Regioregular poly(3-hexylthiophene), (rr-P3HT), is widely regarded as an archetypical semiconducting electron-donor polymer in bulk heterojunction solar cells, mainly due to its notable exciton and charge transport properties. Conversely, its use in photonic/lighting devices received little interest owing to its low emission quantum yield in the solid state, related to its large tendency to self-organize into lamellar domains with highly nonemissive character. We present experimental evidence pointing to a large miscibility of P3HT in poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), accompanied by mutual π–π interactions. Concomitant to this effect, the emission properties of P3HT:F8BT blends, which are attributed to heterogeneous interchain complexes, experience a significant improvement, with photoluminescence quantum efficiency (PLQE) values approaching 25% (almost a 7-fold efficiency enhancement with respect to neat/aggregated rr-P3HT). Our results open up new prospects for improved photonic properties through appropriate control of interchain interactions