Triplet Harvesting with a Simple Aromatic Carbonyl

The efficiency of organic light-emitting diodes crucially depends on triplet harvesters. These accept energy from triplet correlated electron hole pairs and convert it into light. Here, experimental evidence is given that simple aromatic carbonyls, such as thioxanthone, could serve this purpose. In these compounds, the emissive 1ππ* excitation may rapidly equilibrate with an upper triplet state (3nπ*). This equilibrium may persist for nanoseconds. Population of the 3nπ* state via energy transfer from an electron hole pair should result in fluorescence emission and thereby triplet harvesting. To demonstrate the effect, solutions of 1,4-dichlorobenzene (triplet sensitizer) and thioxanthone (harvester) were excited at 266 nm with a nanosecond laser. The emission decay reveals a 100 ns decay absent in the thioxanthone only sample. This matches predictions for an energy transfer limited by diffusion and gives clear evidence that thioxanthone can convert triplet excitations into light

Pyroelectric luminescence via internal Poole-Frenkel effect

Pyroelectric luminescence is a universal property of pyroelectric materials under changing temperature. In all compounds studied, inorganic and organic, it has two possible manifestations: Short-emission bursts caused by discharge of the pyroelectric field through the surrounding atmosphere and a smooth luminescence slowly varying with temperature. This latter emission, characteristic for the specific material, still waits for an explanation. With \chem{Sn_2P_2S_6} as a sample pyroelectric we show that this luminescence arises from a release of trapped charge carriers, triggered by the pyrolectric field via the Poole-Frenkel effect

Growth conditions influence on thermally stimulated luminescence of sapphire single crystals

The results of investigation of thermally stimulated and photoluminescence as well as absorption spectra of a-sapphire grown by a technique of horizontally directed crystallization in a protective gas medium of varying composition and pressure are presented. This technique has an advantage of minimization of W and Mo erosion, thus providing a considerably reduced prime cost. At the same time, there are some problems connected with emergence of considerable number of anion vacancies, whose in-axis concentration gradient is directed oppositely to that of dopants (mainly Ti). The correlations are found between the conditions of -sapphire growing and the data of absorption and luminescent spectroscopy. An interaction between the anion vacancies and Ti dopants, which induces a transition from electric-charge state Ti⁴+ to Ti³+, is discussed