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Industrial n-Type Silicon Solar Cells with Co-Diffused Boron Emitters

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FREY, Alexander, 2018. Industrial n-Type Silicon Solar Cells with Co-Diffused Boron Emitters [Dissertation]. Konstanz: University of Konstanz

@phdthesis{Frey2018Indus-42516, title={Industrial n-Type Silicon Solar Cells with Co-Diffused Boron Emitters}, year={2018}, author={Frey, Alexander}, address={Konstanz}, school={Universität Konstanz} }

2018-06-11T05:54:17Z eng Frey, Alexander terms-of-use Industrial n-Type Silicon Solar Cells with Co-Diffused Boron Emitters 2018-06-11T05:54:17Z In this thesis, metallization-induced power losses of bifacial n-PERT Si solar cells were studied in detail since screen-printing metallization still limits the energy conversion efficiency η of these solar cells. Thereby the focus was set on formation of Ag/Al metal spikes during contact firing and their detrimental influence on the IV-characteristics of bifacial n-PERT solar cells. For the first time, a detailed study on the impact of contact firing conditions on the formation of Ag/Al metal spikes was presented. It was demonstrated in this work that the area density (size and amount) of the Ag/Al metal spikes increases strongly for increasing set peak firing temperature T<sub>S</sub>. Moreover, the presented microscopic SEM analysis revealed that the Ag/Al metal spikes are deep enough to easily penetrate the SCR of the solar cells even for low T<sub>S</sub>, due to their average depth of at least 1 µm. This work shows that the detrimental influence of the Ag/Al metal spikes on the IV-characteristics of the solar cells depends strongly on the area density of the Ag/Al metal spikes. The increasing area density of the Ag/Al metal spikes for increasing T<sub>S</sub> agrees well with the decreasing specific contact resistance ρ<sub>c</sub> of screen-printed Ag/Al metal contacts on boron emitters for increasing T<sub>S</sub>. The impact of the Ag/Al metal spikes, in dependence on the boron emitter profile, was studied on bifacial n-PERT solar cell with different co-diffused boron emitters. It turned out in this study that the Ag/Al spike formation is independent of using an Al<sub>2</sub>O<sub>3</sub>/SiN<sub>x</sub>:H passivation layer stack or an SiO<sub>2</sub>/SiN<sub>x</sub>:H passivation layer stack. As expected, the detrimental influence of the Ag/Al metal spikes on the saturation current density of the first diode j<sub>01</sub> of the solar cells for increasing T<sub>S</sub> is less distinct for solar cells with boron emitters with lower R<sub>sh</sub> (deeper emitter profiles with higher surface doping concentration) than for solar cells with boron emitters with higher R<sub>sh</sub>, because of the better electrical shielding of the metal contacts. The slope of the j<sub>01</sub>-T<sub>S</sub>-curve is proportional to R<sub>sh</sub>, therefore the detrimental influence of the Ag/Al spikes is less pronounced for boron emitters with lower R<sub>sh</sub>. Since j<sub>01</sub> mainly limits the open-circuit voltage V<sub>oc</sub> of the solar cells, it is expected from these findings that V<sub>oc</sub> of solar cells with boron emitters with lower R<sub>sh</sub> decreases less than V<sub>oc</sub> of solar cells with higher R<sub>sh</sub> for increasing T<sub>S</sub>. However, the opposite behavior was observed. This finding was explained by a substantial influence of the Ag/Al metal spikes, penetrating the SCR, on the saturation current density of the second diode j<sub>02</sub> of the solar cells. It turned out that the absolute values of j<sub>02</sub>, as well as its increase for increasing T<sub>S</sub>, is much more distinct for solar cells with boron emitters with lower R<sub>sh</sub> than for solar cells with boron emitter with higher R<sub>sh</sub>. This explains the observed trend of V<sub>oc</sub>, since such high j<sub>02</sub> values also affect V<sub>oc</sub> significantly. The V<sub>oc</sub> loss induced by j<sub>02</sub> was determined to be in the range of 5 – 30 mV, strongly depending on the boron emitter profile and T<sub>S</sub>, being a noticeable part to the total V<sub>oc</sub> loss of the solar cells of 20 – 90 mV. Although the series resistance R<sub>S</sub> of the solar cells is continuously decreasing for increasing T<sub>S</sub>, which should result in a continuously increasing fill factor FF, it was observed that FF is decreasing for high T<sub>S</sub>. This FF decrease is even stronger for the boron emitter profiles with lower R<sub>sh</sub>. This phenomenon can be explained by the strong impact of the Ag/Al metal spikes on j<sub>02</sub>, mainly limiting FF of the solar cells and thereby overcompensating the effect of the decreasing R<sub>S</sub>. The Ag/Al spikes represent direct Schottky-contacts between the highly doped boron emitter and the metal. The width of the potential barrier is lower for a higher boron doping, resulting in a higher probability for charge carriers to tunnel through this potential barrier. Thus, parasitic leakage currents are induced. It is assumed that the detrimental leakage currents are proportional to the doping concentration, also explaining the lower ρ<sub>c </sub>ob-served for the boron emitters with lower R<sub>sh</sub>. These findings agree well with existing analytical models for the calculation of ρ<sub>c</sub> published in [157, 163]. A potentially higher initial defect density in the boron emitters with lower R<sub>sh</sub> may additionally enhance parasitic leakage currents. Moreover, diffusion of paste components to the SCR and differences in the peak wafer temperature during contact firing for the different boron emitter profiles cannot be excluded. The saturation current densities of the first and second diode in the two-diode model attributed to screen-printed Ag/Al metal contacts on boron emitters with R<sub>sh</sub> = 64 Ω/sq and Al<sub>2</sub>O<sub>3</sub>/SiN<sub>x</sub> passivation layer stack (fired at T<sub>S</sub> = 800°C) were calculated to