Synthesis and Photovoltaic Properties of Benzotrithiophene-Based Polymer/Non-fullerene Solar Cells

We designed and synthesized two benzotrithiophene (BTT)-based copolymers, poly(2,5-bis(2-butyloctyl)-3-(5-(4,6-dioctylbenzo[1,2-b:6,5-b′:3,4-c″]trithiophen-2-yl)thiophen-2-yl)-6-(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione) (P1) and poly(4-(5-(4,6-bis(2-octyldodecyl)benzo[1,2-b:6,5-b’:3,4-c”]trithiophen-2-yl)thiophen-2-yl)-5,6-difluoro-7-(thiophen-2-yl)benzo[c][1,2,5]thiazole) (P2) by engineering the spectral range adopting diketopyrrolopyrrole (DPP) and difluorobenzothiadiazole (DFBT) as acceptors. The absorption spectra of P1 and P2 display an absorption maximum (λ_max) at 799 nm and at 610 nm as a film, respectively. The highest occupied molecular orbital (HOMO) levels of P1 and P2 were found to be -5.45 and -5.73, respectively, by optical and electrochemical measurements. Two inverted organic photovoltaic (OPV) devices of P1/ITIC-4F and P2/ITIC-4F were fabricated based on traditional non-fullerene architecture using 3,9-bis(1-oxo-2-methylene-3-(1,1-dicyanomethylene)-5,6-difluoro-indanone)-5,5,11,11-tetrakis(4-n-hexylphenyl)-dithieno[2,3d:2′,3′d′]-s-indaceno[1,2b:5,6b′]dithiophene (ITIC-4F) as an acceptor. After optimizing by polymer concentration, additive concentration, annealing, and film thickness, the best power conversion efficiencies (PCEs) of the devices fabricated from P1 and P2 were measured as 1.62% and 1.24%, respectively. Atomic force microscopy (AFM) study revealed that addition of 1,8-diiodooctane (DIO) to the blend film of P1 changed its morphology, from a big granular structure to a smooth and nano-scale phase separation, leading to great improvement of the PCE. However, DIO addition to P2 did not have a positive effect on the blend film morphologies, which became more rough and granular, leading to inferior PCEs. These AFM results proved the close correlations between the polymer blend morphologies and the corresponding device performances. Graphic abstract: [Figure not available: see fulltext.]