Investigating the origin of the high photoconductivity of rubrene single crystals

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Hikmat Najafov, Byunggook Lyu, and Ivan Biaggio
Department of Physics, Lehigh University, Bethlehem, Pennsylvania 18015, USA

Vitaly Podzorov
Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA

Received 29 May 2007; revised 11 December 2007; published 6 March 2008

Therubrene molecular crystal has the unique property of showing astrong photoconductivity for light wavelengths that are close to theabsorption edge. We studied the microsecond dynamics of the photoconductivityinduced by short light pulses to characterize the way inwhich photoinduced excitons efficiently ionize to produce free charge carriers.We found that the photoconductivity is produced by carriers releasedafter an average time of 100 µs from a “reservoir state”that originates from the photoexcited molecular excitons. The conversion ofphotoexcited excitons into this reservoir state happens only at thesurface of the crystal within a depth of the orderof a few micrometers, but in this region close tothe surface, a photoexcited molecular exciton has a probability ofthe order of unity to ultimately lead to a mobilecharge carrier. This high carrier photoexcitation efficiency leads to apronounced shortening of the photocurrent rise time for decreasing wavelengthor increasing energy of the excitation pulses because of theeffect of quadratic recombination of the photocarriers.

URL:	http://link.aps.org/doi/10.1103/PhysRevB.77.125202
DOI:	10.1103/PhysRevB.77.125202
PACS:	72.20.Jv; 31.70.Ks; 72.40.+w; 71.35.-y 72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping (semiconductors/insulators) 31.70.Ks Molecular solids (electronic structure) 72.40.+w Photoconduction and photovoltaic effects 71.35.-y Excitons and related phenomena YEAR: 2008
KEYWORDS:	carrier mobility, electron-hole recombination, excitons, high-speed optical techniques, organic compounds, photoconductivity, photoexcitation

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