Probing the nanoscale phase separation in binary photovoltaic blends of poly(3-hexylthiophene) and methanofullerene by energy transfer
ABSTRACT The generation of charge carriers in organic photovoltaic devices requires exciton diffusion to an interface of electron donor and acceptor materials, where charge separation occurs. We report a time resolved study of fluorescence quenching in films of poly(3-hexylthiophene) containing a range of fractions of the electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). We show that energy transfer from P3HT to PCBM helps to bring excitons to the interface, where they dissociate into charge carriers. Fluorescence quenching in blends with < or = 50 wt% of PCBM is controlled by exciton diffusion in P3HT. This allows us to estimate the average size of PCBM domains to be about 9 nm in the 1:1 blend. The implications for polymer solar cells are discussed.
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ABSTRACT: The femtosecond-resolved evolution of the emission spectrum of the important conjugated polymer poly(3-hexylthiophene) (P3HT) is presented. Detailed fluorescence up-conversion spectroscopy was performed on P3HT solid-state films and on P3HT in chlorobenzene solution. Two excitation wavelengths and several emission wavelengths, covering the entire fluorescence spectrum, were used. The data were complemented by polarization-sensitive measurements. Our global analysis allowed a reconstruction of the time-resolved emission spectra with 200 fs temporal resolution, so that spectral changes due to the early relaxation processes following π–π* interband absorption in the pristine polymer could be comprehensively characterized. Absorption occurs in isolated polymer chains in solution and in the solid state (including interchain interactions) for the film. In both cases, we find evidence of delocalization of the electrons and holes formed in the energy bands directly after photoexcitation with excess energy. This is followed by ultrafast (100 fs) self-localization of the primary photoexcitation and by relatively slow exciton formation (1 ps). Further relaxation occurs with time constants ranging from hundreds of femtoseconds to tens of picoseconds, due to exciton hopping to sites with lower energy and to a slow conformational planarization of the polymer backbone. Depolarization, a spectral red shift, and important changes in the vibronic structure are observed as a consequence of this relaxation. Finally, relaxed intrachain and interchain singlet excitons are formed in solution and film, respectively, on a 100–200 ps time scale. They decay with a 500 ps time constant, by intersystem crossing in solution and by nonradiative recombination in the film. Our results are consistent with and strongly support the conclusions we obtained from a similar time-resolved fluorescence study of the polymer PCDTBT (J. Am. Chem. Soc.2010, 132, 17459): ultrafast charge separation in polymer:fullerene blends seems to occur before localization of the primary excitation to form a bound exciton.The Journal of Physical Chemistry C 04/2011; 115(19). DOI:10.1021/jp1119348 · 4.77 Impact Factor
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ABSTRACT: Exciton quenching dynamics has been systematically studied in pristine P3HT and nano phase separated P3HT/PCBM blend films under various excitation intensities by femtosecond fluorescence up-conversion technique. The behaviors of excitons in the films can be well described by a three-dimensional diffusion model. The small diffusion length and large charge transfer radius indicate that excitons reach the interface most likely by the delocalization of the excitons in P3HT fibrillar at a range of 4.8-9 nm so that the excitons can quickly delocalize in the P3HT domain to reach the interface (instead of by diffusion).Nanoscale 05/2011; 3(5):2280-5. DOI:10.1039/c0nr01002b · 7.39 Impact Factor
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ABSTRACT: Popularize the concept: A novel piezochromic luminescence (PIEL) system has been developed from a donor-acceptor binary complex. The complex is non-fluorescent, but pressure-induced phase separation leads to a crystalline fluorescent donor. This quenching-recovering mechanism ensures a high contrast ratio and popularizes the concept of PIEL.Chemistry - A European Journal 09/2011; 17(38):10515-9. DOI:10.1002/chem.201101284 · 5.73 Impact Factor