PLASMA AND MATERIAL CHARACTERIZATION TECHNIQUES OF MICROCRYSTALLINE SILICON FOR SOLAR CELLS: THE ROLE OF STRESS FOR HIGH QUALITY INTRINSIC MATERIAL

摘要

Hydrogenated microcrystalline silicon (μc-Si:H) growth by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) is studied in an industrial-type parallel plate KAI reactor. Both plasma and material characterization techniques allow the assessment and control of critical deposition parameters for the fabrication of high quality material. In particular the silane depletion conditions of all the plasma regimes were studied. The role of the intrinsic stress of the μc-Si:H active layer is discussed after reviewing two different series results done in low and high growth rates regimes. It is shown that stress value on its own cannot be a determinant factor for the material quality. Results highlight the difficulties to assess the material quality differences above a certain threshold when the material is actually incorporated into a solar cell device, as neither defect density through Fourier-transform photocurrent spectroscopy (FTPS), infrared spectroscopy (FTIR) nor stress measurement could discriminate between them. Although different plasma conditions could provide the growth of μc-Si:H material with very similar electrical bulk properties, some would differ and tend to favor the formation of cracks on rough substrates, becoming then the main limiting factor of the overall cell efficiency.