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.