Showing papers on "Pentacene published in 2020"

Fast-Response, Highly Air-Stable, and Water-Resistant Organic Photodetectors Based on a Single-Crystal Pt Complex

Abstract: Organic semiconductors demonstrate several advantages over conventional inorganic materials for novel electronic and optoelectronic applications, including molecularly tunable properties, flexibility, low-cost, and facile device integration. However, before organic semiconductors can be used for the next-generation devices, such as ultrafast photodetectors (PDs), it is necessary to develop new materials that feature both high mobility and ambient stability. Toward this goal, a highly stable PD based on the organic single crystal [PtBr2 (5,5'-bis(CF3 CH2 OCH2 )-2,2'-bpy)] (or "Pt complex (1o)") is demonstrated as the active semiconductor channel-a material that features a lamellar molecular structure and high-quality, intraligand charge transfer. Benefitting from its unique crystal structure, the Pt-complex (1o) device exhibits a field-effect mobility of ≈0.45 cm2 V-1 s-1 without loss of significant performance under ambient conditions even after 40 days without encapsulation, as well as immersion in distilled water for a period of 24 h. Furthermore, the device features a maximum photoresponsivity of 1 × 103 A W-1 , a detectivity of 1.1 × 1012 cm Hz1/2 W-1 , and a record fast response/recovery time of 80/90 µs, which has never been previously achieved in other organic PDs. These findings strongly support and promote the use of the single-crystal Pt complex (1o) in next-generation organic optoelectronic devices.

Transparent and Colorless Polyimides Containing Multiple Trifluoromethyl Groups as Gate Insulators for Flexible Organic Transistors with Superior Electrical Stability.

Abstract: A suitable insulating polymer material that is compatible with the fabrication process of organic transistors and has excellent electrical properties is critically required for the next-generation flexible organic electronics. In this study, using a one-step polymerization method, we synthesized two different solution-processable polyimides (PIs) incorporated with abundant trifluoromethyl groups. Not only were the two resulting PIs-termed 6FDA-6FDAM-PI and 6FDA-TFMB-PI-well soluble in organic solvents, but also they showed transparent and colorless optical properties. The fluorinated PI films showed smooth surface topographies and surface energy values that were appropriate for their use in bottom-gate organic transistors. Organic transistors separately fabricated with 6FDA-6FDAM-PI and 6FDA-TFMB-PI as the gate insulators showed excellent device performance and electrical stability under various testing conditions, especially for pentacene-based devices. The excellent performance of the devices with fluorinated PIs was attributed to the enhanced microstructure of the organic semiconductor and the fluorine-rich characteristic of the underlying gate insulator. Furthermore, organic complementary circuits including the basic logic gates of NOT, NOR, and NAND were demonstrated using these devices.

Green solvents for organic thin-film transistor processing

Abstract: In this study, we explored a wide range of green solvents to process the semiconductor layer TIPS-PEN (6,13-bis(triisopropylsilylethynyl)pentacene), as well as to demonstrate potential generality, using several p- and n-type organic semiconductors, for the fabrication of organic thin-film transistors (OTFTs). Our data demonstrate that several different solvents enable good semiconductor film-forming morphologies and optimized TIPS-PEN TFT mobilities of ∼0.5–2 cm2 V−1 s−1, thus surpassing those of toxic chlorinated options. Furthermore, we utilized a green cellulose cinnamate-gate dielectric to fabricate TIPS-PEN OTFTs, where both the semiconductor and the dielectric were processed using green solvents demonstrating the feasibility of a more sustainable OTFT technology.

Conjugated Polymer Controlled Morphology and Charge Transport of Small-Molecule Organic Semiconductors.

