Molecular architecturing of a small two dimensional A-D-A molecule for photovoltaic application
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Molecular architecturing of a small two dimensional A-D-A molecule for photovoltaic application Vinila Nellissery Viswanathan, Arul Varman Kesavan, Praveen C Ramamurthy Department of Materials Engineering, Indian Institute of Science, Bangalore, 560012. ABSTRACT A-D-A architectured quinoxaline and bezodithiophene based small molecule exhibiting low band gap was designed and synthesized for bulk heterojunction solar cell application. Two dimension conjugation to the molecule was imparted by using alkylated aromatic units. This effective extension of conjugation broadens the absorption spectra. Optical and electrochemical properties suggest that the synthesized molecule has a low band-gap and well matching frontier molecular orbital energy levels with that of PCBM. A photovoltaic device using this A-D-A molecule as the active layer was fabricated to evaluate the optoelectronics properties. INTRODUCTION Donor-Acceptor–Donor (D-A-D) and A-D-A architectured molecules have attracted the library of organic photovoltaic molecules, because of its versatility of synthesis and application. Solution processable small molecules are the emerging new type of materials as the active layer in bulk heterojunction (BHJ) solar cells 1. A variety of small molecular combinations such as D-A 2, A-D-A 3, D-A-D 2, star shaped molecules 4,5 have been evaluated in the past. Small molecules based BHJ solar cells have the benefits than that of polymer BHJs such as ease of purification, similar molecular weight from batch to batch synthesis, high hole mobility and high Voc6,1. Criteria of a small molecule to have application in the solar cell is same as that of polymer materials, such as optimum band gap, crystallinity, miscibility with acceptor moiety and solubility 7. Engineering of the material properties can be done by careful selection and combination of donor and acceptor moieties. Substitution with various functional groups can be used to alter the energy levels, band gap, morphology, solubility and packing of material in film.8 A novel A-D-A architectured molecule, based on quinoxaline acceptor and benzodithiophene donor has been designed and synthesized. Benzodithiophene is a strong donor which has a rigid and coplanar structure with active positions for functionalization. In addition to enhance the solubility, the conjugation of the molecule has is extended with the two dimensional substitution on the BDT moiety 9 . Two thiophene units and two alkylated thiophene groups has been incorporated to increase the conjugation as well as the solubility of acceptor moiety. This extended conjugation of acceptor moiety and donor moiety will broaden the absorption spectra. Also the structural modifications with aromatic rings will enhance the interchain overlapping through π-π stacking. Hence hole mobility will increase 6 . Optical electrochemical and photovoltaic properties of these molecules have been evaluated. EXPERIMENT 2-hexyl thiophene (1) To a solution of thiophene (1) (3g, 0.0357 mol) in 30 ml THF at -78°C, 25ml (0.0392 mol) n-BuLi (1.6M
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