Polycrystalline Pentacene Films Research Articles

Two temperature regimes of triplet transfer in the dissociation of the correlated triplet pair after singlet fission

The ability to undergo spin-allowed exciton multiplication makes singlet fission materials promising for photovoltaic applications. Here, we examine the separation of correlated triplet pairs, 1(T…T), in polycrystalline pentacene films via temperature-dependent transient absorption spectroscopy. Single wavelength analysis reveals a profound delay in 1(T…T) dynamics. Moreover, the dynamics of 1(T…T) exhibit temperature dependence, whereas other features show no discernable temperature dependence. Previous literatures have suggested that correlated triplet separation is mediated by a thermally activated hopping process. Surprisingly, we found that the time constants governing triplet pair separation display two distinct temperature-dependent regimes of triplet transport. The high temperature regime follows a thermally activated hopping mechanism. The experimentally derived reorganization energy and electronic coupling is verified by density matrix renormalization group quantum chemical calculations. In addition, we evaluated the low temperature regime and show that the trend can be modelled by a Miller–Abrahams-type model that incorporates the effects of energetic disorder. We conclude that the correlated triplet pair separation is mediated by thermally activated hopping or a disorder driven Miller–Abrahams-type mechanism at high and low temperature, respectively. We observe that crossover between two regimes occurs ∼226 K. We find the time constant for triplet–triplet energy transfer to be 1.8 ps at ambient temperature and 21 ps at 77 K.

Subthreshold characteristics of pentacene field-effect transistors influenced by grain boundaries

Grain boundaries in polycrystalline pentacene films significantly affect the electrical characteristics of pentacene field-effect transistors (FETs). Upon reversal of the gate voltage sweep direction, pentacene FETs exhibited hysteretic behaviours in the subthreshold region, which was more pronounced for the FET having smaller pentacene grains. No shift in the flat-band voltage of the metal-insulator-semiconductor capacitor elucidates that the observed hysteresis was mainly caused by the influence of localized trap states existing at pentacene grain boundaries. From the results of continuous on/off switching operation of the pentacene FETs, hole depletion during the off period is found to be limited by pentacene grain boundaries. It is suggested that the polycrystalline nature of a pentacene film plays an important role on the dynamic characteristics of pentacene FETs.

Photophysics of pentacene thin films: The role of exciton fission and heating effects

There is evidence that the photo-generated singlet exciton in polycrystalline pentacene films undergoes rapid and efficient fission to form two triplet excitons. However, the role of exciton fission in pentacene has been controversial, with previous studies putting forward alternate relaxation pathways for the singlet exciton, such as excimer or charge formation, or internal conversion to a doubly excited exciton. We report temperature- and angular-dependent ultrafast transient optical absorption measurements in a wide spectral and temporal window. Angular-dependent transient spectra identify a common origin to photo-induced absorptions at 530 and 860 nm, which we associate with triplet excitons. These constitute the dominant relaxation channel for singlet excitons. Other features, particularly near 620 nm, previously assigned to excimers or electronic charges, are shown to be caused by thermal modulation from the optical pump.

Grain Boundary Effect on Charge Transport in Pentacene Thin Films

We report on charge transport properties of polycrystalline pentacene films with variable average grain size in the range from 0.1 to 0.3 µm controlled by the preparation technology. We illustrate with the organic field-effect transistors decrease of the effective mobility and presence of traps with decrease of the grain size. Analysis of the charge transfer excitons reveals decrease of the mobile charge density and the steady-state voltammetry showed significant increase of oxygen- and hydrogen-related defects. We also briefly discuss accumulation of the defects on the grain boundary and show relation between the defect density and grain boundary length.

Thickness-dependent threshold voltage in polycrystalline pentacene-based thin-film transistors

We have studied the current–voltage characteristics and threshold voltage behaviors in polycrystalline pentacene-based organic thin-film transistors (OTFTs) with gold top-contact electrodes for different thickness of the pentacene films. The study uses a number of techniques to measure threshold voltage including: square-root method, constant-current method, maximum transconductance method and capacitance–voltage measurements. The results of our experiments suggest that the space–charge and trapping effects of the pentacene bulk film plays an important role in current–voltage characteristics. In the presence of trapping centers within the polycrystalline pentacene film, we have provided an analytical model explaining the effect of bulk traps on the thickness-dependent threshold voltage. Square-law of threshold voltage versus pentacene thickness in OTFTs was derived during our study providing consistent results with experimental data. Furthermore, using the square-law, we estimated trap density of about 10 17 cm −3 in experimental pentacene films. The orders of magnitude in trap density of polycrystalline pentacene films are similar to the values found in other literature source.

