ELSEVIER Synthetic Metals 75 (1995) 127-131 S¥111TH|TIIC RII|TRLS Picosecond transient photoconductivity in a soluble derivative of poly(p- phenylene vinylene) C.H. Lee a, G. Yu a, N.S. Sariciflci a, A.J. Heeger a,., C. Zhang b "Department of Physics and lnstitate for Polymers and Organic Solids, University of California, Santa Barbara, CA 93106, USA b UNIAX Corporation, 5375 Overpass Road, Santa Barbara, CA 93111, USA Received 20 February 1995; revised 2 June 1995; accepted 10 July 1995 Abstract We report the results of picosecond transient photoconductivity (PC) measurements in films of poly [ 2-methoxy-5-(2'-ethyl-hexyloxy)- p-phenylene vinylene] (MEH-PPV), a soluble derivative of poly (p-phenylene vinylene) (PPV). The onset of picosecond transient PC and the onset of steady-state PC are both found to coincide with that of optical absorption. The peak transient PC is proportional to the electric field, independent of temperature, and proportional to the light intensity. Since the transient PC is an order of magnitude smaller than that of PPV, early recombination is more probable due to the large side groups of MEH-PPV. This has been checked through photoinduced charge- transfer studies; both the magnitude and lifetime of the transient PC increase substantially on addition of dilute concentrations of C6o into MEH-PPV. Thus, ultrafast photoinduced electron transfer reduces early recombination and enhances the quantum yield for photogeneration of free charge carriers. These results indicate direct photogeneration of free carriers via an interband transition. Keywords: Photoconductivity; Luminescence; Poly(p-phenylene vinylene) 1. Introduction Conjugated polymers have been investigated as novel semiconductors with rich photophysics [ 1 ] and with poten- tial for use in applications such as in nonlinear optical devices [2] and light-emitting devices [3-5]. These conjugated polymers have anisotropic, quasi-one-dimensional electronic structures with w-electrons coupled to the polymer backbone via the electron-phonon interaction [ 1]. Among ~r-conju- gated polymers, poly(phenylene vinylene) (PPV) and its soluble derivatives have been extensively studied as the pro- totypical luminescent and electroluminescent (EL) poly- mers. There is controversy, however, over the nature of the electronic excitations of PPV upon photoexcitation or upon electron and hole injection [6-15]. The central issue which remains unresolved is the magnitude of the exciton binding energy Eexc. The closely related issue which remains unre- solved is the photogeneration mechanism of free carriers: are free carriers generated directly via an interband transition [ 6- 8 ] or as a secondary process through dissociation ofexcitons? [9-II1. A traditional method of determining the exciton binding energy is to compare the photoconductivity (PC) spectrum * Corresponding author. 0379-6779/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSD!0379-6779 (95)03404-8 with the optical absorption spectrum; one expects the PC onset energy to occur at higher photon energy than the onset energy for absorption by approximately Eexc. This splitting between the onset of absorption and the onset of PC has been clearly established, for example, in the polydiacetylenes [16]. In an earlier report, we demonstrated that the onset of the steady-state PC and the onset of the transient PC of PPV coincide with the onset of optical absorption, and that the PC spectrum can be quantitatively understood within the semi- conductor energy-band picture [7,8]. Moreover, Eckhardt et al. [ 17] found that the electrochemically-derived band gaps for PPV, poly (thienylene vinylene) and their alkoxy-substi- tuted derivatives agree well with the band gaps obtained from optical absorption, indicating that in all cases Ee,c is within the measurement error ( Eexc < 0.1 eV). Subsequently, Hagler et al. [ 12] showed that the instantaneous excited state wavefunctions in MEH-PPV are delocalized over a minimum of 50 unit cells (400 ,~). This extensive delocalization was inferred from the polarization anisotropy ( > 150:1) of the field-induced absorption in highly oriented (and structurally ordered) MEH-PPV. Hagler et al. [ 13,14] have also shown that the absorption and electroabsorption lineshapes in this well-ordered MEH-PPV are highly anisotropic and can be accurately described by a Gaussian-broadened square-root