Organic Heterostructures Research Articles

Fourier Spectroscopy as a Method of Investigation of Photoelectric Properties of Organic Systems

A new method of investigation of photoelectric properties of layered thin-film structures based on broadband Fourier spectroscopy exhibiting a harmonically modulated optical delay is proposed. In contrast to traditional approaches to study photoelectric properties, which are based on application of dispersive spectral devices, the proposed method allows not only simultaneously covering the ultraviolet, visible, and infrared spectral ranges, while demonstrating a wide dynamic range and high spectral resolution, but also easily varying low-frequency modulation of the action of light. The capabilities of the method are demonstrated using a polycrystalline organic heterostructure as an example. Its spectral sensitivity, speed, and specific detectivity are measured. A model and an equivalent electric circuit are proposed for explanation of the results of the measurements.

Centimetre-scale electron diffusion in photoactive organic heterostructures.

The unique properties of organic semiconductors, such as flexibility and lightness, are increasingly important for information displays, lighting and energy generation. But organics suffer from both static and dynamic disorder, and this can lead to variable-range carrier hopping, which results in notoriously poor electrical properties, with low electron and hole mobilities and correspondingly short charge-diffusion lengths of less than a micrometre. Here we demonstrate a photoactive (light-responsive) organic heterostructure comprising a thin fullerene channel sandwiched between an electron-blocking layer and a blended donor:C70 fullerene heterojunction that generates charges by dissociating excitons. Centimetre-scale diffusion of electrons is observed in the fullerene channel, and this can be fitted with a simple electron diffusion model. Our experiments enable the direct measurement of charge diffusivity in organic semiconductors, which is as high as 0.83 ± 0.07 square centimetres per second in a C60 channel at room temperature. The high diffusivity of the fullerene combined with the extraordinarily long charge-recombination time yields diffusion lengths of more than 3.5 centimetres, orders of magnitude larger than expected for an organic system.

Correction: An evidence for an organic N-doped multiwall carbon nanotube heterostructure and its superior electrocatalytic properties for promising dye-sensitized solar cells

Correction for ‘An evidence for an organic N-doped multiwall carbon nanotube heterostructure and its superior electrocatalytic properties for promising dye-sensitized solar cells’ by Alvira Ayoub Arbab et al., J. Mater. Chem. A, 2018, DOI: 10.1039/c8ta00535d.

An evidence for an organic N-doped multiwall carbon nanotube heterostructure and its superior electrocatalytic properties for promising dye-sensitized solar cells

Mechanism of synthesis of an organic N-doped MWCNT heterostructure for Pt-free DSSCs.

Three-dimensional directed assembly of organic charge-transfer heterostructure.

Multicomponent crystalline heterostructures are a powerful approach to integrate different functional materials into the ordered structures. Here we describe three-dimensional spherical assembly of binary organic solids that consist of electron donor and acceptor molecules. A mechanistic study of heterostructure formation reveals that the dewetting and drying-mediated assembly processes are responsible for the spherical crystallite formation. The assembled spherical heterostructures are highly tunable, crystalline and chemically stable, exhibiting phase separation controlled optoelectronic behavior. This simple, generalizable three-dimensional assembly can be modified for the formation of ordered functional organic multicomponent heterostructures for emerging applications.

Polythiophene coated CuBi2O4 networks: A porous inorganic–organic hybrid heterostructure for enhanced photoelectrochemical hydrogen evolution

A thin porous film inorganic-organic heterostructure photocathode has been successfully fabricated and applied in photoelectrochemical (PEC) water splitting. Our work optimized the best structure of CuBi2O4 (CBO) base on FTO (fluorine doped tin oxide). Furthermore, it was passivated with polythiophene (PTh), a kind of conducting polymers of excellent environmental and thermal stability, to decrease the capture ability of surface state for photogenerated charge. Benefiting from the high light capture rate of porous structure of CBO and low electron-hole recombination rate of CBO/PTh heterojunctions, the as-fabricated FTO/CBO/PTh electrode delivers excellent PEC performance with a ultimate photocurrent density of 0.41 mA cm−2 at 0.3 V vs RHE (reversible hydrogen electrode) doubled to that of pure FTO/CBO electrode, which would be due to the significantly improved light absorption and rapid separation efficiency.

Electronic structure, transport, and collective effects in molecular layered systems.

