Organic Heterostructures Research Articles

Organic Heterojunction Devices Based on Phthalocyanines: A New Approach to Gas Chemosensing.

Organic heterostructures have emerged as highly promising transducers to realize high performance gas sensors. The key reason for such a huge interest in these devices is the associated organic heterojunction effect in which opposite free charges are accumulated at the interface making it highly conducting, which can be exploited in producing highly sensitive and faster response kinetics gas sensors. Metal phthalocyanines (MPc) have been extensively studied to fabricate organic heterostructures because of the large possibilities of structural engineering which are correlated with their bulk thin film properties. Accordingly, in this review, we have performed a comprehensive literature survey of the recent researches reported about MPc based organic heterostructures and their application in gas sensors. These heterostructures were used in Organic Field-Effect Transistor and Molecular Semiconductor—Doped Insulator sensing device configurations, in which change in their electrical properties such as field-effect mobility and saturation current in the former and current at a fixed bias in the latter under redox gases exposure were assessed to determine the chemosensing performances. These sensing devices have shown very high sensitivity to redox gases like nitrogen dioxide (NO2), ozone and ammonia (NH3), which monitoring is indispensable for implementing environmental guidelines. Some of these sensors exhibited ultrahigh sensitivity to NH3 demonstrated by a detection limit of 140 ppb and excellent signal stability under variable humidity, making them among the best NH3 sensors.

Molecular- and Structural-Level Organic Heterostructures for Multicolor Photon Transportation.

The rational design and the fine synthesis of organic heterostructures (OHSs) are the key steps toward integrated organic optoelectronics. Herein we have demonstrated a self-assembly approach of combining a molecular-level heterostructure with a structural-level heterostructure and regulating the noncovalent intermolecular interactions for the precise construction of OHSs: a vertical type of anthracene-TCNB heterostructure and a horizontal type of benzopyrene-TCNB heterostructure. The excellent structural compatibility and the low lattice mismatch rate of ∼5.8% between single-component microplates and cocrystal microwires allow anthracene and benzopyrene molecules to grow epitaxially on the cocrystal. Significantly, integrating the multicolor emission and the distinctive dimensional-dependent photon transportation properties of low-dimensional micro/nanostructures, the multicolor optical outputs are achieved via modulating the active/passive optical waveguides in OHSs. Our work exhibits the utilization of the multilevel heterostructure strategy, which boosts the rational design of OHSs for organic photonics.

A generalized self-consistent approach to study position-dependent mass in semiconductors organic heterostructures and crystalline impure materials

Abstract The purpose of this study is to generalize the Coordinate Integral Transform recently introduced in literature for the self-consistent solution of Schrodinger equation in one-dimensional systems with position-dependent mass. It was revealed that the present generalized approach reveals several motivating properties in material sciences and may be used to model and solve a large number of problems arising in solid state sciences ranging from semiconductors heterostructures to organic semiconductors and crystalline solids subject to impurities.

Schottky junction photodiode based on graphene—organic semiconductor heterostructure

The isolation of graphene on an insulating substrate has provided new opportunities for the fabrication of electronic devices with radically new geometries and structures. The use of single-layer graphene as an electrode has enabled the development of novel electronic device architectures that exploit the unique atomically thin structure of the material, which also includes a low density of states at its charge neutrality point. In this work, we present the first example of a vertical Schottky junction photodiode based on the graphene—organic semiconductor–metal heterostructure. The n-type N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) organic semiconductor was thermally deposited onto chemical vapor deposition (CVD)-grown single-layer graphene. The tunable Schottky injection barrier permitted tuning of the diode rectification ratio by more than two orders of magnitude upon application of gate biases, which increased the photocurrent but suppressed the dark current of the photodiodes. Tuning of the photovoltaic properties of the devices was also confirmed, indicating that the device architecture based on the work function tunability of graphene could provide a versatile strategy for enhancing the performance of organic photodiodes.

Few‐Layer Organic Crystalline van der Waals Heterojunctions for Ultrafast UV Phototransistors

AbstractOrganic UV photodetectors using a transistor architecture can yield higher photoresponsivity than diode‐based devices because of the presence of a gate electrode. However, a long‐term issue of these phototransistor devices is the slow response speed, which hinders their practical applications. Here, organic UV phototransistors are constructed using few‐layer organic crystalline van der Waals (vdW) heterojunctions as the photoactive layers. The thickness of the photoactive layers is even less than the exciton diffusion length, thus removing the exciton‐diffusion bottleneck and giving rise to the confinement of charge separation and recombination within the adjacent molecular layers across the heterojunction interface. Hence, the phototransistor devices can exhibit a remarkably enhanced response speed (rise and decay times as short as only ≈4 and 6 ms, respectively). The layer‐dependent photoresponse characteristics are also observed, reinforcing the great importance of few‐layer organic heterostructures in phototransistor devices. This work not only provides a promising avenue toward fast response optoelectronic devices but also presents an in‐depth understanding on the microscopic nature of photogenerated charge carriers at the precision of molecular layers.

