Bis-indene fullerene (ICBA) is a highly symmetric molecule that tends to diffuse and aggregate into larger clusters, resulting in a huge horizontal phase separation when the master solvent vaporizes, in addition to a significant drop in power conversion efficiency (PCE). We found that Poly(3-hexylthiophene) (P31-IT): indene-C-60 bisadduct (ICBA) blended with a high-molecular-weight polyvinylcarbazole (PVK) film could effectively prevent the clustering of ICBA crystalline molecules, thereby improving the phase separation. Furthermore, PVK dispersed the ICBA:P3HT active layer in a more uniform manner, increasing open circuit voltage (V-oc) and reducing photocurrent loss. As a result, the PCE of the device can be improved from 2.2% to 4.5%. (C) 2013 Elsevier B.V. All rights reserved.