Diffusion in Organic Film Stacks Containing Solution-Processed Phosphorescent Poly(dendrimer) Dopants

Abstract

Efficient organic light-emitting diodes (OLEDs) consist of an emissive layer comprising a blend of a light-emitting and host material in contact with one or more charge transporting layers. The distribution of the active material in the guest–host emissive layer blend and the changes that may occur upon thermal annealing are two important factors in determining the stability and efficiency of OLEDs. We have combined neutron reflectometry and photoluminescence measurements to investigate the structures of films comprising an emissive layer containing a phosphorescent poly­(dendrimer) material blended with 4,4′-N,N′-di­(carbazolyl)­biphenyl. This combination has been shown to give rise to highly efficient OLEDs. Here, we show that the emissive poly­(dendrimer) material was not uniformly distributed in the host, but formed a concentration gradient within the emissive layer. Upon heating, the adjacent electron transport layer was found to intermix with the emissive layer, accompanied by changes in the material distribution in the emissive layer. The intermixing of the materials led to a decrease in the photoluminescence from the poly­(dendrimer) within the film. The decrease in the photoluminescence was ascribed to an increase in interchromophore interactions that could arise from a conformational change of the poly­(dendrimer) or phase separation leading to aggregation. The results indicate that, while uniform mixing of the guest and host is not essential for efficiency, the thermal stabilities of both host and charge transport materials are important for device durability.

Probing impact of interface mixing on the charge carrier dynamics of a solution-processed organic light emitting diode via impedance spectroscopy

“…In particular, NR offered a quantitative evaluation of the composition of O-O interfaces in angstrom-ordered resolution and revealed that the onset temperature of thermal diffusion at O-O corresponds to the glass transition temperature (T g ) of the materials. [14][15][16][17] Based on such depth profile measurements, several studies asserted that the intermixing at the HTL/EML interface improved current injection or suppressed triplet polaron quenching and thereby enhanced the device performance. 11,14 However, those performance enhancements might predominantly be due to the thermally modified morphologies of organic layers rather than interface mixing.…”