Synthesis of semiconducting model polymers and their characterization before and after cyclic electrical fatigue
Prof. Dr. M. Rehahn (Institute of Macromolecular Chemistry)
Processes that might be relevant to fatigue can proceed during operation of a device as well as during switching processes, and they may continue in off-stages as well. Moreover, fatigue happening at the various interphases and phase boundaries of a device might be different from those observed in the bulk phases, and they might be chemical or physical in nature.
Chemically driven fatigue is assumed to start predominantly from impurities, defects within the polymer chains, and chain end-groups. Also, reactive species such as oxygen (from the ITO anode) or metals (from anode or cathode) can migrate into the functional organic layer(s) and cause destructive chemical reactions there.
Simultaneously, physical fatigue has to be ad-dressed. For example, frozen non-equilibrium states (such as chain orientation, caused by the spin-coating procedure applied for device preparation) might be mobilized – and thus enabled to relax – at the elevated temperatures of the operating device. Inhomogeneities of the layer thickness may be the result. This again causes spatially modulated charge-carrier transfer through the OLED, and thus potentially a local overstressing of the sensitive organic material. Similar effects might be the result of gel fractions, phase separation and crystallization proc-esses.
The effective relevance of all possible processes to the phenomenon of electrical fatigue of OLED devices has to be analyzed in detail. Based on the created knowledge, strategies have to be deviated, which minimize the OLED’s susceptibility to fatigue. Appropriate measures might be, for example, improved molecular design of the involved polymer chains, modified preparation strategies and/or purification procedures, and adding of functional additives.
Last but not least, the most appropriate device architecture has to be identified for each functional material under investigation. All mentioned goals are pursued in close cooperation with other SFB sub-projects, in particular with projects A3, D3, D4, C2, and C5.