Phthalocyanine-Based Organic Thin-Film Transistors: A Review of Recent Advances

Finely modified crystallites of unsubstituted zinc phthalocyanine for film deposition purposes

The modified dye crystallites are elastic, thus being capable of spontaneous spreading and seamless fusion.

Engineering Cannabinoid Sensors through Solution-Based Screening of Phthalocyanines

Organic thin-film transistors (OTFTs) have shown promise for a range of sensing applications, with phthalocyanine-based OTFTs demonstrated as sensors for atmospheric parameters, volatile gases, and small organic molecules including cannabinoids. However, the process of fabricating, testing, and optimizing OTFTs in a laboratory setting requires highly specialized equipment, materials, and expertise. To determine if sensor development can be expedited and thus reduce manufacturing burden, spectroelectrochemistry is applied to rapidly screen for molecular interactions between metal-free phthalocyanines and a variety of metal phthalocyanines (MPcs) and the cannabinoids Δ⁹-tetrahydrocannabinol (THC) or cannabidiol (CBD), with and without a cannabinoid-sensitive chromophore (Fast Blue BB). Spectral analyses are corroborated by 2D-NMR and related to measured OTFT performance. Spectroelectrochemical changes to the Q band region of the phthalocyanine spectra in the presence of analytes can be used to predict the response of OTFTs. Thus, with spectroelectrochemistry, a range of potential materials for OTFT small organic molecule-sensing applications can be quickly analyzed, and phthalocyanines with a preferred response can be selected.

Two-Dimensional Metal-Polyphthalocyanine Conjugated Porous Frameworks as Promising Optical Limiting Materials

Two-dimensional transition-metal-containing polyphthalocyanine conjugated porous frameworks are synthesized, and transition-metal (TM) ions ranging from Fe, Co, Ni, Cu to In are chosen to combine in phthalocyanine centers to tune their delocalized electronic structure. The fully closed planar delocalized π-conjugated frameworks exhibit efficient nonlinear optical absorption and excellent optical limiting performance under ns pulsed laser. The metal ion (Co, Ni) with ferromagnetism in phthalocyanine center manifests its contribution in enhanced nonlinear optical response through resonance enhancement of the nonlinear excited-state absorption.

Evolution of Electronic Structures of Polar Phthalocyanine-Substrate Interfaces

The electronic structures and core-level spectra of chlorogallium phthalocyanine (ClGaPc) molecules of different thicknesses (submonolayer to multilayer) adsorbed on a polycrystalline Au substrate and a highly oriented pyrolytic graphite (HOPG) substrate, before and after thermal annealing, were investigated using photoelectron spectroscopic techniques for better understanding the charge-transfer properties. The energy-level diagrams (ELDs) of the ClGaPc thin films are found to evolve with film thickness, substrate nature, and thermal annealing. The interfacial dipole moment in the active Au substrate and the molecular dipole moment in the inactive HOPG substrate mainly dictate the ELD. Annealed monolayer films on both the substrates seem to adopt a similar well-ordered Cl-up orientated molecular organization, which is quite interesting, as it certainly indicates a substrate-nature-independent energy minimum configuration. The strong interaction of the active Au substrate gives rise to additional charge transfer and state transfer (of Ga) as evident from the formation of a former lowest unoccupied molecular orbital (F-LUMO) level in the highest occupied molecular orbital (HOMO) region and a low binding energy peak in the Ga 2p_3/2 core level. The presence of strong F-LUMO and molecular-dipole-related HOMO_d levels in the predicted monolayer of well-ordered Cl-up oriented molecules on the Au and HOPG substrates, respectively, creates the optimum energy-level alignment (ELA) for both the systems, while the opposite shift of the vacuum levels in two different substrates makes the ionization potential (IP) for such a monolayer either minimum (on the Au substrate) or maximum (on the HOPG substrate), which is useful information for tuning the charge injection across the interface in organic semiconductor-based devices.

Control of Porphyrin Planarity and Aggregation by Covalent Capping: Bissilyloxy Porphyrin Silanes

Porphyrins are cornerstone functional materials that are useful in a wide variety of settings, ranging from molecular electronics to biology and medicine. Their applications are often hindered, however, by poor solubilities that result from their extended, solvophobic aromatic surfaces. Attempts to counteract this problem by functionalizing their peripheries have been met with only limited success. Here, we demonstrate a versatile strategy to tune the physical and electronic properties of porphyrins using an axial functionalization approach. Porphyrin silanes (PorSils) and bissilyloxy PorSils (SOPS) are prepared from porphyrins by operationally simple κ⁴N-silylation protocols, introducing bulky silyloxy "caps" that are central and perpendicular to the planar porphyrin. While porphyrins typically form either J- or H-aggregates, SOPS do not self-associate in the same manner: the silyloxy axial substituents dramatically improve the solubility by inhibiting aggregation. Moreover, axial porphyrin functionalization offers convenient handles through which optical, electronic, and structural properties of the porphyrin core can be modulated. We observe that the identity of the silyloxy substituent impacts the degree of planarity of the porphyrin in the solid state as well as the redox potentials.