Organic heterostructures composed of one- and two-dimensional polymorphs for photonic applications

Organic heterostructures (OHSs) consist of organic micro/nanocrystals are of essential importance for the construction of integrated optoelectronics in the future. However, the scarcity of materials and the problem of phase separation still hinder the fine synthesis of OHSs. Herein, based on the α phase one-dimensional (1D) microrods and the β phase 2D microplates of one organic compound 3,3′-((1 E ,1′ E )-anthracene-9,10-diylbis(ethane-2,1-diyl))dibenzonitril ( m -B2BCB), we facilely synthesized the OHSs composed of these two polymorph phases, whose growth mechanism is attributed to the low lattice mismatch rate of 5.8% between (001) plane of α phase (trunk) and (010) crystal plane of β phase (branch). Significantly, the multiport in/output channels can be achieved in the OHSs, which demonstrates the structure-dependent optical signals with the different output channels in the OHSs. Therefore, our experiment exhibits the great prospect of polymorphism in OHSs, which could provide further applications on multifunctional organic integrated photonics circuits.

Fine Synthesis of Longitudinal/Horizontal‐Growth Organic Heterostructures for the Optical Logic Gates

AbstractOrganic micro/nanocrystals have been intensively studied and great progress has been achieved in both the modulation of optical/electronical property and the controllable synthesis of single component of low‐dimensional materials. In particular, organic heterostructures (OHSs) are one of the key factors for the multifunctional optoelectronic devices to construct integrated circuits. However, the fine synthesis of OHSs remains a challenge due to the easy occurrence of homogeneous nucleation raised by the weak intermolecular interaction. A facile solution “bottom‐up” fabrication approach is demonstrated for the longitudinal‐ and horizontal‐growth OHSs through building up blocks of MPY (1,4‐phenylene‐bis(3‐(2‐methoxylpyridin‐6‐yl)acrylonitrile)) nanowires and polymorphic TBPe (2,5,8,11‐tetra‐tert‐butylperylene) microcrystals by controlling the evaporation rate of solvents. The growth directions of longitudinal and horizontal OHSs are demonstrated to be driven by the face‐selective principle and the lowest energy principle, respectively. The directional controllable fabrication method provides a novel strategy for the fine synthesis of OHSs. In addition, three models of optical logic gates “100,” “110,” and “111” based on these as‐prepared OHSs with multiple outputs are constructed, which are the potential candidates for the multifunctional integrated optoelectronics.

Low energy excited state vibrations revealed in conjugated copolymer PCDTBT.

Low energy vibrations in the excited state have been hypothesized to play an important role in quickly and efficiently generating free charges in bulk heterojunctions of some conjugated polymer systems. While time-resolved vibrational spectroscopies seemingly are well poised to address the relationship between kinetics and vibrational motions after initial photoexcitation, uncertainty in the measurement arises due to overlapping signals and difficulties in assigning observed oscillatory signals to the molecular response. Here, we demonstrate a high sensitivity strategy to distinguish between signal oscillations originating from lab noise and those molecular in origin in order to isolate the low energy excited-state vibrations in the model conjugated copolymer PCDTBT. Furthermore, to distinguish modes that may be implicated in different kinetic pathways, coherent signal oscillations extracted from 2-dimensional electronic spectroscopy (2DES) are compared for the polymer in two solvents with different polarities resulting in different kinetics. We observe that the change in solvent affects dynamics on the >2 ps scale but not on the time scale required for free charge generation in heterojunctions (∼200 fs time scale). By the same token, the excited state vibrational modes that appear and disappear based on solvent polarity may also be associated with the slower kinetic process. The observation of low energy vibrational motions coupled to the excited state manifold that persists through the solvent change and thus can be associated with the fast kinetic process supports the hypothesis that direct polaron formation, rather than exciton formation and diffusion followed by interfacial charge separation, is a more likely route toward free charges in organic heterostructures.

