Effects of Annealing and Residual Solvents on Amorphous P3HT and PBTTT Films

Abstract

Organic thin film transistors (OTFT) are metal-insulator-semiconductor field-effect transistors in which the semiconductor is a conjugated organic material. They are the subject of intense industrial research because their fabrication process is less expensive when compared with inorganic TFTs. Among the others, the organic material mostly employed in their construction consists of two semiconductor polymers, namely poly(3-hexylthiophene) (P3HT) and poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT). Despite the large amount of experimental efforts in the characterization of the electronic properties of these devices, several questions regarding their morphological arrangement in bulk and at interfaces remain wide open. Here, we report results obtained by classical molecular dynamics simulations of P3HT and PBTTT inspired by OTFT fabrication techniques. In particular, we investigate how the annealing fabrication process and the presence of residual solvent molecules left over after spin coating might modify the morphology and the dynamics of the amorphous phase of these two polymers. Simulations of both polymer deposits at 300 K after annealing show an increase in the number of interdigitation events between the alkyl chains of two polymeric macromolecules; moreover, we find that the increased stability of the pi-pi stacking is caused by an improved layering of the films, which may account for the better charge transport properties reported in experiments. Our results strongly suggest that thin semiconductor films are required to boost the performances of the devices and that a minimal presence of residual solvent does not alter dramatically the microscopic structure and stability of the polymeric films

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“…This probably explains why pBTTT has better performances [102,103]. According to the work of Alberga et al [53], spin-coated then annealed P3HT displays three different zones: an interfacial zone close to the substrate (thickness of 10 Å), an intermediate zone 30 Å thick atop and an OSC/electrolyte interfacial zone over this distance; the first layers display the highest degree of order [53,[87][88][89].…”

“…pBTTT has recently attracted attention as another promising solution-processed semiconducting polymer. pBTTT shows better performances than P3HT (hole mobility up to 1.0 cm 2 /Vs for pBTTT versus 0.1 cm 2 /Vs for P3HT [97,98]), less electrochemical doping and much better stability in aqueous environment [53,99].…”

“…During this operation, the solvent evaporates, leaving a thin film of semiconducting material. An additional annealing step is needed to crystalize the semiconductor molecules, which improve the electrical performances [53,54]. Very roughly, the film thickness depends on the concentration of the solution (the more concentrated, the thicker), the rotation speed (the faster, the thinner) and the evaporation rate of the solvent (the faster, the thicker).…”

“…It has been shown that this results in a lamellar structure formed by interchain stacking ( Figure 5). It has also been shown that the hole mobility varies by two orders of magnitude depending on orientation (parallel or normal to the substrate) of the lamellae [53,54,[81][82][83][84][85][86], the normal one giving the highest mobility. …”

“…It has been shown that this results in a lamellar structure formed by interchain stacking ( Figure 5). It has also been shown that the hole mobility varies by two orders of magnitude depending on orientation (parallel or normal to the substrate) of the lamellae [53,54,[81][82][83][84][85][86], the normal one giving the highest mobility. According to the work of Alberga et al [53], spin-coated then annealed P3HT displays three different zones: an interfacial zone close to the substrate (thickness of 10 Å), an intermediate zone 30 Å thick atop and an OSC/electrolyte interfacial zone over this distance; the first layers display the highest degree of order [53,[87][88][89].…”