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.

Fluorous phthalocyanines and subphthalocyanines

Incorporating fluorine atoms into a molecule can endow it with various unique properties that enable materials applications. Selective solubility in fluorous solvents is achieved by a high fluorine content and selective partitioning into perfluorinated liquids over organic and aqueous phases provides orthogonal opportunities for chemistry and materials assembly. Although there is a growing number of partially fluorinated molecules, there are insufficient structural design principles to produce diverse fluorous soluble dyes. Herein, we report the synthesis of six fluorous phthalocyanine and subphthalocyanine dyes, and study their properties in the fluorous phase. Phthalocyanines generally display limited solubility and we also observed apparent aggregation in the fluorous phase. However, the nonplanar subphthalocyanines showed greater solubility. Subphthalocyanines also displayed fluorescence in selected solvents, and their emissive properties were investigated. The materials described expand the library of fluorous dyes and provide insights for the design of new molecules with fluorous solubility.

Temperature-Independent Polarization of Ultrathin Phthalocyanine-Based Hybrid Organic/Inorganic Heterojunctions

The combination of organic and inorganic materials at the nanoscale to form functional hybrid structures is a powerful strategy to develop novel electronic devices. The knowledge on semiconductor thin-film polarization brings direct benefits to the hybrid organic/inorganic electronics, becoming primordial for the development of devices such as electromechanical logic gates, solar cells, miniaturized valves, organic diodes, and molecular supercapacitors, among others. Here, we report on the dielectric polarization of ultrathin organic semiconducting films-ca. 5 nm thick metal phthalocyanine ensembles (viz., CuPc, CoPc, F16CuPc)-employed to build up hybrid metal/oxide/molecule heterojunctions. Such hybrid heterostructures are fully integrated into self-rolled nanomembrane-based capacitors and further investigated by impedance spectroscopy measurements as a function of temperature (from 6 to 300 K). The dielectric polarization of the metal phthalocyanines is found to be thermally activated above a specific threshold temperature, which depends on the molecular structure. Below this threshold, the current leakage across the system is suppressed, thus evidencing intrinsic-like polarization mechanisms. The temperature-independent permittivities of the ultrathin molecular films are found to be strongly dependent on the organic/inorganic hybrid interfaces, while the calculated relaxation times are more likely related to each single-molecule polarization. Beyond the advances in determining the temperature dependence of the permittivity for ultrathin phthalocyanine films integrated within solid-state electronics, our results also support the deterministic design of novel functional devices based on nanoscale hybrid organic/inorganic heterojunctions.

Novel Strategy for Organic Cocrystals of n-Type and p-Type Organic Semiconductors with Advanced Optoelectronic Properties

Cocrystallization has been applied widely for material synthesis. Recently cocrystal of organic molecules has been developing rapidly, taking the advantages of the flexibility and self-assembly of organic molecules. Here we report an experimental study of a cocrystal of copper-phthalocyanines and fluorinated ones. We have grown the samples via the vapor-phase deposition of the mixture with different mass ratios from 1:13.5 to 6:1. As suggested by our scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy, new crystal structures and morphologies through our novel strategy for the cocrystallization of these molecules have been found. Our work will provide a solid foundation to systematically synthesize the cocrystal of phthalocyanine molecules with new crystal structures, thus providing the opportunity to advance material properties.

[1]Benzothieno[3,2-b][1]benzothiophene-Phthalocyanine Derivatives: A Subclass of Solution-Processable Electron-Rich Hole Transport Materials

The [1]benzothieno[3,2-b][1]benzothiophene (BTBT) planar system was used to functionalize the phthalocyanine ring aiming at synthesizing novel electron-rich π-conjugated macrocycles. The resulting ZnPc-BTBT and ZnPc-(BTBT)₄ derivatives are the first two examples of a phthalocyanine subclass having potential use as solution-processable p-type organic semiconductors. In particular, the combination of experimental characterizations and theoretical calculations suggests compatible energy level alignments with mixed halide hybrid perovskite-based devices. Furthermore, ZnPc-(BTBT)₄ features a high aggregation tendency, a useful tool to design compact molecular films. When tested as hole transport materials in perovskite solar cells under 100 mA cm^-2 standard AM 1.5G solar illumination, ZnPc-(BTBT)₄ gave power conversion efficiencies as high as 14.13 %, irrespective of the doping process generally required to achieve high photovoltaic performances. This work is a first step toward a new phthalocyanine core engineerization to obtain robust, yet more efficient and cost-effective materials for organic electronics and optoelectronics.