Effect of ITO electrode patterning on the properties of organic heterostructures based on non-fullerene acceptor prepared by MAPLE

This paper presents some studies on the organic heterostructures realized by Matrix Assisted Pulsed Laser Evaporation in both bi-layer and mixed layer configurations on glass substrates covered by flat or nano-patterned ITO. The donor, a star-shaped arylenevinylene compound, 4,4′,4″-tris[(4′-diphenylamino) styryl] triphenylamine, and acceptor, a non-fullerene compound, N,N′-bis-(1-dodecyl)perylene-3,4,9,10 tetracarboxylic diimide, were blended in three weight ratios: 1:2, 1:3 and 1:4.A grating of cylindrical pillars with a periodicity of 1.1 μm has been developed by UV-Nanoimprint Lithography in a polymer layer. The shape of the nanostructures changed to cone trunk by the Pulsed Laser Deposition of ITO on this nanostructured surface.The effect of the nanostructures and composition on the optical and electrical properties of the heterostructures was analyzed.The nano-patterning affected both the UV–Vis transmission and photoluminescence through the multiple reflections inside the cavities and at interfaces and the particularities of the molecular arrangement. The patterning was preserved independently of composition, but the roughness increased with increasing acceptor amount.The I-V characteristics drawn at room temperature in dark revealed an ohmic contact behavior for all heterostructures. The nano-patterning had a similar effect on the current in the heterostructures with mixed layer (1:2) and stacked bi-layer.

A feasible heterostructure of P(VDF-TrFE)/semiconductor for a stable multi-state memory

Abstract Ferroelectric random-access memory is widely concerned because of its nearly infinite writing cycles and fast reading and writing ability. Since ferroelectric memory is not as dense as dynamic random-access memory and static random-access memory, which cannot store so much data in the same space, it is difficult to replace these technologies in practical applications. One possible solution to this dilemma is to increase the storage capacity per memory cell. This work uses a feasible capacitor structure of P(VDF-TrFE)/organic semiconductor heterostructure to improve the storage capacity. Both ferroelectric layer and semiconductor layer are prepared using a simple spin coating method. Due to the intervention of semiconductor, three polarization states are demonstrated owing to the splitting of a displacement current peak, where the semiconductor-dielectric transformation of the semiconductor layer plays an important role. The measurement of electrical properties shows that the ferroelectric thin film about this structure with three polarization states can be used to represent the storage 0, 1, 2, and the polarization of P1, P2, P3 are 0, 3.4 and 9.2 μC cm−2, respectively. According to the fitting results, the data retention capacity of this structure is excellent, that the residual polarizations of P1, P2 and P3 are 0, 3.2 and 9.2 μC cm−2 over 3 months, respectively.

Hierarchical self-assembly of organic heterostructure nanowires

Organic heterostructures (OHSs) integrating the intrinsic heterostructure characters as well as the organic semiconductor properties have attracted intensive attention in material chemistry. However, the precise bottom-up synthesis of OHSs is still challenging owing to the general occurrence of homogeneous-nucleation and the difficult manipulation of noncovalent interactions. Herein, we present the rational synthesis of the longitudinally/horizontally-epitaxial growth of one-dimensional OHSs including triblock and core/shell nanowires with quantitatively-manipulated microstructure via a hierarchical self-assembly method by regulating the noncovalent interactions: hydrogen bond (−15.66 kcal mol−1) > halogen bond (−4.90 kcal mol−1) > π-π interaction (−0.09 kcal mol−1). In the facet-selective epitaxial growth strategy, the lattice-matching and the surface-interface energy balance respectively facilitate the realization of triblock and core/shell heterostructures. This hierarchical self-assembly approach opens up avenues to the fine synthesis of OHSs. We foresee application possibilities in integrated optoelectronics, such as the nanoscale multiple input/out optical logic gate with high-fidelity signal.

Controllable Fabrication of In‐Series Organic Heterostructures for Optical Waveguide Application

AbstractOrganic heterostructures have been demonstrated to be excellent candidates for application in photonic components such as modulators and optical switches, but the controllable fabrication of organic heterostructures remains a considerable challenge due to the lattice mismatch and weak intermolecular forces. In this study, a photochromic strategy is demonstrated to fabricate in‐series organic heterostructures with high controllability and designability by selectively covering a portion of the organic microcrystals. Radiative energy transfer is found in the blue‐green fluorescent 2‐block of the organic heterostructure, which acts as the basic component unit, leading to the unique passive/active mixed waveguide mode. Furthermore, optical waveguides with switchable output light wavelength‐bands can be constructed based on the 3‐block and 5‐block heterostructures. By changing the input ports on the heterostructures, the blue, blue/green, or green wavelength band can be selectively outcoupled, thus indicating the potential of the organic heterostructures to be utilized in future photonic systems.

