Optimisation of NaOH texturisation process of silicon wafers for heterojunction solar-cells applications

The formation of a pyramidal structure on the surface of h100i-oriented monocrystalline-silicon wafers is an effective and well known method to reduce reflection losses from the front surface of both silicon solar cells and silicon-heterojunction solar cells (SHJs). The consequence of this texturisation is an important optical gain, with a subsequent increase of the short-circuit current density (Jsc) and thus of the conversion efficiency of the devices. On the other hand, silicon-heterojunction solar cells are critically affected by the surface quality of the c-Si substrates, so the right combination of optimum texturisation- and cleaning steps previous to emitter (a-Si:H) deposition are indispensable in the fabrication process. The main goal of this work has been to perform a systematic and comprehensive analysis aimed at optimising the texturisation process based on the use of alkali solutions of NaOH with de-ionised water (DIW) and isopropyl alcohol (IPA) in different types of monocrystalline-silicon wafers for silicon-heterojunction solar-cell (a-Si:H/c-Si) applications. Three types of h100i silicon substrates have been used: polished float-zone (FZ) wafers and rough- (as-cut) and polished Czochralski (CZ) wafers. The texturisation process has been evaluated from images obtained by Scanning Electron Microscopy (SEM) and from hemispherical-reflectance spectra. Different etching times, temperatures and NaOH concentrations of the solutions as well as cleaning treatments of the wafers prior to the texturisation process have been analysed. Results show different conditions of the optimum texturisation process for each type of silicon wafers. An effective texturisation of FZ and CZ substrates has been achieved. Finally, SHJ solar cells have been obtained from FZ and CZ silicon wafers textured by the chemical processes optimised in this work. 2011 Elsevier Ltd. All rights reserved