Anodic oxidations: Excellent process durability and surface passivation for high efficiency silicon solar cells

N.E Grant,T.C Kho,K.C Fong,E Franklin,K.R Mcintosh,M Stocks,Y Wan,Er-Chien Wang,N Zin,J.D Murphy,A Blakers

doi:10.1016/j.solmat.2019.110155

Abstract

We investigate the versatility of anodically grown silicon dioxide (SiO2) films in the context of process durability and exceptional surface passivation for high efficiency (>23%) silicon solar cell architectures. We show that a room temperature anodic oxidation can achieve a thickness of ~70 nm within ~30 min, comparable to the growth rate of a thermal oxide at 1000 °C. We demonstrate that anodic SiO2 films can mask against wet chemical silicon etching and high temperature phosphorus diffusions, thereby permitting a low thermal budget method to form patterned structures. We investigate the saturation current density J0 of anodic SiO2/silicon nitride stacks on phosphorus diffused and undiffused silicon and show that a J0 of <10 fA cm−2 can be achieved in both cases. Finally, to showcase the anodic SiO2 films on a device level, we employed the anodic SiO2/silicon nitride stack to passivate the rear surface of an interdigitated back contact solar cell, achieving an efficiency of 23.8%.

Highlights

The development of versatile dielectric coatings is of great interest for photovoltaics, especially as the industry is seeking methods to boost the efficiency of solar cells while maintaining or even reducing manufacturing costs
In this work we examine whether anodically grown silicon dioxide films can mask against phosphorus during a thermal diffusion process, which could be useful in the context of high efficiency cell designs, e.g. interdigitated back contact (IBC) solar cells, where masking is required to protect locally diffused regions
We have examined the versatility and robustness of anodically grown silicon dioxide layers for high efficiency silicon solar cells

Summary

Introduction

The development of versatile dielectric coatings is of great interest for photovoltaics, especially as the industry is seeking methods to boost the efficiency of solar cells while maintaining or even reducing manufacturing costs. The high temperatures pose a risk of permanently degrading the bulk minority carrier lifetime and the high thermal budget can be expensive for commercial solar cells. For these reasons, there has been research on developing alternative approaches of growing SiO2 that retain the same versatile qualities of thermal SiO2, but which are grown at much lower temperatures [7]. The voltage typically increases with oxide thickness, and in some cases can exceed 100 V [11] This type of oxidation can form very thick oxides (> 100 nm), but is generally quite aggressive and susceptible to pit/defect formation [10]. One could speculate that a less aggressive oxidation in this case would limit the pit/defect formation

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Conclusion