We have utilized room-temperature solvent vapor treatment followed by thermal annealing to process bulk heterojunction (BHJ) photovoltaic devices based on blends of poly (3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) of varied active layer thickness. The morphological and photovoltaic performance characteristics of the cells subject to these treatments were found to be dependent on active layer thickness. The devices were characterized using atomic force microscopy (AFM) and opto-electrical and external quantum efficiency measurements in order to analyze the mechanism underlying the observed trend. Performance indicators including fill factor, short-circuit current and power conversion efficiency were correlated to the ordering of device active layers and morphology. The maximum power conversion efficiency achieved was 4.1 %.