Effect of Molecular Weight on the Photovoltaic Performance of a Low band gap Copolymer blended with ICBA
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Effect of Molecular Weight on the Photovoltaic Performance of a Low band gap Copolymer blended with ICBA Olzhas Ibraikulov1, Rony Bechara2, Patrick Lévêque2, Nicolas Leclerc3, Galymzhan Koishiyev1 and Thomas Heiser2 1
Nazarbayev University Research and Innovation System, Department of Renewable Energy, 53 Kabanbay Batyr Ave., Block 9, Astana, 010000, Kazakhstan Laboratoire des Sciences de l'Ingénieur, de l'Informatique et de l'Imagerie (ICUBE), Département Electronique du Solide, Systèmes & Photonique, Université de Strasbourg, BP20, 67037 Strasbourg Cedex 2 2
Institut de Chimie et Procédés pour l'Energie, l'Environnement et la Santé, Universite de Strasbourg, Ecole Europeenne de Chimie, Polymeres et Materiaux, 25 rue Becquerel, 67087 Strasbourg, France 3
ABSTRACT An increase in molecular weight of the polymer generally impedes solubility in common solvents and may influence the polymer optoelectronic properties as well. Indeed, higher molecular weights are expected to increase charge carrier mobilities and therefore give rise to better photovoltaic performances of bulk heterojunction solar cells. In this work, we use copolymers based on 2,1,3-benzothiadiazole, thiophene and thieno[3,2-b]thiophene units of various fractions differing in molecular weights almost by a factor of 4 with a fullerene based acceptor material Indene-C60 Bisadduct (IC[60]BA) to elaborate bulk heterojunction solar cells. We investigate the influence of post-deposition annealing temperatures and polymer:fullerene ratios on the final cell performances. We use IC[60]BA as an acceptor to enhance the open circuit voltage due to its high lying LUMO level [1]. Additionally, charge carrier mobilities were probed using bottom contact organic field-effect transistors. As expected, higher molecular weights (as long as homogeneity was maintained) resulted in an increase of the hole field-effect mobility (up to 7x10-3 cm2V-1s-1). Consequently, the power conversion efficiencies of bulk heterojunction solar cells could be improved by increasing the copolymer molecular weight. A power conversion efficiency of 2.4% with an open circuit voltage of 0.82V was reached in a standard device configuration with aluminum as a cathode after post-deposition thermal annealing. INTRODUCTION Organic solar cells (OSCs) attract much attention in the field of renewable energies mainly due to their easiness of fabrication, device flexibility and low cost of production [2-4]. The photoactive layers of these bulk-heterojunction (BHJ) types of cells are usually blends of electron donor and electron acceptor materials. One of the standard devices for BHJ OSCs is based on poly-3-hexylthiophene (P3HT) as donor and the fullerene derivative, [6,6]-phenyl-C61butyric acid methyl ester (PCBM) as an acceptor [5-7]. Recently, the use of Indene-C60Bisadduct (ICBA) as acceptor material [8, 9], has allowed to increase the power conversion efficiency of P3HT based solar cells up to 6.5% after device optimization[11]. The higher lying LUMO level of ICBA (0.17eV higher than that of PCBM [1])
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