The great potential of organic heterostructures for organic device applications is exemplified by the targeted engineering of the electronic properties of phthalocyanine-based systems. The transport properties of two different phthalocyanine systems, a pure copper phthalocyanine (CoPc) and a flourinated copper phthalocyanine–manganese phthalocyanine (F16CoPc/MnPc) heterostructure, are investigated by means of density functional theory (DFT) and the non-equilibrium Green’s function (NEGF) approach. Furthermore, a master-equation-based approach is used to include electronic correlations beyond the mean-field-type approximation of DFT. We describe the essential theoretical tools to obtain the parameters needed for the master equation from DFT results. Finally, an interacting molecular monolayer is considered within a master-equation approach.

Improving the Performance of Graphene Phototransistors Using a Heterostructure as the Light-Absorbing Layer.

Interfacing light-sensitive semiconductors with graphene can afford high-gain phototransistors by the multiplication effect of carriers in the semiconductor layer. So far, most devices consist of one semiconductor light-absorbing layer, where the lack of internal built-in field can strongly reduce the quantum efficiency and bandwidth. Here, we demonstrate a much improved graphene phototransistor performances using an epitaxial organic heterostructure composed of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and pentacene as the light-absorbing layer. Compared with single light-absorbing material, the responsivity and response time can be simultaneously improved by 1 and 2 orders of magnitude over a broad band of 400-700 nm, under otherwise the same experimental conditions. As a result, the external quantum efficiency increases by over 800 times. Furthermore, the response time of the heterostructured phototransistor is highly gate-tunable down to sub-30 μs, which is among the fastest in the sensitized graphene phototransistors interfacing with electrically passive light-absorbing semiconductors. We show that the improvement is dominated by the efficient electron-hole pair dissociation due to interfacial built-in field rather than bulk absorption. The structure demonstrated here can be extended to many other organic and inorganic semiconductors, which opens new possibilities for high-performance graphene-based optoelectronics.

Electronic Properties of a 1D Intrinsic/p-Doped Heterojunction in a 2D Transition Metal Dichalcogenide Semiconductor.

Two-dimensional (2D) semiconductors offer a convenient platform to study 2D physics, for example, to understand doping in an atomically thin semiconductor. Here, we demonstrate the fabrication and unravel the electronic properties of a lateral doped/intrinsic heterojunction in a single-layer (SL) tungsten diselenide (WSe2), a prototype semiconducting transition metal dichalcogenide (TMD), partially covered with a molecular acceptor layer, on a graphite substrate. With combined experiments and theoretical modeling, we reveal the fundamental acceptor-induced p-doping mechanism for SL-WSe2. At the 1D border between the doped and undoped SL-WSe2 regions, we observe band bending and explain it by Thomas-Fermi screening. Using atomically resolved scanning tunneling microscopy and spectroscopy, the screening length is determined to be in the few nanometer range, and we assess the carrier density of intrinsic SL-WSe2. These findings are of fundamental and technological importance for understanding and employing surface doping, for example, in designing lateral organic TMD heterostructures for future devices.

QM/MM calculations combined with the dimer approach on the static disorder at organic‐organic interfaces of thin‐film organic solar cells composed of small molecules

AbstractA QM/MM approach using ωB97X‐D combined with AMOEBA calculations was used to analyze the energetic disorder in the vicinity of interfaces in amorphous organic heterostructures. Distributions of ground‐, excited‐, and cationic‐state energies as well as of ionization potentials and excitation energies, all being relevant quantities for the transport properties of thin films, were calculated. As already found for bulk amorphous organic semiconductors, local densities of states at molecular interfaces possess Gaussian‐shaped profiles with a significant amount of disorder. Assuming a disorder‐limited activation of exciton and charge transport, a decrease in the amount of disorder could improve the transport properties. Relating calculated disorders to molecular parameters revealed that in line with the Bässler model, especially the molecular polarity, its change upon electronic excitation, and the molecular polarizability are relevant quantities leading to energetically largely disordered films. Moreover, because of the different mechanisms of exciton and polaron delocalization, a given morphology with disordered charge‐transport levels gives not necessarily rise to disordered exciton‐transport levels and vice versa.

Enhanced visible light photocatalytic hydrogen evolution over porphyrin hybridized graphitic carbon nitride

Tetra (4-carboxyphenyl) porphyrin (TCPP) was loaded on the surface of Pt/g-C3N4 via a simple adsorption process, and the microstructure and chemical structure of the composites were characterized by high resolution transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, UV–visible diffused reflectance spectroscopy and photoluminescence spectroscopy. Loading TCPP onto Pt/g-C3N4 enhanced the visible-light-driven photocatalytic evolution of H2 from water. The TCPP/Pt/g-C3N4 composite with a TCPP loading of 1wt% had the highest photoactivity, which was 2.1 times higher than that of Pt/g-C3N4. This improvement is attributed to enhanced visible light utilization by the TCPP/Pt/g-C3N4 resulting from the strong visible light response of TCPP. In addition, the formed organic heterostructure between TCPP and g-C3N4 with overlapping bad gaps accelerates the electron transfer and inhibits the recombination of the photogenerated electrons and holes on g-C3N4.

