Solid Pentacene Research Articles

Phonon-Induced Localization of Excitons in Molecular Crystals from First Principles.

The spatial extent of excitons in molecular systems underpins their photophysics and utility for optoelectronic applications. Phonons are reported to lead to both exciton localization and delocalization. However, a microscopic understanding of phonon-induced (de)localization is lacking, in particular, how localized states form, the role of specific vibrations, and the relative importance of quantum and thermal nuclear fluctuations. Here, we present a first-principles study of these phenomena in solid pentacene, a prototypical molecular crystal, capturing the formation of bound excitons, exciton-phonon coupling to all orders, and phonon anharmonicity, using density functional theory, the abinitio GW-Bethe-Salpeter equation approach, finite-difference, and path integral techniques. We find that for pentacene zero-point nuclear motion causes uniformly strong localization, with thermal motion providing additional localization only for Wannier-Mott-like excitons. Anharmonic effects drive temperature-dependent localization, and, while such effects prevent the emergence of highly delocalized excitons, we explore the conditions under which these might be realized.

Davydov-type excitonic effects on the absorption spectra of parallel-stacked and herringbone aggregates of pentacene: Time-dependent density-functional theory and time-dependent density-functional tight binding.

Exciton formation leads to J-bands in solid pentacene. Describing these exciton bands represents a challenge for both time-dependent (TD) density-functional theory (DFT) and for its semi-empirical analog, namely, for TD density-functional tight binding (DFTB) for three reasons: (i) solid pentacene and pentacene aggregates are bound only by van der Waals forces which are notoriously difficult to describe with DFT and DFTB, (ii) the proper description of the long-range coupling between molecules, needed to describe Davydov splitting, is not easy to include in TD-DFT with traditional functionals and in TD-DFTB, and (iii) mixing may occur between local and charge transfer excitons, which may, in turn, require special functionals. We assess how far TD-DFTB has progressed toward a correct description of this type of exciton by including both a dispersion correction for the ground state and a range-separated hybrid functional for the excited state and comparing the results against corresponding TD-CAM-B3LYP/CAM-B3LYP+D3 results. Analytic results for parallel-stacked ethylene are derived which go beyond Kasha's exciton model [M. Kasha, H. R. Rawls, and A. El-Bayoumi, Pure Appl. Chem. 11, 371 (1965)] in that we are able to make a clear distinction between charge transfer and energy transfer excitons. This is further confirmed when it is shown that range-separated hybrids have a markedly greater effect on charge-transfer excitons than on energy-transfer excitons in the case of parallel-stacked pentacenes. TD-DFT calculations with the CAM-B3LYP functional and TD-lc-DFT calculations lead to negligible excitonic corrections for the herringbone crystal structure, possibly because of an overcorrection of charge-transfer effects (CAM refers to Coulomb attenuated method). In this case, TD-DFT calculations with the B3LYP functional or TD-DFTB calculations parameterized to B3LYP give the best results for excitonic corrections for the herringbone crystal structure as judged from comparison with experimental spectra and with Bethe-Salpeter equation calculations from the literature.

Origins of Singlet Fission in Solid Pentacene from an abinitio Green's Function Approach.

We develop a new first-principles approach to predict and understand rates of singlet fission with an abinitio Green's-function formalism based on many-body perturbation theory. Starting with singlet and triplet excitons computed from a GW plus Bethe-Salpeter equation approach, we calculate the exciton-biexciton coupling to lowest order in the Coulomb interaction, assuming a final state consisting of two noninteracting spin-correlated triplets with finite center-of-mass momentum. For crystalline pentacene, symmetries dictate that the only purely Coulombic fission decay process from a bright singlet state requires a final state consisting of two inequivalent nearly degenerate triplets of nonzero, equal and opposite, center-of-mass momenta. For such a process, we predict a singlet lifetime of 30-70fs, in very good agreement with experimental data, indicating that this process can dominate singlet fission in crystalline pentacene. Our approach is general and provides a framework for predicting and understanding multiexciton interactions in solids.

(Invited) Strain Sensing Inverter and Photovoltaic Circuits Based on Novel Heptazole Organic Thin Film

Following solid pentacene films and other p-type small molecules, in the present work, we show a new small molecule, 8,16-dihydrobenzo[a]benzo[6,7]indolo[2,3-h]carbazole (heptazole; C26H16N2) which has almost the same HOMO level as that of pentacene but with a higher HOMO-LUMO gap of ~2.95 eV, so as to be photostable under visible light except deep blue-to-violet range. The field-effect mobility of our new OTFT turns out to be ~0.21 cm2/V s which is lower than that of the pentacene OTFT on the same dielectric layer, however our new OTFT on flexible substrate is found to work as a strain sensor because organic heptazole layer changes its electric resistance under tensile strain but such change appears elastically recoverable by removing the strain. We here display an inverter which is composed of two OTFTs but with 90 drgree different channel orientations each other: one would work as a strain sensor while the other works as a load under static and dynamic tensile bending strain conditions. Moreover, we also found that our heptzole organic film has very small exciton binding (~40 meV) properties under deep blue photons (wave length~410 nm). Taking advantages of such small exciton binding energy, we developed a Schottky diode circuit which can dynamically measure the photovoltaic effects without any assistacnce of electric power. Based on above-mentioned benefits of our heptazole layer in TFT and Schottky diode, we demonstrate in the meeting the dynamic strain sensing of the organic inverter on flexble substrate, and dynamic photovoltaic switching of the Schottky diode circuits as briefly shown below. Figure 1

Locally Broken Crystal Symmetry Facilitates Singlet Exciton Fission.

