IPA-free Texturization of n-type Si Wafers: Correlation of Optical, Electronic and Morphological Surface Properties

The application of an Isopropanol(IPA)-free potassium hydroxide (KOH) solution was evaluated in order to prepare random pyramids on as-cut crystalline n-type Si wafers to reduce reflection losses of substrates for high-efficiency hetero-junction solar cells. The influence of saw damage removal and texturization processes on the resulting pyramid morphology and the corresponding interplay between optical and electronic properties are revealed. It is shown that both the depth of the saw damage etching (SDE) and the duration of the texturization etching have crucial influence on the resulting pyramid size distribution. Reflection losses can be reduced with decreasing fraction of small pyramids. By intermediate saw damage removal and texture etching times in (IPA)-free KOH solution the densities of electronic interface states were found to be strongly decreased (Dit,min < 5 10-11 cm-2eV-1), in comparison to pyramids prepared in IPA containing solutions. For the purpose of fabricating amorphous/crystalline (a-Si:H/c-Si) heterojunction solar cells the Si substrate surfaces were passivated with an intrinsic layer of amorphous silicon (a- Si:H(i)) leading to minority charge carrier lifetimes τeff of 2 to 4 ms, depending on the preceding texturization process.