Mechanical properties of poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend films, prepared under different processing conditions, were evaluated by nanoindentation. Photovoltaic devices fabricated using above active layers presented the highest power conversion efficiencies for blend films having lowest Young's modulus (20.73 GPa) and hardness (649 MPa), as measured by a nanoidentator under optimized conditions of blend proportion (1 :1), film drying rate (slow) and annealing temperature and time (110 degrees C and 10 min). It implies that the degree of nanoscale phase separation for the P3HT:PCBM blend is strongly correlated with the mechanical properties in the nanodimension. The nanoindentation is a method to estimate nano-scale mechanical properties of blend films and the performance of photovoltaic cells. (C) 2011 Elsevier B.V. All rights reserved.