Thickness‐Dependent Structural Evolutions and Growth Models in Relation to Carrier Transport Properties in Polycrystalline Pentacene Thin Films

AbstractThickness‐dependent crystal structure, surface morphology, surface energy, and molecular structure and microstructure of a series of polycrystalline pentacene films with different film thickness ranging from several monolayers to the several hundred nanometers have been investigated using X‐ray diffraction (XRD), atomic force microscopy (AFM), contact angle meter, and Raman spectroscopy. XRD studies indicate that thin film polymorphs transformation behaviours are from the orthorhombic phase to the thin‐film phase and then to the triclinic bulk phase as measured by the increased tilt angle (θtilt) of the pentacene molecule from the c‐axis toward the a‐axis. We propose a growth model that rationalizes the θtilt increased along with increasing film thickness in terms of grain size and surface energy varying with film growth using AFM combined with contact angle measurements. The vibrational characterizations of pentacene molecules in different thickness films were investigated by Raman spectroscopy compared to density functional theory calculations of an isolated molecule. In combination with XRD and AFM the method enables us to distinguish the molecular microstructures in different thin film polymorphs. We proposed a methodology to probe the microscopic parameters determining the carrier transport properties based on Davydov splitting and the characteristics of aromatic C–C stretching modes in Raman spectra. When compared to the triclinic bulk phase at a high thickness, we suggest that the first few monolayer structures located at the dielectric surface could have inferior carrier transport properties due to weak intermolecular interactions, large molecular relaxation energy, and more grain boundaries.

Using atomic steps to induce texture in polycrystalline pentacene films

The authors have studied the effect of substrate atomic steps on the azimuthal alignment of crystals in polycrystalline pentacene films. The substrates used were Si (111) with a low miscut angle and these were annealed at high temperature in ultrahigh vacuum before the pentacene deposition to produce surfaces with atomically flat terraces and arrays of parallel atomic steps. By depositing pentacene on these heated samples, at a low deposition rate, the authors have succeeded in obtaining significant azimuthal alignment of the pentacene crystals relative to the atomic steps.

Efficient organic solar cells are achieved using polycrystalline pentacene films

Efficient organic solar cells are achieved using polycrystalline pentacene films

Cover Picture: Microscopic Evidence for Spatially Inhomogeneous Charge Trapping in Pentacene (Adv. Mater. 11/2005)

AbstractTrapped charge has been observed in polycrystalline pentacene films by electric force microscopy in work reported by Muller and Marohn on p. 1410. The cover shows an optical fiber watching the motion of a metal‐coated cantilever hovering over a polycrystalline pentacene film. The film occupies the 6.5 μm gap in a working field‐effect transistor. Trapped charge (dark blue and black areas in the pentacene) appears as large patches randomly distributed throughout the device, implying that long‐lived traps in polycrystalline pentacene are not correlated with grain boundaries.

Ambipolar pentacene field-effect transistors with calcium source-drain electrodes

Field-effect transistors consisted of vacuum-sublimed polycrystalline pentacene films and calcium source-drain electrodes were prepared and device characteristics were evaluated in an oxygen-free condition. The field-effect transistor showed typical ambipolar characteristics and field-effect hole mobility of 4.5×10−4cm2/Vs and field-effect electron mobility of 2.7×10−5cm2∕Vs were estimated from saturation currents. Appearance of an electron enhancement mode in pentacene field-effect transistors was ascribed to the lowering of barrier for electron injection at source-drain electrodes. Effective elimination of electron traps using an oxygen-free condition was found to be another requirement for the observation of ambipolar behavior in pentacene.

Potential imaging of pentacene organic thin-film transistors

Scanning Kelvin probe microscopy (SKPM) has been used to simultaneously obtain high-resolution topographical and potential images of pentacene organic thin-film transistors (OTFTs) during device operation. SKPM images of OTFTs show large potential drops at the source with the magnitude dependent on contact metallurgy and relatively small potential drops at grain boundaries in polycrystalline pentacene films.

Efficiency of optical second harmonic generation from pentacene films of different morphology and structure

Submicron thick polycrystalline pentacene films of different morphology and molecular configuration are prepared by varying the conditions during vacuum deposition on quartz, sapphire and Si substrates. The films are characterized by X-ray diffraction, atomic force microscopy, optical absorption, and Raman spectroscopy. The efficiency of optical second harmonic generation (SHG) is studied as a function of the film properties using picosecond laser pulses at a fundamental wavelength of 1064 nm and femtosecond pulses at a fundamental wavelength of 870 nm. The second-order nonlinearity originates from the lack of inversion symmetry at the surfaces of the crystallites and is practically independent of the structure and morphology of the films. An abnormally low efficiency of SHG from films deposited at room temperature is explained by scattering of the second harmonic at crystals of a size comparable to the wavelength of the second harmonic.

Second-order nonlinearities of polycrystalline molecular films studied on a 20-fs time scale

Second-harmonic generation in polycrystalline pentacene films of submicrometer thickness by pulses of 18-fs duration is studied. We observe a second-order nonlinearity of as much as d(eff) = 0.045 pm/V, which is resonantly enhanced at wavelengths near 790 nm. Off-resonance autocorrelation studies demonstrate a quasiinstantaneous nonlinear response of the material, whereas experiments on resonance point to a finite response time T(2) of the order of 15 fs.

Spectroscopy of low-energy electrons backscattered from an organic solid surface: pentacene

The energy spectrum of electrons backscattered from polycrystalline pentacene films has been measured for incident elecron energies E i between 5 and 40 eV, with an energy resolution ≈150 meV and an angular resoluton ≈5°. Energy losses Δ E corresponding to the excitation in the film of intramolecular vibrations, triplet and singlet excitons, and σ electrons, have been found, as well as “true secondaries”. The dominant loss at small E i corresponds to excitation of backbone vibrations; triplet excitons are readily excited at 5 < E i < 15 eV. The secondary electron yield of pentacene is comparable to that of other organics.