This research is to synthesize polyaniline (PANI) thin film for polymer solar cells as a hole transport layer by using potentiostatic deposition of electrochemical method. In our previous studies, we have shown that the power conversion efficiencies of the solar cell device were improved for the slow polymerization rate. We choose the potentiostatic deposition method to improve the polymerization rate of PANI for the application in industry. In this study, we investigated optical transmittance, absorption spectrum, Highest Occupied Molecular Orbital (HOMO), surface roughness, and surface morphology of the PANI thin film by changing voltages and to discuss the factors on device efficiency. Then, we compared the device structures with hole transport layer PEDOT: PSS by spin-coating process. We found PANI thin films synthesized from different voltages, and the transmittance measurement results were similar. In addition, we found HOMO, surface roughness, and surface morphology of PANI thin film that varies with different voltages. The power conversion efficiencies of the device mainly were affected by the surface roughness and morphology of PANI thin film surface. Comparing to other parameters, PANI thin film polymerized at 0.8V owns the most appropriate surface roughness and surface morphology. The power conversion efficiency was up to 1.52% under AM 1.5G illumination based on ITO (150 nm) / PANI (75 nm) / P3HT: PCBM (100 nm) / Al (200 nm), and the device area of 0.16 cm2.