ORIGINAL PAPER Synthesis and photovoltaic performance of a fluorene-bithiophene copolymer Isabel R. Grova & Douglas J. Coutinho Roberto M. Faria & Leni Akcelrud Received: 16 October 2012 / Accepted: 4 March 2013 # Springer Science+Business Media Dordrecht 2013 Abstract We present the synthesis of a copolymer structure, poly(9,9 ′ - n -di-hexyl-2,7-fluorene -alt -2,5- bithiophene), referred to herein as LaPPS43, and its physico-chemical characterization. Thin films of this polymer mixed with phenyl-C61-butyric acid methyl ester (PCBM) were used as the active layer in photo- voltaic devices using the ITO/PEDOT:PSS/LaPPS43: PCBM/Ca/Al bulk heterojunction structure. The devices of different active layer thicknesses were electrically studied using J-V curves and the Photo-Celiv technique. The obtained results show that LaPPS43 combined with PCBM is a promising system for photovoltaic devices. Device performance is discussed in terms of the mean drift distance x for charge carriers. Photophysical data showed that the excitonic species are all localized in the aggregated forms. The mechanism of exciton formation and dissociation is also discussed. Keywords Synthesis . Polythiophene derivatives . Photovoltaic properties . Solar cells Introduction Conjugated polymers are now definitively considered active materials for electronic and optoelectronic devices, such as electroluminescent devices, transistors and photovoltaics [1–3]. They have a combination of good semiconducting properties with good thermal and mechanical properties that allow for the production of efficient devices at low cost. Among these devices, organic photovoltaics are one of the most promising; several new polymeric structures have been specially synthesized for this application. Recently, the com- bination of fluorene and thiophene groups to form a single repeating unity has produced donor-copolymers, which when combined with acceptor species, yield high performing solar cells. The incorporation of the fluorine unit has been widely used because it has good open circuit voltage (V oc ) values due to its relatively low HOMO (Highest Occupied Molecular Orbital) level. For charge generation, the insertion of an acceptor species is needed, such as phenyl-C61-butyric acid methyl ester (PCBM) that has been employed in the majority of the published data [4]. Copolymerization, on the other hand, is a classical polymer science tool for incorporating the properties of parent homopolymers into a single material, and in the present application, for inserting electron withdraw- ing units into a fluorene polymer backbone to produce low gap materials [5]. The overall efficiency of organic solar cells is dictated by four main processes: absorption (the creation of bound electron–hole pairs known as excitons), charge gener- ation (the dissociation of excitons into free carriers), recombi- nation and/or collection as carriers to their respective electrodes. The close proximity of the donor and acceptor components is a major advantage of bulk heterojunction de- vices (BHJ). Compared to the bilayer configuration, higher overall results were obtained with the BHJ architecture [6]. Polythiophene and its derivatives have been shown to possess better charge carrier mobilities as hole transporters than other conjugated materials. The combination of I. R. Grova : L. Akcelrud (*) Paulo Scarpa Polymer Laboratory (LaPPS), Federal University of Parana, CP 19081, CEP 81531-990, Curitiba, Paraná, Brazil e-mail: leni@leniak.net I. R. Grova e-mail: isabelgrova@yahoo.com.br I. R. Grova : L. Akcelrud Integrated Graduate Program in Engineering (PIPE), Federal University of Parana, CP 19011, CEP 81531-990, Curitiba, Paraná, Brazil D. J. Coutinho : R. M. Faria Group of Polymers “Bernhard Gross”– Physics Institute of São Carlos, Institute of Physics, São Paulo University (USP), P.O. Box 369, CEP 13084-971, São Carlos, São Paulo, Brazil J Polym Res (2013) 20:119 DOI 10.1007/s10965-013-0119-3