Organic Heterostructures Research Articles

Investigation of Carrier Collection Capability in Organic Heterostructure with Conductive Polymer Nanofiber

Investigation of Carrier Collection Capability in Organic Heterostructure with Conductive Polymer Nanofiber

Templating Effect for Organic Heterostructure Film Growth: Perfluoropentacene on Diindenoperylene

We present a systematic study of templating effects in organic heterostructure growth, that is, where the growth behavior of the top layer is significantly influenced by the structural and morphological properties of the bottom layer. We prepared thin films of the donor material diindenoperylene (DIP) differing in crystal structure, grain size, and grain orientation on indium tin oxide (ITO). The acceptor material perfluoropentacene (PFP) was chosen to be evaporated on DIP to form the heterostructure. We employed X-ray reflectivity and grazing incidence X-ray diffraction as well as atomic force microscopy to investigate the heterostructures. The structure and morphology of the PFP layers depend on the properties of the DIP layer underneath. Two main effects were observed: PFP molecules in the top layer tend to adopt the orientation of the DIP molecules in the bottom layer and the crystalline quality of the PFP layer correlates with the crystalline quality of the DIP layer underneath in terms of the in-pla...

Interfacial energy level bending in a crystalline p/p-type organic heterostructure

A conduction channel was observed at the heterointerface of the crystalline p-type organic films copper phthalocyanine (CuPc) and 2,5-bis(4-biphenylyl) bithiophene (BP2T). Energy level bending at the interface is confirmed by photoemission spectroscopy, which verifies a charge transfer between CuPc and BP2T. This provides a further route to utilize interfacial electronic properties in functional devices and also documents the importance of reconsidering the interfacial electronic structure of organic heterostructures.

Maple prepared organic heterostructures for photovoltaic applications

In this study, we present the deposition of ZnPc, Alq3, and PTCDA thin films using Matrix Assisted Pulsed Laser Evaporation (MAPLE) technique. We also report the realisation of multilayer structures, made by the successive application of MAPLE. The films have been characterized by spectroscopic (UV–VIS and Photoluminescence) and microscopic (SEM and AFM) methods, and the effect of different deposition conditions such as fluence, number of pulses, and target concentration on the properties has been analysed. This paper also presents some investigations on the electrical conduction in sandwich type structures ITO or Si/organic layer/Au or Cu and ITO/double organic layer/Cu, emphasising the dominant effect of the height of the energetic barriers at the inorganic/organic and organic/organic interfaces.

Layer inversion in organic heterostructures

Thermally activated layer inversion of ultrathin pentacene/para-sexiphenyl organic heterostructures is observed using a combination of reflectance difference spectroscopy and scanning tunneling microscopy. The heterostructures are formed by deposition of sub-monolayer pentacene (PEN) on top of well ordered para-sexiphenyl (p-6P) layers on Cu(110) at 15 K. When the sample temperature is raised, these heterostructures invert, with pentacene molecules diffusing through the para-sexiphenyl buffer layer and getting in direct contact with the substrate. The observed irreversible inversion demonstrates that the p-6P/PEN/Cu(110) is energetically preferred over PEN/p-6P/Cu(110). Furthermore, the onset temperature of the inversion increases with the layer thickness of para-sexiphenyl indicating a corresponding increase of the kinetic barrier for the inversion. Our results demonstrate the strong influence of the configuration of organic heterostructures on their thermal stability, especially for the very thin layers.

Field-enhanced charge flow in nanorod heterostructure solar cells

Recent studies on organic heterostructure solar cells have indicated that interface morphology plays an important role in determining the quantum efficiency. Hybrid heterostructure mixing donor and acceptor semiconductors appear to offer the best opportunity in achieving superior performance and there are indications that a network of percolated heterojunctions can be quite effective in promoting light absorption and exciton quenching. Charge transport and collection efficiency, however, appear to be more complex in the bulk heterostructure and the nature of charge flow depends largely on the type of current paths in existence. We report in this work the possible existence of field-assisted charge flow in the nanorod heterostructure solar cells when carriers of different polarity move in close proximity. The field-effect associated with the charge density gradients can exert a force on the nearby carriers resulting in an increase in the short-circuit current. The model is used to explain data reported in the literature on solar cells composed of TiO2 nanorods embedded in a conjugated polymer.

