Polymorphisms and morphological studies of a difluorobenzothiadiazole conjugated copolymer with 7.8% polymer solar cell efficiency

Abstract

This study presents the structural characterization of a 5,6-difluorobenzo-2,1,3-thiadiazole (FBT)–quaterthiophene (Th₄) alternating copolymer (PTh₄FBT) and how the crystalline nature of PTh₄FBT affects the PTh₄FBT–PC₇₁BM morphology and the device performances. The single crystal structure of 5,6-difluoro-4,7-di(thiophen-2-yl)benzothiadiazole (Th₂FBT) first confirms the low conformational preference of the FBT containing molecule. Since the crystallization process does not assist in unifying the conformation of Th₂FBT, both intrachain conformation and interchain c-shifts in the crystalline state of PTh₄FBT have to be scrutinized. By comparing the 2D WAXS pattern of PTh₄FBT and the simulated patterns generated from Cerius² molecular modeling, it was found that the diffraction pattern generated from the lattice containing PTh₄FBT with anti-conformation and limited interchain c-shift matches best with the experimental one. The face-to-face interchain stacking of PTh₄FBT rendered strong crystalline nature to the polymer and caused large segregation in the PTh₄FBT–PC₇₁BM blend system, which was indicated by the TEM and GI WAXS morphological studies. The aggregation size was reduced via the use of the 1-chloronaphthalene additive. The optimized morphology led to improved J_sc and FF. The single-junction PTh₄FBT–PC₇₁BM based polymer solar cells deliver a high PCE of 7.75% with a V_oc of 0.76 V, a J_sc of 14.36 mA cm⁻², and a FF of 71.0%.