Dresden 2014 – scientific programme
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HL: Fachverband Halbleiterphysik
HL 28: Organic electronics and photovoltaics I (organized by DS)
HL 28.3: Talk
Monday, March 31, 2014, 18:30–18:45, CHE 91
Temperature dependent exciton diffusion length in ZnPc — •Bernhard Siegmund1, Johannes Widmer1, Simone Hofmann1, Moritz Riede2, and Karl Leo1 — 1Institut für Angewandte Photophysik, Dresden, Germany — 2Current address: Clarendon Laboratory, Oxford, England
The photo-current of organic solar cells is the result of a multi-step process. It includes the generation and diffusion of excitons as well as their separation into free charge carriers, the transport to the electrodes, and their final extraction. One bottleneck for highly efficient devices is the short exciton diffusion length in organic materials.
In this work, the singlet exciton diffusion length ℓdiff in the absorber material ZnPc is studied. For this purpose, the photo-current of organic solar cells, incorporating ZnPc and C60 in a flat heterojunction architecture, is measured and modelled as a multi-step process. ℓdiff is extracted from a thickness variation of the absorber layer, as not yet encountered in the context of modelling the photo-current to determine ℓdiff before. Measurements at varying temperature between 200 K and 370 K reveal a thermal activation of the diffusion length above 310 K. This is interpreted as promotion of the excitons to higher energies with a density of states allowing for enhanced hopping transport. The activation energy is considered as a measure for the energetic disorder of the excitonic states. These investigations aim for a better understanding of exciton migration in order to design materials with longer exciton diffusion lengths for highly efficient organic solar cells.