Bandlike transport in pentacene and functionalized pentacene thin films revealed by subpicosecond transient photoconductivity measurements

Abstract

We observe bandlike transport in pentacene and functionalized pentacene thin films using time-resolved terahertz pulse spectroscopy. The measured transient photoconductivity exhibits fast ͑Ͻ400 fs͒ photogeneration of mobile charge carriers and reveals a transient carrier mobility that increases as the temperature decreases from 300 K down to 10 K, indicative of bandlike transport over subpicosecond time scales. A wavelength-independent photoconductive signal is observed. The transient photoconductivity in the thin-film samples exhibits a single-exponential decay, whereas a power-law decay is seen in single-crystal samples.

Ultrafast carrier dynamics in pentacene, functionalized pentacene, tetracene, and rubrene single crystals

“…Subpicosecond charge photogeneration has been previously observed in FPc and Pc single crystals 7,8 and thin films. 9,10 The similar trend observed in all crystals studied here ͑inset of Fig. 1͒ suggests that subpicosecond photogeneration of free carriers, [7][8][9][10]16 and not only of excitons, 3 in the absence of a static electric field is an intrinsic property of oligoacenes.…”

“…3͑a͔͒. 7,9,10 While the decay dynamics in FPc are weakly temperature dependent ͓i.e., ␤ ϳ 0.5-0.7 from 5 -297 K in Fig. 3͑c͒ and in Refs.…”

“…[3][4][5] However, the exact nature of charge carrier photogeneration and transport in organic semiconductors is still not completely understood due to difficulties in assessing intrinsic properties that are often masked by impurities, contact effects, or grain boundaries. Recently, ultrafast optical pump-terahertz probe techniques 6 have been used to perform noncontact measurements of transient photoconductivity in organic single crystals and polycrystalline thin films over picosecond time scales, [7][8][9][10] making it possible to probe the intrinsic nature of mobile charge carriers before they are trapped at defect sites. In this letter, we compare the transient photoconductive properties of pentacene, functionalized pentacene, tetracene, and rubrene single crystals assessed using time-resolved terahertz pulse spectroscopy.…”

“…Briefly, if the phase shift between the terahertz pulses transmitted through unexcited and optically excited sample is small, as was the case for all our samples, then −⌬T / T 0 is directly proportional to the transient photoconductivity. [7][8][9] From the peak value of the photoconductive transient, the product of charge carrier mobility and charge carrier photogeneration efficiency can then be calculated using ͑−⌬T / T 0 ͒ max = F a Z 0 / ͓hc͑1+N͔͒, where F a is the absorbed fluence, Z 0 is the free space impedance, is the optical wavelength, h is Planck's constant, c is the speed of light, and N is the refractive index of the unexcited sample. 9,10,15 In all measurements reported here, the electric field of the optical and terahertz pulse was kept parallel to the highest mobility axis of the crystals.…”

“…[7][8][9] From the peak value of the photoconductive transient, the product of charge carrier mobility and charge carrier photogeneration efficiency can then be calculated using ͑−⌬T / T 0 ͒ max = F a Z 0 / ͓hc͑1+N͔͒, where F a is the absorbed fluence, Z 0 is the free space impedance, is the optical wavelength, h is Planck's constant, c is the speed of light, and N is the refractive index of the unexcited sample. 9,10,15 In all measurements reported here, the electric field of the optical and terahertz pulse was kept parallel to the highest mobility axis of the crystals. Upon azimuthal rotation of the crystals, we observed a photoconductivity ana͒ Present address: Department of Physics, Oregon State University; electronic mail: oksana@science.oregonstate.edu isotropy within a factor of ϳ2 -4, a detailed study of which will be reported elsewhere.…”