j<sub>01c,Ag/Al</sub> ≈ 2,856 fA/cm<sup>2</sup> and j<sub>02c,Ag/Al</sub> ≈ 684 nA/cm<sup>2</sup>. These values are much higher than the ones determined for screen-printed Ag metal contacts on the phosphorus FSF with R<sub>sh</sub> = 40 Ω/sq (j<sub>01c,Ag</sub> ≈ 500 fA/cm<sup>2</sup> and j<sub>02c,Ag</sub> ≈ 20 nA/cm<sup>2</sup>), demonstrating the much higher recombination activity of screen-printed Ag/Al metal contacts. Based on these findings, the ideal Ag/Al metallization H-grid pattern was computed. The computation revealed the ideal parameters w<sub>F</sub> ≈ 60 µm and d<sub>F</sub> = 1.6 – 2.0 mm. These values are similar to the ones computed for the Ag metallization H-grid pattern on the front side. This indicates that the power losses caused by recombination at the Ag/Al metal contact are in the same range as the power losses due to shading of the Ag metal contacts. Using the optimized H-grid pattern, the industrial feasibility of sequential co-diffusion was confirmed on large area (156.25 cm<sup>2</sup>) bifacial n-PERT Si solar cells with η = 19.7%. This value is slightly lower than the values reported in [58, 173], which can mainly be explained by a non-advanced single screen-printing step metallization process used in this work. To overcome the metallization-induced power losses attributed to the Ag/Al metal contacts, an alternative metallization scheme was additionally studied in this work. Contact formation of mono-facial n-PERT solar cells was established by local contact opening (LCO) by means of laser ablation. By this means the dielectric passivation layer stack is locally removed by ps-laser pulses with a wavelength of 532 nm. Subsequently, the metallization is conducted by full-area physical vapor deposition of Al layer with a thickness of around 1 µm. With this method f<sub>c</sub> can be reduced to a level of below 1%, in comparison to f<sub>c</sub> of screen-printed metal contacts in the range of 4 – 8%. Furthermore, the evaporated Al metallization does not require contact firing, thus obviating the formation of deep metal spikes. Therefore, this metallization scheme does not suffer from the issues related to Ag/Al metal contacts. However, it turned out that LCO can also induce damage in the Si substrate beneath the local contacts, also reaching the SCR. This results also in an enhanced recombination activity, mainly limiting FF of the mono-facial n-PERT cells. The main loss of FF can be attributed to defects located in the SCR, inducing a high j<sub>02</sub>. It could be shown that this FF loss, due to an enhanced j<sub>02</sub>, is independent of the boron emitter profile, in contrary to the findings on the Ag/Al metal spikes where a strong dependence of the boron emitter profile was observed. For the first time, a model for the distribution of these laser-induced defects was presented in this work. This model allows to successfully simulate the detrimental influence of these defects on the IV-curves of the solar cells and accurately reproduces the measured IV-characteristics pFF, FF and V<sub>oc</sub>. It was demonstrated that the use of a closely packed point structure with a spot size d<sub>S</sub> = 20 µm is beneficial in terms of a high η of the solar cells. Moreover, it was proven in this work that the defects in the base and damage of the passivation layer outside of the local contact spots result in an enhanced j<sub>01</sub>, mainly lowering V<sub>oc</sub>. In contrary to the impact of the defects in the SCR on FF, the influence of these defects on V<sub>oc</sub> is dependent on the boron emitter profile. The V<sub>oc</sub> loss for increasing f<sub>c</sub> is stronger for boron emitters with higher R<sub>sh</sub>. Due to the strongly increasing recombination activity of the defects for increasing f<sub>c</sub>, only a narrow range of f<sub>c</sub> = 0.5 - 1.0% shows up to be ideal for d<sub>S</sub> = 20 µm. This ideal f<sub>c</sub> is almost independent of the boron emitter profile, since the influence of the defects in the SCR dominates. Higher η and a wider range of f<sub>c</sub> are expected from the simulation without defects underneath the local contacts. Although these defects can be partly attributed to a not yet fully optimized laser process used in this work, they might not be completely avoidable, even with a more optimized process. As far as the author knows, no such study has yet been performed. A detailed free energy loss analysis was presented to clarify the relevant loss mechanism in the mono-facial n-PERT solar cells. It was demonstrated that the considerable difference in the IV-characteristics with/without defects underneath the local contact spots cannot be explained by the increased recombination rate R of holes at these defects, which is almost compensated by the decreased R of holes in the bulk. The physical origin of this deviation is the essentially increased Joule loss of minority charge carriers (holes) in the base for increasing f<sub>c</sub>, which is also a consequence of increased recombination at the defects. The industrial feasibility of the alternative approach for boron emitter formation by means of sequential co-diffusion from doping layers manufactured via ICP-PECVD was again confirmed on large area (243.36 cm<sup>2</sup>) mono-facial n-PERT solar cells with η = 20.2%, using potentially low-cost n-type mono-like Si (Cz-Si reference: η = 20.5%). The measured V<sub>oc</sub> = 670 mV shows the high electrical quality of co-diffused boron emitters. The higher η of mono-facial n-PERT solar cells reported in [42, 184] on a similar process sequence can mainly be explained by a more advanced front side metallization scheme. Indeed, the power losses attributed to the front side metallization are the biggest part of the total power loss of the n-PERT solar cells presented in this work. In conclusion, sequential co-diffusion shows up as a promising candidate for further reduction of production cost and is also applicable for advanced solar cell concepts like IBC. 2018 Frey, Alexander

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