Showing papers on "Pentacene published in 2016"
TL;DR: This work demonstrates the integration of an organic small molecule pentacene with a 2D n-type semiconductor, MoS2, and demonstrates the p-n heterojunction, which is gate-tunable and shows asymmetric control over the antiambipolar transfer characteristic.
Abstract: The recent emergence of a wide variety of two-dimensional (2D) materials has created new opportunities for device concepts and applications. In particular, the availability of semiconducting transition metal dichalcogenides, in addition to semimetallic graphene and insulating boron nitride, has enabled the fabrication of “all 2D” van der Waals heterostructure devices. Furthermore, the concept of van der Waals heterostructures has the potential to be significantly broadened beyond layered solids. For example, molecular and polymeric organic solids, whose surface atoms possess saturated bonds, are also known to interact via van der Waals forces and thus offer an alternative for scalable integration with 2D materials. Here, we demonstrate the integration of an organic small molecule p-type semiconductor, pentacene, with a 2D n-type semiconductor, MoS2. The resulting p–n heterojunction is gate-tunable and shows asymmetric control over the antiambipolar transfer characteristic. In addition, the pentacene/MoS2 ...
278 citations
TL;DR: It is found that the charge transport is dominated by hopping in the first conductive layer, but transforms to bandlike in subsequent layers, attributed to strong modulation of the molecular packing by interfacial vdW interactions, as corroborated by quantitative structural characterization and density functional theory calculations.
Abstract: One of the basic assumptions in organic field-effect transistors, the most fundamental device unit in organic electronics, is that charge transport occurs two dimensionally in the first few molecular layers near the dielectric interface. Although the mobility of bulk organic semiconductors has increased dramatically, direct probing of intrinsic charge transport in the two-dimensional limit has not been possible due to excessive disorders and traps in ultrathin organic thin films. Here, highly ordered single-crystalline mono- to tetralayer pentacene crystals are realized by van der Waals (vdW) epitaxy on hexagonal BN. We find that the charge transport is dominated by hopping in the first conductive layer, but transforms to bandlike in subsequent layers. Such an abrupt phase transition is attributed to strong modulation of the molecular packing by interfacial vdW interactions, as corroborated by quantitative structural characterization and density functional theory calculations. The structural modulation becomes negligible beyond the second conductive layer, leading to a mobility saturation thickness of only ∼3 nm. Highly ordered organic ultrathin films provide a platform for new physics and device structures (such as heterostructures and quantum wells) that are not possible in conventional bulk crystals.
215 citations
TL;DR: This work proposes a model, based on the accumulation of photogenerated charges and photoconductive gain, able to describe the magnitude as well as the dynamics of the X-ray-induced photocurrent, and fabricate and test a flexible 2 × 2 pixelated X-rays detector.
Abstract: The application of organic electronic materials for the detection of ionizing radiations is very appealing thanks to their mechanical flexibility, low-cost and simple processing in comparison to their inorganic counterpart. In this work we investigate the direct X-ray photoconversion process in organic thin film photoconductors. The devices are realized by drop casting solution-processed bis-(triisopropylsilylethynyl)pentacene (TIPS-pentacene) onto flexible plastic substrates patterned with metal electrodes; they exhibit a strong sensitivity to X-rays despite the low X-ray photon absorption typical of low-Z organic materials. We propose a model, based on the accumulation of photogenerated charges and photoconductive gain, able to describe the magnitude as well as the dynamics of the X-ray-induced photocurrent. This finding allows us to fabricate and test a flexible 2 × 2 pixelated X-ray detector operating at 0.2 V, with gain and sensitivity up to 4.7 × 104 and 77,000 nC mGy-1 cm-3, respectively.
115 citations
TL;DR: The origin of the decrease in contact resistance in terms of increase of the work function of the modified Au electrodes, Fermi-level pinning effects, and decrease of bulk resistance by electrically doping the organic semiconductor films in the vicinity of the source/drain electrodes are discussed.
Abstract: We report on the reduction of contact resistance in solution-processed TIPS-pentacene (6,13-bis(triisopropylsilylethynyl)pentacene) and PTAA (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]) top-gate bottom-contact organic field-effect transistors (OFETs) by using different contact-modification strategies. The study compares the contact resistance values in devices that comprise Au source/drain electrodes either treated with 2,3,4,5,6-pentafluorothiophenol (PFBT), or modified with an evaporated thin layer of the metal–organic molecular dopant molybdenum tris-[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd)3), or modified with a thin layer of the oxide MoO3. An improved performance is observed in devices modified with Mo(tfd)3 or MoO3 as compared to devices in which Au electrodes are modified with PFBT. We discuss the origin of the decrease in contact resistance in terms of increase of the work function of the modified Au electrodes, Fermi-level pinning effects, and decrease of bulk resistance by e...
