Ultrafast Photoconductivity in Organic Semiconductors
Oksana OstroverkhovaDavid G. CookeSvitlana ShcherbynaR.F. EgertonFrank A. HegmannRik R. TykwinskiJohn E. AnthonyVitaly PodzorovM. E. GershensonOana D. JurchescuT. T. M. Palstra
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Abstract:
We report on sub-picosecond charge photogeneration and band-like charge transport in a variety of organic molecular crystals (pentacene, functionalized pentacene, tetracene and rubrene) and thin films (pentacene and functionalized pentacene) revealed by time-resolved terahertz pulse spectroscopy.Keywords:
Pentacene
Tetracene
Rubrene
Organic semiconductor
Photoconductivity
Picosecond
Charge carrier
The molecular and supramolecular structure and energetics of tetracene and rubrene were investigated by a combined experimental and theoretical study. Accurate equilibrium vapour pressures at various temperatures were measured for both compounds. For rubrene the energetic analysis evidences lower crystal packing efficiency, strong molecular destabilization and confirms the non-planar twisted equilibrium structure in the gas phase. The results also indicate that phenyl internal rotation in rubrene is highly hindered. The intra- and intermolecular interactions in crystal tetracene and rubrene were evaluated by computational methods. The representative stacked dimer of the tetracene⋅⋅⋅tetracene interaction in rubrene has lower ionization energy than the one modelling the same interaction in tetracene, due to stronger cation⋅⋅⋅π interactions in the cation-radical of rubrene. Charge distribution in the cation dimer is symmetrical in rubrene, whereas in tetracene it is largely localized on the C−H⋅⋅⋅π donor partner of the herringbone dimer. These findings highlight the impact of cation⋅⋅⋅π interactions on the semiconducting properties of OSCs.
Rubrene
Tetracene
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Pentacene
Tetracene
Organic semiconductor
Tetracyanoquinodimethane
Crystal (programming language)
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Singlet fission (SF), the photophysical process converting an excited singlet exciton into two triplet excitons, is a promising approach to boost solar cell efficiencies. It is controlled by various parameters such as intermolecular interactions, energetics, entropy and vibronic coupling and a controlled modification of these parameters is key to a fundamental understanding. Blends of organic semiconductors present an interesting alternative to established methods of chemical functionalization and their potential for the study of SF pathways will be discussed using acene blends as example. Mixed thin films of SF chromophores and weakly interacting, high-bandgap spacer molecules allow one to study the impact of a replacement of nearest neighbors on the SF rates. While the SF rate in pentacene, for which SF is a coherent process, is unaffected by the introduction of spacer molecules into the film, we observe a significant decrease in the SF rate in tetracene, indicating incoherent SF. Mixing the two SF chromophores pentacene and tetracene with low pentacene concentrations leads to heterofission of a singlet on pentacene into two triplets on pentacene and tetracene, respectively, when selectively exciting pentacene. This heterofission process is outcompeted by pentacene homofission if the pentacene concentration exceeds 5%. Photoexcitation above the tetracene band gap additionally allows for energy transfer from tetracene to pentacene and results in complex dynamics.
Pentacene
Tetracene
Singlet fission
Acene
Chromophore
Organic semiconductor
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Tetracene
Pentacene
Organic semiconductor
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Abstract We investigated vacuum‐evaporated sandwich systems formed by two organic layers of pentacene and tetracene. We measured spectral dependences of photovoltaic short‐circuit currents and photocurrents as well as current‐voltage dependences. In the case of systems equipped with Au and CuI electrodes the structure of spectral characteristics is determined by exciton release of trapped charge carriers either in the bulk of tetracene or near tetracene‐pentacene interface and by photogeneration in pentacene layer. For systems with two Au electrodes we can also observe an influence of exciton injection of holes into tetracene on spectral characteristics. The measurements of dark current‐voltage characteristics allow to observe the presence of a potential barrier between tetracene and pentacene.
Tetracene
Pentacene
Organic semiconductor
Photoelectric effect
Rubrene
Acene
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The polarizable environment surrounding charge carriers in organic semiconductors impacts the efficiency of the charge transport process. Here, we consider two representative organic semiconductors, tetracene and rubrene, and evaluate their polarization energies in the bulk and at the organic-vacuum interface using a polarizable force field that accounts for induced-dipole and quadrupole interactions. Though both oligoacenes pack in a herringbone motif, the tetraphenyl substituents on the tetracene backbone of rubrene alter greatly the nature of the packing. The resulting change in relative orientations of neighboring molecules is found to reduce the bulk polarization energy of holes in rubrene by some 0.3 eV when compared to tetracene. The consideration of model organic-vacuum interfaces highlights the significant variation in the electrostatic environment for a charge carrier at a surface although the net change in polarization energy is small; interestingly, the environment of a charge even just one layer removed from the surface can be viewed already as representative of the bulk. Overall, it is found that in these herringbone-type layered crystals the polarization energy has a much stronger dependence on the intralayer packing density than interlayer packing density.
Rubrene
Tetracene
Organic semiconductor
Pentacene
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Tetracene
Rubrene
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Pentacene
Tetracene
Singlet fission
Rubrene
Organic semiconductor
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We measure the transient photoconductivity in pentacene, functionalized pentacene, tetracene, and rubrene single crystals using optical pump-terahertz probe techniques. In all of the samples studied, we observe subpicosecond charge photogeneration and a peak photoconductive response that increases as the temperature decreases from 297 down to 20K, indicative of bandlike transport. Similar decay dynamics are observed at room temperature, but at low temperatures the decay dynamics measured in pentacene, rubrene, and tetracene crystals are much faster than those observed in functionalized pentacene crystals, revealing different charge trapping properties.
Pentacene
Tetracene
Rubrene
Photoconductivity
Organic semiconductor
Acene
Charge carrier
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