## 3 Present PV Technologies
### PERC PV Cell Technology  
The increase in the efficiency of BSF cells is limited by both volume recombination and the relatively high recombination rate at the PP\({}^{+}\) contact. To reduce recombination in volume, it was necessary to switch to monocrystalline material. The problem of surface recombination could be solved (after finding a productive and cheap technology) using the passivated emitter rear contact (PERC) structure, as demonstrated in Figure 9. The solution is covering a significant part of the back surface with a dielectric film containing positive charges, thereby electrically passivating it. The contact of the back of the cell is made through a series of holes (with fixed distances and diameters) whose total area is only about 1-4% of the total area of the PV cell. PERC cells with 23% efficiency [24] were demonstrated as early as 1989, but the technology used was too expensive at the time to be accepted into series production.  
After 2010, the development of Al\({}_{2}\)O\({}_{3}\) passivation layer deposition and the realization of the contact hole pattern by local laser ablation decreased the technology cost increment connected with three additional technological steps (rear surface etching, passivating Al\({}_{2}\)O\({}_{3}\) layer combined with SiN\({}_{x}\):H layer deposition, and local windows opening by laser ablation) to an acceptable level.  
The development of production equipment, increasing throughput and decreasing PERC workshop investment costs [25], as demonstrated in Figure 10, accelerated the transition from BSF to PERC technology. While in 2015 the PERC technology share was less than 10% [26], in the year 2020 it represented more than 80% [27].  
As already mentioned, one of limiting factors for PV cells from P-type starting material was forming B-O complexes (LID), decreasing the PV cell's efficiency. The ways to eliminate this effect are either the reduction of the interstitial oxygen content or the substitution of the boron by a different dopant. Fabricating industrial boron-doped Cz silicon or Fz silicon with low oxygen content (<3 ppm) is very difficult and expensive. Attention was focused on using gallium as an acceptor [28]. The difficulty of applying gallium-doped silicon wafers lies in the control of the resistivity in the ingot [29], because there is a significant difference between the segregation coefficient of boron (0.75) and gallium (0.008). With  
Figure 9: PERC structure.  
process optimization at the ingot pulling, it has been possible to deliver Ga-doped wafers at similar prices to B doped wafers and PV cells made with Ga-doped wafers demonstrated an efficiency improvement [30]. The process of replacing boron doping with gallium doping was very fast. It started in 2018; in 2019, 10% of P-type monocrystalline wafers produced were gallium-doped [31]; and in 2020, 60% of P-type monocrystalline wafers produced were gallium-doped [27]. Gallium-doped monocrystalline wafers have become mainstream.  
Introduction of a selective emitter process in low-cost production, the reduction of contact resistance for weakly phosphorus-doped silicon, the reduction of front metallization finger widths from approximately 100 \(\upmu\)m to less than 30 \(\upmu\)m in large-scale production, and contact grid optimization were important steps in achieving the current high-efficiency PV cells in bulk production. The latest comers in silicon PV manufacturing benefit from cheaper production tools with higher productivity: larger cells (from 125 mm square P-type wafers in 2010 to 182 mm or even 210 mm in 2021), thinner wafers, less kerf-loss in slicing, more wafers per batch in operations, higher technology tool throughput, and more automation increasing labour productivity [27; 32].