Photovoltaic yield is normally limited to at most two charge carriers per photon. In solid pentacene this limit may be potentially bypassed owing to singlet exciton fission into a pair of triplets. The process occurs via a superexchange mechanism mediated by charge-transfer (CT) configurations and is sensitive to their energies. As demonstrated recently, these strongly depend on the local environment of the two molecules on which the charges reside. Using a multiscale model, here we show that in the crystal bulk approximate local symmetry affects CT state energetics in a way unfavorable for fission, so that at the places where this symmetry is broken the fission probability is enhanced by up to an order of magnitude. These fission-favorable locations entail the vicinity of vacancies, specific impurities, and interfaces, such as crystallite boundaries. Hence, photovoltaic yield might be substantially increased by using nanoscopically disordered pentacene rather than highly ordered specimens.

Low-Energy Charge-Transfer Excitons in Organic Solids from First-Principles: The Case of Pentacene

The nature of low energy optical excitations, or excitons, in organic solids is of central relevance to many optoelectronic applications, including solar energy conversion. Excitons in solid pentacene, a prototypical organic semiconductor, have been the subject of many experimental and theoretical studies, with differing conclusions as to the degree of their charge-transfer character. Using first-principles calculations based on density functional theory and many-body perturbation theory, we compute the average electron–hole distance and quantify the degree of charge-transfer character within optical excitations in solid-state pentacene. We show that several low-energy singlet excitations are characterized by a weak overlap between electron and hole and an average electron–hole distance greater than 6 Å. Additionally, we show that the character of the lowest-lying singlet and triplet excitons is well-described with a simple analytic envelope function of the electron–hole distance.

Optical properties of pentacene thin films and single crystals

The optical properties of pentacene single crystals and thin films are systematically investigated by variable angle spectroscopic ellipsometry. In combination with x-ray diffraction and atomic force microscopy the method allows us to distinguish between the complex optical constants ($n$ and $k$) of the different polymorphs formed in thin films during the growth process. On the biaxial anisotropic single crystal it is possible to determine the optical constants along the respective crystallographic orientations. Temperature-dependent measurements in the range from $T=5\phantom{\rule{0.3em}{0ex}}\mathrm{K}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}350\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ confirm the assignment of the optical spectra. The results are in good agreement with ab initio calculations of the optical properties of solid pentacene by Tiago et al. [Phys. Rev. B. 67, 115212 (2003)].

Ab initio calculation of electron effective masses in solid pentacene

We have calculated the band structures of solid pentacene. One of our models makes use of experimental data obtained for a single crystal [R. B. Campbell et al., Acta. Crystallogr. 14, 705 (1961); R. B. Campbell and J. M. Robertson, Acta. Crystallogr. 15, 289 (1962)] and the other one considers a thin film fabricated by the present authors. Our calculations indicate that the electronic properties of a pentacene thin film are markedly different from those of a single crystal, although the densities of pentacene molecules in these solids are not significantly different. The energy band gaps of the single crystal and the thin film have been estimated to be 0.73eV and between 0.26 and 0.72eV, respectively. The bandwidths of the highest valence band and the lowest conduction band of the film are wider than those of the single crystal. These results suggest that the electron or hole carriers in the film have effective masses lighter than those of the single crystal, and in order to estimate the carrier mobility we first calculate the effective mass.

Threshold voltage change due to organic-inorganic interface in pentacene thin-film transistors

We have constructed pentacene-based organic thin-film transistors (OTFTs) using 270nm thick Al2O3+x gate dielectric deposited on indium tin oxide glass. Two different deposition techniques have been employed for the solid pentacene channel of our OTFTs: traditional thermal evaporation (TE) and energetic cluster evaporation (ECE). The TE-deposited pentacene channel appeared superior to the ECE-deposited pentacene in regard to crystallinity and hole mobility. However, the ECE-prepared OTFT showed an earlier turn on with smaller threshold voltage than the TE-prepared OTFT. This threshold voltage shift appeared more prominent with proper chemical treatment on the surface of our Al2O3+x gate oxide.

Correlation between photoelectric and optical absorption spectra of thermally evaporated pentacene films

We have measured the spectral photoresponse of Al/pentacene Schottky junction photodiodes and optical absorption spectra of pentacene films thermally evaporated on glass. The photoelectric response exhibited the genuine highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) transition at 1.97 eV and interband absorption peaks at 2.3 and 2.5 eV. These peaks are also identified in the optical absorption spectra, but they are dominated by additional strong exciton peaks at 1.82 and 2.1 eV. By comparing these complementary measurements, we determine the HOMO–LUMO gap energy of 1.97 eV and the fundamental exciton binding energy of 0.15 eV for thin solid pentacene.

Pentacene-based photodiode with Schottky junction

We have fabricated a metal/organic semiconductor Schottky photodiode based on Al/pentacene junction. Since the energy band gap of thin solid pentacene was determined to be 1.82 eV, as characterized by direct absorption spectroscopy, we measured spectral photoresponses on our Schottky photodiode in the monochromatic light illumination range of 325–650 nm applying a reverse bias of −2 V. The main features of photo-response spectra were found to shift from those of direct absorption spectra toward higher photon energies. It is because the direct absorption spectra mainly show exciton level peaks rather than the true highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gaps while the photo-response spectra clearly represents the true HOMO–LUMO gap. Our photo-response spectra reveal 1.97 eV as the HOMO–LUMO gap.

Fluorescence of Solids

THE following observations may be of interest in view of the present interest in problems of fluorescence of solids. Anthracene (I) in the solid state fluoresces blue with a high efficiency, the absorption band lying at wave-lengths shorter than 3800 A. Solid naphtha-cene (II) is yellow in colour and fluoresces very faintly, and solid pentacene (III) is dark bluish and does not fluoresce appreciably. In dilute solid solution in anthracene, however, the latter hydrocarbons fluoresce vividly, the former greenish-yellow and the latter red.