## 2 Experiments and Methods  
In this work, the Al-BSF silicon solar cell and PERC silicon solar cell were fabricated by typical industrial processes and production equipment, respectively. The material used in this work was 156.75 mm \(\times\) 156.75 mm boron-doped Czochralski-grown pseudo-square silicon wafers with 1-2 \(\Omega\)cm base resistivity and an initial wafer thickness of 190 um. The area of silicon wafer was 244.3155 cm\({}^{2}\). After standard cleaning and alkaline texturing, a wet chemical single-sided polishing step, which removes about 4-6 \(\upmu\)m of silicon from the rear surface, was carried out.  
The emitter layer of the Al-BSF solar cell was formed in a closed tube furnace using a POCl\({}_{3}\) liquid source at 800 \({}^{\circ}\)C. The phosphorus diffusion resulted in a 90-100 \(\Omega\)/sq emitter with a peak doping concentration of about 2 \(\times\) 10\({}^{19}\) cm\({}^{-3}\). The p-n junction depth was about 0.4 \(\upmu\)m. After the POCl\({}_{3}\) diffusion, laser doping was carried out so as to form a front selective emitter. The phosphosilicate glass removal, edge isolation, and rear side polishing were fulfilled simultaneously in a Rena wet bench. After a cleaning step, the plasma-enhanced chemical vapor deposition of SiNx:H thin film was used for passivation and anti-reflective purposes on the front side of the solar cell. At last, the screen-printed silver paste back contact, screen-printed aluminum paste back surface field, and silver paste front contact were prepared and co-fired rapidly in a belt furnace made by Centrotherm photovoltaics AG.  
For the PERC solar cell, the formation of the emitter layer was the same as that for the Al-BSF solar cell. As the next step in the process, the rear side of the solar cell was passivated with an atomic-layer deposition of AlOx capped with a plasma-enhanced chemical vapor deposition of a SiNx:H thin film. An 80 nm SiNx:H antireflection coating layer was deposited by plasma-enhanced chemical vapor deposition on the front side of the solar cell. Then, the rear contact pattern was formed by laser ablation with a 532 nm ps laser. Screen printing and co-firing were used for front and rear side metallization of the PERC solar cell.  
Finally, the electrical performance of the Al-BSF solar cell and the electrical performance of the PERC solar cell were tested by a Gsolar testing system (X)CC-10, AAA class pulsed solar simulator, in accordance with IEC 60904-9, 200-1200 W/m\({}^{2}\) by steps of 100 W/m\({}^{2}\)).The light injection level can be calibrated by the reference cell, and can be set via the target voltage. Table 1 lists the measured parameters of the Al-BSF solar cell and the PERC solar cell under standard test conditions (STC: 1 kW/m\({}^{2}\) irradiance, 25 \({}^{\circ}\)C module temperature, and AM1.5 global spectrum). In Table 1, \(\upmu\)s is the conversion efficiency of the solar cell; V\({}_{oc}\) is the open-circuit voltage; \(I_{sc}\) is the short-circuit current density; \(V_{m}\) is the maximum power point voltage; \(J_{m}\) is the maximum power point current density; \(P_{m}\) is the optimal power delivered by the solar cell under STC; FF is the fill factor; and \(R_{s}\) and \(R_{sh}\) are the series resistance and the shunt resistance in ohms, respectively. The schematic structures for the Al-BSF solar cell and the PERC solar cell are shown in Figure 1.