Naphthobistriazole-based wide bandgap donor polymers for efficient non-fullerene organic solar cells: Significant fine-tuning absorption and energy level by backbone fluorination

Abstract

In this work, two wide bandgap polymers of PDTT-TZNT and PDTF-TZNT were developed by Stille-coupling of naphtho[1,2-c:5,6-c]bis(2-octyl-[1,2,3]triazole) (TZNT) acceptor unit with bithiophene (DTH) and fluorinated bithiophene (DTF), respectively. These polymers exhibited a wide bandgap over 1.84 eV. The fluorinated PDTF-TZNT had lower highest occupied molecular orbital HOMO level (- 5.24 eV), higher molar absorption coefficient (1.28 x 10(5) M-1 cm(-1)), and higher molecular packing order. Using a low bandgap 3,9-bis(2-methylene-(5 & 6-methyl-(3-(1,1-dicyanomethylene)-indanone)))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']s-indaceno[1,2-b:5,6-b']-dithiophene (IT-M) as the electron acceptor, the PDTF-TZNT: IT-M devices generated a higher power conversion efficiency (PCE) of 10.05%. To make up the weak absorption of above binary blend of PDTF-TZNT:IT-M in the short wavelength region and increase the device performance further, a large bandgap small molecular acceptor of 5,5,10,10,15,15-hexabutyl-2,7,12-tri(4-(3-ethylhexyl-4-oxothiazolidine-2-yl)dimalononitrile-benzothiadiazole)- truxene (meta-TrBRCN) was added as the second acceptor material to fabricate ternary blend PSCs. The meta-TrBRCN could not only expand the absorption range but also fine-tune the blend morphology by stepwise changing its content. When 0.2 of meta-TrBRCN was added, the PCE of PDTF-TZNT:ITM devices was improved to 11.48%.