p⁺ poly-Si(O_x) passivating contacts for double-side textured c-Si solar cell

Abstract

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.