Abstract: In this study, we report an effective approach to tune the crystallization, microstructure and charge transport of solution-processed organic semiconductors by blending with a conjugated polymer additive poly(3-hexylthiophene) (P3HT). When 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) was used as a model semiconductor material to mix with different amount of P3HT, their intermolecular interactions led to distinctive TIPS pentacene film morphologies, including randomly-oriented crystal ribbons, elongated needles with enhanced long-range order, and grass-like curved microwires with interlinkages. Each type of morphology was found to further correlate to considerably different charge transport and device performance. As compared to pristine TIPS pentacene devices, bottom-gate, top-contact OTFTs with 2% in weight P3HT additive showed a 2-fold and 5-fold improvement of average field-effect mobility and performance consistency (defined as the ratio of average mobility to the standard deviation), respectively. The improvement in transistor electrical performance can be attributed to the combined effect of enhanced crystal orientation and uniformity, as well as increased areal coverage. This work can be applied beyond the particular example demonstrated in this study and to tune the charge transport of other small-molecule organic semiconductors in general.

Enhancement of organic/inorganic hybrid photodetector based on pentacene/n-Si by surface plasmonic effect of gold and silver nanoparticles: A comparative study

Abstract: Herein, Au-NPs (108 nm) and Ag-NPs (96 nm) are spun coated on the surface of thermally evaporated pentacene thin films Ag-NPs induced a more pronounced impact on the crystallinity, morphology and optical properties of pentacene rather than Au-NPs The absorption edge of pentacene is blue-shifted from 199 eV to 206 eV and 269 eV after adding Au-NPs and Ag-NPs, respectively Pentacene's absorption at wavelength 6542 nm is enhanced by 566% and 343% after decorating by Au-NPs and Ag-NPs, respectively The NPs over the pentacene layer boost the quality of pentacene/n-Si heterojunction and improved the photocurrent by one and two orders of magnitudes after adding Au-NPs and Ag-NPs, respectively The surface plasmonic resonance of Au and Ag enhanced the photoresponsivity of the fabricated photodetector by 1475% and 4387% and subsequent enhancement of efficiency from 785% to 360% and 9154%, respectively at wavelength 650 nm

Optoelectronic properties of Co/pentacene/Si MIS heterojunction photodiode

Abstract: The Co/pentacene/n-Si/Al device with the Co/n-Si/Al device was fabricated and characterized by current-voltage measurements to understand the effect of the pentacene on electrical properties. The Co/pentacene/n-Si/Al device exhibited better rectification properties than reference device. The characteristic parameters such as barrier height, ideality factor, interface states density and series resistance values of the devices were calculated and discussed in details. Furthermore, I–V measurements of the Co/pentacene/n-Si/Al device were performed both in dark and under illumination conditions. The results revealed that the reverse current increased with increasing illumination intensity, and the device exhibited photodiode behaviors as well as good photocunducting characteristics. The C–V characteristics of the Co/pentacene/n-Si/Al were studied for various frequencies to determine the donor carrier concentration and fermi energy level. Experimental results highlights that the Co/pentacene/n-Si/Al device is a good candidate for optoelectronic device applications such as solar cells, photodiode, photodetector due to exhibiting good photoconducting characteristics.

Supertransport of excitons in atomically thin organic semiconductors at the 2D quantum limit.

