Matthew Benjamin Boreland

Electrical and Microstructural Analysis of Contact Formation on Lightly Doped Phosphorus Emitters Using Thick-Film Ag Screen Printing Pastes

Screen printing of the metallization of phosphorus diffused emitters is a well-established process for industrial silicon wafer-based solar cells. Previously, screen printed silver pastes typically required a very high phosphorus surface doping concentration to ensure a low-resistance ohmic contact. Recently, paste manufacturers have focused on the development of silver pastes capable of contacting phosphorus emitters with progressively lower surface concentrations, to minimize surface recombination losses and enable higher cell conversion efficiencies. In this paper, we report on the progress of contacting inline-diffused phosphorus emitters, of which the surface concentrations have been reduced by an etch-back process, using two different pastes. Solar cells with emitter surface concentrations ranging from 4.0 × 1020 to 1.7 × 1020 phosphorus atoms/cm 3 were made using two different silver pastes. We present a microstructural analysis of the contact formation, which indicates the possible dominant current transport mechanisms for the two pastes. A high density of silver crystallites formed with a very narrow interfacial glass layer makes the Sol 9600 paste suitable for contacting lowly doped phosphorus emitters. Efficiency gains of 0.2%-0.3% (absolute) were achieved, reaching a maximum efficiency of 18.6% on 156 mm × 156 mm p-type pseudo-square Cz mono-crystalline silicon solar cells.

Single-Component Damage-Etch Process for Improved Texturization of Monocrystalline Silicon Wafer Solar Cells

A new saw damage-etch process based on a hot sodium hypochlorite (NaOCl) solution is reported here. This process performs simultaneous damage removal and oxide masking of raw c-Si wafers in a single step. NaOCl is a strong oxidizing agent, and during the NaOCl damage-etch process, the oxide grown remains present even after the completion of the process. This oxide layer acts as protective mask during alkaline texture to form uniform and small (~2-4 μm height) pyramids on the 〈1 0 0〉 Si wafer surface. Unlike chemical vapor deposited silicon nitride or silicon dioxide protective masking processes reported by other researchers, this new damage-etch process is cost effective. It is also a single-component damage-etch process using only NaOCl solution. Thus, it involves easy bath preparation and performs in situ chlorine cleaning. Using the new damage-etch process, optimized texturing of the wafers is ascertained by electron microscopy and reflectivity studies of the textured surfaces. This new process is applied in the industrial R&D pilot line of the Solar Energy Research Institute of Singapore (SERIS) to fabricate screen-printed 156-mm pseudosquare p-type solar cells with tube-diffused emitters to yield efficiencies of over 18%.

Excellent Passivation of p+ Silicon Surfaces by Inline Plasma Enhanced Chemical Vapor Deposited SiOx/AlOx Stacks

Excellent surface passivation of boron emitters is demonstrated for industrial plasma-enhanced chemical vapor deposited (PECVD) SiOx/AlOx stacks. Emitter saturation current densities of 39 and 34 fA/cm2, respectively, were achieved at 300 K on 80 Ω/sq boron emitters after activation by (i) a standard industrial firing process and (ii) a forming gas anneal followed by industrial firing. We find that the surface passivation by SiOx/AlOx stack can be effectively controlled by varying the SiOx layer thickness. This stack is directly applicable to certain high-efficiency solar cell structures, by optimising the SiOx thickness accordingly.