The ability to control the chemical concentration of etching baths is a critical step in being able to achieve uniform and repeatable texturization of solar cells. It also maximizes the chemical usage by being able to extend the usable bath life of the process. This in turn, cuts down on waste and reduces the overall cost of ownership (COO). The use of conductivity as a means of control has long been in use for oxide etching baths. The ability to extend that technology to other chemicals has been explored. This paper demonstrates the use of pH and conductivity sensors to monitor and control the concentration of KOH solutions. The effects of various additives as well as silicate buildup on pH and conductivity have been investigated and the results showed that the bath life can be extended and stable processes obtained.
Global demand for photovoltaic energy has reached more than 30GW and is still growing. This rapid growth is primarily due to the radical reduction in the price of various components of the value chain, including manufacturing steps [1-5]. In order to obtain stable and reproducible manufacturing processes, a reliable and accurate real-time measurement of the etching constituents becomes necessary [1-3]. The chemical mixtures of interest in the photovoltaic industry include KOH/IPA and/or KOH/IPA-free additives. While standard analytical techniques (e.g. NIR, UV spectroscopy) are sometimes used for concentration monitoring, conductivity cells and pH electrodes can provide fast and very cost-effective real-time control for such baths.
The industry has routinely used conductivity as a concentration control method for such chemicals as HF and HCl. Any mixture that can transmit electricity or heat is considered conductive. Therefore, a mixture such as an acid or base, which dissociates in solution will conduct electricity due to the dissolved ions in solution. The degree of dissociation as well as the bath concentration determines the overall conductivity of the mixture. Strong acids and bases dissociate completely and, therefore, have higher conductivities than weak acids and bases that only partially dissociate in water.
The etchants used in the photovoltaic industry (e.g. KOH, NaOH) dissociate in water, therefore, the conductance of these solutions can be measured and monitored during the etching process. Figure 1 shows the conductivity of several acids and bases versus concentration. It can be seen that the conductivity increases with increasing concentration until it reaches a maximum.
Fig1
When a solution becomes sufficiently concentrated, the degree of dissociation slows down. Any further increase in concentration results in the interaction of the ions in solution, which causes a decrease in the conductivity. It should be noted, however, that the concentrations used in the photovoltaic industry do not reach this maximum, therefore, conductivity can be used to detect chemical concentrations in a fairly accurate manner.
The dissociation of acids and bases also lends itself to measuring the pH of a solution. By considering the hydrogen ion (H+ ) concentration, the pH of an aqueous solution can be measured. This is typically accomplished using standard pH probes and meters. For basic solutions (pH >7), the pH increases as the concentration increases due to a decrease of H+ ions in solution. This is demonstrated in Figure 2 for various common bases.
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