Characteristics Of The Aluminum Back Surface Field Silicon Solar Cells

The back ohmic contact of the aluminum back surface field (Al-BSF) of crystalline silicon solar cells greatly impacts the output properties of solar cells. And so it is imperative to improve the back ohmic contact property by novel technology in order to reduce the series resistance. With the solar cell substrate thickness decreasing gradually, it is difficult for the screen printing to fabricate Al-BSF so as to continue exerting its advantages. So it is also necessary to explore the intrinsic relations among the quality characteristics parameters of silicon substrate and the Al-BSF characteristic.Firstly, the aluminum films and aluminum paste were sputtered and printed on the back surface of the semifinished silicon solar cells by DC magnetron sputtering and screen-printing technique, respectively, and then after rapid thermal annealing (RTA) the Al-BSF was prepared. The experimental results show that when the sputtering Al film is about 3μm and then is annealed rapidly at 800℃, the sheet resistance of Al-BSF back surface has been almost equal to that by screen printing and sintering process commonly used in present industrialized production. Compared with the screen printing process, the grain of sputtering aluminum film after RTA at 800℃tends to be larger and more uniform and more compact, and its surface is smoother, all of which are conducive to further decreasing back ohmic contact resistance and improving the absorption of the middle- and long-wave sun light. The back ohmic contact resistance of Al-BSF by DC magnetron sputtering is downtrend with the annealing temperature increasing, and less than that by the screen printing process. When the annealing temperature is not less than 800℃, the contact resistance by the sputtering process tends to be stable gradually, while via annealing at 900℃the back ohmic contact resistance by the printing process shows an uptrend.To further study the Al-BSF characteristics, the physical model of crystalline silicon solar cells with an n+pp+ structure was established in the simulation software PC1D, and the intrinsic relations among Al-BSF characteristics and silicon wafers parameters, such as substrate thickness, doping concentration and minority bulk lifetime, as well as the Al-BSF parameters which is mainly doping concentration profile, average doping concentration, thickness and so on, were systematically simulated and studied. The emulational results show that the effect of the silicon substrates thickness on the output performance of n+pp+ solar cells depends on the diffusion length of minority carrier in substrates. When the substrate thickness approximately equals the diffusion length of minority carrier in substrate, the role of Al-BSF on improving the output performance of solar cell enhances gradually with the substrate thickness decreasing. When the ratio of minority carriers'diffusion length to the substrate thickness is about 2.5 to 3, the crystalline silicon cells with an Al-BSF can gain the optimal output performance. In addition, When the doping concentration of the p-type silicon substrate ranges from 5×1015 to 1×1017cm-3, corresponding to its resistivity between 0.2 and 3Ω·cm, the crystalline silicon cells with an Al-BSF can also get an better output performance. Besides, the output performance of the crystalline silicon cells with an Al-BSF is almost independent of doping concentration gradient of its Al-BSF, and to a certian extent Al-BSF of solar cells shields its output performance from impacting by the back surface recombination velocity (BSRV). When the average doping concentration in Al-BSF region is in excess of 6.56×1018cm-3, the optimum thickness of Al-BSF is determined by the diffusion length of minority carriers in Al-BSF region and its optimum value generally is in the range of 1 to 2 times as long as the diffusion length of minority carriers in Al-BSF region. When the average doping concentration and thickness of Al-BSF are about 6.56×1018cm-3 and 10μm, the Al-BSF can play its full role to improve the output performance of solar cells.