Performance Of Organic Devices Research Articles

Growth Mode Transition from Monolayer by Monolayer to Bilayer by Bilayer in Molecularly Flat Titanyl Phthalocyanine Film

To avoid defects associated with inhomogeneous crystallites and uneven morphology that degrade organic device performance, the deposition of ultraflat and homogeneous crystalline organic active layers is required. The growth mode transition of organic semiconducting titanyl phthalocyanine (TiOPc) molecule from monolayer-by-monolayer to bilayer-by-bilayer can be observed on highly ordered pyrolytic graphic (HOPG), while maintaining large and molecularly flat domains. The first monolayer of TiOPc lies flat on HOPG with a ∼98% face-up orientation. However, as the thickness of the TiOPc increases to over 15 monolayers (ML), the growth mode transitions to bilayer-by-bilayer with the repeated stacking of bilayers (BL), each of which has face-to-face pairs. Density functional theory calculations reveal that the increasing of thickness induces weakening of the substrate effect on the deposited TiOPc layers, resulting in the growth mode transition to BL-by-BL. The asymmetric stacking provides the driving force to ...

A Two-Dimensional Supramolecular Carpet with Widely Uniform Interfacial Structure

Thin films composed of organic molecules are receiving much attention for their practical uses in various organic devices such as organic light-emitting diodes (OLEDs), organic thin film transistors (OTFTs), and organic photovoltaics (OPVs). Particularly, it has been recognized that interfaces between organic thin films and metal electrodes play crucial roles in operation characteristics of organic devices in recent years [1-3]. The interfaces between organic thin films and metal electrodes are where charge-carriers are injected into organic active layers, and various interactions at the interfaces (i.e., surface-molecule and intermolecular interactions) are of great importance in the formation of intermolecular networks and organic epitaxy which have a strong correlation with their electronic structures [1,3]. Both geometric and electronic structures of the interfaces between organic thin films and metal electrodes have profound effects on the performance of organic devices, and it is necessary to understand and control such interfacial structures for further development and enhancement of organic devices. Despite extensive studies, however, the formation of organic thin films with widely uniform interfacial structures on metal surfaces has been suffered from the limitation of molecular ordering imposed by irregular surface structures, due to the relatively weak intermolecular interactions between organic molecules compared to metallic, covalent, and coordinate bonds. Thus, a new approach is required to achieve the formation of widely uniform organic thin films without breaking structural integrity and to enhance the film uniformity. Here we demonstrate that well-balanced interfacial interactions, based on a careful design of the system, enable the formation of a two-dimensional supramolecular carpet with widely uniform interfacial structure and high adaptability on a metal surface, which can be extended over multiple surface steps and terraces without breaking structural integrity [4]. Strong intermolecular interactions in a planar fashion, symmetry of compounds, and localized surface-molecule interactions were considered in our strategies and principles of the system design. For this work, bis([1,2,5]thiadiazolo)-tetracyanoquinodimethane (BTDA-TCNQ) was used as a key element to form the two-dimensional supramolecular carpet [5], and its geometric and electronic structures were investigated using scanning tunneling microscopy and scanning tunneling spectroscopy under ultrahigh-vacuum conditions. Tetracyanoquinodimethane (TCNQ) is one of the strongest organic electron acceptors which is a widely used class in organic electronics, and it forms a strongly bonded donor-acceptor complex with tetrathiafulvalene (TTF). TCNQ and TTF are self-organized into ordered domains of the strongly bonded donor-acceptor complexes if TCNQ and TTF satisfy a 1:1 stoichiometry. Based on these molecular properties, we designed a model system of BTDA-TCNQ, which integrates both structural properties of TCNQ and TTF with a single molecule. The strong non-bonding intermolecular interactions induced by two electrostatically opposite symmetry axes of the molecule, balanced with surface-molecule interactions and site-specific rearrangements of the BTDA-TCNQ molecules near surface step edges, enables “seamless” growth mode for the formation of supramolecular carpet. It results in widely uniform interfacial structures covering over multiple surface steps and terraces even on the pre-annealed amorphous gold (Au) surface prepared on a glass substrate. In addition, the electronic structures projected onto the supramolecular carpet on the Au surface show different types and dimensionalities corresponding to the energy region and the local position of the system. These result suggest that the supramolecular carpet has great potential for applications in organic electronics, and also provide important guidelines to develop novel materials for seamless growth mode on various surfaces. [1] S. R. Forrest, Chem. Rev. 97, 1793 (1997). [2] H. Ishii, K. Sugiyama, E. Ito, and K. Seki, Adv. Mater. 11, 605 (1999). [3] D. E. Hooks, T. Fritz, and M. D. Ward, Adv. Mater. 13, 227 (2001). [4] J.-H. Kim, J.-C. Ribierre, Y. S. Yang, C. Adachi, M. Kawai, J. Jung, T. Fukushima, and Y. Kim, Nat. Commun. 7, 10653 (2016) [5] Y. Yamashita, T. Suzuku, T. Mukai, and G. Saito, J. Chem. Soc. Chem. Commun. 1985, 1044 (1985).

