Assessing the role of surface electric fields on the interfacial degradation in silicon solar cells

Thesis

Enhancing the reliability and longevity of solar modules is critical for expanding solar power to a multi-terawatt scale target worldwide. A major problem in maintaining high efficiency is the recombination of the photoexcited charge carriers at different interfaces. Within industrial silicon solar cells, there are three main device architectures in actual use: passivated emitter and rear cell (PERC), tunnel oxide passivated contact solar cells (TOPCon), and silicon heterojunction solar cells (SHJ). All these architectures suffer from different instabilities of efficiencies under prolonged illumination. This work investigates the mechanisms involved in interface-related degradation in these three architectures, specifically associated with hydrogen in silicon, especially considering the effects of surface electric fields.

Through the current-voltage measurement for the PERC and TOPCon cells during bias annealing, I have shown the first bias-controlled hydrogen-induced contact resistance change in TOPCon cells. I demonstrate that the degradation occurs purely at the n-type Si / Ag interface on both cell architectures.

Through the application of the surface electric fields on TOPCon and SHJ lifetime specimens during light soaking, I show that light-induced instabilities can be varied by the polarity and strength of the surface polarisation on the dielectric layer. Here, I demonstrate that the charged hydrogen ions, which respond readily to electric fields, are responsible for these observed instabilities. In addition, three mechanistic models are proposed to explain the hydrogen dynamics in these advanced solar cell architectures with surface polarisation.

This work contributes to the body of evidence aimed at understanding hydrogen kinetics at different interfaces and providing valuable insights into the mechanisms behind these light-induced instabilities of these architectures. Besides that, applying surface electric fields via the corona charge or direct bias shows a potential method for detecting and controlling the kinetics of hydrogen in actual PV systems.

Authors: Liu, D

• DOI: 10.5287/ora-54aengbeq
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: TOPCon; hydrogenation; silicon solar cell; SHJ cell; surface-related degradation; surface polarisation; PERC
• Subjects: Interficial degradation in silicon solar cells with surface polarisation