Degradation Mechanisms in Silicon Solar Cell Materials

Student thesis: Phd

Abstract

The anticipated standing of solar photovoltaic (PV) technology based on crystalline silicon (Si) in the future green energy mix, drives the need for further device and material optimization in order to push towards the fundamental efficiency limits. This thesis is concerned with the use of electrical and optical characterization techniques to improve the understanding of defects responsible for the degradation of minority carrier lifetime as well as of fundamental properties of dopant atoms in Si. The motivating aspects behind the use of PV, a wider background revolving around Si, defects in semiconductors and the degradation mechanisms as well as a detailed explanation of the methods used to conduct this work are discussed in the first three chapters. Studies of oxygen-related defects in Si show that Ga-doped Si is not susceptible to light induced degradation (LID), explained by the different electronic properties of GaO2 and BO2 defects. Further, an examination of the hydrogen interaction with the BO2 defect led to the explanation of the regeneration mechanism of LID. The investigation ends with an identification of the possible route by which oxygen-related thermal double donors (TDDs) can degrade the efficiency of n-type Si solar cells. The focus is then turned towards hydrogen and its role in the formation of detrimental defect centres. Here, a comprehensive set of results covering hydrogen interactions with dopant atoms and the most abundant impurities in Si is presented. A particular consideration of the direct reaction of hydrogen dimers with B and Ga atoms is given to establish a connection with light and elevated temperature induced degradation (LeTID). The last set of results in this thesis relies on the analysis of excitonic emissions in Si. The ionization energy of Ga and P atoms in compensated Si samples are extracted from the study of donor-acceptor pair (DAP) luminescence. The impurity-related luminescence peaks from different impurities are described and compared. Also, a method for the determination of Ga concentration in Si from low-temperature PL measurements is presented. The main outcomes of the thesis are then summarized in the final chapter, and a discussion of the potential for further work stimulated by this study is presented.
Date of Award1 Aug 2024
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorMatthew Halsall (Supervisor) & Iain Crowe (Supervisor)

Keywords

  • DLTS
  • Gallium
  • Hydrogen
  • LID
  • Photoluminescence
  • LeTID
  • Degradation
  • Solar cells
  • Silicon

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