Efficient Charge Injection in Organic Field‐Effect Transistors Enabled by Low‐Temperature Atomic Layer Deposition of Ultrathin VOx Interlayer

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.

Simulation of the performance of organic electronic devices based on a two-dimensional drift-diffusion approach

Organic electronic devices will play an important role in future technical applications because of their easy production and low costs. Besides intrinsic material properties, the performance of organic solar cells, light emitting diods and thin-film transistors depends strongly on the morphology, the occurring hetero-junctions, as well as on achieving an appropriate alignment of transport levels. To optimize the efficiency of organic devices it is, therefore, important to understand what limits their performance. The paper supports this concern by providing a general model for simulating the exciton and charge carrier transport in such building blocks. Due to the complex internal structure and the large aspect ratios of the active device regions, the procedure is based on drift-diffusion equa- tions. All of the necessary source, generation and recombination terms are included in the developed model. The applied numerics is based on finite difference approaches and especially on the Scharfetter-Gummel algorithm. Three examples, which prove the appli- cability of the established formalism to enhance the performance of organic devices are given. The performed simulations reveal interesting particularities of the carrier transport in these devices.

Seamless growth of a supramolecular carpet.

Organic/metal interfaces play crucial roles in the formation of intermolecular networks on metal surfaces and the performance of organic devices. Although their purity and uniformity have profound effects on the operation of organic devices, the formation of organic thin films with high interfacial uniformity on metal surfaces has suffered from the intrinsic limitation of molecular ordering imposed by irregular surface structures. Here we demonstrate a supramolecular carpet with widely uniform interfacial structure and high adaptability on a metal surface via a one-step process. The high uniformity is achieved with well-balanced interfacial interactions and site-specific molecular rearrangements, even on a pre-annealed amorphous gold surface. Co-existing electronic structures show selective availability corresponding to the energy region and the local position of the system. These findings provide not only a deeper insight into organic thin films with high structural integrity, but also a new way to tailor interfacial geometric and electronic structures.

Comparative contributions of singlet and triplet excitons in the performance of organic devices

AbstractThe properties of singlet and triplet excitons are compared in organic materials for applications in optoelectronic organic solar cells (OSCs) and organic light emitting devices (OLEDs). It is found that by managing the organic material designs both singlet and triplet excitons can be beneficially used in OSCs and OLEDs equally well. A model of dissociation of excitons excited in the acceptor material of the bulk heterojunction organic solar cells is presented. It is shown that the dissociation of acceptor excitons also can contribute to the photovoltaic performance of OSCs. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A comparative study about electronic structures at rubrene/Ag and Ag/rubrene interfaces

The contact between the electrode and the organic semiconductor is one of the most crucial factors in determining the organic device performance. The development and production technology of different organic devices require the understanding of different types of metal/organic semiconducting thin film interfaces. Comparisons about the electronic structures at Rubrene/Ag and Ag/Rubrene interfaces have been studied using photoemission spectroscopy. The Ag on rubrene interfaces is found to show more interesting and complex natures than its counterpart. The vacuum level (VL) was shifted about 0.51 eV from push back effect for deposition of 5 Å rubrene onto Ag film whereas the electronic features of silver was only suppressed and no energy shift was resulted. While the deposition of 5 Å Ag onto rubrene film leads to the diffusion of the Ag atoms, as a cluster with quantum size effect, inside the film. Angle dependent XPS measurement indicates that diffused metal clusters were present at entire probed depth of the film. Moreover these clusters dope the uppermost surface of the rubrene film which consequences a shift of the electronic states of thick organic film towards higher binding energy. The VL was found to shift about 0.31 eV toward higher binding energy whereas the shift was around 0.21 eV for the electronic states of rubrene layer.

