Optically anisotropic and photoconducting Langmuir–Blodgett films of neat poly(3-hexylthiophene)
Introduction
The Langmuir–Blodgett (LB) technique has been attracted attention as a possible method to prepare ordered ultra thin films with well-defined architecture and also as a way for building molecular devices [[1], [2], [3]]. Conjugated polymers are materials with unique optical and electrical properties extensively investigated for the fabrication of such devices [4]. Organic devices produced from LB technique have superior performance due to the anisotropy of the films [5]. Among of organic materials, the polythiophenes are well-known polymers that exhibit good thermal and environmental stability [6], [7]. Devices made from polythiophenes have been used as light emitting diodes, solar cells, sensors, photoelectric and electrochromic devices, optical switches, etc. [8]. The introduction of alkyl side chains at the 3-position of the thiophene rings generated the most studied polythiophene derivatives namely poly(3-alkylthiophenes) (P3ATs). Attempts to produce neat P3ATs LB films generally failed because the polymers do not form stable Langmuir films at the air-water that could be transferred to solid substrates with a suitable quality [[9], [10], [11]]. Some authors show that the transferability of the P3ATs LB films could be improved by using film-forming aiding materials such as long chain fatty acids. Other authors used liquid crystals to improve the film formation and transferability [[9], [12], [13], [14]]. However, these methods could result in deterioration of electrical properties of these mixed films, making not viable the use of these films in organic devices [15]. In this work, we report on the fabrication and characterization of Langmuir and Langmuir–Blodgett films of a neat P3AT, the poly(3-hexylthiophene) (P3HT). Although the LB technique is not suitable to obtain LB films with a great number of layers, good quality films, with thicknesses suitable for fabrication of opto-electronic devices could be easily built by using the proper deposition conditions.
Section snippets
Experimental details
The P3HT was synthesized via oxidative polymerization using ferric trichloride under nitrogen atmosphere. Solid FeCl3 (12 mmol) was added by a powder addition funnel to the 3-hexylthiophene (3 mmol) dissolved in 4.6 mL of CH3NO2, with the mixture being kept under stirring. Chloroform (30 mL) was added and the resulting mixture was stirred for 4 h and then the reaction was quenched by pouring into 1 L of methanol. The solid polymer was dissolved in chloroform and insoluble products were removed by
Results
Fig. 1 shows the π–A isotherm of pure P3HT film. At a surface pressure of ca. 60 mN m− 1, the film “collapsed”, being visually cracked. The extrapolated area was approximately 2 Å2 per repeating unit for the condensed phase in the π–A isotherm, calculated using the molecular weight of the polymer-repeating unit. This calculated area indicates that P3HT did not form a true monolayer, but probably a multilayer. The polymer molecular arrangements in the air–water interface can change upon compression
Conclusion
The fabrication and characterization of neat P3HT LB films were described and these films could be use in organic electronic devices. These films present a degree of optical anisotropy since the chains appear to be aligning in the dipping direction. The electrical measurements demonstrated the semiconductor characteristic of P3HT film, and in the sandwich configuration it was estimated the hole barrier height by the Fowler–Nordheim theory. This value seems to be lower than those reported by
Acknowledgments
The authors are grateful for the financial support of CNPq, FAPESP, INEO/CNPq and LNLS (Brazil) for providing interdigitated electrodes (project LMF 11325).
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