Polymer films filled with organic molecules by CO2 laser evaporation in vacuum

A new p-type of material for organic field-effect transistors (OFETs) is 5,5′-di(9,9′-di-(butyl)-fluorene′)-2,2′-bithiophene (DBFBT). DBFBT films have been prepared by casting, by thermal evaporation in vacuum, and by means of CO2 laser evaporation. The photon energy from a CO2 laser is too small to break chemical bonds of the DBFBT molecule, but the heating rate is extremely high. Pressed pellets of powders composed of DBFBT and polychlorotrifluoroethylene (PCTFE) were used as targets for the fabrication of films. These targets were irradiated by a laser beam of 40 W cw-power and 0.5 cm diameter. The deposition rate was 30-50 nm/s. The films have been characterised by optical spectroscopy and by AFM. The structure of DBFBT films produced by casting from solution and thermal deposition is composed of elongated crystals, while the laser deposited layers show cluster structure. By FTIR the films show the absorption band of pure DBFBT, with intensity proportional to the DBFBT concentration in the PCTFE matrix. The absorption bands for a pure DBFBT film, for a film with 22 vol.% and with 34 vol.% of DBFBT in the PCTFE matrix are situated at 274, 298 and 402 nm, where the latter is the most intense one. No significant difference in the band positions of all samples has been found. We concluded that no interaction between chromophore and matrix occurs. The DBFBT molecules in the PCTFE matrix did not oxidize during storage under ambient condition over a period of a year.