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-C₆₁-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.