## 1 Introduction  
Organic electronics offer numerous advantages over their inorganic counterparts, including a lower cost, greater flexibility, lighter weight, lower toxicity, and a greater functionality. Among organic electronic devices, organic solar cells (OSCs) are considered to be next-generation energy sources with the aforementioned advantages. Power conversion efficiencies (PCEs) greater than 16% have recently been reported as a result of the development of materials (e.g., fused-ring-based, small-molecule acceptors) and the optimization of device fabrication methods [1, 2, 3]. In 2020, more than 18% efficiency was reported by using a donor-acceptor (D-A) copolymer donor (D18) and a low bandgap acceptor (Y6) [4]. The active layers of OSCs have generally been composed of a polymer donor (D) and a small-molecule (or polymer) acceptor (A). To overcome the organic semiconductors' disadvantage of a limited light absorption range (they absorb only a part of the solar spectrum), researchers have widely used ternary or tandem devices to increase light absorption, thereby improving solar cell performance [5, 6, 7]. Ternary devices are more widely used than tandem cells because ternary devices are easier to manufacture. A photoactive organic semiconductor that exhibits complementary absorption is additionally introduced into a binary active layer, resulting in a ternary device (i.e., a D:D:A or D:A:A device). The important issues to be considered in the fabrication of ternary devices are (1) the selection of the third material to be introduced into a given binary D:A system and (2) systematical control of the blend ratios. The selection of a third material with complementary light absorption is important, and the optimum blend ratio to prevent additional phase separation needs to be determined [8; 9; 10; 11; 12]. Recently, different working mechanisms and models of ternary systems have been proposed to explain the improved device characteristics: the cascade energy-level structure and the alloy-like structure. The alloy-like donors or acceptors, which are formed when two components are well mixed, result in tuned open-circuit voltage (\(V_{\text{OC}}\)) values that vary according to the blend ratios [13; 14; 15; 16] and often offer a robust and stable morphology [17; 18]. By contrast, the cascade energy-level alignment can provide a cascade driving force for electron (or hole) transfer, which enhances charge transfer and the short-circuit current density (\(J_{\text{SC}}\)) [19; 20].  
In our previous work, we reported three diketopyrrolopyrrole (DPP)-based small-molecule isomers for use as acceptors for poly(3-hexylthiophene) (P3 HT)-based OSCs [21]. They were designed to have the same molecular backbone but differ in the position of the cyanide substituents (ortho, meta, or para), which greatly affects the molecules' packing behavior. In particular, the para-substituted molecule, \(p\)-DPP-PhCN, exhibited strong molecular aggregation, as confirmed by UV-vis absorption measurements, and we have reported how this different packing behavior affects the device performance of P3 HT-based binary solar cells. Herein, we applied the \(p\)- and \(o\)-DPP-PhCN isomers in a ternary system to determine whether the different packing behaviors of the third component affect the device performance and the working mechanism of the ternary cells.