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

Abstract

Low contact resistivity (ρ 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/TiO x /low work function metal (ATOM) structure could satisfy these criteria. In this work, to achieve strong downward band bending, an Yb (Φ = 2.5−2.6 eV)/Ag stack is used. Moreover, the impact of (1) substrate topography (flat or textured), (2) TiO x 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 TiO x . However, the TiO x thickness has a significant impact on the ρ c and marginally on the J 0,metal of the ATOM contact. For all topography cases and Ti precursors, 1 nm thick TiO x results in the lowest ρ c value, most likely due to E F,metal depinning. In the silicon heterojunction solar cell, this ATOM contact (i-a-Si:H/TiO x /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°C for 30 min thanks to the TiO x layer behaving as a diffusion barrier. 98% of the initial solar cell efficiency is preserved even after successive annealing treatments at 150 and 175°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.

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

“…However, after the silicidation annealing treatment, a promising trend was found (see Figure 6). Unlike the usual increasing trend of ρc with annealing treatment observed in MIS contacts 37,60 , the ρc of the i-a-Si:H/Yb contact was drastically reduced upon annealing for just 10 min at 150 °C by more than an order of magnitude compared to the initial value. During this anneal, presumably silicidation at the a-Si:H/Yb interface occurs, resulting in the formation of YbSix.…”

“…The J0,front of samples with the structure as shown in Figure 2 All ρc values for the electron contact were determined using the two-contact-two-terminal method in order to include all resistive components of the contact in the measured ρc 34,54 . More details on the methods used to determine ρc and J0 can be found elsewhere 34,37,38 .…”

“…For the hole contact, high work function metal oxides, especially MoOx, have been investigated intensively [16][17][18][19][20][21][22] . For the electron contact, low work function metal oxides [23][24][25][26] , metal nitrides 27,28 , rare-earth metal fluorides 29,30 and low work function metals 22,[31][32][33][34][35][36][37][38] have been studied.…”

Dopant‐Free Bifacial Silicon Solar Cells

“…[ 12,30 ] In that case, the contact resistance is potentially low to form a partial‐area heterocontact for dopant‐free bifacial silicon solar cells with a metal contact fraction between 10% and 50%. [ 31 ] In contrast to most electron transporting layers (0.5–10 nm), [ 6,9,13,20–24,27,32–34 ] a thick ZnO (60–140 nm) electron‐selective layer can additionally serve as a transparent antireflective coating (ARC), thus simplifying the fabrication process.…”

Thermal stability improvement of metal oxide-based contacts for silicon heterojunction solar cells

“…LiF) [34][35][36] or a combination of those layers with low work function metals (e.g. Ca or Yb) [37][38][39][40][41][42][43][44] have been studied. In theory, when a metal is directly in contact with c-Si or a-Si:H, a large number of surface states are created at the interface.…”