Organic photovoltaics (OPVs) have experienced continued interest over the last 25 years as a viable technology for the generation of power. Phthalocyanines are among the oldest commercial dyes and have been utilized in some of the earliest examples of OPVs. In recent years, the use of boron subphthalocyanines (BsubPcs) and silicon phthalocyanines (SiPcs) has attracted a flurry of interest with some examples of fullerene‐free devices reaching power conversion efficiencies >8 %. Unlike other more common divalent phthalocyanines such as copper or zinc, BsubPcs and SiPcs contain additional axial groups that can easily be functionalized without significantly affecting the optoelectronic properties of the macrocycle. This handle facilitates our ability to tune the solid‐state arrangement and other physical characteristics such as solubility ultimately giving us the ability to improve the thin film processing and final device performance. This review covers recent studies on the development of BsubPcs and SiPcs for use as active materials in organic photovoltaics.
Additive manufacturing is the process of creating an object from a digital three-dimensional (3D) design using hardware that deposits and adds materials successively until the desired object is formed. Vat polymerization is a type of additive manufacturing technique that offers an attractive strategy for fabricating 3D objects due to its high resolution, versatility of feedstock materials, and high dimensional accuracy. However, the ability to spatially control the placement of multiple materials during vat polymerization is limited by mechanical designs that can restrict printing to a single photocurable resin, making it difficult to create complex geometries with embedded functionality. In this review, we highlight work-arounds that have been employed, such as manually switching resins, as well as two key areas of innovation: mechanical hardware design and the chemistry of materials. These methods include systems which automatically switch resin vats, volumetric additive manufacturing which overprints a secondary 3D structure on top of an existing object, and specially formulated, unique material chemistries, such as those which enable dual wavelength light to initiate orthogonal chemistry mechanisms, control of the degree of cross-linking with variation in light parameters, and phase separation of functional materials. Perspectives on the interplay of printer type and material chemistry, the constraints they impose on one another, and the future of multimaterial vat polymerization are described. In order for vat polymerization to emerge as a viable technology for multimaterial additive manufacturing, both mechanical hardware design and chemistry of materials must be codeveloped.
Abstract3D printing has enabled materials, geometries and functional properties to be combined in unique ways otherwise unattainable via traditional manufacturing techniques, yet its adoption as a mainstream manufacturing platform for functional objects is hindered by the physical challenges in printing multiple materials. Vat polymerization offers a polymer chemistry-based approach to generating smart objects, in which phase separation is used to control the spatial positioning of materials and thus at once, achieve desirable morphological and functional properties of final 3D printed objects. This study demonstrates how the spatial distribution of different material phases can be modulated by controlling the kinetics of gelation, cross-linking density and material diffusivity through the judicious selection of photoresin components. A continuum of morphologies, ranging from functional coatings, gradients and composites are generated, enabling the fabrication of 3D piezoresistive sensors, 5G antennas and antimicrobial objects and thus illustrating a promising way forward in the integration of dissimilar materials in 3D printing of smart or functional parts.
An extensive study of the electrochemical and spectroelectrochemical properties of 14 boron subphthalocyanine (BsubPc) derivatives with various axial and peripheral substituents was performed in 1,2-dichloromethane (CHCl) containing 0.1 M tetra- n-butyl-ammonium perchlorate (TBAP) as the supporting electrolyte. From the cyclic voltammetry results, all compounds exhibit one oxidation and at least two reduction processes within the solvent potential window of +1.6 to -1.8 V vs SCE. It was found that the reversibility of the redox reactions depends on the axial and peripheral substituents and the dipole moment of the boron-to-axial substituent. In general, UV-vis absorption spectra of the singly reduced BsubPc derivatives exhibit three equal intensity peaks in the 450 to 650 nm region that are derived from the maximum BsubPc absorbance peak upon reduction. Axial substituents affect the intensity of the three peaks upon reduction, while peripheral substituents shift the position of the peaks to higher wavelengths. Upon oxidation, the UV-vis absorption profile flattens considerably with only a single broad (∼300 nm) band apparent. Understanding the effect of substituents on the stability of the redox processes of BsubPcs will aid in further development of these materials for applications in organic electronic devices.
Six different boron subphthalocyanines with fluorophenoxy axial substituents were synthesized to explore and fine-tune the HOMO and LUMO energy levels based on past observations of the unique levels of pentafluorophenoxy-boron subphthalocyanine (F5BsubPc). Electrochemistry reduction potentials, ionization energies from ultraviolet photoelectron spectroscopy (UPS) measurements, and energy levels calculated using semiempirical methods reveal finely variable values between phenoxy-BsubPc (PhO-BsubPc), without any fluorine atoms, and F5BsubPc, with all five fluorines. There is no trend between the number of fluorines on the phenoxy group and the electronic properties, but there is an influence of meta fluorine(s) altering the redox potentials and ionization energy, leading us to categorize the fluorophenoxy-BsubPcs into two “buckets”: with and without meta fluorines. We then applied the fluorophenoxy-BsubPcs as electron acceptors in planar heterojunction organic photovoltaic devices. The categorization of these “buckets” was confirmed by the fine change in open-circuit voltage between 1.15 and 1.21 V, which is an exceptionally high value. This level of fine-tuning of properties and device metrics is a unique handle enabled by the phenoxy axial substituent of BsubPcs.
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