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Photogeneration of Charge Carriers in (Phenyl-C61-butyric Acid Methyl Ester) Mixed with a Small Amount of Polymers

Jung, Jaroslaw, Stefaniuk-Grams, Anna, Ulanski, Jacek
The Journal of Physical Chemistry C 2017 v.121 no.38 pp. 20650-20661
electric field, electrons, energy, fullerene, models, photons, physical chemistry, polymers
Photogeneration of charge carriers in closely packed phenyl-C61-butyric acid methyl ester (PCBM) was investigated by means of the xerographic discharge technique. It was found that the position of the center of mass of the thermalized electron is positioned at a distance that is integral to the multiplicity of the center-to-center distance between the adjacent PCBM molecules. For a low electric field, 10⁴–10⁷ V/m, the most photogenerated electrons originate from the electrons thermalized at the molecule next but one to the geminate recombination center. For the threshold electric field (ca. 2 × 10⁸ V/m) the expected value of the thermalization length, ca. 1.9 nm, is independent of the assumed distribution functions of thermalization distance of the electron–hole pairs. The Coulomb binding energy for this characteristic of PCBM electron–hole separation length is correlated with the threshold energy value separating the extrinsic and intrinsic photogeneration in fullerenes. The model predicts the observed experimentally threshold value of photon energy 2.25 eV, below which the photogeneration yield in fullerene-based materials decreases abruptly; then, taking into account the average electron–hole pair Coulomb binding energy 0.15 eV, one can calculate the electric gap energy 2.4 eV, consistent with the literature data. It explains also how the presence of poly(3-hexylthiophene) influences the photogeneration process.