Ambipolar field-effect transistor based on α,ω-dihexylquaterthiophene and α,ω-diperfluoroquaterthiophene vertical heterojunction

Organic field-effect transistors (OFETs) are alternative emerging device structures for efficient light generation, that could provide a novel architecture to address open issues like exciton-contact and exciton-charge quenching, that still limit the OLEDs efficiency and brightness. Recently, it has been introduced by our research group the model of a tri-layer organic heterostructure implemented in a field-effect configuration, that allows preventing at one time the exciton-metal as well as the exciton-charge quenching in an organic electroluminescence generating device. The device active region is formed by a central optical layer sandwiched between an electron and a hole field-effect conducting film. In order to understand the complex phenomena that happens at the interfaces, with the target to fabricate the most balanced ambipolar structure with high morphological compatibility and high mobility in a vertical heterojunction geometry, we made a preliminary study of a single layer and bi-layer OFET structure composed by α,ω-dihexylquaterthiophene (DH4T) and α,ω-diperfluoroquaterthiophene (DHF4T). By means of this study we showed a new highly balanced ambipolar OFET made of these materials, a first step toward their implementation in a more complex structure as the tri-layer is.

New Insight into the Role of the Interfacial Molecular Structure on Growth and Scaling in Organic Heterostructures

In this combined atomic force microscopy and X-ray diffraction study, we explore the microscopic origin of the scaling properties of the growth of organic−organic heterostructures formed here by F16CoPc and diindenoperylene (DIP) molecules as representative systems for electron and hole transporting materials. We evaluate the influence of the morphological properties of the first organic layer (DIP) on further temporal evolution of the morphology and structure of F16CoPc films. From the derived scaling exponents, we conclude that the morphology evolution is dominated by mound formation. The microscopic origin of such morphology is ascribed to structural changes occurring during the first stages of F16CoPc growth as revealed by grazing incidence X-ray diffraction (GIXD). The microstructure of organic materials should be taken into account, the challenging task of modeling growing surfaces and interfaces of organic materials.

Small gap semiconducting organic charge-transfer interfaces

We investigated transport properties of organic heterointerfaces formed by single-crystals of two organic donor-acceptor molecules, tetramethyltetraselenafulvalene and 7,7,8,8-tetracyanoquinodimethane (TCNQ). Whereas the individual crystals have unmeasurably high resistance, the interface exhibits a resistivity of few tens of megohm with a temperature dependence characteristic of a small gap semiconductor. We analyze the transport properties based on a simple band diagram that naturally accounts for our observations in terms of charge transfer between two crystals. Together with the recently discovered tetrathiafulvalene–TCNQ interfaces, these results indicate that single-crystal organic heterostructures create functional electronic systems with properties relevant to both fundamental and applied fields.

Organic heterostructures in organic field-effect transistors

A heterojunction is an interface between two semiconductor materials of differing energy gap, and inorganic heterojunctions have been studied as the basis of electronic devices for over half a century. Organic heterojunctions have also been successfully applied in organic electronic devices over the past two decades. However, a theoretical understanding comparable to that of inorganic heterojunctions has yet to be developed for organic heterojunctions. Organic heterojunctions have been drawing increasing attention following the discovery of high conductivity in organic heterojunction transistors constructed with active layers of p-type and n-type thin crystalline films. In contrast with the depletion layers that form in inorganic heterojunctions, electron- and hole-accumulation layers have been observed on both sides of organic heterojunction interfaces. Heterojunction films with high conductivity have been used as charge injection buffer layers and as a connecting unit for tandem diodes. Ambipolar transistors and light-emitting transistors have also been realized using organic heterojunction films as active layers. This review highlights the organic heterojunction effect and the application of organic heterostructures in organic field-effect transistors. The category of heterojunction, including organic and inorganic semiconductor heterojunctions, is given on the basis of the work function of the constituent materials, and it is shown that a rich variety of organic heterojunctions are possible based on the formation of molecular pairs.

