Bulk Heterojunction Solar Cells: Insight into Ternary Blends from a Characterization of the Intermolecular Packing and Electronic Properties in the Corresponding Binary Blends

While adding a third component to a binary blend in the active layer of an organic solar cell is a promising approach to improve device performance, the control of active-layer morphology also becomes more complex. Here, a combination of molecular dynamics simulations and long-range corrected density functional theory calculations is used to examine the intermolecular packing and electronic properties in two polymer donor/small-molecule acceptor binary blends, D/A1 (donor/acceptor1) and D/A2 (donor/acceptor2), in order to gain insight into the D/A1/A2 ternary blend. The focus is on the blends of the 4-(3-(2-decyltetradecyl)-5 '-(2,3-difluoro-4-(5-methylthiophen-2-yl)phenyl)-[2,2 '-bithiophen]-5-yl)-7-(4-(2-decyltetradecyl)-5-methylthiophen-2-yl)-5,6-difluoro-2-propyl-2H-benzo[d][1,2,3]triazole (PTFB-O) polymer donor with the 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(5-hexylthienyl)-dithieno[2,3-d:2 ',3 '-d ']-s-indaceno[1,2-b:5,6-b ']dithiophene (ITIC-Th) and 2,2 '-((2Z,2 ' Z)-(((4,4,9,9-tetrakis(5-hexylthiophen-2-yl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b ']dithiophene-2,7-diyl)bis(4-octylthiophene-5,2-diyl))-bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))-dimalononitrile (IEIC-Th) acceptors. The intermolecular packings and extent of mixing between the polymer donor and the acceptor in both binary blends are found to be similar, which is consistent with the well-mixed nature of the ITIC-Th:IEIC-Th phase and the quasi-linear evolution of open-circuit voltage as a function of ITIC-Th concentration in the PTFB-O:ITIC-Th:IEIC-Th ternary blend. The intermolecular packing patterns and electron-transfer rates among the acceptors are explored to rationalize the higher electron mobility found in the PTFB-O:ITIC-Th blend. The energetic distribution of the charge-transfer electronic states and non-radiative recombination rates are also evaluated to understand the difference in voltage losses between the binary blends.