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 ∼30 ps after photoexcitation at room temperature and at applied electric field of 1.2× 104 V/cm, values obtained for the product of mobility and photogeneration efficiency, μ, in ADT-tri-isoproplysilylethynyl-(TIPS)-F, Pc-TIPS, and DCDHF films are ∼0.018-0.025, ∼0.01-0.022, and ∼0.002-0.004 cm2 /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 ∼1 μs after photoexcitation, in the best samples. In contrast, in DCDHF amorphous glass, most of the photogenerated carriers are trapped within ∼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.