(Invited) Electrochemistry for Sustainable Solar Photovoltaics

Abstract

Commercial solar photovoltaic technologies such as Si and CdTe are traditionally considered to be in a separate domain from electrochemistry. Their device operation is governed by semiconductor physics and their production involves non-electrochemical processes such as diffusion, screen printing, fractional distillation, etc. However, electrochemistry will likely play an indispensable role in sustaining the commercial solar technologies. This talk will discuss three roadblocks to sustainable solar photovoltaics and how electrochemistry can remove these roadblocks: 1) storage of intermittent solar electricity, 2) scarce Ag used in today’s Si solar cells, and 3) high energy input in producing solar Si wafers. An off-grid route is proposed for solar electricity storage based on a closed loop of Zn↔ZnO [1], in which Zn rods are produced in a solar electrolyzer from ZnO. The Zn rods are shipped to homes, offices, factories, and electric vehicles to be inserted into mechanically-recharged Zn/air batteries, for electricity on demand. The spent Zn anodes are collected for regeneration of Zn in the solar electrolyzer. This Zn↔ZnO loop is advantageous over the H2↔H2O loop in terms of theoretical performance and technical readiness. Electroplated Al on Si is proposed to replace the screen-printed Ag electrode in Si solar cells [2]. 18% efficiency has been demonstrated in a Si solar cell with an electroplated Al front electrode and a screen-printed Al back electrode, i.e., an Ag-free all-Al Si solar cell. To overcome the high resistivity of the solar Si wafer, direct Al plating on Si without any seed layer is developed through light-induced Al plating. Direct Al plating on Si drastically simplifies the metallization process for Al, resulting in a significantly-lower cost than competing technologies. Finally, electrochemical refining of metallurgical-grade Si for solar-grade Si has been unsuccessful. While metals up to 99.99% purity are readily produced by electrolysis, solar-grade Si requires a purity of at least 99.99999%. This is coupled with non-electrochemical difficulties in Si purification such as oxidation of the Si anode and crystallinity of the Si cathode. An analysis will be presented on ultrapure materials by electrolysis [3]. The reason for unsuccessful Si electrochemical refining will be discussed through an analogy between electrochemical refining and fractional distillation. A revised method for electrochemical refining of Si is proposed. [1] M. Tao, A Zn↔ZnO loop for terawatt-scale storage of solar electricity, 43rd IEEE Photovoltaic Specialist Conference (Portland, OR, 2016), p. 2011[2] 1. L. Wang, W.-H. Huang, W. J. Shin, M. Tao, B. Deng, and D. Wang, Light-induced plating of aluminum on silicon in a Lewis acidic chronoaluminate ionic liquid, Journal of the Electrochemical Society, 165, D381 (2018)[3] M. Tao, Impurity segregation in electrochemical processes and its application to electrorefining of ultrapure silicon, Electrochimica Acta, 89, 688 (2013)