Double B←N bridged bipyridine-containing polymer acceptors with enhanced electron mobility for all-polymer solar cells

• Published in: Materials chemistry frontiers 2019, Vol.3 (1), p.70-77
• Main Authors: Long, Xiaojing, Yao, Jiuyong, Cheng, Fangfang, Dou, Chuandong, Xia, Yanzhi
• Format: Article
• Language: English
• Subjects: Chemical synthesis; Donors (electronic); Electron mobility; Electrons; Energy conversion efficiency; Energy levels; Molecular orbitals; Morphology; Photovoltaic cells; Polymers; Solar cells; Substitutes; Thermodynamic properties
• ISSN: 2052-1537; 2052-1537
• DOI: 10.1039/C8QM00492G

With the aim of developing polymer electron acceptors with high electron mobilities ( μ e ) for all-polymer solar cells (all-PSCs), we synthesized two novel polymer acceptors ( P-BNBP-2f and P-BNBP-4f ) containing alternate double B←N bridged bipyridine (BNBP) and 2,2′-(2,5-difluoro-1,4-phenylene)dithiophene/2,2′-(perfluoro-1,4-phenylene)dithiophene with extended conjugated structures. In comparison to P-BNBP-4f , P-BNBP-2f exhibits a small π–π stacking distance of 3.60 Å, leading to a sufficient electron mobility of 5.40 × 10 −4 cm 2 V −1 s −1 (measured by the space-charge-limited current method). This μ e value is among the highest values of the conventional polymer acceptors, and is close to the hole mobilities ( μ h ) of high-efficiency polymer donors. Moreover, P-BNBP-2f possesses high-lying LUMO/HOMO energy levels of −3.42/−5.81 eV, which match well with that of the reported excellent polymer donors of 2D-conjugated bithienyl-benzodithiophene- alt -fluorobenzotriazole (J61) and thienyl-substituted BDT with alkoxycarbonyl-substituted thieno[3,4- b ]thiophene (PBDTTT-E-T). In addition, the blend films based on P-BNBP-2f exhibit intermixed morphologies, which are beneficial for efficient excition dissociation. As a result, all-PSCs with P-BNBP-2f as an electron acceptor work very well and exhibit a power conversion efficiency of 5.46%. These results demonstrate that high electron mobility of a polymer electron acceptor is very important to produce efficient all-PSCs.