Biomimetic Design of Hollow Flower-Like g-C3N4@PDA Organic Framework Nanospheres for Realizing an Efficient Photoreactivity.

Organic framework polymers have attracted much interest due to the enormous potential design space offered by the atomically precise spatial assembly of organic molecular building blocks. The morphology control of organic frameworks is a complex issue that hinders the development of organic frameworks for practical applications. Biomimetic self-assembly is a promising approach for designing and fabricating multiple-functional nanoarchitectures. A bioinspired hollow flower-like organic framework nanosphere heterostructure comprised of carbon nitride and polydopamine (g-C3N4@PDA) is successfully synthesized via a mild and green method. This heterostructure can effectively avoid the agglomeration of nanosheets to better access the hollow nanospheres with high open-up specific surface area. The electron delocalization of g-C3N4 and PDA under visible light can largely promote photoelectron transfer and enhance the photocatalytic activity of the g-C3N4@PDA. Furthermore, the g-C3N4@PDA can effectively enhance the generation of reactive oxygen species under irradiation, which can lead to cell apoptosis and enhance the performance for cancer therapy. Therefore, the as-prepared g-C3N4@PDA provides a paradigm of highly efficient photocatalyst that can be used as nanomedicine toward cancer therapy. This study could open up a new avenue for exploiting more other potential hollow nanosphere organic frameworks.

Epitaxial growth of dual-color-emitting organic heterostructures via binary solvent synergism driven sequential crystallization.

The controlled construction of organic heterostructured architectures derived from molecules with similar nucleation thresholds and concentrations has been rare and remains a great challenge. Herein, we report a sequential epitaxial growth to synthesize dual-color-emitting organic heterostructures with 9,10-bis(phenylethynyl)anthracene (BPEA) microwire trunks and tris-(8-hydroxyquinoline)aluminium (Alq3) microstructure branches by an anti-solvent induced sequential crystallization strategy. During the epitaxial growth process, the hydrogen-bonding interactions of the anti-solvent and solvent cause a large change in the solubility and crystallization rate of BPEA and Alq3 molecules in the mixed system, which facilitates sequential crystallization of organic molecule pairs with similar nucleation thresholds and concentrations into desired heterostructures by manipulating the synergism of anti-solvents and solvents. The Förster resonant energy transfer process in heterostructures could be modulated by varying the structure of heterostructures, such as the shape, amount and angles of the branches. The present synthesis strategy provides a unique insight into the detailed formation mechanism of complex organic heterostructures, further guiding the construction of more functional heterostructure materials.

Controllable co-assembly of organic micro/nano heterostructures from fluorescent and phosphorescent molecules for dual anti-counterfeiting

Two types of branched organic heterostructures with both a fluorescer and a metal-free ultralong phosphor were prepared for dual anti-counterfeiting applications.

Investigation of recombination in organic heterostructures by i-CELIV

In this work, we propose the experimental technique for the estimation of the recombination rate at the interface between hole and electron transporting layers. This technique could be used in dye sensitized solar cells where a very thin generation layer is placed between transporting layers. The proposed technique is based on the extraction of injected charge carriers and the determination of the dependence of the amount of the extracted charge on the density of injection current. We present the analytical solution of this dependence for two different cases: low recombination rate, when charge carriers accumulate at the interface, and fast recombination rate. The dependencies for moderate recombination rates and in the presence of the intermediate layer have been investigated by the numerical modeling. With the help of the proposed technique, the interface recombination rates were determined experimentally in the sandwich structures of TiO2/P3HT and TiO2/Spiro-MeOTAD. The technique has also been used in the investigation of recombination processes in TiO2/perovskite/Spiro-MeOTAD solar cells.

Flexible organic heterostructures obtained by MAPLE

Organic heterostructures based on zinc phthalocyanine (ZnPc) and perylene tetracarboxylic dianhydride (PTCDA) were deposited by matrix-assisted pulsed laser evaporation (MAPLE) technique on conductive flexible substrate (ITO/PET) in three configurations: ZnPc/PTCDA (stacked layers), ZnPc:PTCDA (blend) and ZnPc/ZnPc:PTCDA/PTCDA. The effect of the configuration on the optical and electrical properties of the obtained heterostructures was investigated. For all heterostructures was observed an improved optical absorption in visible domain. The I–V characteristics recorded under illumination, revealed higher short circuit current (ISC) values for the ZnPc:PTCDA and ZnPc/ZnPc:PTCDA/PTCDA structures in comparison with that of the ZnPc/PTCDA structure. The results proved that by MAPLE can be obtained flexible organic heterostructures (in different configurations) with properties adequate for applications in flexible electronics and solar cell fields.