Abstract: Long-range and fast transport of coherent excitons is important for the development of high-speed excitonic circuits and quantum computing applications. However, most of these coherent excitons have only been observed in some low-dimensional semiconductors when coupled with cavities, as there are large inhomogeneous broadening and dephasing effects on the transport of excitons in their native states in materials. Here, by confining coherent excitons at the 2D quantum limit, we first observed molecular aggregation-enabled ‘supertransport’ of excitons in atomically thin two-dimensional (2D) organic semiconductors between coherent states, with a measured high effective exciton diffusion coefficient of ~346.9 cm2/s at room temperature. This value is one to several orders of magnitude higher than the values reported for other organic molecular aggregates and low-dimensional inorganic materials. Without coupling to any optical cavities, the monolayer pentacene sample, a very clean 2D quantum system (~1.2 nm thick) with high crystallinity (J-type aggregation) and minimal interfacial states, showed superradiant emission from Frenkel excitons, which was experimentally confirmed by the temperature-dependent photoluminescence (PL) emission, highly enhanced radiative decay rate, significantly narrowed PL peak width and strongly directional in-plane emission. The coherence in monolayer pentacene samples was observed to be delocalised over ~135 molecules, which is significantly larger than the values (a few molecules) observed for other organic thin films. In addition, the supertransport of excitons in monolayer pentacene samples showed highly anisotropic behaviour. Our results pave the way for the development of future high-speed excitonic circuits, fast OLEDs, and other optoelectronic devices. Scientists have detected excited quasi-particles, called excitons, moving far and fast in a two-dimensional organic semiconductor, paving the way for applications in quantum computing and fast light emitting diodes. Yuerui Lu of the Australian National University and colleagues conducted tests in a 2D sample of pentacene, an organic semiconducting compound made of benzene rings fused into a chain. Light particles are absorbed by pentacene, generating excitons that can be used to develop tiny, very fast, energy-efficient optoelectronic devices. The team found that excitons move through the crystal depending on the temperature and pentacene’s molecular arrangements and order of crystallinity. The long-range and fast migration of excitons in a monolayer of pentacene was at least an order of magnitude higher than in other materials. The excitons in the pentacene monolayer also generated sharp, strong ‘superradiant’ light emissions.

Super-transport of Excitons in Atomically Thin Organic Semiconductors at the 2D Quantum Limit

Abstract: Long-range and fast transport of coherent excitons is important for development of high-speed excitonic circuits and quantum computing applications. However, most of these coherent excitons have only been observed in some low-dimensional semiconductors when coupled with cavities, as there are large inhomogeneous broadening and dephasing effects on the exciton transport in their native states of the materials. Here, by confining coherent excitons at the 2D quantum limit, we firstly observed molecular aggregation enabled super-transport of excitons in atomically thin two-dimensional (2D) organic semiconductors between coherent states, with a measured a high effective exciton diffusion coefficient of 346.9 cm2/sec at room temperature. This value is one to several orders of magnitude higher than the reported values from other organic molecular aggregates and low-dimensional inorganic materials. Without coupling to any optical cavities, the monolayer pentacene sample, a very clean 2D quantum system (1.2 nm thick) with high crystallinity (J type aggregation) and minimal interfacial states, showed superradiant emissions from the Frenkel excitons, which was experimentally confirmed by the temperature-dependent photoluminescence (PL) emission, highly enhanced radiative decay rate, significantly narrowed PL peak width and strongly directional in-plane emission. The coherence in monolayer pentacene samples was observed to be delocalized over 135 molecules, which is significantly larger than the values (a few molecules) observed from other organic thin films. In addition, the super-transport of excitons in monolayer pentacene samples showed highly anisotropic behaviour. Our results pave the way for the development of future high-speed excitonic circuits, fast OLEDs, and other opto-electronic devices.

Ultra-low misorientation angle in small-molecule semiconductor/polyethylene oxide blends for organic thin film transistors

Abstract: In this work, we report for the first time the use of a nonconjugated semicrystalline polymer as a film-forming agent to control the crystallization and tune the charge transport of solution-processed, small-molecule organic semiconductors. When 6,13-bis (triisopropylsilylethynyl) pentacene (TIPS pentacene) was demonstrated as a representative material to blend with polyethylene oxide (PEO) polymer, it crystallized into uniformly-aligned needles with reduced random orientation, enhanced long-range order and elevated areal coverage. Specifically, an ultra-low misorientation angle of 7.9° ± 3.5° was obtained with 10% PEO additive, beneficial for charge transport in the TIPS pentacene/PEO hybrid film. Bottom-gate, top-contact organic thin film transistors (OTFTs) based on TIPS pentacene/PEO mixture were found to show a field-effect mobility up to 2.5 × 10−2 cm2/Vs. This work may be universally applied to other organic semiconductors to regulate their crystal formation, enhance film forming and improve device performance of OTFTs. It contributes to the utilization of flexible substrates for future-generation high-performance organic electronics.

