Synthesis and Characterization of Benzotriazole-Based Polymer Donors with Good Planarity for Organic Photovoltaics
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Article www.springer.com/13233 pISSN 1598-5032 eISSN 2092-7673
Synthesis and Characterization of Benzotriazole-Based Polymer Donors with Good Planarity for Organic Photovoltaics Suha Lee Jong-Woon Ha Hea Jung Park Do-Hoon Hwang*
Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 46241, Korea Received October 31, 2019 / Revised May 13, 2020 / Accepted May 16, 2020
Abstract: Two benzotriazole-based polymer donors, poly[4-(5-(2,5-bis((2-hexyldecyl)oxy)-4-(thiophene-2-yl)-phenyl)thiophene-2-yl)-5,6-difluoro-2-octyl-2H-benzo [d][1,2,3]triazole] (PPBTA-2HD) and poly[4-(5-(2,5-bis((2-hexyltridecyl)oxy)-4(thiophene-2-yl)-phenyl)thiophene-2-yl)-5,6-difluoro-2-octyl-2H-benzo[d][1,2,3] triazole] (PPBTA-5HD), were synthesized, and their physical, optical, and electrochemical properties were characterized. The backbones of two polymers contain fluorine in the benzotriazole and alkoxy substituents in the phenylene units, resulting in molecular planarity by exploiting S···O and S···F noncovalent interactions. PPBTA2HD and PPBTA-5HD were used as electron donors and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an electron acceptor in the active layer of the bulk-heterojunction solar cells. PPBTA-5HD exhibited better device performance than PPBTA-2HD, with a power conversion efficiency of 2.82% and a short-circuit current of 6.52 mA/cm2. Analysis of the microstructural film morphologies of two polymers by 2D-grazing incident X-ray diffraction revealed that PPBTA-5HD had higher crystallinity than PPBTA-2HD, leading to better device performance. Keywords: D-A copolymer, benzotriazole, linear alkyl spacer, organic photovoltaics.
1. Introduction The generation of electric power using organic photovoltaic solar cells (OPVs)has drawn attention as a way to replace the use of fossil fuels and prevent environmental pollution. OPVs have several advantages such as flexibility, lightweight, low-cost production, and a tunable light absorption range.1-8 However, OPVs have relatively low efficiencies and stabilities compared to those of inorganic photovoltaic solar cells (IPVs) such as Si-based solar cells. Currently, research has focused on developing donor and acceptor materials to improve OPV device performance by controlling the range of light absorption wavelengths, film crystallinity, morphology, and the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels.9-13 OPV materials are usually designed with an alternating donoracceptor (D-A) copolymer structure, where the push-pull driving forces between the donor and acceptor units facilitate electron delocalization and increase hole and electron mobility.14 Research into D-A type copolymers has revealed that the benzotriazole (BTA) unit, contains three nitrogen atoms in a heterocyclic system, is a useful acceptor moiety because of its high electron transporting ability induced by the electron withdrawing imine (-C=N-) on the backbone.15-18 An alkyl side chain can be introduced to a
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