78 citations
TL;DR: In this article, a detailed comparison of the semiconductor thin-film morphology, of the current-voltage characteristics of transistors with channel lengths ranging from 100 to 1μm, and of the contact resistances is provided.
77 citations
TL;DR: A new processing method is reported to systematically tune the intermolecular order and crystalline structure in films of a model singlet fission chromophore, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn), without the need for chemical modifications.
Abstract: The multiplication of excitons in organic semiconductors via singlet fission offers the potential for photovoltaic cells that exceed the Shockley-Quiesser limit for single-junction devices. To fully utilize the potential of singlet fission sensitizers in devices, it is necessary to understand and control the diffusion of the resultant triplet excitons. In this work, a new processing method is reported to systematically tune the intermolecular order and crystalline structure in films of a model singlet fission chromophore, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn), without the need for chemical modifications. A combination of transient absorption spectroscopy and quantitative materials characterization enabled a detailed examination of the distance- and time-dependence of triplet exciton diffusion following singlet fission in these nanocrystalline TIPS-Pn films. Triplet-triplet annihilation rate constants were found to be representative of the weighted average of crystalline and amorphous phases in TIPS-Pn films comprising a mixture of phases. Adopting a diffusion model used to describe triplet-triplet annihilation, the triplet diffusion lengths for nanocrystalline and amorphous films of TIPS-Pn were estimated to be ∼75 and ∼14 nm, respectively. Importantly, the presence of even a small fraction (<10%) of the amorphous phase in the TIPS-Pn films greatly decreased the ultimate triplet diffusion length, suggesting that pure crystalline materials may be essential to efficiently harvest multiplied triplets even when singlet fission occurs on ultrafast time scales.
76 citations
TL;DR: In this article, the performance of p-type VOFETs with pentacene as an active material can be significantly enhanced by the addition of the common p-dopant C60F36 as a thin injection layer underneath the source electrode, resulting in an increase of On-state current and On/Off ratio by one order of magnitude.
Abstract: Doping is a powerful tool to overcome contact limitations in short-channel organic field-effect transistors (OFETs) and has been successfully used in the past to improve the charge carrier injection in OFETs. The present study applies this familiar concept to the architecture of vertical organic field-effect transistors (VOFETs), which are often severely limited by injection due to their very short channel lengths. The present study shows that the performance of p-type VOFETs with pentacene as an active material can be significantly enhanced by the addition of the common p-dopant C60F36 as a thin injection layer underneath the VOFET source electrode, resulting in an increase of On-state current and On/Off ratio by one order of magnitude. The present study further investigates mixed injection layers of pentacene and the p-dopant and finds that the improvement is less pronounced than for the pure dopant layers and depends on the concentration of dopant molecules in the injection layer. Through application of the transfer length method to equivalent OFET geometries, the present study is finally able to link the observed improvement to a decrease in transfer length and can thus conclude that this length is a crucial parameter onto which further improvement efforts have to be concentrated to realize true short-channel VOFETs.
72 citations
TL;DR: In this paper, the authors investigated contact-induced nucleation in the context of small-molecule OSCs and OSC:polymer blends prepared by blade coating, a printing process capable of mimicking large area batch and roll-to-roll manufacturing.
Abstract: Scalable manufacturing of small-molecule organic thin film transistors (OTFTs) with performance approaching single crystals requires extraordinary control over microstructures and morphologies of organic semiconductors (OSCs). Here, contact-induced nucleation in the context of small-molecule OSCs and OSC:polymer blends prepared by blade coating, a printing process capable of mimicking large area batch and roll-to-roll manufacturing, is investigated. Using polarized optical microscopy, microbeam grazing incidence wide angle X-ray scattering, and energy-filtered transmission electron microscopy, it is revealed that previous design rules drawn from spin coating of OSCs and contact-induced nucleation may have to be revisited in the context of blade coating. It is shown that blade coating achieves texture purity in case of 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES-ADT), irrespective of whether the contact is chemically treated with a halogenated self-assembled monolayer (SAM) or not, in contrast to spin coating which requires an SAM. Here, it is demonstrated that OSC–contact interactions increase the nucleation density and can disrupt the vertical stratification in polymer:OSC blends with great detrimental effects on carrier transport. Using these lessons, we demonstrate bottom-contact bottom-gate OTFTs without chemical surface modification achieving hole mobilities of 4.6 and 3.6 cm2 V−1 s−1, using 6,13-bis(triisopropylsilylethynyl)pentacene and diF-TES-ADT, respectively, blended with an insulating polymer.
