Effect of the Temperature of Annealing on the Performance of Fluorene and Bithiophene Copolymer in Bilayer Solar Cells
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Effect of the Temperature of Annealing on the Performance of Fluorene and Bithiophene Copolymer in Bilayer Solar Cells Natasha A. D. Yamamoto1, Daniel C. Silva1, Isabel L. Grova2, Andreia G. Macedo1, Cleber F. N. Marchiori1, Marlus Koehler1, Leni Akcelrud2 and Lucimara S. Roman 1 1 Laboratório de Dispositivos Nanoestruturados, Departamento de Física, Universidade Federal do Paraná, Curitiba, Paraná, Brazil 2 Laboratório de Polímeros Paulo Scarpa, Departamento de Química, Universidade Federal do Paraná, Curitiba, Paraná, Brazil Corresponding author: Natasha A. D. Yamamoto [email protected] ABSTRACT We have investigated the effect of temperature annealing on bilayer heterojunction solar cells based on poly[9,9’-hexyl-fluorene-alt-bithiophene] as active layer. Film morphology for different temperature annealing was probed by atomic force microscopy (AFM) and the values of roughness range from 0.59 up to 2.15 nm. The best photovoltaic performance was found for devices with active layer annealed at 200°C with power conversion efficiency (η) of 2.8 % while devices without annealing presented only 0.4%. This performance enhancement is attributed to the reduction of traps and increased hole mobility after the thermal annealing. INTRODUCTION Fluorene-thiophene copolymers have been widely studied due their good optical, electrical and photovoltaic properties [1-6]. Bulk heterojunction (BHJ) solar cell prepared with poly(9,9´-dihexylfluorene-alt-thiophene) (PFT1) and PCBM displayed open circuit voltage (Voc) of 0.77 V and η of 0.62 % [1]. Moreover, devices prepared with fluorene and two thiophene units have resulted in higher Voc and efficiencies. For instance, BHJ prepared with poly(9,9´dihexylfluorene-alt-bithiophene) (F6T2) or poly[9,9´-dioctyl-fluorene-co-bithiophene] (F8T2) displayed Voc around 1 V,η of 2.7 % [2] or 2.13 % [3,4], respectively. These results were both attributed to the high hole mobility and absorption coefficient in these polymers that produces an efficient photocurrent generation. A bilayer solar cell prepared with F8T2/C70 displayed the highest η of 3.4 %, after thermal annealing at 200 ºC [5]. The increase in η in this device was attributed to a better chain organization and higher roughness of the film, achieving a larger donor-acceptor heterojunction area for exciton dissociation. Recently, we reported a bilayer PV cell prepared with poly[9,9’-hexyl-fluorene-alt-bithiophene] (LaPPS43) and fullerene (C60) displaying an increase of η from 0.4 to 2.8 % after annealing at 200 ºC. Comparing these results and the behavior of the current versus voltage characteristics we concluded that the π-π stacking in solid state is enhanced after thermal annealing with a reduction of traps and thus reflecting in higher hole mobility in this polymer after annealing [6]. Another aspect related to these copolymers is that the spin coated films present nanophase segregation. The F6T2:PCBM films displayed domains with size around 5 nm [2], while for F8T2:PCBM a continuous network of channels with dimensions of 100-150 n
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