Exploiting extrinsic passivation on thin film dielectrics for high efficiency solar cells

The development of high efficiency solar cells is critical for the expansion of solar power capacity across the world. A major limitation to achieving high efficiency is the recombination of electrons and holes at the silicon surface. A common method to reduce recombination is to deposit a dielectric thin film, such as SiO2 or SiNx, upon the silicon surface. This serves to chemically passivate the surface, while the dielectric’s intrinsic charge provides a surface electric field to control the charge carrier population. The passivation performance of such dielectrics can be further improved by extrinsic methods that modify the film properties after deposition. This thesis explores how the intrinsic properties of dielectrics can be supplemented by extrinsic methods to provide enhanced surface passivation.
Dielectric thin films are a fundamental part of solid-state devices providing the means for advanced structures and enhanced operation. Ion-charged dielectrics are a particular kind of thin film in which ions are embedded to create a static electric field. Such charge can add functionality and improve the performance of electronic devices. To date, the electric field has been primarily demonstrated using embedded potassium and sodium cations. While the field effect provided by such ions have shown promising results in surface passivation, it is possible that alternative ions can provide greater long-term durability while enabling fine tailoring of the electric fields. Alongside potassium ions, this work demonstrates the migration kinetics, passivation performance, and stability of two new alkali ions, rubidium and caesium, inside of a SiO2 thin film. A comprehensive model of ion injection and transport has been developed, and a detailed investigation of the kinetics of potassium, rubidium and caesium ions is presented. It is shown that the concentration of charged ions within the film can be tuned by controlling the embedding process, leading to charge densities between 0.1-10 x 1012 q cm-2. Through the use of ion-charged SiO2, this thesis demonstrates that the effective surface recombination velocities can be reduced from ∼100 cm s-1 to as a low as 2.23 cm s-1 on 1 Ω cm n-type FZ silicon. The role of dielectric charge in producing high efficiency solar cells is demonstrated through Sentaurus TCAD modelling of PERC, IBC, TOPCon cell architectures. It is shown that by optimising the charge concentration within the dielectric, the recombination activity at defect-heavy interfaces can be successfully mitigated by field effect passivation. The exploitation of dielectric charge is predicted to achieve efficiencies >24% in PERC and TOPCon cells, and >25 % in IBC cells.
Additionally, this thesis demonstrates that surface electric fields can influence the chemical passivation provided by a SiO2 + SiNx dielectric stack. It is shown that an electric field present within a dielectric not only modifies the charge carrier concentrations at the silicon surface, but also induces a chemical change in the interface properties upon annealing. By tailoring the surface electric field in the dielectric stack prior to annealing, it is shown that the capture rates at the Si-SiO2 interface can be modified depending on the field polarity and magnitude. Understanding the effect of surface electric fields on chemical passivation can lead to novel and unexplored methods of surface passivation.

Keywords: high efficiency solar cells
silicon surface passivation
silicon solar cells
dielectric thin films
Advisor: Bonilla, RS
Wilshaw, P