## 3 Present PV Technologies
### Higher Efficiency PV Cell Technologies  
Although in N-type silicon the bipolar diffusion coefficient is three times smaller than in P-type, the lifetime of N-type silicon carriers is in the order of milliseconds (i.e., an order of magnitude higher than in boron-doped P-type Si), and longer diffusion lengths can be achieved [33]. For this reason, N-type monocrystalline wafers are suitable as a starting material for high-efficiency solar cells. N-type-based crystalline silicon technologies--interdigitated back contact (IBC), heterojunction technology (HJT), and tunnel oxide passivated contact (TOPCon)--have higher efficiencies (23-25%) and a lower temperature coefficient in comparison with boron-doped P-type-based technologies [33; 34]. However, their market share remains relatively low (5% in 2015, 5% in 2019, and 5% in 2020), probably due to the higher costs of fabrication [35] (especially in the case of IBC and HJT) and consequently, they come with a higher price. The progress of industrialization is impacted by numerous factors, including the improvement in efficiency, the progress in cost reduction, and the initial investment in production lines, where the key lies in the follow-up on cost reduction for primary segments. However, an increase in the market share of N-type-based crystalline silicon technologies is expected in  
Figure 10: Development of single tools throughput (after [25]). the future [27], especially in applications with higher area-dependent costs. Prospective developments in N-type PV cell technologies are discussed in [36; 37; 38]. Advances in gallium doping make it possible to produce a P-type starting material with a carrier lifetime comparable to that of the N-type. These advances make it possible to produce highly efficient cells from both types of materials. For PERC cells, efficiency is limited by the series resistance associated with current contraction into small area contacts. The TOPCon structure solves this problem by means of a 1-2 nm thick SiO\({}_{2}\) layer, which allows the tunnelling of major carriers into a several hundred nm thick layer of heavily doped microcrystalline silicon, provided with full-area contact. The front side of TOPCon cells is designed in the same way as the PERC structure, so the investment requirements for the transition from PERC to TOPCon are not very high. The TOPCon structure is shown in Figure 11. The theoretical efficiency limit for TOPCon cells is about 28% [39]. For the N-type starting material, 25.21% efficiency was reached; for P-type starting material, 25.19 % efficiency was reached [40]. High-efficient cells can be obtained also using heterojunction technology (HJT). Cells on the base of the heterojunction, between the amorphous and c-Si, utilize very thin (10-20 nm) a-Si:H layer stacks of N-type (or P-type) wafers to provide surface passivation, emitter formation, and a back surface field [41; 42], as demonstrated in Figure 12. The a-Si:H layers are deposited on c-Si by PECVD using a low-temperature process (below 200 \({}^{\circ}\)C). On both doped layers, transparent conductive oxide layers are formed by sputtering, and metal fingers are screen printed. However, the a-Si:H layers cannot be exposed to a temperature above 200 \({}^{\circ}\)C, thus special low-temperature curing pastes are required. PV cells have the ability to capture light from both sides and thus to increase the excessive generation of carriers and the subsequently generated current, if the electrical  
Figure 11: TOPCon structure. Figure 12: HJT structure. contacts are designed accordingly. Such cells can collect diffuse radiation and radiation reflected from the surroundings (albedo) by the back surface, thus increasing the overall efficiency. An example of a simple bifacial cell structure is shown in Figure 13. It is necessary to ensure that the passivation dielectric layer on the rear also acts as an anti-reflective layer. In 2020, bifacial cells represented about 30% of total production [27] and this share is expected to grow significantly in following years [43].