Ternary-blend active layers composed of one donor and two acceptors have been actively studied to enhance the power conversion efficiency (PCE) of organic solar cells (OSCs), and numerous advances in miscible ternary blends have been reported. In this study, we report an unusual behavior of immiscible ternary blends with respect to the enhancement of PCE and stability of OSC devices. The active layer consists of three relatively easily synthesizable materials: poly[(4,8-bis(5-(2-ethylhexyl)thiophen2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)-alt-((5-bromo-4-octylthiazol-2-yl)thiophene-2,5-diyl)] (POTz) as a donor, 2,2′-((2Z,2′Z)-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IDIC) as a non-fullerene acceptor, and [6,6]-phenyl-C₇₀-butyric acid methyl ester (PC₇₀BM) fullerene. The POTz:IDIC blends showed a typical bicontinuous and uniform nanomorphology; however, the addition of PC₇₀BM to the POTz:IDIC blends led to immiscible domains. When small amounts of PC₇₀BM (8.3–16.7 wt %) were incorporated into POTz:IDIC:PC₇₀BM blends, the POTz:IDIC formed a continuous phase, and small PC₇₀BM domains were uniformly dispersed into the continuous phase, which resembled raisin bread. The dispersed PC₇₀BM domains enhanced the electron mobility of the POTz:IDIC:PC₇₀BM film and promoted charge balance between holes and electrons, resulting in an enhanced short-circuit current density and fill factor. Notably, the ternary blends with PC₇₀BM exhibited the improved long-term stability in light-soaking tests by reducing the burn-in loss of the devices.