Rectification ratio and direction controlled by temperature in copper phthalocyanine ensemble molecular diodes

Organic diodes and molecular rectifiers are fundamental electronic devices that share one common feature: current rectification ability. Since both present distinct spatial dimensions and working principles, the rectification of organic diodes is usually achieved by interface engineering, while changes in molecular structures commonly control the molecular rectifiers' features. Here, we report on the first observation of temperature-driven inversion of the rectification direction (IRD) in ensemble molecular diodes (EMDs) prepared in a vertical stack configuration. The EMDs are composed of 20 nm thick molecular ensembles of copper phthalocyanine in close contact with one of its fluorinated derivatives. The material interface was found to be responsible for modifying the junction's conduction mechanisms from nearly activationless transport to Poole-Frenkel emission and phonon-assisted tunneling. In this context, the current rectification was found to be dependent on the interplay of such distinct charge transport mechanisms. The temperature has played a crucial role in each charge transport transition, which we have investigated via electrical measurements and band diagram analysis, thus providing the fundamentals on the IRD occurrence. Our findings represent an important step towards simple and rational control of rectification in carbon-based electronic nanodevices.

Developing and Comparing 2,6-Anthracene Derivatives: Optical, Electrochemical, Thermal, and Their Use in Organic Thin Film Transistors

Anthracene-based semiconductors have attracted great interest due to their molecular planarity, ambient and thermal stability, tunable frontier molecular orbitals and strong intermolecular interactions that can lead to good device field-effect transistor performance. In this study, we report the synthesis of six anthracene derivatives which were di-substituted at the 2,6-positions, their optical, electrochemical and thermal properties, and their single crystal structures. It was found that 2,6-functionalization with various fluorinated phenyl derivatives led to negligible changes in the optical behaviour while influencing the electrochemical properties. Furthermore, the choice of fluorinated phenyl moiety had noticeable effects on melting point and thermal stability (ΔTm < 55 °C and ΔTd < 65 °C). Bottom-gate top-contact (BGTC) organic thin transistors (OTFTs) were fabricated and characterized using the 2,6-anthracene derivatives as the semiconducting layer. The addition of fluorine groups on the phenyl groups led to a transition from p-type behaviour to n-type behaviour in BGBC OTFTs.

Improving Thin-Film Properties of Poly(vinyl alcohol) by the Addition of Low-Weight Percentages of Cellulose Nanocrystals

The increased demand for electronic devices, combined with a desire to minimize the environmental impact, necessitates the development of new eco-friendly materials. One promising approach is the incorporation of renewable and green materials that possess the desired mechanical and electrical properties while allowing for more ecologically friendly disposal of these devices. The addition of low-weight percentages (0.25-0.75 wt %) of cellulose nanocrystals (CNCs) was investigated as an environmentally friendly additive in aqueous dispersions of poly(vinyl alcohol) (PVA). It was found that these low CNC loadings were sufficient to induce a favorable increase in viscosity, which in turn dramatically enhanced the film quality of the PVA blends through an improvement in the critical radius of the spun film, overall film thickness, and homogeneity of the thin film. This corresponded to an increase in the number of functioning organic electronic devices that could be fabricated by spin coating, including metal-insulator-metal (MIM) capacitors and organic thin-film transistors (OTFTs). Most importantly, the incorporation of CNCs into PVA did not significantly alter the native dielectric properties of the polymer thin films when incorporated into both MIM capacitors and OTFTs.

Bis(trialkylsilyl oxide) Silicon Phthalocyanines: Understanding the Role of Solubility in Device Performance as Ternary Additives in Organic Photovoltaics

The use of ternary additives in organic photovoltaics is a promising route for improving overall device performance. Silicon phthalocyanines (SiPcs) are ideal candidates due to their absorption profile, low cost, and ease of synthesis and chemical tunability. However, to date, only a few examples have been reported and specific strategies for aiding in the design of improved ternary additives have not been established. In this study, we report a relationship between ternary additive solubility and device performance, demonstrating that device performance is maximized when the SiPc additive solubility is similar to that of the donor polymer (P3HT, in this case). This improved performance can be attributed to the favored interfacial precipitation of the SiPc when its solubility matches that of the other components of the thin film. The power conversion efficiency (PCE) varied from 2.4% to 3.4% by using axially substituted SiPcs with different solubilities, where the best ternary additive led to a 25% increase in PCE compared to that of the baseline device.

Cyber-Physiochemical Interfaces

Living things rely on various physical, chemical, and biological interfaces, e.g., somatosensation, olfactory/gustatory perception, and nervous system response. They help organisms to perceive the world, adapt to their surroundings, and maintain internal and external balance. Interfacial information exchanges are complicated but efficient, delicate but precise, and multimodal but unisonous, which has driven researchers to study the science of such interfaces and develop techniques with potential applications in health monitoring, smart robotics, future wearable devices, and cyber physical/human systems. To understand better the issues in these interfaces, a cyber-physiochemical interface (CPI) that is capable of extracting biophysical and biochemical signals, and closely relating them to electronic, communication, and computing technology, to provide the core for aforementioned applications, is proposed. The scientific and technical progress in CPI is summarized, and the challenges to and strategies for building stable interfaces, including materials, sensor development, system integration, and data processing techniques are discussed. It is hoped that this will result in an unprecedented multi-disciplinary network of scientific collaboration in CPI to explore much uncharted territory for progress, providing technical inspiration-to the development of the next-generation personal healthcare technology, smart sports-technology, adaptive prosthetics and augmentation of human capability, etc.

