Recomibiation Center Defects Induced by TCO Reactive Plasma Deposition in Carrire Selective Contact Solar Cells

Yoshio Ohshita, Tomohiko Hara, Taichi Tanaka, Keita Kimura, Yuto Ifuji Toyota Technological Institute, Nagoya, Japan

Solar cells with a carrier selective contact (CSC) structure is one of the candidates to realize the theoretical limit conversion efficiency of crystalline Si cell, but some processes of cell fabrication deteriorate the cell performance and limit the conversion efficiency. In this paper, the authors discuss the properties of defects generated by the RPD process for TCO deposition. The defects are induced  both at SiO2/Si interface and in the silicon bulk near the interface. The electrical characteristics, thermal stability, and formation mechanism of process-induced crystal defects are studied by DLTS using machine learning for spectrum analysis and high-low capacitance - voltage (C-V) method. The defects at the interface and three types of electron traps in  the bulk are induced by the process. The interface states and the defects with deep energy around 0.5eV in the Si crystal near the interface act as recombination centers. Also, the fixed charges are induced in SiO2 and/or at the interface. They change the surface potential at the inteface layer of Si, which affect the recomination velocity at the interfafce between SiO2 and Si. These fixed charge disappeared by 10min annealing at 200 °C. There are two ahihilation processes of recombination centers, which have differenent activaion energies.  At the initial stage of annealing, the density of defect swiftly decreases and then the concentration becomes lower with the reltively high activation energy. With regards to the degradation mechansim, it is suggested that the interaction between oxygen in the Si crystal and UV light during RPD process plays an important role in the defect formation.