Plasticized conjugated polymers: a possible route to higher voltage solar cells
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Plasticized conjugated polymers: a possible route to higher voltage solar cells Paul Wentzel, Aurelien Du Pasquier Energy Storage research group Department of Ceramics and Materials Engineering Rutgers, The State University of New Jersey 10, Knightsbridge Road, Piscataway NJ 08854 ABSTRACT The introduction of propylene carbonate (PC) into the poly (3-alkyl)thiophenes (P3AT) poly(octyl)thiophene (P3OT) and poly(hexyl)thiophene) (P3HT) is studied. In both cases, we observe a diminution of their melting temperature, which demonstrates a plasticizing effect. We also observe an increase of photovoltage and photocurrent in sandwich devices M//P3AT+PC//ITO-PET where M=Al, Mg and ITO-PET is Indium thin oxide coated on polyester. We attribute this effect to the introduction of oxygen from the PC. When mixed with an electron acceptor PCBM, the photocurrents only slightly increase and the photovoltages are similar. In this case, the presence of oxygen is detrimental to the system. INTRODUCTION Blends of conjugated polymers and fullerene derivatives have been used successfully as photoactive layers to build organic solar cells. This is because they create efficient dissociation centers for excitons. This effect relies on the ultra fast electron transfer from conjugated polymers to fullerenes [1,2,3], with long-lived charge separation [4] and transport of the separated holes and electrons to metallic electrodes of different work functions. Such donor-acceptor bulk heterojunction results in charge separation quantum efficiency near unity. In order to improve the film formation and the homogeneity of the blends, the use of a plasticizer such as polystyrene or poly (vinyl) carbazole has been investigated. It was found that no decrease in performance was observed up 10 Wt.% added plasticizer [5]. Further, other conjugated plasticizer such as poly-3-butyl-co-3,4-dibutylthiophenes have been found to improve homogeneity and dispersion of the fullerene C60 in P3OT polymer, as evidenced by SEM [6]. There are other potential benefits to plasticizers, which include facilitating the fabrication of solar cells by lamination [7] and increasing the diffusion coefficient of small molecules in the blend, which can act as redox shuttles. In his context, we have investigated the addition of propylene carbonate on the thermal and photochemical properties of poly (alkyl) thiophenes blended or not with a fullerene derivative. EXPERIMENTAL DETAILS Regioregular poly (3-octylthiophene) was purchased from Aldrich. Regioregular poly (3-hexyl) thiophene was kindly provided by Plextronics. [6,6]- phenyl-C61-butyric acid methyl ester) (PCBM) was purchased from American Dye Source. The polymers and PCBM were dissolved at room temperature in chloroform to make 5 g/L solutions.
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Various amount of 5 g/L propylene carbonate (PC hereafter) solution in chloroform were added to their solution, resulting after casting and solvent evaporation in plasticized films with PC contents ranging from 0 to 100 Wt.%. Differential scanning calorimetry (DSC herea
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