Constructing the nanointerpenetrating structure of PCDTBT:PC70BM bulk heterojunction solar cells induced by aggregation of PC70BM via mixed-solvent vapor annealing

Abstract

In this paper, mixed-solvent vapor annealing (M-SVA) was proposed to improve polymer crystallinity and inhibit fullerene intercalation into polymer side chains in a poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT):[6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₀BM) blend system. Experiments showed the optimal volume ratio of a mixed solvent, i.e., for tetrahydrofuran (THF) and carbon disulfide (CS₂) vapor it is 1 : 1, and the appropriate annealing time is 5 min. After the M-SVA process, out-of-plane grazing incidence X-ray diffraction (GIXD) and spectroscopic ellipsometry (SE) data show that PCDTBT crystals with larger size and higher crystallinity were formed due to enhanced π–π interaction of polymer chains; the aggregation of PC₇₀BM was promoted as well, which inhibited the intercalation behavior, resulting in a continuous electron path and improved phase purity. The morphology transition improved the carrier mobility, in particular electron mobility. Also, the recombination of a charge transfer (CT) state was reduced and the lifetime of mobile carriers was increased, which could be verified by transient-absorption (TA) spectra. As a result, the power conversion efficiency (PCE) of the BHJ reached 6.60%, an increase of more than 40% compared with the reference device (PCE = 4.54%), even though the thickness of the active layer is up to 220 nm and the PC₇₀BM content is as low as 71.4 wt% by weight.