We present a high efficiency (η ∼ 18.5%) mono-crystalline Si solar cells (A=15.6×15.6cm2) with a selective emitter structure using a conventional diffusion process (POCl3) and screen printed contact. Especially, in this paper, we focus on the improvement of Voc by selective emitter pattern optimization. As a result, we improve about 4mV Voc as a function of heavy emitter area, investigate the cause of the Voc increase by internal quantum efficiency and life time. The fabricated solar cell with selective emitter has accomplished Voc 635.78 mV, Jsc 37.03 mA/cm2, FF 78.35%, Eff 18.45%. The result shows improvement of Voc 8 mV, Jsc 0.57 A/cm2, Eff 0.5 % in comparison with the reference cell (homogeneous emitter structure) and improvement of Voc 3.7 mV, Eff 0.1 % as a function of heavy emitter size.