Transient Photocurrent in Organic Photocells Assisted By Electric Double Layers in Electrolytes

Recently, there has been increasing interest in photoelectric conversion by organic semiconductors. Organic materials have many advantages over inorganic materials, such as the possibility of low temperature, and thus low cost processing, which allows for flexible architectures to be realized. However, to date, organic device performance still lags behind that of inorganic devices. It is our aim to use the intrinsic properties of organic materials to develop novel operation principles for organic devices, to rival the performance of inorganic devices. In previous works, our group has reported on anomalous transient photocurrents induced by a multi-layer photocell architecture, which incorporates a polarizable insulating layer to aid charge separation in the active semiconductor layer. More recently, we reported the use of ionic liquids as the insulating layer. Such architectures generate a time-varying (transient) response to continuous illumination, which may have an application to optical communication. In these devices, The electric double layers in ionic liquid enhance the charge separation in the active layers and generate an anomalous transient photocurrent. In this work, we adopted the KCl aqueous solution to the insulating layer, instead of ionic liquids. Figure.1 shows the transient photocurrent with several concentration KCl aqueous solutions. The transient photocurrent was obtained ever with a small amount of KCl. Furthermore, there is a concentration dependence of the peak values and the relaxation times of the transient photocurrents. Figure 1

Effects of temperature and clay content on water absorption characteristics of modified MMT clay/cyclic olefin copolymer nanocomposite films: Permeability, dynamic mechanical properties and the encapsulated organic device performance

In the present study, amino-silane modified layered organosilicates were used to reinforce cyclic olefin copolymer to enhance the thermal, mechanical and moisture impermeable barrier properties. The optimum clay loading (4%) in the nanocomposite increases the thermal stability of the film while further loading decreases film stability. Water absorption behavior at 62°C was carried out and compared with the behavior at room temperature and 48°C. The stiffness of the matrix increases with clay content and the recorded strain to failure for the composite films was lower than the neat film. Dynamic mechanical analysis show higher storage modulus and low loss modulus for 2.5–4wt% clay loading. Calcium degradation test and device encapsulation also show the evidence of optimum clay loading of 4wt% for improved low water vapor transmission rates compared to other nanocomposite films.

Quinacridone on Ag(111): Hydrogen Bonding versus Chirality

Quinacridone (QA) has recently gained attention as an organic semiconductor with unexpectedly high performance in organic devices. The strong intermolecular connection via hydrogen bonds is expected to promote good structural order. When deposited on a substrate, another relevant factor comes into play, namely the 2D-chirality of the quinacridone molecules adsorbed on a surface. Scanning tunneling microscopy (STM) images of monolayer quinacridone on Ag(111) deposited at room temperature reveal the formation of quasi-one-dimensional rows of parallel quinacridone molecules. These rows are segmented into short stacks of a few molecules in which adjacent, flat-lying molecules of a single handedness are linked via hydrogen bonds. After annealing to a temperature of T = 550–570 K, which is close to the sublimation temperature of bulk quinacridone, the structure changes into a stacking of heterochiral quinacridone dimers with a markedly different intermolecular arrangement. Electron diffraction (LEED) and photoelectron emission microscopy (PEEM) data corroborate the STM findings. These results illustrate how the effects of hydrogen bonding and chirality can compete and give rise to very different (meta)stable structures of quinacridone on surfaces.

Crystalline Organic Heterostructures Engineering Based on Vanadyl Phthalocyanine and Rod‐Like Conjugated Organic Semiconductors with Selected Central Groups

AbstractThe effect of groups in conjugated molecules on films’ interface morphology, electronic structure, and charge transport behavior is explored utilizing a series of crystalline organic heterostructures that are constructed by four rod‐like molecules and vanadyl phthalocyanine (VOPc). The four rod‐like molecules, which possess the same biphenyl endgroup but different central groups, present similar growth behavior along with different thin film structural parameters and electronic structures. The changes of the structure parameters result in diverse morphology of the VOPc/rod‐like molecule heterostructure because of the different lattice mismatch. The electronic structure differences in rod‐like molecules cause different interface electronic structure of the heterostructure. Under the conjunct effect of morphology and interface electronic structure, the transistors based on the crystalline heterostructures present diverse charge transport behavior and field‐effect mobilities. These results provide a clue for the development of crystalline organic heterostructure engineering and tailoring overall organic device performance through molecular design in local inducing layer.

