectra of a unipolar pentacene OFET in the channel region under various V GS . Under the hole accumulation condition (V GS < 0 V), three large positive peaks were observed at 1.86, 1.98, and 2.15 eV, respectively.…”

“…On the basis of the transfer I-V characteristics, the modulations under the hole (V GS < 0 V) and electron (V GS > 0 V) injection conditions were attributed to the charge and field modulated effects, respectively, by Haas et al 20 Nevertheless, it was suggested by Harima et al that the charge modulation of the pentacene cation was in the near infrared (NIR) region, and the modulation in the visible region should be attributed to the quadratic Stark effect. 21 Here, it should be noted that although Haas et al used an FET structure, only the area beneath the source electrode was probed, which was similar to a metal-insulator-semiconductor (MIS) structure. On the other hand, in our experiments, the channel region was probed, which should be much simpler to analyze in that the carrier behaviour has been well studied along the channel.…”

Microscopic hole-transfer efficiency in organic thin-film transistors studied with charge-modulation spectroscopy

“…Charge-modulation spectroscopy (CMS) is another spectroscopic means to gain information of intrinsic carrier mobility of OSC in TFTs under working conditions. In CMS, the absorption spectra of injected carriers are recorded by applying an alternating voltage to the gate electrode of a device; the charge density modulations induced by the alternating voltage allow us to detect carriers with high sensitivity [18][19][20][21][22][23][24][25][26]. Trapped charges that are not affected by the alternating voltage do not contribute to CM signals.…”

Spatially Mapping Charge Carrier Density and Defects in Organic Electronics Using Modulation‐Amplified Reflectance Spectroscopy

“…Recent charge modulated spectroscopy studies have tracked electronic interactions and carrier formation in conjugated organic molecules by probing the optical changes that occur when a charge‐carrying species is formed . A handful of complementary experiments have shown that these and similar tools can provide spatial mapping of electronic and charge‐related phenomena .…”