67 citations
TL;DR: From the temperature-variable current-voltage measurement, a space-charge-limited conduction and a variable range hopping conduction at a low temperature were suggested as the gate-tunable charge transport characteristics of these hybrid p-n heterojunctions.
Abstract: We investigated the trap-mediated electronic transport properties of pentacene/molybdenum disulphide (MoS2) p-n heterojunction devices. We observed that the hybrid p-n heterojunctions were gate-tunable and were strongly affected by trap-assisted tunnelling through the van der Waals gap at the heterojunction interfaces between MoS2 and pentacene. The pentacene/MoS2 p-n heterojunction diodes had gate-tunable high ideality factor, which resulted from trap-mediated conduction nature of devices. From the temperature-variable current-voltage measurement, a space-charge-limited conduction and a variable range hopping conduction at a low temperature were suggested as the gate-tunable charge transport characteristics of these hybrid p-n heterojunctions. Our study provides a better understanding of the trap-mediated electronic transport properties in organic/2-dimensional material hybrid heterojunction devices.
56 citations
TL;DR: In this article, flexible organic field effect transistors with high electrical stability upon bending are demonstrated on indium tin oxide coated polyethylene terephthalate substrates with TIPS-Pentacene semiconductor crystals formed by drop casting on a hybrid gate dielectric consisting hafnium dioxide grown by atomic layer deposition and spin coated poly(4-vinylphenol).
55 citations
TL;DR: Pentacene diodes with 2,3,4,5,6-pentafluorobenzenethiol-coated Au anodes show high current densities of 100 A cm−2 at 3 V with rectification ratios of 107.8 V to be achieved from a 10 V sinusoidal input at 1 GHz as discussed by the authors.
Abstract: Pentacene diodes with 2,3,4,5,6-pentafluorobenzenethiol-coated Au anodes show high current densities of 100 A cm−2 at 3 V with rectification ratios of 107. Using such diodes in rectifier circuits allows an output voltage of 3.8 V to be achieved from a 10 V sinusoidal input at 1 GHz.
TL;DR: In this paper, organic and inorganic thin film transistors (TFTs) are fabricated, simulated, and tested for circuit applications and two-dimensional finite element simulation methodology is used.
TL;DR: It could be demonstrated that the substrate not only determines the geometric and electronic characteristics of molecular monolayer films but also plays a crucial role for multilayer film growth.
Abstract: We measured the electronic and the molecular surface structure of pentacene deposited on the (1 1 1)-surfaces of coinage metals by means of ultraviolet photoelectron spectroscopy (UPS) and low-energy electron diffraction (LEED). Pentacene is almost flat-lying in monolayers on all three substrates and highly ordered on Au(1 1 1) and on Cu(1 1 1). On Ag(1 1 1), however, weak chemisorption leads to almost disordered monolayers, both, at room temperature and at 78 K. On Cu(1 1 1) pentacene is strongly chemisorbed and the lowest unoccupied molecular orbital becomes observable in UPS by a charge transfer from the substrate. On Ag(1 1 1) and Cu(1 1 1) multilayers adopt a tilted orientation and a high degree of crystallinity. On Au(1 1 1), most likely, also in multilayers the molecular short and long axes are parallel to the substrate, leading to a distinctively different electronic structure than on Ag(1 1 1) and Cu(1 1 1). Overall, it could be demonstrated that the substrate not only determines the geometric and electronic characteristics of molecular monolayer films but also plays a crucial role for multilayer film growth.
TL;DR: In this paper, an insulating polymer, poly(α-methyl styrene) (PαMS) was blended with TIPS pentacene to relieve the thermal stress and effectively prevent the generation of thermal cracks.
TL;DR: A 50 nm-wide 6,13-bis(triisopropylsilylethynyl) pentacene nanowire (NW) array is fabricated on a centimeter-sized substrate via a facile nanograting-assisted pattern-transfer method, and a NW-based organic field-effect transistor shows high field- effect mobility.