MAPLE-deposited polymer films for improved organic device performance

Matrix-assisted pulsed-laser evaporation (MAPLE) provides a mechanism for layer-by-layer growth to control the polymer–dielectric interface in organic metal–insulator–semiconductor (MIS) diodes and field-effect transistors (FETs). MAPLE-deposited copolymers of polyfluorene (PF) and polythiophene maintain their structural and optical properties, as determined by Raman spectroscopy, absorption, and photoluminescence. These films are further utilized in MIS and FET structures with SiO2 and other polymer dielectrics. Since common polymer dielectrics prevent spin coating of solution processable polymers due to solubility effects, MAPLE is one of the only deposition techniques for investigating all polymer semiconductor-insulator interfaces. In this paper we present optical and electrical studies of MAPLE-deposited PF and polythiophene films in FETs and MIS structures. The FET carrier mobilities of these devices compare well with spin-coated devices. Capacitance–voltage and conductance–voltage from MIS structures with MAPLE-deposited PF copolymer films yield interface trap densities in the low 1012 eV−1 cm−2 range.

Polysilane based ultraviolet light-emitting diodes with improved turn-on voltage, stability and color purity

We demonstrate ultraviolet organic light-emitting diodes (OLEDs) with improved stability, low turn-on voltage and color purity by changing the cathode and annealing temperature of the polymer film. The electron injection process, which limits the electroluminescent performance of organic devices, has been enhanced tremendously by inserting a layer of LiF with appropriate thickness between the cathode and a poly( n-butylphenylsilane) (PS-4) layer, whose device structure is ITO/PEDOT:PSS/polysilane (PS-4)/LiF/Al. Devices with a LiF (6 Å) have the turn-on voltage of 4 V, which is lower than 7 V of devices made with Ca/Al layer. By inserting LiF as the anode interfacial layer, there is increase in the injection of electrons from Al (cathode) side due to tunneling effect and also act as hole blocking layer which enhance the recombination of electron and hole in the emissive layer. PS-4 is spin coated and annealed in vacuum for 1 h at different temperatures (90–120 °C). EL Spectra from these devices consists of white emission along with the UV peak. White emission is significantly suppressed when PS-4 is annealed at higher temperature and threshold voltage is lowest at 110 °C annealing temperature.

A particularly strong organic acceptor for tuning the hole-injection barriers in modern organic devices

A better understanding of metal-organic interfaces combined with means to control their properties is crucial for the further improvement of organic (opto)electronic devices. In this context, the use of organic acceptors is an efficient tool to modify metal work functions and hole-injection barriers, which has the potential to considerably improve the performance of organic devices. Here, we use density functional theory based calculations to discuss a particularly potent acceptor suitable for that purpose, 3,5-difluoro-2,5,7,7,8,8-hexacyanoquinodimethane (F2HCNQ), which clearly outperforms the frequently applied and in the meantime prototypical system 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). Comparative calculations for a single monolayer of the two molecules adsorbed on an Ag(1 1 1) surface reveal that (i) the work-function increase induced by F2HCNQ is more than 20% higher than for F4TCNQ and that (ii) at the same time the adsorption energy basically is unaffected, while (iii) the electronic structure is slightly modified. In the end of the day, F2HCNQ is a highly promising candidate for applications in organic devices.

Dicyanovinyl-quinquethiophenes with varying alkyl chain lengths: Investigation of their performance in organic devices

We compare between two derivatives of dicyanovinyl-quinquethiophenes with different alkyl side chain lengths. Both materials show comparable open circuit voltages Voc in organic solar cells with fullerene C60 as acceptor, as expected since they have the same highest occupied molecular orbital energy. However, differences in the current-voltage-characteristics, particularly in the fill factor, are observed. We analyze both derivatives in hole-only devices and find a difference in the hole injection between the doped hole transport layer and the oligothiophenes. Additionally, we determine the hole mobility of the two materials and explain the different behaviors of the two materials in solar cells.

Optimal negative electrodes for poly(3-hexylthiophene): [6,6]-phenyl C61-butyric acid methyl ester bulk heterojunction photovoltaic devices

The role of the work function and interfacial chemistry on organic device performance was investigated by using a series of contact materials. The active layer was a standard blend of poly(3-hexylthiophene) and [6-6]-phenyl C61-butyric acid methyl ester. Over 100 devices were fabricated and measured to obtain good statistics. Ba∕Al and Ca∕Al electrodes performed best, with similar open-circuit voltages and power conversion efficiencies. Device stability studies showed devices with these two electrodes remained similar after six weeks with degradation of 11%–16% in net conversion efficiency observed. The incorporation of silver into the electrodes led to considerably more degradation than other electrode types.