Room-Temperature Operation of a p-Type Molecular Spin Photovoltaic Device on a Transparent Substrate

The coupling of diverse degrees of freedom opens the door to physical effects that go beyond each of them individually, making multifunctionality a much sought-after attribute for high-performance devices. Here, the multifunctional operation of a single-layer p-type organic device, displaying both spin transport and photovoltaic effect at the room temperature on a transparent substrate, is shown. The generated photovoltage is almost three times larger than the applied bias to the device which facilitates the modulation of the magnetic response of the device with both bias and light. The device shows an increase in power conversion efficiency under magnetic field, an ability to invert the current with magnetic field and under certain conditions it can act as a spin photodetector with zero power consumption in the standby mode. The room-temperature exploitation of the interplay among light, bias, and magnetic field in the single device with a p-type molecule opens a way toward the development of efficient high-performance spin photovoltaic cells.

Functionalization of commercial pigment Hostasol Red GG for incorporation into organic thin-film transistors

We explored the functionalization of the inexpensive commercial dye Red GG through a simple one-pot Grignard reaction leading to novel substituted derivatives that were incorporated into organic thin film transistors.

Supramolecular organization and optical properties of BODIPY derivatives in Langmuir–Schaefer films

Formation of thin-film nanomaterials, promising for nanoelectronic applications, with a given structure based on four new BODIPY dyes.

Optoelectronic parameters of peripherally tetra-substituted copper(ii) phthalocyanines and fabrication of a photoconductive diode for various conditions

In this study, the molecular structure of 4-(4-(trifluoromethoxy)phenoxy)phthalonitrile (1) has been elucidated and its supra-molecular dynamics have been revealed by the analysis of single crystal X-ray diffraction measurements.

Self-organization of octa-phenyl-2,3-naphthalocyaninato zinc floating layers

Formation of thin-film nanomaterials, promising for nanoelectronic applications, with a given structure based on octa-phenyl-2,3-naphthalocyaninato zinc.

Phthalocyanines core-modified by P and S and their complexes with fullerene C60: DFT study

Phthalocyanines (Pcs) and their derivatives have attracted a lot of attention because of their both biological importance and technological applications. The properties of Pcs can be tuned by replacing the central atom, by modifying the periphery of phthalocyanine ring, and by changing the meso-atoms. One more promising pathway for modifying Pcs and their derivatives can be the core-modification, or substitution of the core isoindole nitrogen(s) by other elements. Motivated by the results obtained for some core-modified porphyrins, we investigated computationally complete core-modification of regular Zn phthalocyanine (ZnPc) with P and S. We performed density functional theory studies of the structures, charges, and frontier molecular orbitals of P-core-modified and S-core-modified ZnPcs, ZnPc(P)4 and ZnPc(S)4, using both B3LYP and two dispersion-corrected functionals. Also, we studied computationally formation of complexes between the fullerene C60 and ZnPc(P)4 and ZnPc(S)4. Both ZnPc(P)4 and ZnPc(S)4 show strong bowl-like distortions similar to the results obtained earlier for ZnP(P)4 and ZnP(S)4. The size of the “bowl” cavity of the both core-modified Pcs is essentially the same, showing no dependence on the core-modifying element. For ZnPc(S)4, the HOMO is quite different from those of ZnPc and ZnPc(P)4. When the fullerene C60 forms complexes with the ZnPc(P)4 and ZnPc(S)4 species in the gas phase, it is located relatively far (4.30–5.72 Å) from the one of the P-centers and from the Zn-center of ZnPc(P)4, whereas with ZnPc(S)4 C60 forms relatively short bonds with the Zn-center, varying from ca. 2.0 to ca. 3.0 Å. The very strong deformations of both the ZnPc(P)4 and ZnPc(S)4 structures are observed. The calculated binding energy at the B3LYP/6-31G* level for the C60-ZnPc(P)4 complex is quite low, 1.2 kcal/mol, which agrees with the quite long distances fullerene - ZnPc(P)4, whereas it is noticeably larger, 13.6 kcal/mol, for the C60-ZnPc(S)4 complex which again agrees with the structural features of this complex. The binding energies of the complexes optimized using the dispersion-corrected functionals, CAM-B3LYP and wB97XD, are significantly larger, varying from ca. 14 till 52 kcal/mol which corresponds with the shorter distances between the fullerene and ZnPc(X)4 species.