Abstract: A 50 nm-wide 6,13-bis(triisopropylsilylethynyl) pentacene nanowire (NW) array is fabricated on a centimeter-sized substrate via a facile nanograting-assisted pattern-transfer method. NW growth under a nanoconfined space adopts a lattice-strained packing motif of the NWs for strong intermolecular electronic coupling, and thus a NW-based organic field-effect transistor shows high field-effect mobility up to 9.71 cm(2) V(-1) s(-1) .
TL;DR: In this paper, the authors used the low and high density porous structure of polymethylmethacrylate (PMMA) film as tunneling dielectric layer in the floating-gate organic field effect transistor (OFET) memory devices.
TL;DR: The present experimental findings provide molecular scale insights into the formation mechanisms of the organic p-n heterojunction through an accurate structural analysis of the single-crystalline molecular contact.
Abstract: Designing molecular p–n heterojunction structures, i.e., electron donor–acceptor contacts, is one of the central challenges for further development of organic electronic devices. In the present study, a well-defined p–n heterojunction of two representative molecular semiconductors, pentacene and C60, formed on the single-crystal surface of pentacene is precisely investigated in terms of its growth behavior and crystallographic structure. C60 assembles into a (111)-oriented face-centered-cubic crystal structure with a specific epitaxial orientation on the (001) surface of the pentacene single crystal. The present experimental findings provide molecular scale insights into the formation mechanisms of the organic p–n heterojunction through an accurate structural analysis of the single-crystalline molecular contact.
TL;DR: In this article, a photo-sensitive polymer electret material, spiropyran, containing poly(3,5-benzoic acid hexafluoroisopropylidene diphthalimide) (6FDA-DBA-SP), was presented.
Abstract: Light-responsive spirotype compounds have been receiving attention as attractive smart materials because of their various potential applications in organic optoelectronic devices, based on organic field-effect transistors (OFETs). However, it still remains a challenge to apply the organic flash memory devices using spirotype compounds due to the relatively poor development of new photosensitive electret materials. Here, we report the synthesis of a novel photosensitive polymer electret material, spiropyran, containing poly(3,5-benzoic acid hexafluoroisopropylidene diphthalimide) (6FDA-DBA-SP), and the development of light-responsive flexible memory devices using the 6FDA-DBA-SP electret layer. The charge trapping properties of 6FDA-DBA-SP under light illumination were enhanced by the electron withdrawing properties and lowering energetic barrier of charge trapping between 6FDA-DBA-SP and pentacene analysed by measuring the electronic structures at the pentacene/6FDA-DBA-SP interfaces. The resulting OFETs showed enlarged hysteresis under white-light illumination and exhibited bi-stable current states after the light-assisted programing and erasing processes, and they were utilized in non-volatile flexible memory device applications.
TL;DR: Flexible organic field effect transistors (OFETs) with TIPS-pentacene: polystyrene (PS) blend are demonstrated to exhibit enhanced mobility and significantly improved electrical stability compared to neat TIPS pentacene on poly(4-vinylphenol) (PVP) dielectric (bi-layer OFETs), along with high mechanical stability.
TL;DR: In this paper, the authors describe the fabrication and characterization of Al-O-Ti (AT) nanolaminate gate dielectric films using a PEALD process, and their OFET applications.
TL;DR: A new class of conjugated polycyclic molecules containing a C6–C5–C6-C7–C7-C6 polycylic framework was synthesized, showing different electron structures in comparison to their pentacene isomers.
Abstract: Benzo[4,5]cyclohepta[1,2-b]fluorene (5a), a new π-conjugated polycyclic hydrocarbon containing linearly fused six-, five-, six-, seven- and six-membered rings (C6–C5–C6–C7–C6), was designed and its stable derivatives 5b and 5c were synthesized. With 22 π electrons, 5a is an isomer of pentacene with quinoidal, dipolar ionic and diradical resonance forms. Molecules 5b and 5c were experimentally investigated with cyclic voltammetry, electronic absorption spectroscopy and X-ray crystallographic analysis, and theoretically studied by calculating the NICS value, diradical character and dipole moment. A comparison of 5a–c with pentacene and other pentacene analogues containing linearly fused five- or seven-membered rings was also conducted and discussed. It was found that 5b behaved as a p-type organic semiconductor in solution-processed thin film transistors with a field effect mobility of up to 0.025 cm2 V−1 s−1.
TL;DR: In this paper, the authors investigated the performance of four π-conjugated low molecules, the [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentaene), under various laser light irradiation conditions using THz time-domain spectroscopy.
