Characterization of Fluorene - Thiophene Copolymer Layers by the Surface Photovoltage Method Jirˇı´ Tousˇek, * 1 Petra Pavlacˇkova´, 2 Veˇra Cimrova´, 2 JanaTousˇkova´ 1 Summary: The surface photovoltage method was used for the study of thin polymer layers made of poly{(9,9-dihexylfluorene-2,7-diyl)[-3-(3-methylbutyl)thiophene-2,5- diyl]} prepared by spin-coating on ITO coated glass substrates. The surface photo- voltage (SPV) in a thin polymer layer is a result of dissociation of photogenerated free excitons in electric field of the space charge region (SCR) which forms spontaneously at the surface. The photovoltage signal was formed between a transparent electrode capacitively coupled to the surface of the sample and the back ITO electrode. Photovoltaic spectra were measured under illumination both the free surface (SPV fs ) and the substrate (SPV ITO ) side and for various layer thicknesses. The shape of the SPV ITO spectrum is very similar to that of the absorption spectrum, whereas the SPV fs spectrum maximum is shifted to longer wavelengths. Fitting theory to the exper- imental data, the thickness of the space charge region and the diffusion lengths were evaluated. The results are discussed also in relation to photoluminescence study of thin films. Keywords: conjugated polymers; diffusion length; fluorene-thiophene; surface photovoltage; thin films Introduction The use of semiconducting organic materi- als as an active layer in optoelectronic devices offers some advantages in proces- sing and new possibilities of device fabrica- tion. In molecular materials light absorp- tion generates excitons, which must be dissociated in electric fields and electrons and holes transported to the electrodes. At the surface of materials a space charge region with internal electric field often spontaneously forms. This is employed in the surface photovoltage method yielding parameters essentially influencing solar cell operation. In this paper, a technique previously developed for inorganic materi- als is applied to an organic semiconducting polymer. Preparation of the Fluorene-Thiophene (FIPT) Copolymer Layers The synthesis and characterization of poly{(9,9-dihexylfluorene-2,7-diyl)[-3-(3- methylbutyl)thiophene-2,5-diyl]}, the struc- ture of which is displayed in Scheme 1, are described elsewhere. [1] Thin polymer films were prepared by spin coating from tetra- hydrofuran solutions. All solutions were filtered with 0.45 mm Millex-FH13 Milli- pore syringe filters prior to spin-coating. Thin films for optical studies were spin- coated onto fused silica substrates and for PV study onto indium-tin oxide (ITO) substrate. All polymer films were dried in vacuum (10 3 Pa) at 353 K for 1 h. Layer thicknesses were measured using a KLA- Tencor P-10 profilometer. UV-vis spectra were measured on a Perkin-Elmer Lambda 35 spectrometer and PL spectra using a Perkin-Elmer LS55 spectrofluorometer. Macromol. Symp. 2008, 268, 96–99 DOI: 10.1002/masy.200850820 96 1 Department of Macromolecular Physics, Faculty of Mathematics and Physics, Charles University, V Holesˇovicˇka´ch 2, 180 00 Prague 8, Czech Republic E-mail: jiri.tousek@mff.cuni.cz 2 Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic. E-mail: cimrova@imc.cas.cz Copyright ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim