Pentacene Thin Films Research Articles

Structural Analysis of Pentacene Thin Films by Grazing Incident X-ray Diffraction Measurements

Structural Analysis of Pentacene Thin Films by Grazing Incident X-ray Diffraction Measurements

Three-Dimensional Topological Insulator Bi2Te3/Organic Thin Film Heterojunction Photodetector with Fast and Wideband Response from 450 to 3500 Nanometers.

In the pursuit of broadband photodetection materials from visible to mid-IR region, the fresh three-dimensional topological insulators (3D TIs) are theoretically predicted to be a promising candidate due to its Dirac-like stable surface state and high absorption rate. In this work, a self-powered inorganic/organic heterojunction photodetector based on n-type 3D TIs Bi2Te3 combined with p-type pentacene thin film was designed and fabricated. Surprisingly, it was found that the Bi2Te3/pentacene heterojunction photodetector exhibited a fast and wideband response from 450 to 3500 nm. The optimized responsivity of photodetector reached 14.89 A/W, along with the fast response time of 1.89 ms and the ultrahigh external quantum efficiency of 2840%. Moreover, at the mid-IR 3500 nm, our devices demonstrated a responsivity of 1.55 AW-1, which was several orders of magnitude higher than that of previous 3D TIs photodetector. These excellent properties indicate that the inorganic/organic heterojunction, that is, the combination of 3D TIs with organic materials, is an exciting structure for high performance photodetectors in the wideband detection region. On account of the fact that the device is constructed on mica substrate, this work also represents a potential scenario for flexible optoelectronic devices.

Development of the fragment-based COHSEX method for large and complex molecular systems

We present a fragment-based many-body Green's function method suitable for treating large molecular systems in heterogeneous polarizable environments. The Green's function for a total system is approximated from fragment Green's functions and is expanded up to two-body terms. The screened Coulomb potential is approximated from the sum of intrafragment density-response functions, with interfragment polarization terms being neglected. The approximations for the Green's function and screened Coulomb potential lead to a many-body expansion of the self-energy. This expansion is essentially equivalent to the many-body expansion of the Fock matrix in the fragment molecular orbital method. To handle large molecular systems, the present implementation relies on the Coulomb hole plus screened exchange (COHSEX) approximation for the $GW$ self-energy. The accuracy of the FMO-COHSEX method was demonstrated in comparison to conventional COHSEX results for organic molecular aggregates. We confirmed that the present fragmentation approximation can provide reasonably accurate results, and mean absolute errors for quasiparticle energies of less than 0.1 eV have been achieved for valence orbitals. We also assessed the accuracy of the COHSEX approximation to describe the effects of molecular aggregation of electronic states, by comparing them with the $GW$ method. The COHSEX approximation has been shown to successfully describe the induced polarization and dispersion effects. As an illustrative application of the present method, we considered the electronic states of the pentacene thin film, which contains 1476 atoms. We investigated the impact of the induced polarization effect in the heterogeneous environment, highlighting the gap renormalization and the polarization-induced localization. This application shows that the present fragment-based method is useful for studying electronic structures of molecular aggregates in complex environments.

Sub-50 picosecond to microsecond carrier transport dynamics in pentacene thin films

Carrier transport dynamics from sub-50 ps to ∼μs over five temporal decades in pentacene films was studied by transient photoconductivity. The behavior of the temperature independent photocurrent peak suggests that the photogenerated carriers exhibit pre-trapping transport in extended states upon pulsed laser excitation. From 300 ps to ∼30 ns, the carriers thermalize and fall into shallow band tail states, and multiple-trapping and release transport dominates. From ∼30 ns to ∼μs, the weak temperature dependence of power-law photocurrent decay and the linear relation of logarithmic photocurrent with T−1/4 suggest a variable range hopping transport in deep trap states.

Thermo-evaporated pentacene and perylene as hole transport materials for perovskite solar cells

Thermo-evaporated organic semiconductors with suitable frontier molecular orbitals are potential hole transport materials (HTMs) for perovskite solar cells (PSCs), owing to their high crystallinity associated with high carrier mobility. In this study, we employ typical organic semiconductors, namely pentacene and perylene, as the HTMs in PSCs with an inverted structure. We gradually regulate the crystallographic morphologies of pentacene and perylene thin films by elevating the target substrate temperature from room temperature to 120 °C and 80 °C, respectively. We observe a clear dependence of the photovoltaic performance of inverted PSCs on the substrate temperature. We obtain the optimal power conversion efficiency (PCE) for pentacene and perylene at the substrate temperatures of 40 °C and 60 °C, respectively. The PSCs based on perylene show the best PCE of 13.5%, which is much higher that based on pentacene (5.7%). Perylene-based device shows better stability than PEDOT:PSS-based one. This study substantially deepens our knowledge regarding PSCs based on thermo-evaporated HTMs.

