Low Work Function Ytterbium Silicide Contact for Doping-Free Silicon Solar Cells

Abstract

Metal silicide is a well-known material for contact layers; however, it has not been tested in the context of doping-free carrier selective contacts. Thin film deposition of an appropriate metal with mild annealing treatment is an interesting alternative to the more complex depositions of other compound materials. Reaction of Yb deposited on top an i-a-Si:H passivation layer results in the formation of YbSix on top of a remnant i-a-Si:H, following a low-temperature annealing below 200 degrees C. Such a contact is an interesting candidate as a doping-free electron-selective contact. Detailed investigation of the i-a-Si/YbSix contact shows that Yb thickness, i-a-Si:H thickness and silicidation annealing conditions play a significant role in determining the recombination current density (J(0,metal)) and the contact resistivity (rho(c)). Low J(0,metal) of 5 fA/cm(2) and low rho(c) below 0.1 Omega.cm were independently demonstrated for such i-a-Si:H/YbSix contacts. We also demonstrate that lowtemperature silicidation can be combined with contact sintering (160 degrees C/25 min) or module lamination (160 degrees C/20 min), which are potential pathways for process simplification. Combining the optimized i-a-SEFI/YbSix electron contact with MoOx based hole contact in the MolYSili doping-free cell (i-a-SEH/MoOx + i-a-SEFI/YbSix), we achieved 16.7% in average efficiency and 17.0% for the champion cell. Furthermore, the YbSc contact stability was evaluated at module level and excellent thermal stability of the MolYSili laminate was demonstrated using the damp-heat test method (humidity 85%, 85 degrees C, 1000 h), where the laminated MolYSili cell did not show any degradation in the cell efficiency. This is the first proof-of-concept demonstration of a stable silicide-based contact for low-temperature processed doping-free solar cells.