Experimental evidence of direct contact formation for the current transport in silver thick film metallized silicon emitters

Abstract

Great advances have been achieved in the development of silver pastes. The use of smaller silver particles, higher silver content, and, thus, less glass frit allow modern silver pastes to contact high resistive emitters without the necessity of a selective emitter or subsequent plating. To identify the microscopic key reasons behind the improvement of silver paste, it is essential to understand the current transport mechanism from the silicon emitter into the bulk of the silver finger. Two current transport theories predominate: i) The current flows through the Ag crystallites grown into the Si emitter, which are separated by a thin glass layer or possibly in direct contact with the silver finger.ii) The current is transported by means of multistep tunneling into the silver finger across nano-Ag colloids in the glass layer, which are formed at optimal firing conditions; the formation of Ag crystallites into the Si surface is synonymous with over-firing. In this study, we contact Si solar cell emitters with different silver pastes on textured and flat silicon surfaces. A sequential selective silver-glass etching process is employed to expose and isolate the different contact components for current transport. The surface configurations after the etching sequences are observed with scanning electron microscopy. Liquid conductive silver is then applied to each sample and the contact resistivity is measured to determine the dominant microscopic conduction path system. We observe glass-free emitter areas at the tops of the pyramidal-textured Si that lead to the formation of direct contacts between the Ag crystallites grown into the Si emitter and the bulk of the silver finger. We present experimental evidence that the major current flow into the silver finger is through these direct contacts.

Influence of random pyramid surface texture on silver screen-printed contact formation for monocrystalline silicon wafer solar cells

“…3). This has also been observed in other studies [22]. Now consider, hypothetically, two adjacent pyramids of comparable height.…”

“…Cabrera et al explained this observation by suggesting that direct Ag crystallite contacts to the bulk Ag at pyramid tips are primarily responsible for current conduction to the n þ Si surface and discussing the glass coverage of pyramid tips [20,22]. However, Cabrera et al [20] observed almost no influence of pyramid height on FF for pyramids larger than 300 nm, whereas Han et al [21] observed a variation in FF without a clear trend.…”

“…This implies that the Ag crystallites covered by the interfacial glass layer will not be affected by this step. Conversely, if there are some Ag crystallites not covered by interfacial glass (Ag crystallites in direct contact to bulk Ag metallisation) then these will also be removed by this step, leaving behind characteristic pits in the Si surface [22]. Several of the HNO 3 etched samples were further etched in 5% hydrofluoric acid (HF) at room temperature, to additionally remove the interfacial glass layer and thus more clearly reveal the Ag crystallites grown into the n þ Si surface.…”

Electrical properties of the rear contact structure of MWT silicon solar cells

“…For screen-printed and fired front silver contacts commonly applied for p-type silicon solar cells, the actual mechanisms of contact formation to the highly doped n-type emitter and of current transport are still being discussed [16][17][18]. Tunneling through a thin dielectric layer is assumed to be one of the major current transport mechanisms [14,16,19,20] leading to ohmic contact properties [21].…”

2D-modelling of Metal-Si Emitter Interface Assuming Schottky or Ohmic Contact

“…Presently two main current conduction mechanism models have been proposed in literature: an "Ag-crystallites" model [4][5][6][7] and "nano-Ag colloids assisted tunneling" model [8]. In the former model, after the firing process, few Ag crystallites are grown into the Si emitters.…”