The Rise of Silicon Phthalocyanine: From Organic Photovoltaics to Organic Thin Film Transistors

Axial Phenoxylation of Aluminum Phthalocyanines for Improved Cannabinoid Sensitivity in OTFT Sensors

Cannabis producers, consumers, and regulators need fast, accurate, point-of-use sensors to detect Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD) from both liquid and vapor source samples, and phthalocyanine-based organic thin-film transistors (OTFTs) provide a cost-effective solution. Chloro aluminum phthalocyanine (Cl-AlPc) has emerged as a promising material due to its unique coordinating interactions with cannabinoids, allowing for superior sensitivity. This work explores the molecular engineering of AlPc to tune and enhance these interactions, where a series of novel phenxoylated R-AlPcs are synthesized and integrated into OTFTs, which are then exposed to THC and CBD solution and vapor samples. While the R-AlPc substituted molecules have a comparable baseline device performance to Cl-AlPc, their new crystal structures and weakened intermolecular interactions increase sensitivity to THC. Grazing-incidence wide-angle X-ray scattering (GIWAXS) and atomic force microscopy (AFM) are used to investigate this film restructuring, where a significant shift in the crystal structure, grain size, and film roughness is detected for the R-AlPc molecules that do not occur with Cl-AlPc. This significant crystal reorganization and film restructuring are the driving force behind the improved sensitivity to cannabinoids relative to Cl-AlPc and demonstrate that analyte-semiconductor interactions can be enhanced through chemical modification to create more responsive OTFT sensors.

Structural determination, characterization and computational studies of doped semiconductors base silicon phthalocyanine dihydroxide and dienynoic acids

The chemical doping of silicon phthalocyanine dihydroxide (SiPc(OH)2), with (2E, 4Z)-5, 7-diphenylhepta-2, 4-dien-6-ynoic acids (DAc) with electron-withdrawing (BrDAc) and electron-donating (MeODAc) substituents is the main purpose of this work. Theoretical calculations were carried out on Gaussian16 software, with geometrical optimization of all involved species, and obtention of the highest occupied molecule orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and the respective energy gaps. The theoretical calculations show two hydrogen bridge formations: the first one as a peripheral interaction between the terminal oxygen atoms from the acid unit and hydrogen atoms from the phthalocyanine aromatic rings. The second one as the interaction at the nitrogen atoms of the phthalocyanine, which are compelled to form a new flat plane far from the original flat phthalocyanine deck. These organic semiconductors were deposited as thin films and characterized by IR spectroscopy, atomic force microscopy (AFM), and the optical parameters were gathered from UV-Vis studies. The indirect and direct optical band gap, the onset gap and the Urbach energy were obtained. In order to compare the effect of the acids as dopants of the silicon phthalocyanine, the SiPc(OH)2-DAc films were electrically characterized. The SiPc(OH)2-DAc films exhibit an ambipolar electrical behavior, which is influenced by the incidence of different lighting conditions at voltages above 0.3V. The glass/ITO/SiPc(OH)2-MeODAc/Ag reaches a maximum current of 5.68 × 10-5 A for natural light condition, while the glass/ITO/SiPc(OH)2-BrDAc/Ag, reaches a maximum current of 9.21 × 10-9 A for white illumination condition.

Roles of Singlet Fission in the Photosensitization of Silicon Phthalocyanine

The roles of singlet fission in the triplet generation of silicon phthalocyanine (SiPc), a compound analogous to the IRDye700DX photosensitizer used in near-infrared photoimmunotherapy, are investigated by considering the energetical relation between the excitations of this compound. These excitations are obtained through spin-flip long-range corrected time-dependent density functional theory calculations. To initiate singlet fission, chromophores must meet two conditions: (1) near-degenerate low-lying singlet and quintet (triplet-triplet) excitations with a considerable energy gap of the lowest singlet and triplet excited states and (2) moderate π-stacking energy of chromophores, which is higher than but not far from the solvation energy, to facilitate the dissociation and generation of triplet-state chromophores. The present calculations demonstrate that SiPc satisfies both of these conditions after the formation of π-stacking irrespective of the presence of an axial ligand(s), suggesting that singlet fission plays a crucial role in the triplet generation process, although intersystem crossing occurs simultaneously at a very slow rate.

