Development and Characterization of Inkjet-Printed Poly-Si Passivating Contacts for Silicon Solar Cells
Photovoltaic (PV) technology leads the way among renewable energy options, as silicon_based solar cells dominating the market. A key advancement in this domain is the implementation of polycrystalline silicon (poly_Si) passivating contacts, which consist of a doped poly_Si layer and an ultra_thin SiOx layer. This structure has driven the efficiency of crystalline silicon (c_Si) solar cells to exceed 26%. Further improvements can be achieved by incorporating localized poly_Si passivating contacts, such as in interdigitated back contact (IBC) structures. Despite their promise, the implementation of such localized contacts is currently limited by the complexity and cost of the required patterning processes, posing a barrier to large_scale manufacturing and commercial deployment. This thesis introduces a maskless, scalable and cost_effective inkjet printing technique to fabricate both localized n_ and p_type poly_Si passivating contacts. In the first part of this thesis, n_type poly_Si passivating contacts were fabricated using an inkjet printer with a phosphorus dopant ink. By optimizing the printing parameters and annealing conditions, uniform and effective doping was achieved, resulting in excellent passivation quality with an implied open_circuit voltage (iVoc) of 729 mV and a contact resistivity of 5.4 mohm.cm2 on full_area printed samples. Micro_photoluminescence (uPL) mapping confirmed enhanced PL emissions in a 75 um_wide printed line. The second phase extended the process to p_type passivating contacts using inkjet printing. A pre_oxidation step was introduced to prevent unintended doping in unprinted regions. This approach achieved a promising iVoc of 718 mV and a contact resistivity of 6.1 mohm.cm2. Furthermore, building on these developments, the co_annealing of phosphorus_ and boron_printed lines with feature sizes down to 60 um was demonstrated, highlighting the potential of fabricating tunnel oxide passivated contact back contact (TBC) cells using inkjet printing. In the third part of this study, advanced characterization techniques were employed to evaluate the localized poly_Si passivating contacts. Dynamic secondary ion mass spectrometry (SIMS) revealed unintended doping in unprinted regions adjacent to the dopant lines and cross_doping when dopant lines were co_annealed. Despite this, sharp doping concentration gradients and corresponding drops in PL intensities were detected by SIMS and uPL at the line edges, suggesting effective lateral confinement of dopants. The full width at half maximum (FWHM) values of the SIMS and uPL cross_sectional profiles again agreed well with the line widths observed under the microscope. The results suggested that dopant volatilization from the liquid dopant sources during high_temperature processing is the primary mechanism behind the observed unintended and cross_doping issues. Given that significant unintended and cross_doping effects may lead to shunting and reduced device performance, the fourth part of this study introduced a mitigation strategy. A spin_on SiOx layer was applied after printing and high temperature baking. This barrier layer effectively suppressed dopant diffusion, with SIMS measurements showing unintended and cross_doping concentrations reduced to below 5% of the intended doping level. These results confirm the potential of this approach to enhance the reliability of localized doping. The impact of the remaining unintended and cross_doping on device performance will depend on the specific cell architecture. By systematically developing and optimizing inkjet printing for both n_type and p_type poly_Si passivating contacts and addressing key challenges such as unintended doping and cross_doping, this work established a strong foundation for the implementation of inkjet_printed localized poly_Si passivating contacts in high_efficiency silicon solar cells, including TBC solar cells and front_side localized poly_Si passivating contact designs.
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