Charge Separation and Charge Transfer in the Low-Lying Excited States of Pentacene

Abstract: Pentacene thin films are common constituents of organic photovoltaic materials and a prototypical example of a material that undergoes singlet exciton fission, but significant questions remain rega...

Double Negative Differential Resistance Device Based on Hafnium Disulfide/Pentacene Hybrid Structure.

Abstract: Recently, combinations of 2D van der Waals (2D vdW) materials and organic materials have attracted attention because they facilitate the formation of various heterojunctions with excellent interface quality owing to the absence of dangling bonds on their surface. In this work, a double negative differential resistance (D-NDR) characteristic of a hybrid 2D vdW/organic tunneling device consisting of a hafnium disulfide/pentacene heterojunction and a 3D pentacene resistor is reported. This D-NDR phenomenon is achieved by precisely controlling an NDR peak voltage with the pentacene resistor and then integrating two distinct NDR devices in parallel. Then, the operation of a controllable-gain amplifier configured with the D-NDR device and an n-channel transistor is demonstrated using the Cadence Spectre simulation platform. The proposed D-NDR device technology based on a hybrid 2D vdW/organic heterostructure provides a scientific foundation for various circuit applications that require the NDR phenomenon.

Superhydrogenation of pentacene: the reactivity of zigzag-edges

Abstract: Investigating the hydrogenation of carbonaceous materials is of interest in a wide range of research areas including electronic device development, hydrogen storage, and, in particular, astrocatalytic formation of molecular hydrogen in the universe. Polycyclic Aromatic Hydrocarbons (PAHs) are ubiquitous in space, locking up close to 15% of the elementary carbon. We have used thermal desorption measurements to study the hydrogenation sequence of pentacene from adding one additional H to the fully hydrogenated pentacene species. The experiments reveal that hydrogenated species with an even number of excess H atoms are highly preferred over hydrogenated species with an odd number of H atoms. In addition, the experiments show that specific hydrogenation states of pentacene with 2, 4, 6, 10, 16 and 22 extra H atoms are preferred over other even numbers. We have investigated the structural stability and activation energy barriers for the superhydrogenation of pentacene using Density Functional Theory. The results reveal a preferential hydrogenation pattern set by the activation energy barriers of the hydrogenation steps. Based on these studies, we formulate simple concepts governing the hydrogenation that apply equally well for different PAHs.

Enhanced Intersystem Crossing and Transient Electron Spin Polarization in a Photoexcited Pentacene–Trityl Radical

Abstract: Identifying and characterizing systems that generate well-defined states with large electron spin polarization is of high interest for applications in molecular spintronics, high-energy physics, and magnetic resonance spectroscopy The generation of electron spin polarization on free-radical substituents tethered to pentacene derivatives has recently gained a great deal of interest for its applications in molecular electronics After photoexcitation of the chromophore, pentacene-radical derivatives can rapidly form spin-polarized triplet excited states through enhanced intersystem crossing Under the right conditions, the triplet spin polarization, arising from mS-selective intersystem crossing rates, can be transferred to the tethered stable radical The efficiency of this spin polarization transfer depends on many factors: local magnetic and electric fields, excited-state energetics, molecular geometry, and spin-spin coupling Here, we present transient electron paramagnetic resonance (EPR) measurements on three pentacene derivatives tethered to Finland trityl, BDPA, or TEMPO radicals to explore the influence of the nature of the radical on the spin polarization transfer We observe efficient polarization transfer between the pentacene excited triplet and the trityl radical but do not observe the same for the BDPA and TEMPO derivatives The polarization transfer behavior in the pentacene-trityl system is also investigated in different glassy matrices and is found to depend markedly on the solvent used The EPR results are rationalized with the help of femtosecond and nanosecond transient absorption measurements, yielding complementary information on the excited-state dynamics of the three pentacene derivatives Notably, we observe a 2 orders of magnitude difference in the time scale of triplet formation between the pentacene-trityl system and the pentacene systems tethered with the BDPA and TEMPO radicals

