Luminescence quenching in the presence of polarons is one of the major challenges in organic light emitting devices. In this work, exciton quenching in the presence of polarons is studied using phase sensitive photocurrent measurements on pentacene field effect transistors. The enhancement of conduction in the organic field effect transistors on light illumination is studied using photocurrent spectral response measurements and corresponding optical simulations. The photocurrent is shown to be governed by the polaron mobility and the exciton quenching efficiency, both of which depend on the polaron density in the channel. Two models are proposed on the exciton dynamics in the presence of gate induced polarons in the transistor channel. The first model simulates the steady-state exciton concentration profile in the presence of exciton-polaron interaction. The second one is a three-dimensional steady state exciton-polaron interaction model, which supports the findings from the first model. It is shown that the excitons quench by transferring its energy to polarons, thereby promoting the latter to high energy states in the density of states manifold. The polarons move in the higher energy states with greater microscopic mobility before thermalizing, thereby leading to an enhancement of conduction. It is observed that for the present system, where charge carrier transport is by hopping, all polarons interact with excitons. This implies that for low mobility systems, the interaction is not limited to deep trapped polarons.
The combined effect of deposition rate and substrate temperature on the film crystallinity, morphology, and electronic properties of pentacene is studied. It is shown that the channel mobility in polycrystalline pentacene thin-film transistors is relatively immune to substrate temperature, and the films offer good hole mobility when grown at a high rate. This is advantageous when high throughput with low deviation in electrical parameters over devices are required. The surface morphology is characterized by atomic force microscopy measurements and the crystallinity is studied using x-ray diffraction. The effect of growth parameters on the crystalline phases of pentacene is correlated to the charge carrier transport. It is found that the field-effect mobility is primarily affected by the crystalline phases of the film rather than the grain size. The charge carrier dependence of the hole mobility is used to parameterize the dispersion (width) in the density of states (DOS) of the highest occupied molecular orbital of the films in the transistor channel region. It is found that the presence of multiple phases in the path of the charge carrier flow increases the dispersion of the DOS.
This article adds a new direction
to the functional capability
of protein-protected atomically precise gold clusters as sensors.
Counting on the extensively researched intense luminescence of these
clusters and considering the electron donating nature of select amino
acids, we introduce a dual probe sensor capable of sensing changes
in luminescence and conductivity, utilizing bovine serum albumin-protected
atomically precise gold clusters hosted on nanofibers. To this end,
we have also developed a hybrid nanofiber with a conducting core with
a porous dielectric shell. We show that clusters in combination with
nanofibers offer a highly selective and sensitive platform for the
detection of trace quantities of trinitrotoluene, both in solution
and in the vapor phase. In the solution phase, trinitrotoluene (TNT)
can be detected down to 1 ppt at room temperature, whereas in vapor
phase, 4.8 × 109 molecules of TNT can be sensed using
a 1 mm fiber. Although the development in electrospinning techniques
for fabricating nanofibers as sensors is quite substantial, a hybrid
fiber with the dual properties of conductivity and luminescence has
not been reported yet.
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