High-Performance Nonvolatile Organic Field-Effect Transistor Memory Based on Organic Semiconductor Heterostructures of Pentacene/P13/Pentacene as Both Charge Transport and Trapping Layers.

Nonvolatile organic field‐effect transistor (OFET) memory devices based on pentacene/N,N′‐ditridecylperylene‐3,4,9,10‐tetracarboxylic diimide (P13)/pentacene trilayer organic heterostructures have been proposed. The discontinuous n‐type P13 embedded in p‐type pentacene layers can not only provide electrons in the semiconductor layer that facilitates electron trapping process; it also works as charge trapping sites, which is attributed to the quantum well‐like pentacene/P13/pentacene organic heterostructures. The synergistic effects of charge trapping in the discontinuous P13 and the charge‐trapping property of the poly(4‐vinylphenol) (PVP) layer remarkably improve the memory performance. In addition, the trilayer organic heterostructures have also been successfully applied to multilevel and flexible nonvolatile memory devices. The results provide a novel design strategy to achieve high‐performance nonvolatile OFET memory devices and allow potential applications for different combinations of various organic semiconductor materials in OFET memory.

IZO deposited by PLD on flexible substrate for organic heterostructures

In:ZnO (IZO) thin films were deposited on flexible plastic substrates by pulsed laser deposition (PLD) method. The obtained layers present adequate optical and electrical properties competitive with those based on indium tin oxide (ITO). The figure of merit (9 × 10−3 Ω−1) calculated for IZO layers demonstrates that high quality coatings can be prepared by this deposition technique. A thermal annealing (150 °C for 1 h) or an oxygen plasma etching (6 mbar for 10 min.) were applied to the IZO layers to evaluate the influence of these treatments on the properties of the transparent coatings. Using vacuum evaporation, organic heterostructures based on cooper phthalocyanine (CuPc) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) were deposited on the untreated and treated IZO layers. The optical and electrical properties of the heterostructures were investigated by UV–Vis, FTIR and current–voltage (I–V) measurements. For the heterostructure fabricated on IZO treated in oxygen plasma, an improvement in the current value with at least one order of magnitude was evidenced in the I–V characteristics recorded in dark conditions. Also, an increase in the current value for the heterostructure deposited on untreated IZO layer can be achieved by adding an organic layer such as tris-8-hydroxyquinoline aluminium (Alq3).

Controlled Modification of Superconductivity in Epitaxial Atomic Layer-Organic Molecule Heterostructures.

Self-assembled organic molecules can potentially be an excellent source of charge and spin for two-dimensional (2D) atomic-layer superconductors. Here we investigate 2D heterostructures based on In atomic layers epitaxially grown on Si and highly ordered metal-phthalocyanine (MPc, M = Mn, Cu) through a variety of techniques: scanning tunneling microscopy, electron transport measurements, angle-resolved photoemission spectroscopy, X-ray magnetic circular dichroism, and ab initio calculations. We demonstrate that the superconducting transition temperature (Tc) of the heterostructures can be modified in a controllable manner. Particularly, the substitution of the coordinated metal atoms from Mn to Cu is found to reverse the Tc shift from negative to positive directions. This distinctive behavior is attributed to a competition of charge and spin effects, the latter of which is governed by the directionality of the relevant d-orbitals. The present study shows the effectiveness of molecule-induced surface doping and the significance of microscopic understanding of the molecular states in these 2D heterostructures.

Organic heterostructures deposited by MAPLE on AZO substrate

Organic heterostructures based on poly(3-hexylthiophene) (P3HT) and fullerene (C60) as blends or multilayer were deposited on Al:ZnO (AZO) by Matrix-Assisted Pulsed Laser Evaporation (MAPLE) technique. The AZO layers were obtained by Pulsed Laser Deposition (PLD) on glass substrate, the high quality of the films being reflected by the calculated figure of merit. The organic heterostructures were investigated from morphological, optical and electrical point of view by atomic force microscopy (AFM), UV–vis spectroscopy, photoluminescence (PL) and current-voltage (I–V) measurements, respectively. The increase of the C60 content in the blend heterostructure has as result a high roughness. Compared with the multilayer heterostructure, those based on blends present an improvement in the electrical properties. Under illumination, the highest current value was recorded for the heterostructure based on the blend with the higher C60 amount. The obtained results showed that MAPLE is a useful technique for the deposition of the organic heterostructures on AZO as transparent conductor electrode.

