Microstructural and electrical characterization of high temperature passivating contacts for silicon solar cells

Recombination at metal/semiconductor interfaces represents the main limitation in mainstream c-Si solar cells, primarily based on the passivated emitter and rear cell (PERC) concept. Full-area passivating contacts based on SiOx/poly-Si stacks are a candidate for the next generation of industrial cells, with a potential for higher efficiencies while requiring minimal changes to existing manufacturing processes. In this context, this thesis investigates fired passivating contacts, featuring a thin SiOx layer capped with a nc-SiCx(p) layer and activated through rapid thermal processing (RTP). This approach differs from long annealing processes usually employed for the formation of poly-Si contacts. The objective of the research is to explore the potential of FPCs as rear-side alternatives in PERC cells, focusing on their compatibility with screen-printed metallic grids fired through a silicon nitride layer.The first part of this work examines the feasibility of replacing silver contacts with aluminium, a more cost-effective and abundant alternative. The investigation focuses on understanding the complex interactions between aluminium and various layers within the stack during firing. Minority carrier lifetime and contact resistivity measurements are combined with microstructural analysis by ex situ and in situ transmission electron microscopy (TEM). The results reveal how varying the N content in the SiNx layer affects Al penetration depth during firing. Additionally, the research unveils the reactions between Al and the SiOx layer, leading to a partial degradation of the passivation already before Al spikes formation in the wafer. Nevertheless, the study suggests potential pathways for improvement by tuning the properties of the different layers in the stack.The second part of the thesis addresses the requirements on contact resistivity and lateral conductivity to minimize resistive losses when contacting FPCs with a metal grid, as used at the rear side of industrially relevant structures. The contributions of the SiOx/SiCx(p) and the SiCx(p)/metal interfaces (rC,SiOx and rC,Met, respectively) to the total contact resistivity are decorrelated over a broad range of thermal budgets. Low values of rC,Met can be easily obtained, enabling elevated current densities to flow through a localized metallization at this interface. In contrast, the higher rC,SiOx values only support lower current densities, necessitating a full-area metallization in the case of lower thermal budget. Nevertheless, the results demonstrate that increasing the thermal budget of the RTP step boosts the lateral conductivity of the SiCx sufficiently for lateral transport. As a result, the requirements for contact resistivity are relaxed, leading to a low series resistance, even when localized metallization is used.Consequently, the impact of RTP on passivation quality is also investigated. To improve passivation while increasing RTP thermal budget, an oxide layer based on plasma treatment with N2O gas is introduced. Characterization reveals the benefits of the plasma oxide: enhanced stability at high thermal budget, comparable contact resistivity to conventional UV-O3 and HNO3 oxides, and maintenance of structural integrity at the interface, linked to the higher O and N content compared to other oxide layers. This makes the plasma oxide a promising candidate for fabricating FPCs at high thermal budget and their integration as rear-side contacts in PERC-like solar cells.