P+N and P+NN+ solar cells with the area of 3.36 $ extrm{cm}^2$ were fabricated by thermal diffusion. Under the light intensity of 100 mW/$ extrm{cm}^2$, total area(active area) conversion efficiency was 13.4%(14.7%) for P+N cell fabricated by 15 min boron predeposition at 94$0^{circ}C$ and 20 min annealing at 80$0^{circ}C$, and 14.3%(15.6%) for P+NN+ cell processed by 15 min boron predeposition at 94$0^{circ}C$ and 50 min annealing at 80$0^{circ}C$ after 20 min back phosphorus diffusion at 1,05$0^{circ}C$. The minority carrier lifetime in bulk of P+NN+ cells was increased about 2~3 times comparing with P+N cells because of guttering and BSF effect due to back phosphorus doping. The methods used for efficiency improvement were AR coating, Ag electroplating, back doping and fine grid pattern as well as the control of front doping profile.