On-the-Spot Detection and Speciation of Cannabinoids Using Organic Thin-Film Transistors

Quality control is imperative for Cannabis since the primary cannabinoids, Δ⁹-tetrahydrocannabinol (THC) and cannabidiol (CBD), elicit very different pharmacological effects. THC/CBD ratios are currently determined by techniques not readily accessible by consumers or dispensaries and which are impractical for use in the field by law-enforcement agencies. CuPc- and F₁₆-CuPc-based organic thin-film transistors have been combined with a cannabinoid-sensitive chromophore for the detection and differentiation of THC and CBD. The combined use of these well-characterized and inexpensive p- and n-type materials afforded the determination of the CBD/THC ratio from rapid plant extracts, with results indistinguishable from high-pressure liquid chromatography. Analysis of the prepyrolyzed sample accurately predicted postpyrolysis THC/CBD, which ultimately influences the psychotropic and medicinal effects of the specific plant. The devices were also capable of vapor-phase sensing, producing a unique electrical output for THC and CBD relative to other potentially interfering vaporized organic products. The analysis of complex medicinal plant extracts and vapors, normally reserved for advanced analytical infrastructure, can be achieved with ease, at low cost, and on the spot, using organic thin-film transistors.

Orientational and Crystalline Order of Copper-Phthalocyanine Films on Gold: the Role of Substrate Roughness and Cleanliness

Although metal phthalocyanines are widely used in optoelectronic devices, e.g., as hole-transport and electron-blocking layers, or as UV-stable dyes, their multilayer growth on metal substrates has surprisingly not been studied very systematically. Even for CuPc, one of the most widely studied representatives of phthalocyanines, contradictory structures are reported for films grown on gold, a common electrode material, suggesting that the influence of actual substrate surface properties on film growth has not been sufficiently considered. In this study, we analyze the growth of CuPc films on gold substrates for thicknesses ranging from the initial seed layer to thick multilayers (50 nm) by combining near-edge X-ray absorption spectroscopy with atomic force microscopy and X-ray diffraction. To study the influence of surface roughness, we compare the formation of CuPc films on well-ordered Au(111) and sputter-deposited polycrystalline gold substrates and also investigate the influence of surface contamination by exposing these gold surfaces to air before film growth. While on clean gold substrates, CuPc molecules exclusively adopt a recumbent orientation and form (112̅)-oriented films, they also grow in an upright orientation on contaminated gold surfaces. On Au(111), this leads to the coexistence of (112̅)- and (100)-oriented regions, whereas only (100)-oriented films are formed on contaminated polycrystalline gold. Remarkably, the (112̅)-oriented films consist of extended but isolated crystalline islands, resulting in large overall roughness, whereas the (100)-oriented films consist of rather small domains but have significantly lower film roughness.

Amine-assisted solubilization of unsubstituted zinc phthalocyanine for film deposition purposes

Typical zinc phthalocyanines (ZnPc) exhibit poor solubility in common solvents and, hence, are processed into thin films mostly from the vapor phase. The present work discloses how these limitations can be effectively overcome. Specifically, highly concentrated molecular solutions of unsubstituted ZnPc are prepared by combining a weakly structured ZnPc polymorph with binary liquid systems composed of a π-accepting solvent and a simple nitrogenous base, such as ammonia or tertiary aliphatic amine. The amine-assisted solubilization of ZnPc is rationalized by quantitative analysis of optical spectra and electrostatic potential maps of the dye molecule. A volatile aminoalcohol is proposed in order to rationally modify the habit of ZnPc crystallites and concurrently to produce uniform deposition of the crystallites by drop-casting the dye solutions onto a glass substrate. Finally, a versatile algorithm for wet-processed ZnPc films is declared.

Wet-processing of unsubstituted zinc phthalocyanine into polycrystalline films is mastered.

Evolutionary Computation for Parameter Extraction of Organic Thin-Film Transistors Using Newly Synthesized Liquid Crystalline Nickel Phthalocyanine

In this work, the topic of the detrimental contact effects in organic thin-film transistors (OTFTs) is revisited. In this case, contact effects are considered as a tool to enhance the characterization procedures of OTFTs, achieving more accurate values for the fundamental parameters of the transistor threshold voltage, carrier mobility and on-off current ratio. The contact region is also seen as a fundamental part of the device which is sensitive to physical, chemical and fabrication variables. A compact model for OTFTs, which includes the effects of the contacts, and a recent proposal of an associated evolutionary parameter extraction procedure are reviewed. Both the model and the procedure are used to assess the effect of the annealing temperature on a nickel-1,4,8,11,15,18,22,25-octakis(hexyl)phthalocyanine (NiPc6)-based OTFT. A review of the importance of phthalocyanines in organic electronics is also provided. The characterization of the contact region in NiPc6 OTFTs complements the results extracted from other physical-chemical techniques such as differential scanning calorimetry or atomic force microscopy, in which the transition from crystal to columnar mesophase imposes a limit for the optimum performance of the annealed OTFTs.

