Photoconductivity in organic thin films: From picoseconds to seconds after excitation

We present a detailed study, on time scales from picoseconds to seconds, of transient and continuous wave (cw) photoconductivity in solution-grown thin films of functionalized pentacene (Pc), anthradithiophene (ADT), and dicyanomethylenedihydrofuran (DCDHF). In all films, at temperatures of 285-350 K, we observe fast carrier photogeneration and nonthermally activated charge transport on picosecond time scales. At similar to 30 ps after photoexcitation at room temperature and at applied electric field of 1.2x10(4) V/cm, values obtained for the product of mobility and photogeneration efficiency, mu eta, in ADT-tri-isoproplysilylethynyl-(TIPS)-F, Pc-TIPS, and DCDHF films are similar to 0.018-0.025, similar to 0.01-0.022, and similar to 0.002-0.004 cm(2)/V s, respectively, depending on the film quality, and are weakly electric field dependent. In functionalized ADT and Pc films, the power-law decay dynamics of the transient photoconductivity is observed, on time scales of up to similar to 1 mu s after photoexcitation, in the best samples. In contrast, in DCDHF amorphous glass, most of the photogenerated carriers are trapped within similar to 200 ps. Transport of photoexcited carriers on longer time scales is probed by cw illumination through an optical chopper, with a variable chopper frequency. In contrast with what is observed on picosecond time scales, charge carriers on millisecond and longer time scales are predominantly localized, and are characterized by a broad distribution of carrier lifetimes. Such carriers make the principal contributions to dc photoconductivity. (C) 2008 American Institute of Physics.