Zero bias high responsive visible organic photodetector based on pentacene and C60

Abstract: Organic photodetector (OPD) based on simple bilayer heterojunction of pentacene and fullerene (C60) has been fabricated using thermal evaporation method. Absorption spectra of series of OPD, annealed at different temperatures, have been observed over a spectral range between 400 nm and 700 nm. At 0 V and at wavelength 650 nm, external quantum efficiency (EQE) = 5% and responsivity of 0.025 A/W have been achieved for an OPD annealed at 90 °C. For the same device, the maximum ratio of photocurrent to dark current density is found to be of the order of 2.5 × 103. Furthermore, OPD annealed at 90 °C has exhibited maximum detectivity of 2 × 1010 Jones and response time of 180 ms. Structural characterization confirms formation of small grain size of pentacene and C60 molecules thereby leading to improvement of the device performance. Our experimental signatures seem to indicate that pentacene and C60 based OPD, after suitable engineering, might be promising for red light detection applications.

Van der Waals Bound Organic/2D Insulator Hybrid Structures: Epitaxial Growth of Acene Films on hBN(001) and the Influence of Surface Defects

Abstract: Combining 2D materials with functional molecular films enables the fabrication of van der Waals bound organic/inorganic hybrids that are of interest for future device architectures. Recently, the 2D dielectric hexagonal boron nitride (hBN) has received particular attention since exfoliation allows the preparation of crystalline layers which have been utilized as ultrathin dielectrics in electronic devices. Here, we have studied the formation and structure of molecular films of the prototypical organic semiconductors pentacene (PEN) and perfluoropentacene (PFP) on hBN. Special attention was paid to the influence of substrate surface defects on the film formation by comparing molecular films that were grown on hBN substrates of various quality, including single crystals (representing the most ideal surface), briefly ion bombarded substrates, and exfoliated flakes. While X-ray diffraction (XRD) yields precise information about the crystalline structure of films grown on (large) single crystals, it is hardly applicable to analyze the films formed on exfoliated flakes because of their small size. Here, we demonstrate that in the case of flakes detailed structural analyses of the molecular films are possible by combining atomic force microscopy (AFM) with microspot UV/vis spectroscopy and optical polarization microscopy. On well-ordered hBN single crystal surfaces both acenes form very smooth and epitaxial crystalline films where molecules adopt a recumbent orientation (even in 100 nm thick films). By contrast, both materials adopt an upright molecular orientation and different polymorphs on defective hBN surfaces and reveal distinctly different film morphologies. On exfoliated flakes, PFP shows a film structure similar to that on the hBN single crystals, while PEN films exhibit a structure as on defective hBN substrates. In addition, a pronounced decoration of defect steps, which are probably created by the exfoliation process, was observed for PEN leading to the formation of tall and extended fibers where molecules adopt a recumbent orientation. The present study reveals different robustness in film growth on exfoliated hBN flakes for various molecules, which has to be considered in their device integration, especially with regard to their optoelectronic properties such as light absorption or charge transport, which depend critically on the molecular orientation and crystalline order.

