Heavy phosphorous tube-diffusion and non-acidic deep chemical etch-back assisted efficiency enhancement of industrial multicrystalline silicon wafer solar cells

Abstract

Improvement in emitter and bulk regions of multicrystalline silicon (multi-Si) cells by phosphorus (P) gettering is a well-known technique. Earlier researchers exploited P gettering using a combination of deep emitter formation, complete emitter etching and re-diffusion, or, the use of sacrificial dielectric layers. In this work, our approach consists of heavy P diffusion in a tube diffusion furnace, followed by chemical etch-back of the P diffused layer. The novelty of our work is three-fold. Firstly, for the first time a low-cost, non-acidic emitter etch-back process – the ‘SERIS etch’ is applied on the tube-diffused emitter. Earlier the ‘SERIS etch’ was reported only for the inline-diffused cells. Secondly, a deep etch-back (change in sheet resistance by ∼40 Ω sq⁻¹) is performed to get the advantage of P gettering on heavily diffused emitter without affecting its surface reflectance and doping uniformity. Thirdly, unlike previously reported works, our process does not required additional diffusion or dielectric deposition processes; hence it is cost-effective and industry competitive. For the screen-printed full-area aluminium back surface field multi-Si solar cells, an average cell efficiency gain of 0.5% (absolute) is observed for etched-back cells as compared to reference cells with as-diffused emitter (no etch-back). As both groups of cells are of same sheet resistance, the efficiency gain reflects the positive effect phosphorous diffusion gettering for the etch-back cells using our modified process.