Performance, Morphology, and Charge Recombination Correlations in Ternary Squaraine Solar Cells

Ternary bulk-heterojunction organic solar cells (BHJ-OSCs) are demonstrated by combining two squaraine donors (USQ3OH and IDPSQ) having complementary optical absorption and PC71BM as the acceptor. While the corresponding binary cells exhibit maximum power conversion efficiencies (PCEs) of 4.65% (IDPSQ binary) and 6.85% (USQ3OH binary), the ternary cells of weight composition IDPSQ:USQ3OH:PC71BM = 0.15:1.0:3.0 (15%TB, TB = ternary blend) exhibit a PCE of 7.20%, which is the highest known value to date for a squaraine OSC. Single crystals of both squaraines and space-charge-limited current (SCLC) measurements explain the efficiency difference between the binary cells. SCLC measurements and transmission electron microscopy imaging of the ternary devices indicate that the charge mobility slightly increases and the BHJ domain size optimizes for the 15%TB device vs that based on the USQ3OH blend. Grazing incidence wide-angle X-ray scattering data reveal that enhanced π–π stacking and larger correlation lengths can be achieved after thermal annealing of the ternary blend film. Charge recombination measurements demonstrate that IDPSQ can be incorporated into the blend without increasing charge recombination. Finally, flexible OSCs on PET (polyethylene terephthalate) with a PCE of ∼4.5% were fabricated. This study demonstrates that readily accessible squaraine cores represent a viable choice for the design of new organic solar cell donor materials.