Effect of Polymer Molecular Weight on Morphology and Charge Transport of Small-Molecular Organic Semiconductors

Abstract: The utilization of polymer additives provides an intriguing pathway to tune the electrical performance of solution-grown, small-molecular organic semiconductors. In this study, we report the effect of different polymer molecular weight on the crystal growth, phase segregation and charge transport of the organic semiconductors. A semicrystalline polymer additive polyethylene oxide (PEO) with 8000 and 100 K molecular weight was blended with a well-known organic semiconductor 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene). Distinctively different thin film morphology of the resultant TIPS pentacene/PEO blend film was identified and quantitatively characterized. In particular, PEO with a higher molecular weight of 100 K exerted a stronger effect on enhancing film formation, crystal coverage and likely mechanical properties, whereas PEO with an 8000 molecular weight yielded more superior crystal alignment and larger crystal sizes. Bottom-gate, top-contact TIPS pentacene/PEO OTFTs were fabricated to test the charge transport, which indicated loading the PEO polymer with molecular weight of 8000 demonstrated a five-fold enhancement in the extracted hole mobilities as compared to the 100 K counterpart. This work of using polymer additives with different molecular weight can be used to tune the crystallization of other solution-processed high-mobility small-molecular organic semiconductors.

3d-transition metals (Cu, Fe, Mn, Ni, V and Zn)-doped pentacene π-conjugated organic molecule for photovoltaic applications: DFT and TD-DFT calculations

Abstract: In this study, we have performed a thorough examination of density functional theory (DFT) and time-dependent (TD) DFT to investigate the structural and optoelectronic properties of 3d-transition metals (Cu, Fe, Mn, Ni, V and Zn)-doped pentacene π-conjugated organic molecule. The HOMO energy level of Ni-doped pentacene is − 6.17 eV wide, i.e., about 1.31 eV greater and more negative than pentacene. The bandgap of the pentacene considerable decreases from 2.20 eV to 1.32, 1.35 and 0.37 eV, for Mn, Zn and V-doped pentacene structures, respectively, which affords an efficient charge transfer from HOMO to LUMO. The HOMO–LUMO energy gap is higher (4.44 eV, for Ni-doped pentacene), implying that the kinetic energy is higher and high chemical reactivity. We have examined, additionally, the reactivity and absorption properties of individual undoped and 3d-transition metals-doped pentacene. Pentacene has the largest vertical ionization potential (6.18 eV), corresponding to the highest chemical stability. Our results suggest that the new 3d-transition metals-doped pentacene may significantly contribute to the efficiency of solar cells.

Unilaterally Fluorinated Acenes: Synthesis and Solid-State Properties.

Abstract: The rapid development of organic electronics is closely related to the availability of molecular materials with specific electronic properties. Here, we introduce a novel synthetic route enabling a unilateral functionalization of acenes along their long side, which is demonstrated by the synthesis of 1,2,10,11,12,14-hexafluoropentacene (1) and the related 1,2,9,10,11-pentafluorotetracene (2). Quantum chemical DFT calculations in combination with optical and X-ray absorption spectroscopy data indicate that the single-molecule properties of 1 are a connecting link between the organic semiconductor model systems pentacene (PEN) and perfluoropentacene (PFP). In contrast, the crystal structure analysis reveals a different packing motif than for the parent molecules. This can be related to distinct F⋅⋅⋅H interactions identified in the corresponding Hirshfeld surface analysis and also affects solid-state properties such as the exciton binding energy and the sublimation enthalpy.

Modulating the Electronic and Solid-State Structure of Organic Semiconductors by Site-Specific Substitution: The Case of Tetrafluoropentacenes

Abstract: The properties as well as solid-state structures, singlet fission, and organic field-effect transistor (OFET) performance of three tetrafluoropentacenes (1,4,8,11: 10, 1,4,9,10: 11, 2,3,9,10: 12) are compared herein. The novel compounds 10 and 11 were synthesized in high purity from the corresponding 6,13-etheno-bridged precursors by reaction with dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate at elevated temperatures. Although most of the molecular properties of the compounds are similar, their chemical reactivity and crystal structures differ considerably. Isomer 10 undergoes the orbital symmetry forbidden thermal [4+4] dimerization, whereas 11 and 12 are much less reactive. The isomers 11 and 12 crystallize in a herringbone motif, but 10 prefers π-π stacking. Although the energy of the first electric dipole-allowed optical transition varies only within 370 cm-1 (0.05 eV) for the neutral compounds, this amounts to roughly 1600 cm-1 (0.20 eV) for radical cations and 1300 cm-1 (0.16 eV) for dications. Transient spectroscopy of films of 11 and 12 reveals singlet-fission time constants (91±11, 73±3 fs, respectively) that are shorter than for pentacene (112±9 fs). OFET devices constructed from 11 and 12 show close to ideal thin-film transistor (TFT) characteristics with electron mobilities of 2×10-3 and 6×10-2 cm2 V-1 s-1 , respectively.

