High-resolution structural investigation of passivated interfaces of silicon solar cells

• Investigation of chemical composition, homogeneity, thickness and electrical characteristic of ultrathin oxide layers
• Thickness measurements of oxide layers performed with high resolution TEM, XPS and ellipsometry with accuracy of 0.35 nm.
• Quantitative chemical composition with sub-nm resolution obtained by TEM/EDS, XPS and ToF-SIMS.
• XPS measurements prove the presence of different sub-oxide species in different depths of the layer stack.
• ToF-SIMS analysis show oxygen rich layer below oxide layer before thermal annealing and none after.

High-efficiency solar cell technologies rely on a sound knowledge of interface engineering and characterization on an atomic scale. Electrical interface recombination losses are closely related to structural and chemical properties of passivation layers. Ultra-thin silicon oxide layers (thickness about 1 nm, often referred to as tunnel oxide layers) applied in the recently introduced TOPCon cell concept and ALD passivation techniques require advanced techniques in characterization of layer thickness and homogeneity. Furthermore, the interface properties seem to play an important role for PID solar cell reliability. Within this paper, we have a methodological focus on ultrathin silicon oxide layer characterization both from a preparative and analytical point of view. We show results on atomically resolved imaging of passivation layers and related chemical and electrical properties. The thickness of silicon oxide layers which lies in the low nm range is measured by TEM and angle-resolved XPS. The preparation of TEM samples was done by wedge polishing including final Ar+ ion polishing. Further preparation steps are required to inhibit further oxidation. From the comparison of TEM and XPS for thickness measurement results that layer thickness can be determined with an accuracy of 0.35 nm. These thickness values are compared to data obtained by spectral ellipsometry. The chemical composition of interfaces and stoichiometric composition of SiOx interface layers was analyzed by means of TEM, XPS and ToF-SIMS. The elemental depth resolution of all methods was compared. Finally, the silicon oxidation state was investigated by XPS as a function of layer thickness addressing both interfacial and thin film oxidation states. The presence of subsurface oxides is investigated depending on annealing processes.

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