Considering the rapidly growing energy demand in worldwide and climate deterioration caused by fossil fuel, the remarkable potential of solar energy has captured the attention of individuals and industries alike. Among different techniques, poly-Si based passivating contacts have shown great performance on solar cell application, which enabling a high efficiency of over 26%. Plasma-enhanced chemical vapor deposition (PECVD) as one of the promising technologies in a-Si contacting layer fabrication has gradually replaced the conventional LPCVD method in industry. However, the accompanied severe ion bombardment is not negligible, especially on its underlying fragile tunnelling oxide. In this work, the impact of PECVD a-Si:H contacting layer deposition on poly-Si/SiO_x passivating contact is investigated. 
PECVD radio-frequency (RF) power for the contacting a-Si layer on the underlying SiO_x is the only variable in this project, varying from 5 W to 55 W, and pinhole density acts as a bridge to help analyse the intrinsic principles. Firstly, the results of a-Si:H thin film characterization suggest that with an increasing RF power, the a-Si:H thin film is grown at a higher deposition rate and becomes porous. In addition, the pinholes in tunnel oxide are inspected by applying the concepts of “selective etching” and “pinhole magnification”. With a two-step five-point sampling method, it is shown that the effect of RF power on the pinhole density is not monotonically increasing. The highest value is found at 25 W. To explain this, a concept of “protective layer” is proposed, which is defined as a buffer layer (contacting layer) formed at the very beginning during a-Si:H deposition. It appears to be more effective when higher RF power (> 35 W) is applied. Another influence on tunnel oxide property is discussed according to the result from XPS measurement. The percentage of Si⁴⁺ species is found in the case of 25 W, corresponding to the highest pinhole density. This proves to some extent that the severe particle bombardment brought by strong power would weaken or directly break the Si-O bonds in the PECVD substrate, that is the tunnel oxide in our case.
As a result of thin film characterization, five factors contribute to pinhole formation: (i) Defects in tunnel oxide from imperfect oxidation leave potential for pinhole formation. (ii) Severe ion bombardments in PECVD deposition are allowed to weaken or break Si-O in SiO_x. (iii) Island growth of a-Si:H contacting layer makes the exposed region in tunnel oxide continue to be damaged. (iv) “Buffer layer” formation protects the substrate from ion bombardments. (v) The tensile stress applied by a-Si:H films during annealing intensifies the formation of pinholes. 
Subsequently, an unexpected result from passivation quality assessment is that higher passivation level is presented with higher pinhole density. The best passivation quality is found in the case of 25 W, with J₀ of 3.3 fA/cm² and iV_oc of 714 mV. Further, a large optimal process window for RF power adjustment is found from 25 W to 35 W, which leads to an iV_oc over 710 mV, with single side J₀ below 3.5 fA/cm². The results from specific contact resistivity indicate that it is positively correlated to the pinhole density. Eventually, the champion passivating contact with a selectivity of 14.37 in this project is expected to yield a maximum efficiency of 28.9% in an ideal c-Si solar cell.

Passivating contacts in the front-rear contacted c-Si solar cells enhances the device performance by quenching the contact recombination and enabling sufficient carrier transport. Tunneling oxide passi- vating contact (TOPCon) which consists of an interfacial oxide and a poly-SiO𝑥 layer is one of the most promising contact techniques in passivating both surfaces of the silicon wafer. However, the full area poly-SiO𝑥 layer is absorptive when it is applied at the front side which has become an efficiency limiting factor. In this project, an advanced local poly-SiO𝑥 passivating contact architecture is proposed and presented. The local poly-SiO𝑥 passivating contacts, so-called “poly-SiO𝑥 finger”, is deposited through a hard mask at predefined locations which will be only placed underneath the metal contacts. The formation of poly-SiO𝑥 finger and its application in the solar cells are discussed.

First, a silicon wafer hard mask is fabricated by a lithography process followed by a KOH wet chemical etch-back process. Specified openings down to 38.7 𝜇m are established which isolate sections of the substrate for the poly-SiO𝑥 finger formation. P+ doped a-SiO𝑥 :H is deposited using PECVD with SiO𝑥 p+ diffused emitter commercial Cz wafers through the hard mask. By investigating the influence of deposition parameters during the PECVD, specifically the power and pressure, decent uniformity of the finger deposition is achieved. An optimized deposition condition of the doped a-SiO𝑥 :H is established for the poly-SiO𝑥 finger formation: an RF power source at 15 Watt, a chamber pressure of 1.5 mbar, and a substrate temperature of 180 °C. A narrow poly-SiO𝑥 finger with a width of 40.0 𝜇m has been demonstrated.

