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 sola r cells. Besides the use as an anti-reflection coating, the porous layer al so acts as a light-diffuser. However, major drawbacks are the light absorpt ion within the porous layer and both insufficient as well as unstable surfa ce passivating capabilities. This work deals with a comparative analysis of different PS treatments with the aim of maintaining the light diffusing pr operty while both the absorption losses are reduced and surface passivation is improved and stabilized. In order to obtain a surface layer with a cont rolled 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 th ese different treatments are studied simultaneously on the level of the por ous material as well as on solar cell structures (IQE-analysis). A solar ce ll process is applied which provides an identical emitter for all condition s allowing an analysis of the blue response and an assessment of the most s uited 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 obse rved in the case of nitridation or anodic oxidation. The modified porous ma terial preserves its light diffusing properties and suffers less from light absorption. The conclusions are drawn up as a strength-weakness analysis f or each of the investigated treatments. This balance is not in favour of ap plying any of the PS modification techniques since in all three cases impor tant drawbacks are the presence of an additional process step as well as th e fact that the refractive index decreases which is unfavourable from the v iewpoint of ARC-properties. (C) 1999 Elsevier Science B.V. All rights reser ved.