Old Molecule, New Chemistry: Exploring Silicon Phthalocyanines as Emerging N-Type Materials in Organic Electronics

Efficient synthesis of silicon phthalocyanines (SiPc) eliminating the strenuous reaction conditions and hazardous reagents required by classical methods is described. Implementation into organic thin-film transistors (OTFTs) affords average electron field-effect mobility of 3.1 × 10^-3 cm² V^-1 s^-1 and threshold voltage of 25.6 V for all synthetic routes. These results demonstrate that our novel chemistry can lead to high performing SiPc-based n-type OTFTs.

Straightforward and Relatively Safe Process for the Fluoride Exchange of Trivalent and Tetravalent Group 13 and 14 Phthalocyanines

To avoid the use of hydrofluoric acid, a series of fluorinated trivalent and tetravalent metal-containing phthalocyanines (MPcs) were synthesized using a straightforward one-step halide substitution process using cesium fluoride (CsF) as the fluoride source and by reflux in N,N-dimethylformamide for less than an hour. The resulting fluoro MPcs were characterized and compared to the parent chloro MPcs. In some cases, very little change in properties was observed between the fluoro MPcs and the chloro MPcs. In other cases, such as fluoro aluminum phthalocyanine, a blue shift in the absorbance characteristics and an increase in oxidation and reduction potential of as much as 0.22 V was observed compared to the chloro derivative. Thermo gravimetric analysis was performed on all halo-MPcs, indicating that the choice of halo substitution on the axial position can have an effect on the decomposition or sublimation temperature of the final compound. After initial establishment and characterization of the fluoro MPcs, the halide substitution reaction of difluoro silicon phthalocyanine (F₂-SiPc) was further explored by scaling the reaction up to a gram scale as well as considering tetrabutylammonium fluoride (TBAF) as an additional safe fluoride source. The scaled-up reactions producing F₂-SiPc using CsF and TBAF as fluoride exchange sources were successfully reproducible, resulting in reaction yields of 100 and 73%, respectively. Both processes led to pure final products but results indicate that CsF, as the fluoride exchange reagent, appears to be the superior reaction process as it has a much higher yield.

Silicon Phthalocyanines as Acceptor Candidates in Mixed Solution/Evaporation Processed Planar Heterojunction Organic Photovoltaic Devices

Silicon phthalocyanines (SiPc) are showing promise as both ternary additives and non-fullerene acceptors in organic photovoltaics (OPVs) as a result of their ease of synthesis, chemical stability and strong absorption. In this study, bis(3,4,5-trifluorophenoxy) silicon phthalocyanine ((345F)2-SiPc)) and bis(2,4,6-trifluorophenoxy) silicon phthalocyanine ((246F)2-SiPc)) are employed as acceptors in mixed solution/evaporation planar heterojunction (PHJ) devices. The donor layer, either poly(3-hexylthiophene) (P3HT) or poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), was spin coated followed by the evaporation of the SiPc acceptor thin film. Several different donor/acceptor combinations were investigated in addition to investigations to determine the effect of film thickness on device performance. Finally, the effects of annealing, prior to SiPc deposition, after SiPc deposition, and during SiPc deposition were also investigated. The devices which performed the best were obtained using PCDTBT as the donor, with a 90 nm film of (345F)2-SiPc as the acceptor, followed by thermal annealing at 150 °C for 30 min of the entire mixed solution/evaporation device. An open-circuit voltage (Voc) of 0.88 V and a fill factor (FF) of 0.52 were achieved leading to devices that outperformed corresponding fullerene-based PHJ devices.

Atypical Film-Forming Behavior of Soluble Tetra-3-Nitro-Substituted Copper Phthalocyanine

Thin films of metal phthalocyanines (MPc) are known to exhibit excellent physical properties but poorly controlled morphologies. Therefore, the present work seeks to understand the film growth mechanism of a model compound for potentially usable MPc, specifically, copper tetra(3-nitro-5-tert-butyl)phthalocyanine (CuPc*). The Langmuir-Schaefer (LS) technique was applied to prepare a series of CuPc* films under different processing conditions. The film growth was examined by Brewster angle microscopy (BAM) on the water surface and small-angle X-ray scattering (SAXS) from the solid films. Neutron reflectometry (NR) measurements of the water uptake into the films and computer simulation of hydrated CuPc* were performed to substantiate an idea of colloidal MPc-water aggregates as nanoscale precursors of smooth solid films. This idea appears fruitful in terms of materials chemistry.

A sandwich-type tetrakis(phthalocyaninato) europium–cadmium quadruple-decker complex: structural, spectroscopic, OFET, and gas sensing properties

A new tetrakis(phthalocyaninato) europium(iii)–cadmium(ii) quadruple-decker complex {[Pc(SC₆H₁₃)₈]Eu[Pc(SC₆H₁₃)₈]Cd[Pc(SC₆H₁₃)₈]Eu[Pc(SC₆H₁₃)₈]} (1) has been designed, synthesized, and characterized.