Abstract: Terahertz (THz)-transmission modulation through the Si substrate coated with four π-conjugated materials under various laser light irradiation conditions was investigated using THz time-domain spectroscopy. Two of the π-conjugated low molecules, the [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), and two of the π-conjugated polymer materials, poly[5-(2-ethylhexyloxy)-2-methoxycyanoterephthalyliden] (MEH-CN-PPV) and poly(benzimidazobenzophenanthroline) (BBL), were investigated. Among these materials, PCBM and TIPS-pentacene showed higher modulation efficiencies and it was also shown that thermal annealing is quite effective in obtaining THz modulation efficiencies higher than those previously reported for copper phthalocyanine. Utilizing these solution-processable π-conjugated materials, various types of THz materials and devices could be fabricated by printing technologies. Our findings may open the way to fabricating various types of THz active devices.
TL;DR: In this article, a pentacene-based thin film phototransistors were fabricated with a photocurable polymer insulator and their electrical stability was monitored when the devices were exposed to light sources at different wavelengths.
TL;DR: It is found that pentacene moves along rails parallel and perpendicular to the surface molecules, which extends the molecular level understanding of diffusion in complex organic systems.
Abstract: The realization of efficient organic electronic devices requires the controlled preparation of molecular thin films and heterostructures. As top-down structuring methods such as lithography cannot be applied to van der Waals bound materials, surface diffusion becomes a structure-determining factor that requires microscopic understanding. Scanning probe techniques provide atomic resolution, but are limited to observations of slow movements, and therefore constrained to low temperatures. In contrast, the helium-3 spin-echo (HeSE) technique achieves spatial and time resolution on the nm and ps scale, respectively, thus enabling measurements at elevated temperatures. Here we use HeSE to unveil the intricate motion of pentacene admolecules diffusing on a chemisorbed monolayer of pentacene on Cu(110) that serves as a stable, well-ordered organic model surface. We find that pentacene moves along rails parallel and perpendicular to the surface molecules. The experimental data are explained by admolecule rotation that enables a switching between diffusion directions, which extends our molecular level understanding of diffusion in complex organic systems.
TL;DR: Time-resolved transient absorption measurements revealed the formation of excited triplet states of the pentacene derivatives by singlet fission in high yield by suppressing energy transfer to the gold surface.
Abstract: One of the major drawbacks of organic-dye-modified self-assembled monolayers on metal nanoparticles when employed for efficient use of light energy is the fact that singlet excited states on dye molecules can be easily deactivated by means of energy transfer to the metal surface. In this study, a series of 6,13-bis(triisopropylsilylethynyl)pentacene-alkanethiolate monolayer protected gold nanoparticles with different particle sizes and alkane chain lengths were successfully synthesized and were employed for the efficient generation of excited triplet states of the pentacene derivatives by singlet fission. Time-resolved transient absorption measurements revealed the formation of excited triplet states in high yield (172±26 %) by suppressing energy transfer to the gold surface.
TL;DR: Zirconium tetraacrylate (ZrTA) as discussed by the authors is an organic-inorganic hybrid gate dielectric material, where inorganic Zr elements are embedded in organic acrylate matrix.
Abstract: In this study, we have demonstrated a novel organic–inorganic hybrid gate dielectric material, zirconium tetraacrylate (ZrTA). ZrTA gate dielectric, where inorganic Zr elements are embedded in organic acrylate matrix, takes advantage of the complementary properties of single organic or inorganic gate dielectrics. A simple spin-coating and UV-assisted cross-linking reaction of acrylate moieties allowed ZrTA film to be photopatterned. The cross-linked ZrTA film by UV and heat treatments (UV, 365 nm for 3 min; heat, 120 °C for 30 min) showed high dielectric strength (10–7 A/cm2 at 2 MV/cm), and dielectric constant (5.48). In addition, surface properties of the ZrTA film (surface energy, surface roughness) were favorable for the growth of overlying pentacene organic semiconductor. Consequently, the organic thin-film transistor composed of a pentacene semiconductor and a cross-linked ZrTA gate dielectric displayed a moderately high field-effect mobility of 0.50 cm2/(V·s) with a negligible hysteresis transfer c...
TL;DR: In this paper, the working mechanism of a planar organic heterojunction based on pentacene/C70 under reverse voltage is studied through currentvoltage and capacitance-voltage measurements.