The Adsorption Mechanisms of SF6-Decomposed Species on Tc- and Ru-Embedded Phthalocyanine Surfaces: A Density Functional Theory Study

Designing high-performance materials for the detection or removal of toxic decomposition gases of sulfur hexafluoride is crucial for both environmental monitoring and human health preservation. Based on first-principles calculations, the adsorption performance and gas-sensing properties of unsubstituted phthalocyanine (H2Pc) and H2Pc doped with 4d transition metal atoms (TM = Tc and Ru) towards five characteristic decomposition components (HF, H2S, SO2, SOF2, and SO2F2) were simulated. The findings indicate that both the TcPc and RuPc monolayers are thermodynamically and dynamically stable. The analysis of the adsorption energy indicates that H2S, SO2, SOF2, and SO2F2 underwent chemisorption on the TcPc monolayer. Conversely, the HF molecules were physisorbed through interactions with H atoms. The chemical adsorption of H2S, SO2, and SOF2 occurred on the RuPc monolayer, while the physical adsorption of HF and SO2F2 molecules was observed. Moreover, the microcosmic mechanism of the gas-adsorbent interaction was elucidated by analyzing the charge density differences, electron density distributions, Hirshfeld charges, and density of states. The TcPc and RuPc monolayers exhibited excellent sensitivity towards H2S, SO2, and SOF2, as evidenced by the substantial alterations in the band gaps and work functions of the TcPc and RuPc nanosheets. Our calculations hold significant value for exploring the potential chemical sensing applications of TcPc and RuPc monolayers in gas sensing, with a specific focus on detecting sulfur hexafluoride.

A smart and visible way to switch the aromaticity of silicon(IV) phthalocyanines

Unlike traditional methods of modifying phthalocyanines (Pcs), we herein report a smart and visible way to switch the aromaticity of silicon(IV) phthalocyanines via a reversible nucleophilic addition reaction of the Pc skeleton induced by alkalis and acids, leading to an interesting allochroism phenomenon and the switching of photosensitive activities.

Not Just Surface Energy: The Role of Bis(pentafluorophenoxy) Silicon Phthalocyanine Axial Functionalization and Molecular Orientation on Organic Thin-Film Transistor Performance

Understanding the effect of surface chemistry on the dielectric-semiconductor interface, thin-film morphology, and molecular alignment enables the optimization of organic thin-film transistors (OTFTs). We explored the properties of thin films of bis(pentafluorophenoxy) silicon phthalocyanine (F₁₀-SiPc) evaporated onto silicon dioxide (SiO₂) surfaces modified by self-assembled monolayers (SAMs) of varying surface energies and by weak epitaxy growth (WEG). The total surface energy (γ^tot), dispersive component of the total surface energy (γ^d), and polar component of the total surface energy (γ^p) were calculated using the Owens-Wendt method and related to electron field-effect mobility of devices (μ_e), and it was determined that minimizing γ^p and matching γ^tot yielded films with the largest relative domain sizes and highest resulting μ_e. Subsequent analyses were completed using atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray scattering (GIWAXS) to relate surface chemistry to thin-film morphology and molecular order at the surface and semiconductor-dielectric interface, respectively. Films evaporated on n-octyltrichlorosilane (OTS) yielded devices with the highest average μ_e of 7.2 × 10^-2 cm²·V^-1·s^-1 that we attributed to it having both the largest domain length, which were extracted from power spectral density function (PSDF) analysis, and a subset of molecules with a pseudo edge-on orientation relative to the substrate. Films of F₁₀-SiPc with the mean molecular orientation of the π-stacking direction being more edge-on relative to the substrate also generally resulted in OTFTs with a lower average V_T. Unlike conventional MPcs, F₁₀-SiPc films fabricated by WEG experienced no macrocycle in an edge-on configuration. These results demonstrate the critical role of the F₁₀-SiPc axial groups on WEG, molecular orientation, and film morphology as a function of surface chemistry and the choice of SAMs.

Strong Magnetic Field Annealing for Improved Phthalocyanine Organic Thin-Film Transistors

Thin-film microstructure, morphology, and polymorphism can be controlled and optimized to improve the performance of carbon-based electronics. Thermal or solvent vapor annealing are common post-deposition processing techniques; however, it can be difficult to control or destructive to the active layer or substrates. Here, the use of a static, strong magnetic field (SMF) as a non-destructive process for the improvement of phthalocyanine (Pc) thin-film microstructure, increasing organic thin-film transistor (OTFTs) mobility by twofold, is demonstrated. Grazing incident wide-angle X-ray scattering (GIWAXS), X-ray diffraction (XRD), and atomic force microscopy (AFM) elucidate the effect of SMF on both para- and diamagnetic Pc thin-films when subjected to a magnetic field. A SMF is found to increase the concentration of oxygen-induced radical species within the Pc thin-film, lending a paramagnetic character to ordinarily diamagnetic metal-free Pc and resulting in magnetic field induced changes to its thin-film microstructures. In a nitrogen environment, without competing degradation effects of molecular oxygen, SMF processing is found to favorably improve charge transport characteristics and increase OTFT mobility. Thus, post-deposition thin-film annealing with a magnetic field is presented as an alternative and promising technique for future thin-film engineering applications.