Lattice‐Matched Epitaxial Growth of Organic Heterostructures for Integrated Optoelectronic Application

AbstractDevelopment of nanowire photonics requires integration of different nanowire components into highly ordered functional heterostructures. Herein, we report a sequential self‐assembly of binary molecular components into branched nanowire heterostructures (BNHs) via lattice‐matched epitaxial growth, in which the microribbon backbone of 2,5‐Bis(5‐tert‐butyl‐2‐benzoxazolyl)thiophene (BBOT) functions as blue‐emitting microlaser source to pump the nanowire branches of BODIPY. By constructing Au electrodes on both branch sides and measuring the photocurrent in them, we successfully realize the integration of an organic laser and a power meter in a single device. This work provides a new insight into the integration of 1D organic nanostructures into BNHs for realizing organic multifunctional photonic devices.

Nonvolatile transtance change random access memory based on magnetoelectric P(VDF-TrFE)/Metglas heterostructures

Transtance change random access memory (TCRAM) is a type of nonvolatile memory based on the nonlinear magnetoelectric coupling effects of multiferroics. In this work, ferroelectric P(VDF-TrFE) thin films were prepared on Metglas foil substrates by the sol-gel technique to form multiferroic heterostructures. The magnetoelectric voltage coefficient of the heterostructure can be switched reproducibly to different levels between positive and negative values by applying selective electric-field pulses. Compared with bulk multiferroic heterostructures, the polarization switching voltage was reduced to 7 V. Our facile technological approach enables this organic magnetoelectric heterostructure as a promising candidate for the applications in multilevel TCRAM devices.

Poly complexes based unsaturated β-diketones and rare earth elements for optoelectronics

ABSTRACTThe complexes of samarium(III) with 2-methyl-5-phenylpenten-1-3,5-dione (mphpd) Sm(mphpd)3·2H2O and 2,6-dymethyl-heptene-1-3,5-dione (hdmhpd), Sm(hdmhpd)m were synthesized. The polycomplexes on their basis [Sm(mphpd)3]n, [Sm(hdmhpd)m]n and copolymers with styrene Sm(mphpd)3-co-styrene in ratio 5:95 were obtained by free-radical polymerization and the kinetics of polymerization was studied by dilatometric method at the first time. The method of dynamic light scattering and the results of electronic microscopy showed that the obtained polymer systems are nanoscale. The luminescent spectra of obtained metal complexes in solid state are investigated and analyzed. The electroluminescent spectra of organic planar heterostructure ITO/PEDOT:PSS/{[Sm(mphpd)3]n[Styrene]m}/Al are examined.

Improved photoswitching response times of MoS2 field-effect transistors by stacking p-type copper phthalocyanine layer

Photoswitching response times (rise and decay times) of a vertical organic and inorganic heterostructure with p-type copper phthalocyanine (CuPc) and n-type molybdenum disulfide (MoS2) semiconductors are investigated. By stacking a CuPc layer on MoS2 field effect transistors, better photodetection capability and fast photoswitching rise and decay phenomena are observed. Specifically, with a 2 nm-thick CuPc layer on the MoS2 channel, the photoswitching decay time decreases from 3.57 s to 0.18 s. The p-type CuPc layer, as a passivation layer, prevents the absorption of oxygen on the surface of the MoS2 channel layer, which results in a shortened photoswitching decay time because adsorbed oxygen destroys the balanced ratio of electrons and holes, leading to the interruption of recombination processes. The suggested heterostructure may deliver enhanced photodetection abilities and photoswitching characteristics for realizing ultra-thin and sensitive photodetectors.

Precise, Self-Limited Epitaxy of Ultrathin Organic Semiconductors and Heterojunctions Tailored by van der Waals Interactions.

Precise assembly of semiconductor heterojunctions is the key to realize many optoelectronic devices. By exploiting the strong and tunable van der Waals (vdW) forces between graphene and organic small molecules, we demonstrate layer-by-layer epitaxy of ultrathin organic semiconductors and heterostructures with unprecedented precision with well-defined number of layers and self-limited characteristics. We further demonstrate organic p-n heterojunctions with molecularly flat interface, which exhibit excellent rectifying behavior and photovoltaic responses. The self-limited organic molecular beam epitaxy (SLOMBE) is generically applicable for many layered small-molecule semiconductors and may lead to advanced organic optoelectronic devices beyond bulk heterojunctions.