Investigating the origin of the high photoconductivity of rubrene single crystals 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 manuscript received 11 December 2007; published 6 March 2008 The rubrene molecular crystal has the unique property of showing a strong photoconductivity for light wavelengths that are close to the absorption edge. We studied the microsecond dynamics of the photoconduc- tivity induced by short light pulses to characterize the way in which photoinduced excitons efficiently ionize to produce free charge carriers. We found that the photoconductivity is produced by carriers released after an average time of 100 s from a “reservoir state” that originates from the photoexcited molecular excitons. The conversion of photoexcited excitons into this reservoir state happens only at the surface of the crystal within a depth of the order of a few micrometers, but in this region close to the surface, a photoexcited molecular exciton has a probability of the order of unity to ultimately lead to a mobile charge carrier. This high carrier photoexcitation efficiency leads to a pronounced shortening of the photocurrent rise time for decreasing wavelength or increasing energy of the excitation pulses because of the effect of quadratic recombination of the photocarriers. DOI: 10.1103/PhysRevB.77.125202 PACS numbers: 72.20.Jv, 31.70.Ks, 72.40.+w, 71.35.-y I. INTRODUCTION The physical process by which an absorbed photon may photoexcite mobile charge carriers in a pure single crystal is fundamentally different, compared to standard covalently bound semiconductors, when the crystal consists of large molecules bound only by van der Waals attraction, as is often the case in organic molecular crystals. At a fundamental level, one expects that the weak intermolecular interaction leads to a weak overlap of the electron wave functions be- tween neighboring molecules, which in very general terms tends to decrease the probability that a molecule in an ex- cited state transfers its electron to a neighboring molecule to generate a delocalized electron and hole pair. Instead, the probability for electron transfer to a neighboring molecule could remain significantly lower than the probability for the molecule to return to its ground state by either radiative or nonradiative recombination. In other words, in van der Waals bound molecular crystals, one expects that photoexcitation mostly leads to localized molecular excitons that only rarely autoionize to produce free carriers, leading to a small photo- conductivity even when the optical absorption becomes very high. 1–3 In some cases, however, and in contrast to the above ex- pectations, a signal that can be assigned to free charge carri- ers has been observed within a fraction of a picosecond from photon absorption, 4–6 and there has been a lively discussion in the literature about the nature of photoexcitation in mo- lecular materials and the possibility that photon absorption can directly lead to the creation of delocalized mobile carriers. 7–12 The discussion is farther complicated by the variety of excitonic states that are found in organic materials. 1–3,12–16 In this context, we have recently investigated the photo- excitation mechanism in rubrene single crystals. 17 By vary- ing the energy of the incident photons, we observed how, initially, photon absorption leads to the promotion of the ground state electron onto one of the vibronic sublevels of the first electronic excited state of the rubrene molecule, thus creating the molecular exciton equivalent of a Frenkel exciton. 17 This exciton subsequently decays within a few nanoseconds, leading to a fast photoluminescence signal. Fi- nally, a large delayed photoconductivity grows from zero with a buildup time of up to 100 s after photoexcitation. 17 We assigned this delayed photoconductivity to the decay of an unknown intermediary state a “reservoir” that was popu- lated from the initial photoexcited exciton, a conclusion that was derived mainly from the correspondence between the excitation spectrum of the photocurrent and the excitation spectrum of the luminescence. 17 In the following, we will use the term reservoir to describe this as yet unspecified intermediary state that slowly releases free carriers, thus leading to both a slowly rising and a long-lived photocon- ductivity after pulsed excitation. In this work, we investigate the dependence of the photo- conductivity dynamics in rubrene single crystals from the photon energy, photon density, and temperature. We develop a general model applicable to any case where a pulsed exci- tation results in a localized state that can ionize into free charge carriers. We use the model to analyze the evolution of the photoconductivity in rubrene after exposure to a 20 ps long laser pulse, which allows us to separate the processes of exciton creation, transfer to the reservoir state, and carrier generation, because they occur on different time scales. From this, we obtain information on the reservoir state and on the properties of the photoexcited carriers. Recently, it has been pointed out that oxygen-related de- fects close to the surface of rubrene play an important role in light-induced switching of field-effect transistors 18 and in the photoluminescence. 19 One of the conclusions we will arrive at is that the intermediary reservoir state mentioned above could be the result of an exciton interacting with such an oxygen-related defect state to create a bound state from which holes are later thermally excited. We will also show PHYSICAL REVIEW B 77, 125202 2008 1098-0121/2008/7712/12520212 ©2008 The American Physical Society 125202-1