Difluoroterthiophene as promising block to build highly planar conjugated polymer for polymer photovoltaic cells. (April 2021)

• Record Type: Journal Article
• Authors: Jhang, Rong-Xian; Chen, Guan-Lin; Raja, Rathinam; Chen, Po-Tuan; Hayashi, Michitoshi; Rwei, Syang-Peng; Hsu, Shan-hui; Wang, Leeyih
• Is Part Of: Dyes and pigments. Volume 188(2021)
• Publication Date: 2021-04
• Subjects: Coplanarity -- Benzothiadiazole -- Difluorothiophene -- Polymer solar cells; Dyes and dyeing -- Periodicals; Pigments -- Periodicals; 667.2
• DOI: 10.1016/j.dyepig.2021.109206
• Languages: English
• ISSNs: 0143-7208

Abstract

Abstract: Developing highly planar semiconducting polymer is essential for achieving high mobility and improving the photovoltaic performance of bulk-heterojunction polymer solar cells. In this contribution, two novel low-bandgap donor-acceptor copolymers, P1-3T and P2-3T2F, consisting of electron-accepting benzothiadiazole acceptor segment and electron-donating terthiophene donor segment with and without fluorine atoms were designed and synthesized. The density functional theory calculations and ultraviolet–visible absorption studies demonstrate that the fluorinated P2-3T2F polymer has more planar backbone conformation, deeper HOMO level, broader absorption spectrum with a vibronic shoulder peak, and higher absorption coefficient compared with the non-fluorinated analogue polymer, P1-3T. Furthermore, P2-3T2F exhibits good crystallinity and high hole mobility, presumably due to the well-ordered lamellar packing and the π-π stacking interaction between the backbones of the conjugated polymers. The optimized polymer solar cell based on P2-3T2F:PC61 BM exhibits a short-circuit current density ( J sc ) of 12.77 mA cm −2, a fill factor (FF) of 68.99%, an open-circuit voltage (Voc ) of 0.80 V, and a maximum power conversion efficiency (PCE) of 7.14%, which is approximately 46% higher than that of the P1-3T:PC61 BM device. The enhanced PCE is primarily due to increased light-harvesting ability and interconnected morphology with finely dispersed polymer-rich and PC61 BM-rich domains, which improves the efficiency of exciton dissociation and rises the mobility of charge carriers, thus boosting the short-circuit current density and the FF value. Moreover, the relatively deep HOMO of P2-3T2F effectively increases the V oc . Graphical abstract: Image 1 Highlights: Highly planar conjugated polymer was demonstrated using difluoroterthiophene as building block. Incorporation of two fluorine atoms into thiophene unit down-shifts HOMO and enhances light-harvesting ability of polymer. The difluoroterthiophene improves the compatibility between polymer donor and PC61 BM, thus enhancing cell performance.