Second, the passivation properties of the symmetrical samples are examined by varying the poly-SiO𝑥 and Al2O3 thickness for the hole carrier selective contacts. A full area p+ TOPCon and Al2O3 symmetric samples were coated on the textured diffused p+ surface with an n-type c-Si substrate to investigate the process parameters on the passivation properties. The optimal 𝑖𝑉𝑜𝑐 of p+ TOPCon samples is 662 𝑚𝑉 and a corresponding 𝐽0 of 156 𝑓 𝐴 𝑐𝑚2 which is obtained with a poly-SiO𝑥 thickness of 50 nm when annealed at 850 °C. The optimal thickness for the Al2O3 passivation layer is achieved by depositing a 20 nm thick layer followed by FGA, with an 𝑖𝑉𝑜𝑐 of 692 𝑚𝑉 and a 𝐽0 of 22.4 𝑓 𝐴/𝑐𝑚2.

Finally, a process flowchart has been created to fabricate c-Si solar cells with the local p+ TOPCon on the front p+ diffused emitter and a full area n+ TOPCon on the rear side. A localized polished area is made by a poly-etch process for the alignment between the substrate and the pattern in the lithography mask. Using the ascertained parameters during the optimization, NAOS-SiO𝑥 combining with 50 nm-thick doped p+ a-SiO𝑥 :H fingers are applied on the p+ diffused surface and 100 nm-thick n+a-SiO𝑥 :H on the rear side. A annealing step is performed for the crystallization. A 20 nm-thick Al2O3 followed by a 75 nm SiN𝑥 layer are deposited on the front side and 100 nm SiN𝑥 layer is applied on the rear side. Lithography, wet chemical etch-back together with Al/Ag(front/rear) evaporation are utilized for solar cells metallization. The fabricated c-Si solar cells with poly-SiO𝑥 fingers has a champion efficiency with an efficiency of 7.91 %, with a 𝑖𝑉𝑜𝑐 of 562 𝑚𝑉, 𝑖𝑛𝑡 𝐽𝑠𝑐 of 34.7 𝑚𝐴 𝑐𝑚2 and fill factor of 40.5 %. The limited performance originates from the poor passivation in the poly-SiO𝑥 local contact area and severe 𝑖𝑉𝑜𝑐 drop after the metallization process. For further improvement, good performing c-Si solar cells by minimizing the recombination in the poly-SiO𝑥 local fingers contacts area and optimizing metallization steps are expected to be fabricated .

Silicon solar cells account for about 95% of the total photovoltaic market share. Poly-Si passivating contacts are promising techniques enabling high performance c-Si solar cells with conversion efficiency over 26.0%. The highly absorptive nature of poly-Si materials makes the transparent passivating contacts attractive, such as poly-Si(O_x). However, the research of p⁺ poly-Si(O_x) passivating contacts on a textured surface with an in-situ doping nature is still missing. Therefore, in this thesis, the optimization of p⁺ poly-Si(O_x) carrier selective passivating contacts on double side textured wafers are given and the application in solar cells is demonstrated.
Firstly, the influences of different interfacial tunnelling oxides fabrication methods, nitric acid oxidation of silicon (NAOS-SiO_x), plasma assisted N₂O oxidation (PANO-SiO_x), and thermal oxidation (t-SiO_x) on the passivation of p⁺ poly-Si(O_x) passivating contacts are explored. It is found that Si⁴⁺ stoichiometry in the tunnelling oxide layer is an indicator for its quality. There is a positive correlation between Si⁴⁺ and SiO_x density. The t-SiO_x can be denser with higher Si⁴⁺ compared to its counterparts NAOS-SiO_x and PANO-SiO_x. And fewer boron dopants in-diffuse phenomenon can be observed in the t-SiO_x samples. Then, different intrinsic layer deposition approaches are explored. The intrinsic layer deposited by LPCVD results in higher iV_oc compared to PECVD counterpart. The enhanced iV_oc is given by suppressing the blistering which is caused by hydrogen accumulation at the interface between intrinsic layer and SiO_x. It is assumed that less hydrogen accumulation exists in intrinsic layer deposited by LPCVD. Next, p⁺ doping layer thickness is changed from 0 nm to 200 nm to observe its effect on the passivation quality. The optimum p⁺ doping layer thickness is found to be 100 nm with the highest iV_oc. After that, the hydrogenation process is introduced to enhance chemical passivation by coating SiN_x:H and performing forming gas annealing. The highest iV_oc with the standard hydrogenation process is 674 mV. In order to improve the hydrogen level of p⁺ poly-Si(O_x) passivating contacts, AlO_x:H inserted layer is used for the hydrogen reservoir together with SiN_x:H. It results in an improved iV_oc of 685 mV. It is assumed that the hydrogen in AlO_x:H diffuses into c-Si/p⁺ poly-Si(O_x) interface and enhances the chemical passivation.

