The successful design and synthesis of two styryl-functionalized fullerene derivatives, [6,6]-phenyl-C-61 -butyric acid styryl dendron ester (PCBSD) and [6,6]-phenyl-C 61 -butyric acid styryl ester (PCBS) is presented. The polymerizable PCBS or PCBSD materials are incorporated into a poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) blend to form an active layer of ternary blend. The blending systems are fi rst thermally annealed at 110 C for 10 min to induce optimal morphology, followed by heating at 150 C for 10 min to trigger the in situ polymerization of styrene groups. Through chemical crosslinking of PCBSD, the initial morphology of the blend (P3HT:PCBM:PCBSD = 6:5:1 in weight) can be effectively fi xed and stably preserved. The device based on this blend shows extremely stable device characteristics, delivering an average power conversion effi ciency (PCE) of 3.7% during long-term thermal treatment. By molecular engineering to reduce the insulating portion, PCBS with higher C 60 content (71 wt%) possesses better electron-transport properties than PCBSD (58 wt%). Encouragingly, at a low doping concentration of PCBS in the blend (P3HT:PCBM:PCBS = 6:5:1 in weight), linear-polymerized PCBS can stabilize the morphology against thermal heating. This device exhibits more balanced charge mobility to achieve an average PCE of 3.8% over 25 h heating at 150 degrees C.