Kinetic Barrier Against Standing Up of Pentacene Molecules Upon a Pentacene Monolayer

The growth of pentacene thin films on Cu(110)‐(2 × 1)O surface was investigated in situ and in real time using reflectance difference spectroscopy (RDS). Deposition at a substrate temperature of 230 K results in a lying‐down orientation for all pentacene molecules. Subsequent annealing to a temperature of 350 K induces reorientation for molecules above the first monolayer into a standing‐up configuration. By a quantitative analysis on the corresponding evolution of the RD signals, the kinetic barrier of meV has been deduced for the reorientation of pentacene molecule.

Current Voltage Characteristics and Excess Noise at the Trap Filling Transition in Polyacenes

Experiments in organic semiconductors (polyacenes) evidence a strong super quadratic increase of the current–voltage (I–V) characteristic at voltages in the transition region between linear (Ohmic) and quadratic (trap-free space-charge-limited current) behaviors. Similarly, excess noise measurements at a given frequency and increasing voltages evidence a sharp peak of the relative spectral density of the current noise in concomitance with the strong superquadratic I–V characteristics. Here, we discuss the physical interpretation of these experiments in terms of an essential contribution from field-assisted trapping-detrapping processes of injected carriers. To this purpose, the fraction of filled traps determined by the I–V characteristics is used to evaluate the excess noise in the trap-filled transition (TFT) regime. We have found an excellent agreement between the predictions of our model and existing experimental results in tetracene and pentacene thin films of different length in the range [Formula: see text].

The synergistic role of azeotropic solvent mixtures and atactic polystyrene on the morphology, crystallization and field effect mobility of thin film 6,13-bis(triisopropylsilylethynyl)-pentacene based semiconductors

The effect of anisole/decane binary solvent mixture and the subsequent addition of low wt% aPS on 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) thin film morphology is investigated by optical microscopy, AFM and UV–Vis measurements. We show that, over the composition range anisole/decane of 96/4 to 85/15 wt%, the solution maintains an azeotropic composition with the boiling point of the binary mixture remaining constant at 152 °C, and the solvent composition remaining constant during evaporation and drying. It was found that addition of up to 20 wt% decane has little impact on micro-scale crystal morphology but has a significant influence on the growth mode and terrace roughness. The formation of large crystals is explained in terms of the change in solvent, increase in decane content, weakening the solute–solvent interactions and promoting efficient nucleation of the favoured H-aggregates of TIPS-pentacene. The effect of the TIPS-pentacene—aPS ratio up to 20 wt% for drop-cast thin film was similarly investigated. It is found that addition of aPS has a significant effect on both macroscopic crystal properties such as surface coverage, unity of orientation and long range order. It also changes the surface morphology and layer ordering on the nano-scale.

Improving performance of OFET by tuning occurrence of charge transport based on pentacene interaction with SAM functionalized contacts

Optimized pentacene thin film transistors have been achieved by two technological self-assembled monolayers (SAM) treatments, with surface modification of 3-octadecyltrichlorosilane (OTS) monolayer on SiO2 dielectric layer and 2,3,4,5,6-pentafluorobenzenethiol (PFBT) monolayer on gold source/drain electrodes, respectively. Higher mobility and on/off ratio with 0.36 cm2 V−1 s−1 and >106 are consistently investigated for films deposited on PFBT compared to on OTS, which could be explained by the different occurrence charge transport near the semiconductor/dielectric and semiconductor/electrodes interfaces. It can be further confirmed by XPS and electrical property measurements. In the end, a high mobility increased up to 0.68 cm2 V−1 s−1 was obtained by cumulative treatments of the two SAM. In particular, the improving performance of the organic thin films transistor devices through tuning the charge transport with SAM layers highlighting the utility of surface modulations and controlling of charge transport of interfaces in nanoscale materials.

In Situ Wettability Characterization of Chemically Heterogeneous Surfaces Probed by Ionic Liquid Contact Angle in Vacuum: Pentacene on Single-Crystal SrTiO3(001)

Intrinsic surface wettability of pentacene thin films with different island coverages grown on single-crystal SrTiO3(001) was investigated by an in situ contact angle measurement in a high vacuum using ionic liquid as a probe fluid. The contact angle at the quasi-equilibrium state, which was measured at a certain time after its fast decay depending on the liquid viscosity, increased with the deposition amount of pentacene and saturated when the surface was fully covered with pentacene islands. The observed island coverage dependence of the in situ contact angle was well-fitted by a three-component Cassie equation composed of surface areas of not only the bare substrate and island pentacene but also adsorbed pentacene. The importance of the in situ contact angle measurement was confirmed by the result of an air exposure experiment, where the contact angle took almost irrelevant values to the integral coverage of pentacene, suggesting the significant effects of airborne contaminants on the surface wettability.

