How the interaction between a third component and a donor/acceptor
affects the crystallization and phase separation of the active layer
remains to be explored from the perspective of miscibility. To demonstrate
the miscibility matching in nonfullerene-based ternary organic solar
cells, the poly[(4,4′-bis(2-butyloctoxycarbonyl-[2,2′-bithiophene]-5,5-diyl)-alt-(2,2′-bithiophene-5,5′-diyl)] (PDCBT)
and poly(3-hexylthiophene) (P3HT), showing similar chemical structures
despite their dissimilar side chains, were selected as third components
to incorporate into the poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione))]:3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (PBDB-T:ITIC) system. On the basis of the melting
point depression method, the Flory–Huggins interaction parameters
for PDCBT:PBDB-T(1:9) and P3HT:PBDB-T(1:9) blends were calculated
to be −0.08 and −0.07, respectively, whereas the values
are 0.99, 0.66, and 0.41 for PDCBT:ITIC, PBDB-T:ITIC, and P3HT:ITIC blends, respectively. PDCBT and
P3HT tend to form bimolecular crystals with PBDB-T, as revealed by
grazing incidence wide-angle X-ray scattering (GIWAXS). The more evident
phase separation induced by poor miscibility between PDCBT and ITIC
in addition to bimolecular crystallization between PDCBT and PBDB-T
contributes to the obvious enhancement in power conversion efficiency
(PCE) from 9.40% to 10.97%. Unfortunately, the miscibility mismatch
and the change in charge transport upon the incorporation of P3HT
into PBDB-T:ITIC results in the nonuniform P3HT distribution and energy
level alignment, leading to a serious reduction in the PCE value.
Miscibility matching in the selection of an appropriate third component
should be a reasonable way to guide the fabrication of high-performance
ternary organic solar cells.