Abstract: The working mechanism of a planar organic heterojunction based on pentacene/C70 under reverse voltage is studied through current–voltage (I–V) and capacitance–voltage (C–V) measurements. It is found that the pentacene/C70 heterojunction generates large amounts of charges and the charge generation is a tunneling process. The proposed Fowler–Nordheim (F–N) model theoretically demonstrates the I–V properties of the pentacene/C70 heterojunction-based device at different temperatures. The heterojunction interface energy diagram is also well determined by ultraviolet photoemission spectroscopy (UPS) measurements, further elucidating this tunneling process. Moreover, by taking advantage of the large charge generation property of the pentacene/C70 heterojunction, a high efficiency green tandem organic light emitting diode (OLED) is successfully fabricated, where not only the current efficiency is doubled, but also the power efficiency is greatly enhanced, proving the excellent performance of the pentacene/C70 heterojunction as charge generation layer (CGL). This work highlights the working mechanism of such heterojunction and provides us a new guide to design high performance tandem OLEDs.
TL;DR: In this article, orthogonal photolithography is used for the patterning of the source/drain electrodes of the organic field effect transistors (OFETs) as well as the interface dopant insertion layers for further modifications of the contact resistance.
Abstract: Emerging organic integrated electronics require capability of high speed and the compatibility with high-resolution structuring processes such as photolithography. When downscaling the channel length, the contact resistance is known to limit the performance of the short channel devices. In this report, orthogonal photolithography is used for the patterning of the source/drain electrodes of the organic field-effect transistors (OFETs) as well as the interface dopant insertion layers for further modifications of the contact resistance. Bottom-gate top-contact pentacene OFETs with different thicknesses of the p-dopant 2,2′-(perfluoronaphthalene-2,6-diylidene)dimalononitrile under the Au electrodes show a significant decrease in threshold voltage from −2.2 V to −0.8 V and in contact resistance from 55 k Ω cm to 10 k Ω cm by adding a 1 nm thin dopant interlayer. The influence of doping on charge carrier injection is directly visible in the temperature-dependent output characteristics and a charge-transfer acti...
TL;DR: In this paper, a low-temperature atomic layer deposition (ALD) was proposed to enhance the charge injection by using an ultrathin vanadium oxide (VOx) layer as an efficient hole injection interlayer for organic field effect transistors (OFETs).
Abstract: Charge injection at metal/organic interface is a critical issue for organic electronic devices in general as poor charge injection would cause high contact resistance and severely limit the performance of organic devices. In this work, a new approach is presented to enhance the charge injection by using atomic layer deposition (ALD) to prepare an ultrathin vanadium oxide (VOx) layer as an efficient hole injection interlayer for organic field-effect transistors (OFETs). Since organic materials are generally delicate, a gentle low-temperature ALD process is necessary for compatibility. Therefore, a new low-temperature ALD process is developed for VOx at 50 °C using a highly volatile vanadium precursor of tetrakis(dimethylamino)vanadium and non-oxidizing water as the oxygen source. The process is able to prepare highly smooth, uniform, and conformal VOx thin films with precise control of film thickness. With this ALD process, it is further demonstrated that the ALD VOx interlayer is able to remarkably reduce the interface contact resistance, and, therefore, significantly enhance the device performance of OFETs. Multiple combinations of the metal/VOx/organic interface (i.e., Cu/VOx/pentacene, Au/VOx/pentacene, and Au/VOx/BOPAnt) are examined, and the results uniformly show the effectiveness of reducing the contact resistance in all cases, which, therefore, highlights the broad promise of this ALD approach for organic devices applications in general.
TL;DR: The dynamic nature of the interface is thus critical, with the polarization energy distributions indicating that sites along the interface shift in time between favorable and unfavorable configurations for charge separation.
Abstract: We probe the energetic landscape at a model pentacene/fullerene (C60) interface to investigate the interactions between positive and negative charges, which are critical to the processes of charge separation and recombination in organic solar cells. Using a polarizable force field, we find that polarization energy, i.e., the stabilization a charge feels due to its environment, is larger at the interface than in the bulk for both a positive and a negative charge. The combination of the charge being more stabilized at the interface and the Coulomb attraction between the charges results in a barrier to charge separation at the pentacene/C60 interface that can be in excess of 0.7 eV for static configurations of the donor and acceptor locations. However, the impact of molecular motions, i.e., the dynamics, at the interface at room temperature results in a distribution of polarization energies and in charge separation barriers that can be significantly reduced. The dynamic nature of the interface is thus critic...