An aromatic micelle with bent pentacene-based panels: encapsulation of perylene bisimide dyes and graphene nanosheets

Abstract: For exploitation of a new class of aromatic micelles, we synthesized a bent pentacene-based amphiphilic molecule through Diels-Alder reaction. The amphiphiles bearing two trimethylammonium tethers assemble into a spherical aromatic micelle, with an average core diameter of 1.5 nm, in water at room temperature. The new aromatic micelle efficiently encapsulates perylene bisimide (PBI) dyes and graphene nanosheets (GNS) in water. The encapsulated PBI dyes form a parallel stacked dimer, exhibiting characteristic absorption and emission bands. In addition, the encapsulated GNS are composed of few-layer graphene sheets with an average lateral size of ∼7 nm, as confirmed by Raman spectroscopy. The resultant, aqueous host-guest complexes are stable even after three weeks in water under ambient conditions.

Pyrene-stabilized acenes as intermolecular singlet fission candidates: importance of exciton wave-function convergence.

Abstract: Singlet fission (SF) is a photophysical process considered as a possible scheme to bypass the Shockley-Queisser limit by generating two triplet-state excitons from one high-energy photon. Polyacene crystals, such as tetracene and pentacene, have shown outstanding SF performance both theoretically and experimentally. However, their instability prevents them from being utilized in SF-based photovoltaic devices. In search of practical SF chromophores, we use many-body perturbation theory within the GW approximation and Bethe-Salpeter equation to study the excitonic properties of a family of pyrene-stabilized acenes. We propose a criterion to define the convergence of exciton wave-functions with respect to the fine k-point grid used in the BerkeleyGW code. An open-source Python code is presented to perform exciton wave-function convergence checks and streamline the double Bader analysis of exciton character. We find that the singlet excitons in pyrene-stabilized acenes have a higher degree of charge transfer character than in the corresponding acenes. The pyrene-fused tetracene and pentacene derivatives exhibit comparable excitation energies to their corresponding acenes, making them potential SF candidates. The pyrene-stabilized anthracene derivative is considered as a possible candidate for triplet-triplet annihilation because it yields a lower SF driving force than anthracene.

Pentacene and Tetracene Molecules and Films on H/Si(111): Level Alignment from Hybrid Density Functional Theory

Abstract: The electronic properties of hybrid organic-inorganic semiconductor interfaces depend strongly on the alignment of the electronic carrier levels in the organic/inorganic components. In the present work, we address this energy level alignment from first principles theory for two paradigmatic organic-inorganic semiconductor interfaces, the singlet fission materials tetracene and pentacene on H/Si(111), using all-electron density functional theory calculations with a hybrid exchange-correlation functional. For isolated tetracene on H/Si(111), a type I-like heterojunction (lowest-energy electron and hole states on Si) is found. For isolated pentacene, the molecular and semiconductor valence band edges are degenerate. For monolayer films, we show how to construct supercell geometries with up to 1,192 atoms, which minimize the strain between the inorganic surface and an organic monolayer film. Based on these models, we predict the formation of type II heterojunctions (electron states on Si, hole-like states on the organic species) for both acenes, indicating that charge separation at the interface between the organic and inorganic components is favored. The paper discusses the steps needed to find appropriate low-energy interface geometries for weakly bonded organic molecules and films on inorganic substrates from first principles, a necessary prerequisite for any computational level alignment prediction.