Angelo Pace

Emission Intensity and Degradation Processes of Alq3 Films

Emission Intensity and Degradation Processes of Alq3 Films Giuseppe Baldacchini,* Tommaso Baldacchini, Angelo Pace, and Ramchandra Balaji Pode ENEA, Frascati Research Center, Department of Advanced Physical Technologies, 00044 Frascati, Roma, Italy Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA Department of Physics, Nagpur University, Nagpur 440 010, India

Improvement of environmental stability of aluminum tris(8-hydroxyquinoline) thin films

Abstract It is argued that some organic materials play an important role in preventing the formation of nonemissive species and enhancing the intrinsic stability of Alq3 thin films. In the present work, films (Alq3 pure, Alq3+ORGMAT (top), and Alq3 doped with same) were grown on glass and silica substrates by thermal evaporation. Excitation, fluorescence, and transmission spectra have been measured, and the intensity of the green photoluminescence as a function of time has been monitored. Preliminary results show an improved stability of the emission in the doped Alq3 films compared with the undoped ones.

Effects of Chemical and Physical Agents on the Emission Properties of Alq3 Films

Tris(8-hydroxyquinoline)aluminum (Alq 3 )-based organic light emitting diodes are highly susceptible to environmental conditions which severely limit their lifetime. In particular, moisture and oxygen play an important role in the degradation processes. In order to study those effects more deeply, annealing processes of Alq 3 films have been performed in various atmospheres, and they have confirmed the physical and chemical destructive actions of temperature, oxygen, and water, the last one more effective than the previous two agents. However, also a sizeable increase of the photoluminescence has been discovered by annealing in humidified atmospheres, especially air. Due to the complexity of the matter, it is not yet clear whether this important effect can be ascribed to the temperature alone or to the combined action of temperature and water, but almost certainly it is related to the phase transitions of the molecular material itself.

Photoluminescence and Morphology of Alq3 Films and Four-Components Model

Recently, it has been shown that the photoluminescence (PL) of one Alq 3 film decays in air with four different constant times and spectral features, implying the existence of four different components of molecular aggregation. Now the same behavior has been confirmed by measuring in open and dry air the PL of films fabricated in different physical conditions, which influence to various degrees the amplitude of the components. Moreover, the constant times are longer in Alq 3 films kept in the external dry environment than in the normal atmosphere. The implications of these experimental observations for the morphology of the films is discussed in the light of a phenomenological approach which has been called the four-components model.

Role of Humid Air Annealing on Emission Stability of Alq3

Recent annealing experiments have shown that Alq3 films are highly susceptible to various atmospheres, going to their destruction in particular environments well below 200°C. Moreover, before this abrupt transition a sizable increase in photoluminescence has been observed especially by annealing in humid air. This property, which hints at a new phase of the material with some analogies to the recently discovered δ-phase except for a much lower temperature, displays a remarkable stability in ambient air, a result with important implications both for the development of environmentally stable organic light-emitting diodes devices and for basic understanding of the morphological structure of Alq 3 systems.

Photoluminescence of Alq3 Stabilized by a Phenolic Compound

Over the last 10 years, several organic compounds have been used to realize efficient electroluminescent devices, organic light-emitting diodes (OLED), which have now become competitive with the well-known and older inorganic semiconductor diodes, LED. Among these new luminescent compounds, the metallorganic molecule Alq 3 has an efficient emission band in the green region of the electromagnetic spectrum, and is easily handled. However, OLEDs realized with Alq 3 and other organic compounds display a lifetime defined as the time required by the emission to reduce by half its initial value, equal to about 5000 h, which is considered to be too short for practical applications. Several studies have been performed on organic compounds to understand their degradation, which has been ascribed to both intrinsic and extrinsic effects; among the latter, exposition to atmospheric agents is considered to be very important. Simple experiments have been made to verify the oxidation processes and the possibility to halt or retard them by using appropriate chemical compounds. Butylated hydroxytoluene, a molecule belonging to the big family of phenols, proved to be effective in preserving the photoluminescence of Alq 3 films. Our results support the idea that phenols, which are known to be strong antioxidant products, can increase the lifetime of OLEDs realized with small organic molecules and polymers.

Leaky modes in lithium fluoride thick films thermally evaporated on glass

Polycrystalline thick lithium fluoride (LiF) films have been grown by thermal evaporation on heated glass substrates. Excellent agreement between the values obtained from effective index measurements carried out by the m-line technique and those provided by computer simulations has shown that leaky modes are supported by these dielectric films, as they have a gradually increasing refractive index distribution along the thickness moving from the substrate interface towards the air interface. In some cases the films present also optical anisotropy which results in TM modes having higher effective indices than TE ones.

Electron beam lithography on LiF films for integrated active optical waveguides

Low energy electron beam irradiation of LiF single crystals and polycrystalline films induces efficient formation of stable laser active defects emitting in the visible spectral range at room temperature, together with a consistent increase of the real part of the refractive index in the same wavelength interval. The use of electron lithography techniques look promising for the realization of active channel waveguides.

Point Light Sources Based on Color Centers in LiF Films for Scanning Near‐Field Optical Microscopy

Local fluorescent probes based on low‐energy electron beam irradiated lithium fluoride (LiF) thin films were preparated and tested for their applications in Scanning Near‐Field Optical Microscopy.