Elucidating the Mechanics Behind Light- and Elevated Temperature-Induced Degradation in Silicon Solar Cells

The adoption of passivated-emitter and rear cell (PERC) architectures by the photovoltaic industry in recent years has drastically enhanced the conversion efficiency of commercially produced solar cells. However, rapid technological transitions often lead to novel complications, and for solar cells, these complications commonly take the form of stability and reliability issues. Lately, a new degradation phenomena coined light- and elevated temperature-induced degradation (LeTID) has been identified as a major limiting factor for cell performance and subsequent energy yield, yet its properties remain unexplored. This thesis aims to advance the current understanding of LeTID in silicon and unveil its mechanics and unique behaviors in an effort to identify the root cause.\\ This thesis begins with a thorough examination of the ever-evolving literature, focusing on the known behaviors, electrical properties, characterization techniques and mitigation strategies of LeTID. An investigation into the universal nature of LeTID in silicon is carried out, with results establishing that such defect is present not only in p-type multicrystalline silicon wafers but also in higher-quality monocrystalline silicon. In addition, further analysis unveiled that: defect pre-formation occurs during typical solar cell contact-firing processes; defects can also be formed via annealing in the absence of illumination; and that previously observed deterioration of high-quality float-zoned wafers may also have had identical origins.\\ The techniques established for identifying LeTID are further adapted to n-type silicon wafers, not only demonstrating for the first time, the susceptibility of n-type silicon materials to such degradation, but also revealing the influence of diffused layers near the surface on degradation dynamics. The possibility that hydrogen is responsible for LeTID is then discussed in the context of widely emerging evidence. Moreover, an observation of surface-related degradation with prolonged annealing is evaluated, with analysis indicating a formation of hydrogen-induced defects as the result of hydrogen migration towards the surface.\\ The fractional occupation of different hydrogen charge states in both the silicon bulk and in the vicinity of surface diffused layers were subsequently modeled. It was concluded that the coulombic interactions between charged hydrogen species and dopants can either accelerate or impede hydrogen motion and incidentally, the LeTID degradation or recovery rates. To finalize this thesis, a generalized model of LeTID and hydrogen in silicon is presented, illustrating the various theories and conclusions drawn throughout the literature and within this work