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The charge carrier transport mechanism of passivating contacts which feature an ultra-thin oxide layer is investigated by studying temperature-dependent current-voltage characteristics. 4-Terminal dark J-V measurements at low temperatures reveal non-linear J-V characteristics of passivating contacts with a homogeneously grown silicon oxide, which result in an exponential increase in contact resistance towards lower temperature. The attempt to describe the R(T) characteristic solely by thermionic emission of charge carriers across an energy barrier leads to a significant underestimation of the resistance by several orders of magnitude. However, the data can be described properly with the metal-insulator-semiconductor (MIS) theory if tunneling of charge carriers through the silicon oxide layer is taken into account. Furthermore, temperature-dependent light J-V characteristics of solar cells featuring passivating contacts at the rear revealed a FF drop at T < 205 K, which is near the onset temperature of the exponential increase in contact resistivity. [ABSTRACT FROM AUTHOR]

We present a novel passivating contact structure based on a nanostructured silicon-based layer. Traditional poly-Si junctions feature excellent junction characteristics but their optical absorption induces current losses when applied to the solar cell front side. Targeting enhanced transparency, the poly-Si layer is replaced with a mixed-phase silicon oxide/silicon layer. This mixed-phase layer consists of an amorphous SiO x matrix with incorporated Si filaments connecting one side of the layer to the other, and is referred to as nanocrystalline silicon oxide (nc-SiO x ) layer. We investigate passivation quality, measured as saturation current density, and nanostructural changes, characterized by Raman spectroscopy and transmission electron microscopy, carefully studying the influence of annealing dwell temperature. Excellent surface passivation on n-type and also p-type wafers is shown. An optimum annealing temperature of 950 °C is found, resulting in a saturation current density of 8.8 fA cm −2 and 11.0 fA cm −2 for n-type and p-type wafers, respectively. Even before forming gas annealing, emitter saturation current densities of 27.9 fA cm −2 (n+/n junction) and 32.0 fA cm −2 (n+/p junction) are reached. Efficient current extraction is presented with specific contact resistivities of 86 mΩ cm 2 on n-type wafer and 19 mΩ cm 2 on p-type wafers, respectively. High-resolution transmission electron microscopy reveals that the layer stack consists of intermixed SiO x and Si phases with the Si phases being partly crystalline already in the as-deposited state. Thermal annealing at temperatures ≥850 °C further promotes crystallization of the Si-rich regions. The addition of the SiO x phase enhances the thermal stability of the contact and should allow to tune the refractive index and improve transparency, while still providing efficient electrical transport through the crystalline Si phase, which extends throughout almost the entire contact. [ABSTRACT FROM AUTHOR]

In this work, we report on a spectroscopic ellipsometry study of thin films of silicon nanocrystals embedded in a SiO x matrix that is performed to better understand how substoichiometric SiO x species affect the optical properties of these systems. The silicon nanocrystals are fabricated in a SiO x matrix by thermal annealing of magnetron-sputtered thin films in a wide range of x values (0 < x < 2) in order to produce every possible substoichiometric SiO x species in various fractions. A generic optical model to describe the thin film content with various SiO x volumetric fractions is developed and used to analyze the data obtained from the spectroscopic ellipsometry. Tauc-Lorentz and Bruggeman effective medium approximation models are employed to determine the ellipsometric angles (Ψ and Δ) and the complex dielectric function. We show that the optical properties of the nc-Si embedded in a SiO x matrix are highly dependent on the value of x and on the volumetric fractions of the substoichiometric SiO x species present in the system. [ABSTRACT FROM AUTHOR]