Textured p-type crystalline silicon surfaces obtained by multi-step plasma process for SHJ solar cells

Abstract

P-type monocrystalline silicon wafers were textured by means of a multi-step plasma etching technique with the aim of greatly reducing their reflectance and effectively exploiting the highest possible amount of solar radiation in heterojunction photovoltaic devices. Suitable surface morphologies, characterized by high light scattering, have to be obtained without damaging the c-Si electrical properties. For this reason, some aspects of the plasma etching process have been studied and improved. Modifications have been made to obtain both “cleaner” processes and energetically lower ion bombardment. By operating at high pressure (0.4 mbar), a process controlled by diffusion of reactive species towards the substrate was favoured. Another innovative aspect consisted in two “reconditioning” plasma processes of the Si surface before the actual silicon removal step with SF6 /O2 gas mixtures. A strong reduction in the average optical reflection down to 5% was obtained coupled with a minority carriers lifetime (τeff) of about 950 μs, value comparable to that of the untreated silicon. An interesting surface morphology, consisting in “U" shaped cavities, was obtained. Texture irregularities were easily removed by acid etching. The moderate depth of the cavities is very promising for an effective passivation with a-Si:H thin films.