Polymer solar cells have been fabricated with buffer layers to enhance the charge extraction toward the electrodes. We investigated the active area and the light intensity dependences in poly (3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction solar cells with different hole buffer layers, molybdenum oxide (MoO3) and poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS). The short-circuit current (JSC) for the MoO3-based device is nearly independent of the active area and the light intensity whereas the JSC and light intensity dependence for PEDOT: PSS-based device are strongly affected by the active areas below 20 mm2. These dependences on the active area and the light intensity can be explained by the lateral conductivity characteristics of the hole buffer layers, and compared to PEDOT: PSS, MoO3 can provide efficient and stable hole extraction regardless of active area and light intensity.