Miscibility Matching and Bimolecular Crystallization Affording High-Performance Ternary Nonfullerene Solar Cells

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.