Photoluminescence-Based Characterization of Silicon Materials for Solar Cells

Abstract

The ongoing global transition from fossil fuels to clean renewable energies relies heavily on the developments in crystalline silicon photovoltaics technology, which dominates the solar cell market. In modern high-efficiency crystalline silicon solar cells, such as Tunnel Oxide Passivated Contact and Silicon Heterojunction cells, the effective passivation of defect states has a critical impact on the device performance. During the passivation process, hydrogen is the key agent for both surface and bulk passivation, yet its fundamental transport kinetics within the silicon lattice remains under debate, as shown by significant discrepancies in the reported effective hydrogen diffusivity values.

This thesis employs photoluminescence-based techniques to investigate carrier recombination and hydrogen diffusion in silicon materials. The work is divided into two complementary parts. First, time-resolved photoluminescence is used to measure the carrier lifetimes in heavily-doped silicon wafers under different conditions, in thin intrinsic silicon films, and thin doped polycrystalline silicon films. This experiment reveals the dominance of Auger recombination in heavily-doped silicon wafers and the impact of hydrogenation on different thin silicon films.

The second part of the thesis develops a novel, steady-state spectrally-resolved photoluminescence methodology to quantify the effective hydrogen diffusivity in crystalline silicon. It is emphasized that this measurement specifically probes the effective diffusivity, which reflects the net result of all hydrogen interactions within the silicon lattice. Under thermal equilibrium, this technique reveals a strong doping dependence: while diffusivity in moderately-doped n-type and undoped silicon aligns with literature, it is significantly reduced in heavily-doped n-type and all p-type silicon. Supporting simulations suggest this reduction in effective diffusivity arises from complex interactions, including those between hydrogen charge states and the formation of hydrogen dimers or other metastable interactions.

Furthermore, this research demonstrates the active control of hydrogen diffusion under non-equilibrium conditions. Strong laser illumination is shown to increase the effective hydrogen diffusivity significantly in heavily-doped n-type and moderately-doped p-type silicon. This light-enhanced diffusion is explained by an illumination-induced shift in the dominant hydrogen charge state, which mitigates the metastable interactions within the silicon lattice. In contrast, the illumination has a negligible effect where the metastable interactions remain dominant (heavily-doped p-type) or where the effective diffusivity is already high (moderately-doped n-type silicon).

In summary, this thesis makes two principal contributions to the understanding of defect passivation in silicon photovoltaics. First, it employs time-resolved photoluminescence to quantify the carrier lifetimes and hydrogenation effects in key materials for high-efficiency solar cells. Second, this work develops a novel spectrally-resolved photoluminescence methodology to directly measure the effective hydrogen diffusivity, revealing its complex doping and illumination dependence. These findings provide critical, separate insights: the first into the Auger recombination limits, and the second into the hydrogen diffusion mechanisms. The established methods provide powerful tools for fundamental studies of defect kinetics in silicon photovoltaics. Show more