Low-thermal-budget treatments of porous silicon surface layers on crystalline Si solar cells: A way to go for improved surface passivation?

Porous silicon (PS) has several potential interests for crystalline Si solar cells. Besides the use as an anti-reflection coating, the porous layer also acts as a light-diffusor. However, major drawbacks are the light absorption within the porous layer and both insufficient as well as unstable surface passivating capabilities. This work deals with a comparative analysis of different PS treatments with the aim of maintaining the light diffusing property while both the absorption losses are reduced and surface passivation is improved and stabilized. In order to obtain a surface layer with a controlled and stable structure and composition, rapid thermal oxidation (RTO), plasma-nitridation and anodic oxidation have been selected as potentially interesting pathways with a low thermal budget in common. The effects of these different treatments are studied simultaneously on the level of the porous material as well as on solar cell structures (IQE-analysis). A solar cell process is applied which provides an identical emitter for all conditions allowing an analysis of the blue response and an assessment of the most suited porous Si treatment. An improvement of the blue response is observed for RTO treatments at high temperatures, which is due to the creation of an intermediate oxide at the PS/Si interface. No passivation effects are observed in the case of nitridation or anodic oxidation. The modified porous material preserves its light diffusing properties and suffers less from light absorption. The conclusions are drawn up as a strength-weakness analysis for each of the investigated treatments. This balance is not in favour of applying any of the PS modification techniques since in all three cases important drawbacks are the presence of an additional process step as well as the fact that the refractive index decreases which is unfavourable from the viewpoint of ARC-properties.