A New Pentacene Polymorph Induced by Interaction with a Bi(0001) Substrate

Pentacene, one of the most studied organic semiconductors, crystallizes in different structures depending on the growth conditions. The structure is usually classified into one of the three polymorphs, the so-called single crystal phase, bulk phase, and thin film phase. In this report, we show that epitaxial crystalline pentacene thin films on Bi(0001), which have been believed to be the bulk phase, have a distinct molecular orientation in the same crystal lattice as the bulk phase. The calculated band structures are clearly different from those of the bulk phase and show an excellent agreement with the experimental data for the bandwidth.(21) A tight-binding model indicates that one of the two main intermolecular interactions is almost identical to that of the single crystal phase, while the other one is almost identical to that of the bulk phase. The results implies that polymorphs in the same crystal lattice can be formed by a molecular-substrate interaction.

Robust singlet fission in pentacene thin films with tuned charge transfer interactions

Singlet fission, the spin-allowed photophysical process converting an excited singlet state into two triplet states, has attracted significant attention for device applications. Research so far has focused mainly on the understanding of singlet fission in pure materials, yet blends offer the promise of a controlled tuning of intermolecular interactions, impacting singlet fission efficiencies. Here we report a study of singlet fission in mixtures of pentacene with weakly interacting spacer molecules. Comparison of experimentally determined stationary optical properties and theoretical calculations indicates a reduction of charge-transfer interactions between pentacene molecules with increasing spacer molecule fraction. Theory predicts that the reduced interactions slow down singlet fission in these blends, but surprisingly we find that singlet fission occurs on a timescale comparable to that in pure crystalline pentacene. We explain the observed robustness of singlet fission in such mixed films by a mechanism of exciton diffusion to hot spots with closer intermolecular spacings.

Surface Modification of Solution-Processed ZrO2 Films through Double Coating for Pentacene Thin-Film Transistors

We report the modification of surface properties of solution-processed zirconium oxide (ZrO2) dielectric films achieved by using double-coating process. It is proven that the surface properties of the ZrO2 film are modified through the double-coating process; the surface roughness decreases and the surface energy increases. The present surface modification of the ZrO2 film contributes to an increase in grain size of the pentacene film, thereby increasing the field-effect mobility and decreasing the threshold voltage of the pentacene thin-film transistors (TFTs) having the ZrO2 gate dielectric. Herein, the molecular orientation of pentacene film is also studied based on the results of contact angle and X-ray diffraction measurements. Pentacene molecules on the double-coated ZrO2 film are found to be more tilted than those on the single-coated ZrO2 film, which is attributed to the surface modification of the ZrO2 film. However, no significant differences are observed in insulating properties between the single-and the double-coated ZrO2 dielectric films. Consequently, the characteristic improvements of the pentacene TFTs with the double-coated ZrO2 gate dielectric film can be understood through the increase in pentacene grain size and the reduction in grain boundary density.

Field Effect Transistors Based on Composite Films of Poly(4-vinylphenol) with ZnO Nanoparticles

In order to adjust the characteristic of pentacene thin film transistor, we modified the dielectric properties of the gate insulator, poly(4-vinylphenol), or PVP. PVP is an organic polymer with a low dielectric constant, limiting the performance of organic thin film transistors (OTFTs). To increase the dielectric constant of PVP, a controlled amount of ZnO nanoparticles was homogeneously dispersed in a dielectric layer. The effect of the concentration of ZnO on the relative permittivity of PVP was measured using impedance spectroscopy and it has been demonstrated that the permittivity increases from 3.6 to 5.5 with no percolation phenomenon even at a concentration of 50 vol.%. The performance of OTFTs in terms of charge carrier mobility, threshold voltage and linkage current was evaluated. The results indicate a dramatic increase in both the field effect mobility and the linkage current by a factor of 10. It has been demonstrated that the threshold voltage can be adjusted. It shifts from 8 to 0 when the volume concentration of ZnO varied from 0 vol.% to 50 vol.%.

