Poly Silicon on Oxide Contact Silicon Solar Cells

Abstract

Silicon photovoltaic (PV) is a promising solution for energy shortage and environmental pollution. We are experiencing an era when PV is exponentially increasing. Global cumulative installation had reached 380 GW in 2017. Among which, silicon-based PV productions share more than 90% market. Performance of the first two-generation commercial popular silicon solar cells - Al-BSF and PERC - are limited by metal/Si contacts, where interface defects significantly reduce the open-circuit voltage. In this context, full-area passivation concepts are proposed for c-Si solar cells, with expectation to enhance the efficiency via reducing carrier recombination loss at the contact regions. In this thesis, poly silicon on oxide (POLO) passivating contact is developed for high efficiency c-Si solar cells. We unveiled the working mechanisms of POLO cells and then optimized the device performance based on our conclusion. We use boiling nitric acid to oxidize c-Si surface, which is of significance to determine the POLO working mechanisms. Phosphorus and boron doped silicon films are deposited by plasma enhanced vapor deposition (PECVD) or low-pressure vapor deposition (LPCVD) followed by high temperature (>800°C) annealing. SiOx structural evolution process under different annealing temperature was observed and the corresponding effects on passivation have been elucidated. The carrier transport mechanisms in the POLO contact annealed at high temperature, e.g. 800°C  900°C, were explored. We unveil that carrier transport in POLO structure is a combination of tunneling and pinhole transport, but dominant at varied temperature regions. Phosphorus-doped n-type POLO contact is optimized by several parameters, such as doping concentration, film thickness, annealing temperature, film deposition temperature, film relaxation time during annealing process, etc. We successfully obtained minority carrier lifetime over 10ms and contact resistivity lower than 30 mΩ·cm2. Boron-doped p-type POLO contact is also optimized by changing the doping concentration and annealing temperature. Finally, further hydrogen passivation is applied to enhance p-type POLO contact passivation, achieving an iVoc>690 mV, J0 <5 fA/cm2 and contact resistivity 1.3 mΩ·cm2. With the optimized n-type and p-type POLO contacts, an efficiency over 18% is achieved on n-type c-Si solar cells with a flat front surface.