The Molecular Orientation of para‐Sexiphenyl on Cu(110) and Cu(110) p(2×1)O

Controlling the molecular growth of organic semiconductors is an important issue to optimize the performance of organic devices. Conjugated molecules, used as building blocks, have an anisotropic shape and also anisotropic physical properties like charge transport or luminescence. The main challenge is to grow highly crystalline layers with molecules of defined orientation. The higher the crystallinity, the closer these properties reach their full intrinsic potential, while the orientation determines the physical properties of the film. Herein we show that the molecular orientation and growth can be steered by the surface chemistry, which tunes the molecule-substrate interaction. In addition, the oxygen reconstruction of the surface, demonstrates the flexibility of the organic molecules to adopt a given surface corrugation and their unique possibility to release stress by tilting.

Systematic control of the electrical conductivity of poly (3,4-ethylenedioxythiophene) via oxidative chemical vapor deposition (oCVD)

Systematic variation in the electrical conductivity of poly (3,4 ethylenedioxythiophene) (PEDOT) was achieved by oxidative chemical vapor deposition (oCVD). For oCVD, both the oxidant, Fe(III)Cl 3, and 3,4 ethylenedioxythiophene (EDOT) monomer are introduced in the vapor phase. A heated crucible allows for sublimation of the oxidant directly into the reactor chamber. Spontaneous reaction of the oxidant with the monomer results in the rapid (> 200 nm thick film in 30 min) formation of π -conjugated PEDOT thin films directly onto a temperature controlled substrate. As the substrate temperature is increased from 15 °C to 100 °C, increasing conjugation length and doping level of the PEDOT chains are observed by Raman spectroscopy. The X-ray photoelectron spectroscopy (XPS) study showed that the surface consists of PEDOT polymer chains doped with Cl − ions in the absence of metallic impurities such as Fe. This contrasts to the surface composition of commonly used PEDOT:PSS films, where the PSS segregates to the surface. Scanning transmission electron microscopy (STEM) with energy dispersive X-ray analysis (EDX) study clearly demonstrates that the bulk composition is uniformly maintained throughout thickness of the film and is identical to the surface concentration. Both the substrate and air side interfaces were abrupt. The doping level could also be controlled by changing substrate temperature and accordingly, the work function was tuned from 5.1 to 5.4 eV, a range desirable for optimizing hole transport by lowering the charge injection barrier. The controllability of doping level and work function of oCVD PEDOT offers great potential advantages for optimization of the performance of organic devices.

Influence of the Side Functionalization of Quinquethiophene-S,S-Dioxides on the Morphology of Blends with Poly(3-hexylthiophene): Scanning Force Microscopy Reveals

Blends of an electron donor, i.e. a regioregular poly(3-hexylthiophene) (P3HT), with electron acceptors, a series of soluble quinquethiophene-S,S-dioxides (T5Os) bearing different alkyl side groups were self-assembled at surfaces. Scanning Force Microscopy (SFM) studies revealed that while the T5O symmetrically functionalized with two hexyl groups in the central thiophene (1) self-organizes into micrometer sized crystals embedded in a grainy matrix of P3HT, by substituting the central thiophene of 1 with one hexyl and one methyl unit (2) smaller and less anisotropic crystals of the acceptor having a sub-micrometer scale size were formed. The generation of these crystals is due to the joint effect of different non-covalent intermolecular interactions between the T5Os that self-segregate from the P3HT. By derivatizing the compound 1 with cyclo-hexyl moieties in the four external thiophenes molecule 3 was obtained. Such system was found to assemble into grainy disordered structures when co-deposited with P3HT, providing evidence for the absence of a phase segregation between the two components. Generally, the self-assembly at surfaces is governed by the interplay of intramolecular as well as intermolecular and interfacial interactions. In the present case, the cyclo-hexyl side groups in 3 both induce an intramolecular loss of planarity of the thiophene rings and hinders intermolecular interactions, reducing the tendency of the molecules to self-associate forming large crystals, whereas the symmetrical functionalization of the two central thiophenes with hexyl chains favours the crystallization of the T5O. The reported results demonstrate that subtle differences in the chemical functionalization can lead to different types of molecular architectures at surfaces. This is of importance since controlling the self-organization of pi-conjugated molecules at surfaces towards pre-programmed assemblies is a viable approach to enhance their electronic and luminescent properties, which should help to improve the performance of organic devices.

Heteroepitaxy of Organic−Organic Nanostructures

Highly crystalline organic heteroepitaxial layers with controlled molecular orientations and morphologies are one of the keys for optimum organic device performance. With studies of molecular orientation, structure, and morphology, we have investigated the ability of oriented organic films to act as substrate templates for the growth of a second organic layer. Depending on the molecular orientation in the sexiphenyl substrate, crystalline sexithiophene nanostructures of either pyramidal or needlelike morphology, with either near vertical or parallel molecular orientations, respectively, grow.