Transport characteristics in Au/pentacene/Au diodes

We have used scanning and transmission electron microscopes (SEM and TEM) to study the structure of a pentacene thin film grown on a Au layer with and shown that it consists of randomly oriented amorphous pentacene clusters. We have also investigated the transport properties of amorphous pentacene in a metal–semiconductor–metal (MSM) diode structure and shown that the current is logarithmically proportional to the square root of the applied voltage, which indicates that transport occurs as the result of hopping between localized sites randomly distributed in space and energy.

Effect of thickness on bistable switching characteristics of a pentacene based memory device

In this paper, the effect of thicknesses on vacuum deposited pentacene (C22H14) thin films on ITO coated glass substrate for use as write once read many (WORM) memory devices are reported. RV characteristics of pentacene devices deposited for different thicknesses of 50 nm, 100 nm and 200 nm at a deposition rate of 20 Å/s were studied. Results revealed that the ON/OFF ratio was more for the device with active material deposited at a thickness of 100 nm. This device showed a stable switching with an ON/OFF current ratio as high as nearly 108 with a switching threshold voltage of 1.14 V. The irreversible switching of all the devices makes it suitable for a write once read many memory device applications. The structural studies of these pentacene thin films on glass substrates were also done and the dependence of the grain size of the films with thickness is also reported.

Pentacene Coated Atop of Ultrathin InN Gas Sensor Device for the Selective Sensing of Ammonia Gas for Liver Malfunction Application

An ultrathin InN resistive gas sensor is used for selective sensing of ammonia gas by using (∼5nm) pentacene film coating on the InN epilayer for liver malfunction applications. In order to distinguish between ammonia and acetone gases in the exhaled breath, we used two physically different characteristics of InN epilayer such as bare InN epilayer and thin pentacene film atop InN epilayer. On the bare InN epilayer, current variation responses for 8ppm ammonia and 8ppm acetone were 0.6% and 0.57% respectively. On bare InN epilayer, the current variation response of ammonia gas is only 1.05 times the current variation response of acetone gas. On pentacene coated InN epilayer current variation responses for 8ppm ammonia and 8ppm acetone gases are 4.7% and 1.9% respectively. The current variation response for ammonia is 2.47 folds higher than acetone on the pentacene coated InN epilayer. A thin film pentacene atop InN epilayer is very effective to filter out the acetone gas. Therefore, pentacene coated atop ultrathin InN epilayer is useful for the selective sensing of the ammonia gas in the exhaled breath for liver malfunction applications.

Polarized absorbance and Davydov splitting in bulk and thin-film pentacene polymorphs.

Pentacene is one of the most studied organic materials and in particular its optical properties have been the subject of intense research during the last two decades. In spite of such a widespread interest and of the extensive knowledge achieved so far, a number of issues are still debated. One of the most relevant questions concerns the role of polymorphism and how it affects the lowest-energy exciton, which appears in the visible region and is subject to a sizable Davydov splitting. We address this problem in a combined theoretical and experimental work, where the optical absorption properties of three pentacene polymorphs are investigated within the whole energy range of visible light. Optical spectra computed from first principles in the framework of many-body perturbation theory are directly compared with the polarization-resolved absorbance, measured for three different pentacene phases (the two bulk polymorphs and the thin-film phase). In this way, we unambiguously identify the two Davydov components of the first exciton and the optical fingerprints of each considered phase. With very good agreement between theory and experiment, we show that all polymorphs exhibit common features at the absorption onset, while phase-dependent characteristics appear only above 2 eV. We discuss the character of the lowest-lying singlet and triplet excitons, including dark ones, highlighting the contributions from the electronic bands and the role of the electron-hole interaction and of the local-field effects.

Flexible solution-processed high-voltage organic thin film transistor

Abstract

Quantitative evaluation of spatial scale of carrier trapping at grain boundary by GHz-microwave dielectric loss spectroscopy

Charge carrier mobility is an important primary parameter for the electronic conductive materials, and the intrinsic limit of the mobility has been hardly access by conventional direct-current evaluation methods. In the present study, intra-grain hole mobility of pentacene thin films was estimated quantitatively using microwave-based dielectric loss spectroscopy (time-resolved microwave conductivity measurement) in alternating current mode of charge carrier local motion. Metal-insulator-semiconductor devices were prepared with different insulating polymers or substrate temperature upon vacuum deposition of the pentacene layer, which afforded totally four different grain-size conditions of pentacene layers. Under the condition where the local motion was determined by interfacial traps at the pentacene grain boundaries (grain-grain interfaces), the observed hole mobilities were plotted against the grain sizes, giving an excellent correlation fit successfully by a parabolic function representative of the boarder length. Consequently, the intra-grain mobility and trap-release time of holes were estimated as 15 cm2 V−1 s−1 and 9.4 ps.