Performance and Thermal Stability of an a-Si:H/TiOx/Yb Stack as an Electron-Selective Contact in Silicon Heterojunction Solar Cells

Low contact resistivity (rho(c)) and low recombination current density at the metallized area (J(0,metal)) are the key parameters for an electron-selective contact in solar cells, and an i-a-Si:H/TiOx/low work function metal (ATOM) structure could satisfy these criteria. In this work, to achieve strong downward band bending, an Yb (Phi = 2.5-2.6 eV)/Ag stack is used. Moreover, the impact of (1) substrate topography (flat or textured), (2) TiOx thickness, and (3) Ti precursor (TTIP vs TDMAT) on the ATOM contact performance is investigated. The results show that the ATOM contact is relatively insensitive to the surface topography and to the Ti precursors (TTIP or TDMAT) used for the atomic layer deposition (ALD) of TiOx. However, the TiOx thickness has a significant impact on the rho(c) and marginally on the J(0,metal) of the ATOM contact. For all topography cases and Ti precursors, 1 nm thick TiOx results in the lowest rho(c) value, most likely due to E-F,E-metal depinning. In the silicon heterojunction solar cell, this ATOM contact (i-a-Si:H/TiOx/Yb/Ag) yields a solar cell efficiency of 19.2% with a high V-oc of 723 mV without the need of a doped n-a-Si:H layer. Concerning the thermal stability of these contacts, TEM images confirm that Yb does not diffuse into the i-a-Si:H layer after an annealing at 180 degrees C for 30 min thanks to the TiOx layer behaving as a diffusion barrier. 98% of the initial solar cell efficiency is preserved even after successive annealing treatments at 150 and 175 degrees C, which are values in the same temperature range used in the module lamination and the sintering of the printed Ag. These results in combination with the demonstrated efficiency underline that the ATOM contact is a promising route to realize high-efficiency solar cells.