Organic heterostructures obtained on ZnO/Ag/ZnO electrode

This work is focused on the use of multilayer transparent conductive electrode (TCE) based on ZnO/Ag/ZnO in the fabrication of the organic heterostructures. The ZnO/Ag/ZnO obtained combining sputtering/thermal evaporation/sputtering techniques is featured by a good optical transmittance, a low electrical resistivity and a reduced roughness. All these characteristics recommend it as a viable alternative to indium tin oxide (ITO) for different applications. The organic materials, N,N'−diphenyl−N,N'−bis (1−naphthyl)−1,1´−biphenyl−4,4´−diamine (α-NPD), 5,12-Dihydro-5-12-dimethylquino [2,3-b]acridine-7,14dione (DMQA) and 4,7 diphenyl-1,10-phenanthroline (BPhen) were deposited by vacuum thermal evaporation (VTE) method, the properties of the obtained layers being investigated by FTIR, UV-VIS and PL) spectroscopy. The I-V characteristic (recorded in dark) of the organic heterostructure fabricated on the ZnO/Ag/ZnO electrode shows diode behavior, revealing its potential applications in the organic light emitting devices (OLED).

Bilayer Formation vs Molecular Exchange in Organic Heterostructures: Strong Impact of Subtle Changes in Molecular Structure

Organic heterostructures are a central part of a manifold of (opto)electronic devices and serve a variety of functions. Particularly, molecular monolayers on metal electrodes are of paramount importance for device performance as they allow tuning energy levels in a versatile way. However, this can be hampered by molecular exchange, i.e., by interlayer diffusion of molecules toward the metal surface. We show that the organic–metal interaction strength is the decisive factor for the arrangement in bilayers, which is the most fundamental version of organic–organic heterostructures. The subtle differences in molecular structure of 6,13-pentacenequinone (P2O) and 5,7,12,14-pentacenetetrone (P4O) lead to antithetic adsorption behavior on Ag(111): physisorption of P2O but chemisorption of P4O. This allows providing general indicators for organic–metal coupling based on shifts in photoelectron spectroscopy data and to show that the coupling strength of copper-phthalocyanine (CuPc) with Ag(111) is in between that ...

On the properties of organic heterostructures prepared with nano-patterned metallic electrode

This paper presents a comparative study between the properties of the heterostructures realized with single/multi layer organic (zinc phthalocyanine or/and fullerene) prepared on Si substrate between flat or patterned aluminum (Al) layer metallic electrode and multi layer ZnO/Au/ZnO transparent conductor electrode (TCE). The UV-Nanoimprint Lithography was used for the realization of a 2D array of nanostructures (holes/pillars) characterized by a periodicity of 1.1 μm and cylindrical shape: diameter = 400 nm and depth/height = 300 nm. The effect of the electrode patterning on the properties of the organic heterostructures was analyzed. For the samples with patterned Al electrode was remarked a slight red shift of the peaks in the reflection spectra determined by an increased interaction between the organic molecules in the delimited region of the patterned holes. The shape of the emission spectra at excitation with UV light showed a narrow intense peak around 500 nm associated with the intense resonance phenomena between the energy of the incident light and the surface plasmons in the patterned Al layer. The TCE followed the morphology of the organic film on which it was deposited. The significant differences between the morphology of the top layer in the heterostructures realized on flat and patterned Al are correlated with the total thickness of the successively deposited layers and with the particularities of the molecular arrangement, leading to the preservation or deleting of patterning. An injection contact behavior was evidence for most heterostructures built on flat and patterned Al. The slight increase in current at an applied bias <1 V in the heterostructure Si/Al/ZnPc/TCE is attributed to the larger interfacial area between the patterned Al electrode and ZnPc layer compared to the interface area between flat Al and ZnPc. A buffer layer of 1,4,5,8-naphthalen-tetracarboxylic dianhydride (NTCDA), sandwiched between the flat metallic electrode and organic film in the heterostructure Si/Al/C60/ZnPc/TCE has determined an increase in the current at low applied voltages.