Recent advances in chemistry of phthalocyanines bearing electron-withdrawing halogen, nitro and N-substituted imide functional groups and prospects for their practical application

Herein, we present an overview of the approaches for the synthesis of phthalocyanines bearing electron-withdrawing halogen-, nitro- and N-substituted imide functional groups in different positions of the phthalocyanine macrocycle.

Metal phthalocyanine organic thin-film transistors: changes in electrical performance and stability in response to temperature and environment

Metal phthalocyanines (MPcs) are a widely studied class of materials that are frequently used in organic thin-film transistors (OTFTs), organic photovoltaics (OPVs) and organic light emitting diodes (OLEDs). The stability of these devices and the materials used in their fabrication is important to realize their widespread adoption. Seven P-type MPcs: zinc (ZnPc), magnesium (MgPc), aluminum (AlClPc), iron (FePc), cobalt (CoPc), and titanium (TiOPc) were investigated as the semiconductors in OTFTs under varying temperatures (25 °C to 150 °C) and environmental conditions (air and vacuum, P < 0.1 Pa). Devices using the divalent MPcs (except MgPc) showed significant shifts in threshold voltage and field-effect mobility with rising temperature in both air and vacuum. AlClPc and TiOPc, on the other hand, had more stable electrical properties, making them useful for applications requiring consistent performance. Distinct variations in film morphology as determined by atomic force microscopy may explain the different thermal response between the two groups of MPcs, while thermal gravimetric analysis in air and nitrogen (N₂) provides additional insight into their susceptibility to oxidation at elevated temperature. To demonstrate proof-of-concept thermal sensing under realistic operating conditions, current changes were monitored in response to temperature stimuli using two more sensitive divalent MPcs. This comparative study of the effect of central atom inclusion in MPcs, the resulting material stability and thin-film characteristics will facilitate design of future sensors and other OTFT applications.

A series of metal phthalocyanine based organic thin film transistors were evaluated and their responses to changes in temperature and environmental was determined: the choice of central atom makes a difference.

Synthesis of non-peripherally tetra-substituted copper(ii) phthalocyanines: characterization, optical and surface properties, fabrication and photo-electrical properties of a photosensitive diode

This study describes the synthesis and characterization of a non-peripherally tetra-substituted copper(ii) phthalocyanine bearing 4-(trifluoromethoxy)phenol groups.

P and N type copper phthalocyanines as effective semiconductors in organic thin-film transistor based DNA biosensors at elevated temperatures

Many health-related diagnostics are expensive, time consuming and invasive. Organic thin film transistor (OTFT) based devices show promise to enable rapid, low cost diagnostics that are an important aspect to enabling increased access and availability to healthcare. Here, we describe OTFTs based upon two structurally similar P (copper phthalocyanine – CuPc) and N (hexdecafluoro copper phthalocyanine – F₁₆-CuPc) type semiconductor materials, and demonstrate their potential for use as both temperature and DNA sensors. Bottom gate bottom contact (BGBC) OTFTs with either CuPc or F₁₆-CuPc semiconducting layers were characterized within a temperature range of 25 °C to 90 °C in both air and under vacuum. CuPc devices showed small positive shifts in threshold voltage (V_T) in air and significant linear increases in mobility with increasing temperature. F₁₆-CuPc devices showed large negative shifts in V_T in air and linear increases in mobility under the same conditions. Similar OTFTs were exposed to DNA in different hybridization states and both series of devices showed positive V_T increases upon DNA exposure, with a larger response to single stranded DNA. The N-type F₁₆-CuPc devices showed a much greater sensing response than the P-type CuPc. These findings illustrate the use of these materials, especially the N-type semiconductor, as both temperature and DNA sensors and further elucidate the mechanism of DNA sensing in OTFTs.

This study illustrates the use of an N-type semiconductor, in both temperature and DNA sensors and further elucidates the mechanism of DNA sensing in OTFTs.

Phthalocyanine arrangements on Ag(100): From pure overlayers of CoPc and F16CuPc to bimolecular heterostructure

We have utilized scanning tunneling microscopy (STM) and low energy electron diffraction to determine the structural properties of two types of metal-phthalocyanines (MPcs), i.e., cobalt-phthalocyanine (CoPc) and hexadecafluorinated copper-phthalocyanine (F₁₆CuPc) on the Ag(100) surface. For coverage close to one monolayer, both systems form long-range ordered structures with square unit cells. The size and rotation of the unit cell with respect to the silver lattice depend on the chemical composition of MPc. Both types of molecules prefer adsorption with around a 30° angle between the molecular axis and the [011] silver direction. The CoPcs mainly arrange in a (5 × 5)R0 phase; however, two additional local arrangements, a 26 × 26 R 1 1 ○ and a (7 × 7)R0, were detected by STM. The F₁₆CuPcs form a 29 × 29 R 2 2 ○ structure. The co-adsorption of CoPc and F₁₆CuPc on the Ag(100) surface in a 1:1 ratio leads to the formation of a compositionally ordered chessboard-like 5 2 × 5 2 R 4 5 ○ structure. During filled states imaging, the different appearance of the central part of each MPc allows us to distinguish CoPcs from F₁₆CuPcs. Regardless of the applied voltage polarity, the ligands of F₁₆CuPcs appear brighter than the ligands of CoPcs.

