Increasing photon harvest is an essential issue in improving the efficiency of organic photovoltaics. Here, we study ternary blend polymer solar cells that are composed of a thiazolothiazole-based crystalline semiconducting polymer and a fullerene derivative as the host binary system and a very small content of a series of narrow-band gap non-fullerene acceptors as the third-component sensitizer that is selectively located at the interface between the host donor and acceptor. Surprisingly, the cells give an external quantum efficiency in the sensitizer absorption range that is as high as that in the polymer absorption range despite the fact that the optimal sensitizer content is only 6 wt %, which is far smaller than the host polymer contents. This leads to significantly improved photocurrents and, in turn, high power conversion efficiencies relative to the binary blend cell. Such pronounced sensitization is found to originate in the markedly amplified sensitizer absorption owing to the optical interference effect of more than 300 nm-thick photoactive layers. In parallel, the ternary blend system realizes markedly reduced photon energy loss, which is also important for the power conversion efficiency improvement, and high thermal stability. With such excellent features, we believe that the sensitized ternary blend cells have exceptional possibilities and that exploring more well-matched material combinations would improve the performance further.