Surface plasmon mediated energy transfer of electrically-pumped excitons

We report strong surface plasmon polariton mediated transfer of energy between molecular excitons across the metallic cathode of an electrically-pumped organic heterostructure. The donor molecular excitons at the organic heterojunction resonantly excite surface plasmon modes on both sides of the optically thick metal electrode, which evanescently couple to dye molecules near the electrode's exterior surface. Dye fluorescence in the capping layer on the exterior of the device shows a 6.5-fold increase in intensity due to this effect, far exceeding any enhancement attributable to Purcell or optical microcavity effects. Demonstration of this energy transfer mechanism for electrically-pumped excitons suggests new sensing and imaging applications with high signal to noise ratio and new routes for performance improvement in energy harvesting devices, plasmonic devices, and organic LEDs (including white light emission).

Modulation of electrical conduction through individual molecules on silicon by the electrostatic fields of nearby polar molecules: Theory and experiment

We report on the synthesis, scanning tunneling microscopy (STM) and theoretical modeling of the electrostatic and transport properties of one-dimensional organic heterostructures consisting of contiguous lines of ${\text{CF}}_{3}$- and ${\text{OCH}}_{3}$-substituted styrene molecules on silicon. The electrostatic fields emanating from these polar molecules are found, under appropriate conditions, to strongly influence electrical conduction through nearby molecules and the underlying substrate. For suitable alignment of the ${\text{OCH}}_{3}$ groups of the ${\text{OCH}}_{3}$-styrene molecules in the molecular chain, their combined electric fields are shown by ab initio density-functional calculations to give rise to potential profiles along the ${\text{OCH}}_{3}$-styrene chain that result in strongly enhanced conduction through ${\text{OCH}}_{3}$-styrene molecules near the heterojunction for moderately low negative substrate bias, as is observed experimentally. Under similar bias conditions, dipoles associated with the ${\text{CF}}_{3}$ groups are found in both experiment and in theory to depress transport in the underlying silicon. Under positive substrate bias, simulations suggest that the differing structural and electrostatic properties of the ${\text{CF}}_{3}$-styrene molecules may lead to a more sharply localized conduction enhancement near the heterojunction at low temperatures. Thus choice of substituents, their attachment site on the host styrene molecules on silicon and the orientations of the molecular dipole and higher multipole moments provide a means of differentially tuning transport on the molecular scale.

Thin-film heterostructures based on conducting polymers and organic semiconductors

We have carried out the experimental verification of a possibility to produce organic heterostructures by using films of organic semiconductors (OS) which are photosensitive in a wide spectral region, absorb light, and generate charge carriers in the region of transparency of films of conducting polymers - electrochemically synthesized polyaniline (PAN) and poly-3,4-ethylenedioxythiophene (PEDOT) stabilized by polystyrenesulfone acid (PSS). As components for the fabrication of these heterostuctures, we chose photosensitive organic semiconductors of the n-type, N,N`- dimethyl-3,4,9,10-perylenetetracarboxydiimide (МРР), and of the р-type, pentacene (Pn), which were thermally sprayed on thin layers of PAN and PEDOT. The photovoltage of heterostuctures produced at the temperature of substrates 370 K is several times greater than that of components, reaches its maximum in the region of strong light absorption in OS layers, but is one order less in the region of strong absorption in polymers. This testifies to the appearance of an internal electric field near the polymer/OS boundary which enhances the efficiency of separation of photogenerated charge carriers, whereas the efficiency inherent to photogeneration of carriers in the layers with PAN and PEDOT-PSS is insufficient yet.

Electronic non-equilibrium conditions at C 60–pentacene heterostructures

The electronic structure of vacuum-sublimed layered organic heterostructures of pentacene (PEN) and fullerene (C 60) on conducting polymer substrates was investigated using ultraviolet photoelectron spectroscopy (UPS). The conditions at the PEN/C 60interface changed from thermodynamic non-equilibrium (i.e. the onset of the PEN highest occupied molecular orbital above the substrate Fermi-energy) for thin PEN coverages on C 60 to thermodynamic equilibrium for thicker PEN coverages (i.e. Fermi-level pinning of PEN). This finding is attributed to a coverage-dependent pinhole connection of PEN through the C 60 layer with the substrate. The experiments demonstrate the importance of organic thin film morphology for UPS measurements to assess the energy level alignment at organic/organic heterointerfaces.

