Thermally stable amorphous polymeric semiconductors containing fluorene and thiophene for use in organic photovoltaic cells

We report amorphous polymeric semiconductors containing fluorene and thiophene, specifically poly(2-(5-(9,9-dibutyl-9 H-fluoren-2-yl)-3-hexylthiophen-2-yl)-5-(3-hexylthiophen- 2-yl) thieno[3,2-b] thiophene) (P1) and poly(2-(5-(9,9-dibutyl-9 H-fluoren-2-yl)-4-hexylthiophen- 2-yl)-5-(4-hexylthiophen-2-yl) thieno[3,2-b] thiophene) (P2), for use as electron donor materials in organic photovoltaic cells. The optical properties of the polymers blended with [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) were characterized by UV-Visible absorption and photoluminescence (PL) measurements. A maximum power conversion efficiency (PCE) of 1.8% and a high open circuit voltage (V-OC) of 0.97 eV were obtained from the P2: PCBM blend. Morphological and structural analysis of the polymer: PCBM blend films, by atomic force microscopy (AFM) and transmission electron microscopy (TEM), demonstrated that the range of possible side chain arrangements produced distinctly different nano-morphologies and, therefore, different thermal stabilities of the resulting devices. P2 was characterized by long distance between alkyl chains along the polymer backbone that yielded a uniformly amorphous morphology without PCBM clusters to hinder charge transport to electrode and steady photovoltaic performance exhibiting remarkable thermal stability, even at elevated temperatures. (C) 2010 Elsevier B.V. All rights reserved.