Wet chemical textures for crystalline silicon solar cells

In this work, two alternative solutions to the standard potassium hydroxide (KOH)– isopropyl alcohol (IPA)texturization process of as-cut mono-crystalline silicon (mono-Si) wafers used in photovoltaics are presented.The standard KOH-IPA etch solution suffers from two drawbacks when the texturization process is carried out at 80oC, i.e. near the boiling point of IPA (82.4oC), as it is usually carried out in the photovoltaic community.The first problem corresponds to the constant evaporation of IPA during the etching process. The quick solution here is the re-dosing of IPA, but unfortunately this solution has economic disadvantages, i.e., high costs.The second problem corresponds to the high sensitivity of the KOH-IPA etch solution to the wafer characteristics of the as-cut mono-Si wafers. In other words, it means that different pyramidal results are obtained when different assortments of ascut mono-Si wafers (e.g. from different manufacturers) are textured.In this context, the introduction of new sawing methods for silicon also changes the surface morphology of the as-cut silicon (Si) wafers. This implies a modification of the etching process in order to obtain almost the same pyramidal texture on differently sawn wafers.The solution proposed for the first problem corresponds to the usage of another alcohol. An alcohol with high boiling point in order to avoid constant evaporation of it during the etching process and to reduce etching time. The alcohol which fulfills these demands is polyvinyl alcohol (PVA) and the new solution is now called KOHPVA solution. Due to the characteristics of the new alcohol an etching temperature of 100oC was used.Although the pyramidal texture results produced on as-cut mono-Si wafers by using the new KOH-PVA etch solution are similar to those obtained by using the standard KOH-IPA etch solution, some well-defined differences were observed. For example, the pyramid size of the KOH-PVA texture (4 μm in average) is only about half of the pyramid size of the KOH-IPA texture (8 μm in average). Another important difference is that the KOH-PVA texture shows 1% lower weighted reflectance (between 400-1100 nm) compared to the reflectance of KOH-IPA textured wafers.When trying to explain this difference by means of geometrical optics and the ray tracing software SUNRAYS, no difference was found. This is due to the fact, that in geometrical optical theory it is assumed that the size of the pyramids is bigger than the wavelength impinging on it, and because SUNRYAS does not take into consideration the random nature of the wet chemically produced pyramidal texture. However, the difference in reflectance can be well explained by solving the wave equation, and by considering diffraction phenomena of pyramids (or structures) with sizes between 1-4 μm. With this method it is also possible to take into consideration the random nature of the wet chemically produced pyramidal texture.Although some optical differences are observed between the KOH-IPA and KOHPVA texture, at solar cell level both textures show similar results.The second solution proposed in this work corresponds to the introduction of a closed etching bath in order to recover evaporated IPA (from the standard KOH-IPA etch solution) during the etching process. The closed etching bath was developed by the wet etching company LOTUS. A cooling system was adapted on top of the closed etching bath in order to liquefy evaporated IPA and conduct it again to the reservoir.Furthermore, the new closed etching bath enables the periodic application of low pressure (below atmospheric pressure) inside the closed etching bath during the etching process. This accelerates the etching process, and therefore the etching time can be reduced to almost half of the time of a texturization process taking place at atmospheric pressure (by using the same standard KOH-IPA etch solution).The periodic decrease of the pressure inside the bath, containing a standard KOH-IPA solution and the as-cut mono-Si wafers at constant temperature of 80oC, implies a decrease of the boiling point of IPA. Therefore IPA evaporates periodically when low pressure is applied, thereby removing the hydrogen bubbles and monosilicic acid particles from the surface of the silicon wafers. Thus the surface can be supplied with fresh etch solution and the etching process is accelerated