58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

During the 09/01/2012 – 08/31/2013 period we have performed extensive study of photo-conductivity of rubrene – the highest performance organic semiconductor, in which delocalized band-like carrier transport (see, e.g., [1]) and long-range triplet exciton diffusion [2] have been observed recently. We have obtained a very interesting data set (briefly described below), and we are preparing a paper that we intend to submit to Phys. Rev. Letters in a month.

We report on the steady state photoconductivity of rubrene single crystals measured in a very wide range of excitation light intensities (F), covering about 8 orders of magnitude from ~ 2x10¹⁹ to ~ 2x10²⁷ photons×m^-3s^-1 [3].  We have observed several non-trivial (that is, non-linear) distinct photoconductivity regimes that we interpret in terms of contributions from multi-particle interactions.  Namely, (1) at a very low excitation intensity (F < 10²⁰ photons×m^-3s^-1), the photocurrent (I_PC) is proportional to F (the linear regime), (2) at intermediate excitation densities (F » 10²⁰ - 10²⁶ photons×m^-3s^-1), the photocurrent varies as F^1/3 (the “1/3” regime), and (3) at a high photoexcitation density (F > 10²⁶ photons×m^-3s^-1), the photocurrent is proportional to F^1/4 (the “1/4” regime).  We have investigated the photoconductivity in these different regimes as a function of light polarization, excitation pulse shape and duration, as well as excitation wavelength.  Our measurements strongly suggest that photocurrent in pristine high-purity rubrene single crystals is generated almost entirely at the surface of the crystal's (a,b) facet, even though the excitation light is absorbed deep in the bulk (typical light penetration length for visible light at a normal incidence to (a,b) facet of rubrene is 1-5 mm).  Our studies indicate that the photo-conductivity is highly sensitive to low concentrations of charge trap states at the surface and to the quality of the crystal in general.  We have constructed a physical model that explains the 1/3 regime as occurring due to annihilation of triplet excitons on mobile charge carriers at the surface of the crystals, and the ¼ regime as occurring due to the triplet-triplet fusion that becomes dominant at higher excitation densities.  We have developed a mathematical model that describes these non-trivial phenomena using a set of differential rate equations dealing with the exciton and charge densities and their generation, interaction and recombination rates. 

1. V. Podzorov, Organic Single Crystals - Addressing the Fundamentals of Organic Electronics. MRS Bulletin 38, 15-24 (2013).

2. H. Najafov, B. Lee, Q. Zhou, L. C. Feldman and V. Podzorov, Observation of long-range exciton diffusion in highly ordered organic semiconductors. Nature Mater. 9, 938-943 (2010).

3. P. Irkhin, H. Najafov and V. Podzorov “Non-linear excitation density dependence of photo-conductivity in crystalline rubrene”, in preparation for Phys. Rev. Lett. (2013).