Organic heterostructure device with nonvolatile memory behavior using electrically doped layers

Recently, several memory devices based on organic semiconductors were reported. In most cases, single organic layers were employed, and the switching mechanisms were not fully understood. Here, we report on a novel device structure based on electrically doped organic heterostructures showing highly stable memory behavior. The organic layers are embedded between a bottom indium tin oxide and a top metal contact and form a quantum well-like structure. The device shows a hysteresis in the current-voltage characteristics and well-defined switching behavior. We achieved reproducible bistable electrical switching and stable memory phenomena in these organic multilayer heterostructure devices. The two states were retained for up to several days without noticeable decrease of their on/off ratio. Further, it was found that the ratio of the on/off current depends on the writing voltage, allowing the use as multistate memory.

Defect-induced states in the electronic structure of a Cu(100)-benzenethiolate-pentacene heterostructure

We report on the influence of defects in the electronic properties of a pentacene single-layer grown on a benzenethiolate self-assembled-monolayer (Bt-SAM) adsorbed on Cu(100) at 300 K, using high-resolution ultraviolet photoelectron spectroscopy. The weak interaction between pentacene and the underlying Bt-SAM is deduced by the lineshape and evolution of the highest-occupied molecular orbital (HOMO). Closer inspection of the pentacene HOMO band reveals a skewed feature, where the contribution in the low binding energy side is found originating from states associated to structural defects that can be reduced by controlled mild-temperature annealing (355 K). Presence of such defects influences also the pentacene HOMO onset energy and hole-injection barrier that can be reduced to typical value of this organic heterostructure after the annealing.

Nonlocal Conductance Modulation by Molecules: Scanning Tunneling Microscopy of Substituted Styrene Heterostructures on H-Terminated Si(100)

One-dimensional organic heterostructures consisting of contiguous lines of CF3- and OCH3-substituted styrene molecules on silicon are studied by scanning tunneling microscopy and ab initio simulation. Dipole fields of OCH3-styrene molecules are found to enhance conduction through molecules near CF3-styrene/OCH_{3}-styrene heterojunctions. Those of CF3-styrene depress transport through the nearby silicon. Thus the choice of substituents and their attachment site on host molecules provide a means of differentially tuning molecule and substrate transport at the molecular scale.

Photoelectron processes in heterojunctions based on organic films

The photoconductivity and photo emf have been studied in a heterostructure based on thin films of electron donor and acceptor organic compounds. It is established that the main process responsible for the photogeneration and separation of charge carriers is the decay of photoexcited molecular states at the interface between the donor and acceptor materials. The determining influence on the photoelectron processes in the organic heterostructure is produced by the energy structure of electron states at the interface. The role of the photoinduced intermolecular charge transfer in the development of photoconductivity in the given heterostructure is considered.

Molecular structure and growth morphologies of pentacene/fluorinated copper-phthalocyanine heterostructures

By combining in-situ X-ray diffraction and atomic force microscopy, we investigate the structure and morphology of organic heterostructures formed by deposition of pentacene onto fluorinated copper-phthalocyanine (F 16CuPc) films grown with two different molecular orientations. Well ordered pentacene films grow upon deposition on top of standing-up F 16CuPc molecules ( s-structure) and finally form homogeneous films. When pentacene is deposited on lying-down F 16CuPc ( l-structure), laterally small and high crystallites form. The pentacene preserves the same thin-film structure and excellent alignment along the surface normal in either case, with mosaicities of only 0.015 ± 0.001°. We demonstrate in this work how thin organic buffer layers can be employed to tailor the growth of ordered organic heterostructures.

Optimal film thickness for exciton diffusion length measurement by photocurrent response in organic heterostructures

Exciton diffusion in organic materials is crucial for designing and optimizing organic photovoltaic devices. This paper addresses the important complication of the choice of film thickness affecting measurement accuracy in the method of measuring exciton diffusion length from thickness-dependent photocurrent response in organic bilayer heterostructures. Based on the photocurrent model integrated with optical interference effect, the correlation between the exciton diffusion length and the optimal thickness of organic layer is analytically solved. It shows that the intrinsic nonlinearity of the correlation causes prominent uncertainty of estimated diffusion length. Two approaches were proposed to improve the measurement accuracy: (1) the thickness effect of each layer in this heterostructure on photocurrent response is asymmetric so that certain thickness combination can be chosen to reduce the uncertainty; (2) on account of the limited accuracy of the optimal thickness determined by photocurrent spectra, estimated diffusion length should be verified by spectrum fitting. Taking these improvements into account, the exciton diffusion length in copper phthalocyanine film is determined as 20 ± 5 nm from curve fitting by this method.