Regensburg 2013 – wissenschaftliches Programm
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BP: Fachverband Biologische Physik
BP 4: Symposium Charge Transfer Effects in Molecular Materials (SYCT, joint with CPP, HL and DS)
BP 4.2: Hauptvortrag
Montag, 11. März 2013, 10:00–10:30, H1
Identifying and resolving charge separation in organic solar cells — •Eberhard Riedle — BioMolekulare Optik, LMU München
The charge separation and the charge mobility are essential factors for efficient solar devices. Time resolved spectroscopy can resolve these processes completely. We use sub-50 fs resolution, fully tunable pump pulses, and continuum probing from 300 to 1700 nm. Since solar cells are intended to absorb visible light, particularly probing in the NIR allows to differentiate between the excitonic and polaron states. We show that the primary excitation in P3HT-Si hybrid cells are excitons located on the polymer. They dissociate to polarons in 140 fs. A significant part of the excitation is lost at this early stage due to internal conversion back to the ground state [1]. Using the knowledge from the transient measurements, we optimize the composition and preparation of the cells to an efficiency of 1.1 % [2]. The character and mobility of the polarons is determined by excitation fluence dependent measurements. In a series of polymers - mixed with C60 - with increasing conjugation length, the highly structured transient spectra do not allow the easy identification of polarons. The analysis shows that the spectrum is closely related to the electroabsorption spectrum of the polymer, i.e. the derivative of the absorption spectrum. In this way we can conclude that indeed charge separation and an electric field is generated in the thin film solar cells by the visible excitation.
[1] D. Herrmann et al., J. Am. Chem. Soc. 133, 18220 (2011).
[2] S. Niesar et al., Green 1, 339 (2011).