Besides, metallization methods of p⁺ poly-Si(O_x) passivity contacts are also studied. There are two approaches to complete the metallization process. Firstly, with thin p⁺ poly-Si(O_x) passivating contacts, TCO is required to provide with the lateral and vertical carrier transport as respect to the carrier collection. However, it commonly brings with the TCO introduced sputtering damage. The iV_oc losses are 70-90 mV when TCO is sputtered on p⁺ poly-Si(O_x) passivating contacts. When the thickness of p⁺ doping layer is over 50 nm, sufficient lateral conductivity can be provided with thick p⁺ poly-Si(O_x) passivating contacts. Therefore, it can directly contact with metal which is the second method of metallization. However, when utilizing this metallization method, the metal induced recombinations need to be taken into consideration when contacting with p⁺ poly-Si(O_x) passivating contacts. Thus, the plot of J_o,total along with different metal fractions is fitted to extract J_o,metal of p⁺ poly-Si(O_x) passivating contacts with 100 nm p⁺ doping layer. When contacting with evaporated aluminum, the measured J_o,metal is around 91 fA/cm². In addition, after calculating, there is 24 mV iV_oc loss when p⁺ poly-Si(O_x) passivating contacts contacting with metal. It is smaller than the loss induced by TCO sputtering. Therefore, the thick p⁺ poly-Si(O_x) passivating contacts with 100 nm p⁺ doping layer directly contacting with metal is used as metallization method for the application in c-Si solar cell. In addition, after linear fitting and calculating, ρ_c between p⁺ poly-Si(O_x) passivating contacts with 100 nm boron doped layer and c-Si is about 23 mΩ· cm².

Finally, p⁺ poly-Si(O_x) passivating contact is applied in c-Si solar cells together with n⁺ poly-Si(O_x) passivating contact as front surface field. The poly-poly solar cell of the highest quality has the following electrical performance: V_oc is 648 mV, J_sc is 35.9 mA/cm², FF is 73.1% and η is 17.0%. A roadmap to realize 22% is given by addressing the bottlenecks of poly-Si(O_x) passivating contacts based c-Si solar cells.

In recent decades, there has been increasing concern about the impact of climate change on the earth. Various countries are actively developing sustainable energy technologies, of which solar cell is one of the new energy sources with the most attention. This thesis is based on poly−Si(O_x) cells consisting of SiO_x/poly−Si(O_x) passivating contact. Various methods have been investigated to mitigate the TCO (IWO) deposition induced passivation degradation and to optimize the screen printing process.

First, approaches to reduce the passivation degradation due to IWO deposition were explored. The power density and working pressure in IWO deposition were optimized. With utilizing 1.23 W/cm² power density and 5×10⁻³ mbar working pressure, the 𝑖𝑉_𝑂𝐶 degradation was reduced to 4.7 mV for 𝑛⁺ sample (NAOS-SiO_x) and to 7.9 mV for 𝑝⁺ sample after deposition of 75 nm IWO. Besides, hydrogenated amorphous silicon and AZO were used as buffer layers for the 𝑝⁺ SiO_x/poly−Si(O_x) samples, finding that they were effective in reducing sputtering damage. Then, the optimal post-annealing condition was investigated, which turned out to be vacuum annealing at 400 ^◦𝐶 for 30 min, recovering the 49 mV and 60 mV 𝑖𝑉_𝑂𝐶 for 𝑛⁺ sample (thermal-SiO_x) and 𝑝⁺ sample, respectively. Based on the above experiments, the first cells were prepared with a maximum efficiency of 17.4%.

Second, the existing screen printing process in the lab was optimized. The 0.37% organic solvent was added into the silver paste, reducing the viscosity of paste. The snap-off distance was changed to 0.02 mm in order to improve the continuity of printed grids. Moreover, the squeegee speed was optimized to 30 mm/s, limiting the spreading of silver paste to 57.3 𝜇m, much narrower than the initial spreading distance of 155.3 𝜇m.

Finally, a batch of poly−Si(O_x) cells were prepared by applying the above various optimized conditions. The cell with optimized IWO and optimized screen printing process performed best with an champion efficiency of 18.9%.