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Mechanism of Carrier Photoexcitation in Semiconducting Polymers: The Role of Electron Photoemission in “Photoconductivity” Measurements

Published online by Cambridge University Press:  21 March 2011

Daniel Moses*
Institute for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, CA 93106, USA
Paulo B. Miranda
Departamento de Física, Universidade Estadual Paulista, Caixa Postal 473, Bauru - SP 17015-970, Brazil
Cesare Soci
Current Address INFM – Phys. Deptment, University of Pavia, Italy
Alan J. Heeger
Institute for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, CA 93106, USA
#Corresponding author, E-mail:
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Ultrafast photoinduced absorption by infrared-active vibrational modes (IRAV) is used to detect charged photo-excitations (polarons) in solid films of conjugated luminescent polymers. Experiments, carried out in zero applied electric field, show that polarons are generated within 100 fs with quantum efficiencies of approximately 10%. The ultrafast photoinduced IRAV Absorption, the weak pump-wavelength dependence, and the linear dependence of charge density on pump intensity indicate that both charged polarons and neutral excitons are independently generated even at the earliest times. Measurements of the excitation profile of the transient and steady-state photoconductivity of poly(phenylene vinylene) and its soluble derivatives over a wide spectral range up to hν = 6.2 eV indicate an apparent increase in the “photoconductivity” at hν > 3-4 eV that arises from external currents generated by electron photoemission (PE). After quenching the PE by addition of CO2+SF6 (90%:10%) into the sample chamber, the bulk photoconductivity is nearly independent of photon energy in all polymers studied, in a good agreement with the IRAV spectra. The single threshold for photoconductivity is spectrally close to the onset of π-π* absorption, behavior that is inconsistent with a large exciton binding energy.

Research Article
Copyright © Materials Research Society 2001

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