'Just Clean Enough' Optimization of Wet Chemical Cleaning Processes for Crystalline Silicon Solar Cells
Advanced concepts for photovoltaic silicon solar cells, especially high efficiency n type solar cells, requires appropriate wet cleaning treatment in order to remove metallic contamination prior to high temperature processes like diffusion and passivation [1] The cost of the cleaning process should be as low as possible that requires an optimized usage of the chemicals by increasing process tank lifetimes and developing dedicated feed and bleed recipes. The just clean enough concept has been developed to fulfil the needs of PV industry to minimize the consumption of chemicals. When the dominant contamination metal is identified in quality and quantity, a dedicated wet chemical cleaning process can be applied to remove the metal concentration from the semiconductor surface under a specified limit with the minimum volume on cleaning solution. The paper describes how to optimize a dedicated wet cleaning process for prominent metal impurities like Fe, Cu, Cr, Ti, Co and Zn. For each metal an exchange volume is determined to develop a feed and bleed recipe. The accumulation of the metal impurities in the process tank is calculated and process tank lifetimes are predicted.