Bulk heterojunction polymer solar cells based on the blend of MEH-PPV (poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene]) and PCBM (1-(3-mehyloxycarbonyl)propyl-phenyl[6,6]C61) were fabricated. The thickness of the active layer was controlled by changing the concentration of MEH-PPV : PCBM (1 : 4 in weight ratio) solution and spin speeds. Investigation of the effects of active layer thicknesses on the performance of the photovoltaic devices indicates that, when the spin-coated speeds are lower than 4,000 r x min(-1) (round per minute), the open-circuit voltage (V(oc)) remains almost unchanged at approximately 0.8 V, whereas the short-circuit density (J(sc)) monotonically increases and the fill factor (FF) decreases slightly. The spin speeds that are higher than 5,000 r x min(-1) rpm result in the V(oc) and J(sc) both reduced. The V(oc) decreases from 0. 78 V at the spin-speed of 5,000 r x min(-1) to 0.67 V at 8,000 r x min(-1), and the J(sc) even decreases from 3.96 mA x cm(-2) at 5 000 r min(-1) to 1.76 mA x cm(-2) at 8,000 r x min(-1). J(sc) depends on the mutual impact of light absorption and carrier transport, while a contradicting effect from the two aspects is caused by varying the thickness of the active layer. The thicker the active layer, the more the excitons induced by light absorption. However, the build-in electric field becomes weaker and the pathway becomes longer for transporting the opposite charge carriers derived from exiciton separation to their corresponding electrodes at the same time, which makes the probability of charges collection by respective electrodes lower. With respect to the reduced V(oc), it may be attributed to the increased proportion of exciton dissociation at the interfaces of MEH-PPV and PCBM with the relevant electrodes.