Recent studies in crystalline silicon solar cell fabrication have been focused on high efficiency and low cost. However, additional technologies for high efficiency solar cell deliver more complicate processes which rise of the manufacturing cost. In this paper, we studied on the characteristics of the crystalline silicon solar cell with selective emitter structure formed by one-step diffusion process using a phosphorus paste as a diffusion source. For the selective emitter structure, one-step diffusion process is necessary only additional phosphorus paste screen printing and drying steps, which might be formed heavy and light doped area at the same time. In this work, we studied on the optimization of one-step diffusion process conditions such as the printing and diffusion of phosphorus paste in selective emitter formation. Finally, the electrical properties of thus prepared Si-solar cell were measured and characterized by SIMS, SEM and Suns-Voc.
The spread properties of printed phosphorus paste was affected by the pyramidal size of textured wafer: spreading width was 745 ㎛(avg) in 5 ㎛ pyramid height and 337 ㎛(avg) in 3 ㎛ pyramid height under the printing frame of 100 ㎛. One-step diffusion process mainly consisted of paste oxidation and two drive-in sequences. Paste oxidation sequence was performed to form P2O5 on the wafer surface for effective diffusion. Oxidation has carried out under 500℃ to prevent carbonization of phosphorus paste. Sheet resistance and calculated standard deviation of emitter with phosphorus paste were optimized at the oxidation temperatures of 400℃ in the temperature range of < 790℃. The first stage of drive-in sequence was carried out to make the heavily doped area by phosphorus paste. In this stage, the phosphorus atoms in paste were diffused at high temperature of ～850℃ and then cooled down to 790℃, which formed the 50Ω/sq emitter region. The second stage was formed the lightly doped area of selective emitter with 100Ω/sq using POCl3. at around 815℃. Doping profiles of selective emitter formed by one-step diffusion process could be controlled as a function of drive-in temperature. The current density of the Si-solar cell with selective emitter was improved by 0.6 ㎃/㎠ in comparison to the homogeneous emitter, 35.37㎃/㎠. On the other hand, the fill factor and shunt resistance of Si-solar cell with selective emitter was decreased to 76.1% and 7.42Ω from 79.1% and 17.07Ω in homogeneous one. As a result of the degraded contact parameter, the conversion efficiency of solar cell was decreased to 17.1% in selective emitter from 17.5% in homogeneous emitter. The efficiency decrease of selective emitter solar cell was related to the deterioration of contact properties between electrode and silicon wafer. It was resulted from ununiform emitter on pyramid structures of textured wafer surface due to low viscosity of phosphorus paste. To solve this problem, we printed phosphorus paste by two times. Contact property was improved by double printing of phosphorus paste, but the electrical characteristics such as Voc and Jsc were decreased due to indirect doping by excessive phosphorus paste. The improved properties of phosphorus paste and the optimized diffusion condition would be further studied for the selective emitter by one-step diffusion process, which could be applicable technique for the fabrication of crystalline silicon solar cell.