Influence of bulk defects in p-type Czochralski silicon on reliability of high efficiency solar cells
As the energy demand and average temperature of the Earth increase, the need for renewable, alternative energy sources increases dramatically. Climate change has already made permanent changes to the Earth including coral bleaching, population extinction, and decreased water availability. Due to the alarming consequences of climate change, new energy installations should rely on renewable sources such as solar, wind, geothermal, and biomass. Utilizing the Sun, which supplies ~1000 times more than the global energy demand, photovoltaics (PV) provide a reliable, renewable energy source. However, the passivated emitter rear contact (PERC) solar cell, which dominates PV market, albeit high efficiency, suffers from light induced degradation (LID) and light and elevated temperature induced degradation (LeTID). Even though these degradation phenomena have been studied in depth, the atomic-level defect structure for both LID and LeTID remain elusive. Understanding the defects responsible for both LID and LeTID at the atomic-level, which has been done in this thesis work, can provide insights to implement industrially feasible stabilization processes and make efficiency improvements to PERC solar cells. This will ensure the stability of hundreds of gigawatts of solar to be installed worldwide over the lifetime of the panels, 25 – 50 years. First, we address LID defects. We show that under light exposure, boron-doped Cz Si transitions from paramagnetic to diamagnetic via the creation of ~1016 cm-3 shallow hole traps, in which ~1012 cm-3 are the recombination active LID defect. Additionally, we find that both boron and oxygen are required for the transition from paramagnetic to diamagnetic state to occur upon LID. Then, we address the mechanism and atomic origin of LeTID, which is expected to become the dominant degradation mechanism as the Si PV market is rapidly transitioning to gallium-doped Cz Si. Even though gallium-doped Cz Si is immune to LID, it is susceptible to LeTID. LeTID has been empirically correlated to hydrogen injection into the Si bulk but atomic-level insight of the defect responsible for LeTID has been lacking. We show for the first time, the defect responsible for LeTID is a Si dangling bond within a vacancy agglomerate and hydrogen in the vicinity. We postulate that LeTID can be suppressed with slower pulling rates of the Czochralski Si and/or an optimized contact firing process. We extend our work to solar cells to replicate our results in Chapters 2-5.
degradation, electron paramagnetic resonance, light and elevated temperature induced degradation, light induced degradation, renewable energy, silicon solar cells
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