Optical and transient photoconductive properties of pentacene and functionalized pentacene thin films: Dependence on film morphology

Abstract

We present a comprehensive study of the optical and transient photoconductive properties of pentacene and functionalized pentacene thin films grown by evaporation or from solution onto a variety of substrates. The transient photoconductivity was studied over picosecond time scales using time-resolved terahertz pulse spectroscopy. The structure and morphology of the films were assessed using x-ray diffraction, atomic force microscopy, and scanning electron microscopy. Regular pentacene films grown by evaporation under similar conditions but on different substrates yielded polycrystalline films with similar morphology and similar optical and transient photoconductive properties. Single exponential or biexponential decay dynamics was observed in all of the regular pentacene films studied. Functionalized pentacene films grown by evaporation at two different substrate temperatures ͑as well as from solution͒ yielded significant variations in morphology, resulting in different optical-absorption spectra and transient photoconductivities that could be correlated with film structure. The lower limit of the charge-carrier mobility, estimated from the amplitude of the transient photoconductive response, was ϳ0.02-0.04 cm 2 / V s in the case of regular pentacene films and ϳ0.01-0.06 cm 2 / V s in the case of functionalized pentacene films, depending on the film morphology. The best functionalized pentacene films exhibited transient photoconductivity values reaching ϳ30%-40% of those obtained in functionalized pentacene single-crystal samples, and showed similar power-law decay dynamics. We also report on terahertz pulse generation from voltage-biased pentacene thin films.

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.…”

“…This is supported by our previous observation of the values measured in good quality FPc thin films reaching 30%-40% of those obtained in FPc single crystals, in spite of the abundance of deep traps at the grain boundaries in thin films. 10 Therefore, the differences in the trapping properties of FPc, Pc, Rub, and Tc crystals most likely account only for a factor of up to ϳ2 in the differences in values obtained in these crystals at RT, while further differences are due to factors ͑i͒ and ͑ii͒. For example, the lower transient photoconductivity observed in Rub compared to Pc single crystals at RT at 400 nm could be mostly due to the lower photogeneration efficiency ͓factor ͑ii͔͒ in Rub, in qualitative agreement with Refs.…”

“…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.…”

“…[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.…”

“…[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.…”