Interaction of UV light with amorphous small-molecule organic thin films

There is an ongoing interest in organic materials due to their application in various organic electronic devices. However stability of organic materials limits their potential use. They are prone to degradation both during the working life and storage. One of the main causes is extrinsic degradation, under the influence of oxygen and moisture. This problem can be solved by encapsulation of devices. However no encapsulation is perfect.

We study interaction of thin films of small-molecule organic blue-emitter materials, such as N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)benzidine (TPD) and 4,4′-bis(2,2-diphenylvinyl)-1,1′-biphenyl (DPVBi), with UV light. Films are stable in vacuum, but readily degrade in the presence of oxygen. Thus, the necessary condition for interaction (degradation) is the simultaneous presence of UV light and oxygen. These impurities are responsible for increased morphological stability of irradiated films and quenching of photoluminescence (PL).

In general, there are two possible pathways for chemical reaction of films and oxygen: (1) excited singlet molecule gives an electron to ground-state oxygen molecule to form radical cation and the superoxide anion, which can further react chemically to form new species; (2) host molecule in excited triplet state acts as a sensitizer and transfers its energy to ground-state triplet oxygen to form singlet oxygen and ground-state host molecule. The energy needed for singlet oxygen formation is 0.97 eV. Singlet oxygen is very reactive and may further interact with host molecules to form photo-oxidized species.

Only small amount of impurities, less than 0.5%, causes 50% decrease of PL. This implies a non-trivial mechanism of quenching. The necessary condition for quenching is that the distance between impurities is smaller or equal to exciton diffusion length, which is fulfilled in our films.

Following mechanism of PL quenching is proposed: exciton diffuses by hopping form one host molecule to another through Förster resonant energy transfer in a random walk manner. If, during its lifetime, it comes to proximity of an impurity, a PL quenching process occurs. Even a small amount of oxygen that penetrates a blue emitter layer would impair luminescence efficiency of a device. Moreover, the absorption of its own radiation would additionally contribute to the rate of degradation of a device.

a) Photoluminescence IPL/IPL0 at 458 nm and b) absorbance A/A0 at 355 nm vs. t of 190 nm thick DPVBi films irradiated with 3mWcm-2 recorded in vacuum (10−4 Pa, open downward triangles), nitrogen at 100 kPa (open upward triangles), different oxygen pressures (solid symbols) and air at 100 kPa (solid stars). In a) top x-axis (in minutes) refers to measurements in vacuum and nitrogen (open symbols and indicated by arrow), while bottom x-axis (in seconds) refers to measurements in different oxygen pressures and air. Curves obtained in air and at oxygen pressure of 24 kPa have similar rates due to the fact that the partial pressure of oxygen in air is around 20 kPa.

Micro-rods of oxidized pentacene

Pentacene is hole-type semiconductor used in organic electronic devices, well-known for its high charge carrier mobility as high as 35 cm2V-1s-1 at room temperature. Performance of devices made from organic thin films strongly depends on their morphology and it is often limited by the presence of molecular disorder and grain boundaries, which reduce mobility of charge carriers of a material.

To improve charge transport properties thermal annealing could be used. Prolonged annealing of pentacene films leads to surprising result: nano- and micro-scale rod-shaped structures are forming on film surface. Based on scanning electron microscopy measurements, it is supposed that these structures are crystalline. Their UV-vis absorbance indicates that they are composed of more than one species of oxidized pentacene molecules, including 6,13-pentacenequinone.

Images of crystal-like structures (micro-rods and micro-crystals) formed on the surface of pentacene films obtained by a), d) optical and b), c) scanning electron microscopes.