We report on the characteristics of a host-guest lasing system obtained by co-evaporation of an oligo(9,9-diarylfluorene)
derivative named T3 with the red-emitter
4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dye
(DCM). We demonstrate that the ambipolar semiconductor T3 can be implemented as active matrix in the realization of a
host-guest system in which an efficient energy transfer takes place from T3 matrix to the lasing DCM molecules. We
performed a spectroscopic study on the system by systematically varying the DCM concentration in the T3 matrix.
Measurements of steady-state photoluminescence (PL), PL quantum yield (PLQY) and amplified spontaneous emission
(ASE) threshold are used to optimize the acceptor concentration at which the ASE from DCM molecules takes place
with the lowest threshold.
Organic light-emitting diodes (OLEDs) implementing the DCM:T3
host-guest system as recombination layer are
fabricated for verifying the optical properties of the optimised blend in real working devices.
Indeed, the very low ASE threshold of T3:DCM makes the investigated blend an appealing system for use as active layer
in lasing devices. In particular, the ambipolar charge transport properties of the T3 matrix and its field-effect
characteristics make the host-guest system presented here an ideal candidate for the realization of electrically-pumped
organic lasers.
Organic light emitting transistors (OLET) have been recently demonstrated as innovative architectures combining into an integrated optoelectronic device multi-functional properties namely, the driving ability of a transistor device with the conversion of electrical current into efficient light emission via electronic relaxation of organic semiconductors. State of the art materials, achievement and performances will be presented and discussed in view of possible OLET architectures exploitation as electrically pumped organic nanolasers.
The dynamics of triplet recombination in fluorene trimers have been studied using steady state photoinduced absorption
(PA) spectroscopy. We investigated two type of oligomeric films, deposited by different techniques: thermal evaporation
and spincoating. The different molecular arrangement in both films is manifested in a red-shift of the absorption, PL and
T1-Tn triplet PA spectra of the sublimated film relative to the spincoated one. Triplet recombination dynamics follow a
dispersive bimolecular recombination model away from the trap filling regime.
Moreover we report on the characteristics of a host-guest lasing system obtained by co-evaporation of the most
promising oligofluorene derivative (T3) with the red-emitter
4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dye (DCM). The blend satisfies the necessary condition for an efficient Förster energy transfer to take place
from T3 matrix to DCM molecules.
We report on the influence of the dielectric/organic interface properties on the electrical characteristics of field-effect
transistors based on Poly-phenylenevinylene derivatives. We observe a direct influence of the dielectric surface on the
field-effect mobility as well as on the charge injection at the source electrode, despite the fact that we used a top contact
transistor structure.
We find that the presence of traps at the dielectric surface, decreases the hole mobility and increases the threshold
voltages. By treating the silicon dioxide dielectric surface with gas phase molecules such as octadecyltrichlorosilane
(OTS) and hexamethyldisilazane (HMDS) the hole mobility improves and the threshold voltage slightly increases.
The effects of a dielectric polymer layer spin coated onto silicon dioxide substrates before deposition of the
semiconductor polymer can be related to the density of the oxydryl groups (-OH ), which are the most efficient traps for the charges flowing in the device. We use different polymer species such as polyvinylalchol (PVA),
polymethylmetacrilate (PMMA) and a cyclotene derivative (B-staged bisbenzocyclobutene or BCB). The elimination of
the -OH groups and of other traps, produces the same effect observed with HMDS coupled to a more pronounced
enhancement of the threshold voltage, with the exception of PMMA. The electrical characteristics obtained with HMDS
and PMMA polymer dielectrics are the highest reported to date for PPV-based field-effect transistors.
We confirm that the purification of the active material is crucial to enhance the device performances and to achieve a
better device to device reproducibility.
We also investigated the effect of the dispersion of a phosphorescent dye into the active polymeric material. The
electrical characteristics of OFETs with HMDS or PMMA dielectric with and without dye doping are compared.
Organic light-emitting field-effect transistors are a new class of electrooptical devices that could
provide a novel architecture to address open questions concerning fundamental optoelectronic
phenomena in organic semiconductors, and can be potentially used as key components in optical
communication systems, advanced display technology, solid-state lighting and organic lasers.
The realisation of Organic Light-Emitting Transistors (OLETs) with high quantum efficiency and
fast switching time is crucial for the development of highly integrated organic optoelectronic
systems. Organic molecular materials having intrinsically ambipolar transport and high charge
mobility values are restricted in number and show poor light-emission efficiency. Here, we describe
the device operation principles of OLETs and report on the approach of combining p-type and n-type
molecular materials in a layered structure to achieve ambipolar transport and light emission. Imaging
of the individual layers and a correlation between active layer structure and device electrical
performances is achieved by means of the Laser Scanning Confocal Microscopy.
The phoenix project aims to develop all-optical switches based on
the combination of inorganic and organic materials in hybrid
devices. We present first results in developing low-loss ring
resonators fabricated in silicon-on-insulator (SOI) technology,
with Q-factors as high as 125.000, and losses of α≈3.5dB/cm in the ring.
Organic light-emitting diodes (OLED) are rapidly reaching large-scale marketing figures, driven by attractive features like low cost and fast response, being also suitable for application on flexible substrates. All these aspects enable a wide range of applications such as displays, innovative devices in optoelectronics and novel light sources. Furthermore, the benefits expected from OLEDs based devices, if compared to "classical semiconductors" based devices consist of low production costs, lightweight and geometrical flexibility. Novel OLEDs based light sources fulfilling the above-mentioned requirements, call for a considerable effort both in the production processes and in product innovation. Among the variety of possible applicative OLED applications, we focused our research effort on the Automotive sector. Our envisioned approach enabling control of light distribution from an OLED light source include modeling and patterning of the light source, design and fabrication of suitable micro-optics coupled to the flexible transparent Organic Light Emitting Diode (OLED) substrate.
We report measurements on a multilayer of (alpha) - sexithienyl (T6) and C60 grown in ultra-high vacuum. The photoluminence spectrum of the multilayer is made up to T6 emission at high energies, and C60 emission at low energies. The T6 emission has more structure than that of bulk T6, even after taking into account reabsorption by the C60. However, the spectrum can be simulated quite well by assuming that there is both emission from bulk T6 and isolated molecules of T6, based on the vibronic structure that we would expect from isolated T6 molecules. Molecules of T6 may conceivably become isolated at the C60 interface, if they migrate into gaps between C60 grains, for example. Thus we cannot conclude that any of the new peaks we see are due to charge transfer between the C60 and T6. The low energy fluorescence is that of C60, but its efficiency is orders of magnitude larger than is seen in films of C60. We cannot explain this yet.
The nature of the optical excitations at the dipole forbidden optical gap of crystalline C60 is investigated by linear and non-linear spectroscopic techniques. The resonances observed both in the absorption and emission spectra are identified with Herzberg-Teller vibronic coupling of the electronic states to intramolecular vibrational modes. The highly structured fluorescence spectra of both C60 single crystals and crystalline C60 films are interpreted in terms of Frenkel exciton emission from X-traps.
Two-photon excitation measurements of C60 single crystal at 4 K have been performed. The TPE spectrum shows a sharp band at 1.846 eV which is assigned to the C60 lowest forbidden Frenkel singlet exciton of T1g symmetry. This assignment is supported by the analysis of Herzberg-Teller induced photoluminescence.
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