Octafluoro-Substituted Phthalocyanines of Zinc, Cobalt, and Vanadyl: Single Crystal Structure, Spectral Study and Oriented Thin Films

In this work, octafluoro-substituted phthalocyanines of zinc, vanadyl, and cobalt (MPcF₈, M = Zn(II), Co(II), VO) were synthesized and studied. The structures of single crystals of the obtained phthalocyanines were determined. To visualize and compare intermolecular contacts in MPcF₈, an analysis of Hirshfeld surfaces (HS) was performed. MPcF₈ nanoscale thickness films were deposited by organic molecular beam deposition technique and their structure and orientation were studied using X-ray diffraction. Comparison of X-ray diffraction patterns of thin films with the calculated diffractograms showed that all three films consisted of a single crystal phase, which corresponded to a phase of single crystals. Only one strong diffraction peak corresponding to the plane (001) was observed on the diffraction pattern of each film, which indicated a strong preferred orientation with the vast majority of crystallites oriented with a (001) crystallographic plane parallel to the substrate surface. The effect of the central metals on the electronic absorption and vibrational spectra of the studied phthalocyanines as well as on the electrical conductivity of their films is also discussed.

Low-Cost Silicon Phthalocyanine as a Non-Fullerene Acceptor for Flexible Large Area Organic Photovoltaics

We demonstrate large-area (1 cm²) organic photovoltaic (OPVs) devices based on bis(tri-n-butylsilyl oxide) silicon phthalocyanine (3BS)₂-SiPc as a non-fullerene acceptor (NFA) with low synthetic complexity paired with poly(3-hexylthiophene) (P3HT) as a donor polymer. Environment-friendly nonhalogenated solvents were used to process large area OPVs on flexible indium tin oxide (ITO)-coated polyethylene terephthalate (PET) substrates. An alternate sequentially (Alt-Sq) blade-coated active layer with bulk heterojunction-like morphology is obtained when using (3BS)₂-SiPc processing with o-xylene/1,3,5-trimethylbenzene solvents. The sequential (Sq) active layer is prepared by first blade-coating (3BS)₂-SiPc solution followed by P3HT coated on the top without any post-treatment. The conventional sequentially (Sq) blade-coated active layer presents very low performance due to the (3BS)₂-SiPc bottom layer being partially washed off by processing the top layer of P3HT. In contrast, alternate sequentially (Alt-Sq) blade-coated layer-by-layer film shows even better device performance compared to the bulk heterojunction (BHJ) active layer. Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) and atomic force microscopy (AFM) reveal that the Alt-Sq processing of the active layer leads to a BHJ-like morphology with a well-intermixed donor-acceptor component in the active layer while providing a simpler processing approach to low-cost and large-scale OPV production.

Design of ternary additive for organic photovoltaics: a cautionary tale

Silicon phthalocyanines as ternary additives are a promising way to increase the performance of organic photovoltaics. The miscibility of the additive and the donor polymer plays a significant role in the enhancement of the device performance, therefore, ternary additives can be designed to better interact with the conjugated polymer. We synthesized N-9′-heptadecanyl-2,7-carbazole functionalized SiPc ((CBzPho)₂-SiPc), a ternary additive with increased miscibility in poly[N-90-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT). The resulting additive was included into PCDTBT and [6,6]-phenyl C₇₁ butyric acid methyl ester as bulk (PC₇₁BM) heterojunction OPV devices as a ternary additive. While the (CBzPho)₂-SiPc demonstrated strong EQE >30% contribution in the range of 650–730 nm, the overall performance was reduced because (CBzPho)₂-SiPc acted as a hole trap due to its high-lying HOMO energy level. This study demonstrates the importance of the solubility, miscibility, and energy level engineering of the ternary additive when designing organic photovoltaic devices.

Silicon phthalocyanines with carbazole axial functional groups were synthesized to improve the miscibility in PCDTBT and for use as ternary additives in organic photovoltaics.