Semiconducting Polymer Spherulites—From Fundamentals to Polymer Electronics

Abstract

structures have to be highly regularexamples are given in Figure 1. This includes regular sequences of repeat units, high regioregularity, and defined tacticity (isotactic or syndiotactic). In addition, strong intermolecular interactions such as hydrogen bonds (e.g., in polyamides) or π-π stacking (e.g., in conjugated polymers) can strongly favor crystallization. To get control over the crystallinity of semicrystalline polymers, numerous efforts have been made. For crystallization in general, thermodynamic as well as kinetic aspects must be taken into account. The structure formation of semicrystalline polymers, however, is determined by kinetics rather than equilibrium thermodynamics. This is particularly prominent for the processing from solution, where the rapid solvent removal determines the fast crystallization kinetics. This implies that the time given for crystallization is crucial and that the resulting morphology has a strong dependence on the history imposed by processing. Thus a precise control of crystallization conditions such as temperature and time is required to obtain defined morphologies.This feature article first gives a very short overview of classical crystallization theory including nucleation and growth, where the focus should be on spherulite growth. In the second part, this article will extend toward semicrystalline polymers with additional semiconducting properties that were crystallized in spherulitic morphologies. Spherulites are the most common superstructure in semicrystalline polymers that are also formed during classical processing like injection molding. As the spherulite size affects mechanical properties of the polymeric material such as tensile strength and strain at failure, it is important to be able to control spherulite growth. The growth of spherulites of classical semicrystalline polymers such as PP during processing can, for instance, be controlled with nucleating agents. While spherulites are aimed to be very small in this case, for semiconducting semicrystalline polymers the growth of large spherulites is advantageous. The known radial growth and the defined orientation of lamellae and crystal axes allows for direction dependent measurements of charge carrier mobility or conductivity. Therefore charge transport anisotropy can be studied and structureproperty relations established. Solvent vapor annealing as the most suitable technique to induce the desired crystallization into spherulites is introduced in this article, and the resulting anisotropic optical and electronic properties are presented. The high degree of orientation and improved crystallinity in spherulites also improves the overall charge transport properties. To do so, Conducting PolymersThe control of the morphology of semiconducting semicrystalline polymers is crucial to the performance of various electronic devices. Among other superstructures in semicrystalline polymers, spherulites stand out for various reasons. They are highly ordered, relatively easy to grow, and their underlying molecular structure...

Role of Postdeposition Thermal Annealing on Intracrystallite and Intercrystallite Structuring and Charge Transport in Poly(3-hexylthiophene)

“…Conjugated polymers have demonstrated great potential as active layers in next-generation electronic devices that are low-cost, lightweight, and flexible. , Through iterations and refinement on the design of new materials and optimization of processing conditions, the research community has drastically improved the performance of conjugated-polymer-based devices, with reported mobilities of field-effect transistors comprising them routinely above 1 cm 2 V –1 s –1 , and power-conversion efficiencies of solar cells above 15%. , The performance of electronic devices comprising conjugated polymers depends strongly on their intrinsic materials characteristics, including molecular weight and molecular-weight distribution, regioregularity, and chain rigidity, and also on the details of extrinsic processing conditions. − Owing to the conformational freedom of polymer chains and the weak intermolecular forces that govern their structuring, the solid-state morphology of solution-processed polymers is directly impacted by the specifics of processing conditions, including the choice of solvent from which the polymer is processed, , solution engineering by sonication and aging, − casting techniques and details, , and postdeposition thermal − and solvent-vapor annealing treatments. , The microstructural differences in turn impact charge transport. It has been demonstrated in many conjugated polymers, including poly­(3-hexylthiophene) (P3HT), poly­[4-(4,4-dihexadecyl-4 H -cyclopenta­[1,2- b :5,4- b ′]­dithiophen-2-yl)- alt -[1,2,5]­thiadiazolo-[3,4- c ]­pyridine] (PCDTPT), , and poly­[2,6-(4,4-bis-alkyl-4 H -cyclo-penta-[2,1- b ;3,4- b 0]-dithiophene)- alt -4,7-(2,1,3-benzothiadiazole)] (CDTBTZ), , that the mobility of transistors comprising the same polymer can vary by more than an order of magnitude depending on how the active layer is processed.…”

“…7−10 Owing to the conformational freedom of polymer chains and the weak intermolecular forces that govern their structuring, the solidstate morphology of solution-processed polymers is directly impacted by the specifics of processing conditions, including the choice of solvent from which the polymer is processed, 11,12 solution engineering by sonication and aging, 13−15 casting techniques and details, 16,17 and postdeposition thermal 18−20 and solvent-vapor annealing treatments. 21,22 The microstructural differences in turn impact charge transport. It has been demonstrated in many conjugated polymers, including poly(3-hexylthiophene) (P3HT), 23 4-c]pyridine] (PCDTPT), 24,25 and poly[2,6-(4,4-bis-alkyl-4H-cyclo-penta-[2,1-b;3,4-b0]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (CDTBTZ), 25,26 that the mobility of transistors comprising the same polymer can vary by more than an order of magnitude depending on how the active layer is processed.…”

Gaining control over conjugated polymer morphology to improve the performance of organic electronics

“…70 Electrical properties of CPs can be fine-tuned not only through judicious synthesis planning, as in the case of regioregularity control 66,67 and strategic defect incorporation. 70 Also, the choice of processing conditions, such as solvent engineering, 71 casting techniques, 72 postdeposition thermal, and solvent-vapor annealing, 73 affects the microstructure. All of these methods are capable of changing polymer microstructure and, consequently, charge mobility.…”

Formation of Needle-like Poly(3-hexylthiophene) Crystals from Metastable Solutions

“…The most widely employed strategy to grow P3HT crystals started from an initially homogeneous solution of well-dissolved polymer chains. − Nanowhiskers and nanofibers have been grown by slowly cooling a solution having a P3HT concentration close to its solubility limit. ,,, Besides crystallization induced via a change in temperature, extensive efforts were carried out to form nanofibers and nanowires of P3HT using mixed solvents. − Due to the presence of a large number of defects in the ordered regions surrounded by many dissolved polymers, the morphology of crystals resulting from such an approach showed rarely faceted shapes. The formation of high-quality large single crystals of P3HT has been a major challenge .…”