Low-Voltage, Flexible, and Self-Encapsulated Ultracompact Organic Thin-Film Transistors Based on Nanomembranes

Organic thin-film transistors (OTFTs) are an ever-growing subject of research, powering recent technologies such as flexible and wearable electronics. Currently, many studies are being carried out to push forward the state-of-the-art OTFT technology to achieve characteristics that include high carrier mobility, low power consumption, flexibility, and the ability to operate under harsh conditions. Here, we tackle this task by proposing a novel OTFT architecture exploring the so-called rolled-up nanomembrane technology to fabricate low-voltage (<2 V), ultracompact OTFTs. As the OTFT gate electrode, we use strained nanomembranes, which allows all transistor components to be rolled-up and confined into a tubular-shaped tridimensional device structure with reduced footprint (ca. 90% of their planar counterpart), without any loss of electrical performance. Such an innovative architecture endows the OTFTs high mechanical flexibility (bending radius of <30 μm) and robustness-the devices can be reversibly deformed, withstanding more than 500 radial compression/decompression cycles. Additionally, the tubular device design possesses an inherent self-encapsulation characteristic that protects the OTFT active region from degradation by UV-light and hazardous vapors. The reported strategy is also shown to be compatible with different organic semiconductor materials. All of these characteristics contribute to further extending the potentialities of OTFTs, mainly toward rugged electronics.

Complex momentum behavior of electronic excitations in β-CuPc

The electronic excitation spectrum of β-CuPc has been investigated using electron energy-loss spectroscopy in transmission. Our results demonstrate a rather strong momentum dependence of the lowest exciton features. Both main components show a negative dispersion, and the momentum dependence indicates that this negative dispersion is parallel to the molecular stacks in β-CuPc. In addition, the spectral shape also varies upon increasing momentum transfer indicating a particular momentum dependence of the inter-molecular interactions.

Hole Mobility Modulation in Single-Crystal Metal Phthalocyanines by Changing the Metal-π/π-π Interactions

Weak intermolecular interactions in organic semiconducting molecular crystals play an important role in determining molecular packing and electronic properties. Single crystals of metal-free and metal phthalocyanines were synthesized to investigate how the coordination of the central metal atom affects their molecular packing and resultant electronic properties. Single-crystal field-effect transistors were made and showed a hole mobility order of ZnPc>MnPc>FePc>CoPc>CuPc>H₂ Pc>NiPc. Density functional theory (DFT) and 1D polaron transport theory reach a good agreement in reproducing the experimentally measured trend for hole mobility. Additional detail analysis at the DFT level suggests the metal atom coordination into H₂ Pc planes can tune the hole mobility via adjusting the intermolecular distances along the shortest axis with closest parallel π stackings.

Oxide-organic heterostructures: a case study of charge transfer disturbance at a SnO2-copper phthalocyanine buried interface

Reduced tin dioxide/copper phthalocyanine (SnOx/CuPc) heterojunctions recently gained much attention in hybrid electronics due to their defect structure, allowing tuning of the electronic properties at the interface towards particular needs. In this work, we focus on the creation and analysis of the interface between the oxide and organic layer. The inorganic/organic heterojunction was created by depositing CuPc on SnOx layers prepared with the rheotaxial growth and vacuum oxidation (RGVO) method. Exploiting surface sensitive photoelectron spectroscopy techniques, angle dependent X-ray and UV photoelectron spectroscopy (ADXPS and UPS, respectively), supported by semi-empirical simulations, the role of carbon from adventitious organic adsorbates directly at the SnOx/CuPc interface was investigated. The adventitious organic adsorbates were blocking electronic interactions between the environment and surface, hence pinning energy levels. A significant interface dipole of 0.4 eV was detected, compensating for the difference in work functions of the materials in contact, however, without full alignment of the energy levels. From the ADXPS and UPS results, a detailed diagram of the interfacial electronic structure was constructed, giving insight into how to tailor SnOx/CuPc heterojunctions towards specific applications. On the one hand, parasitic surface contamination could be utilized in technology for passivation-like processes. On the other hand, if one needs to keep the oxide's surficial interactions fully accessible, like in the case of stacked electronic systems or gas sensor applications, carbon contamination must be carefully avoided at each processing step.