Optimization of protocrystalline silicon p-type layers for amorphous silicon n–i–p solar cells

Real time spectroscopic ellipsometry has been applied to develop deposition phase diagrams for p-type hydrogenated silicon (Si:H) films prepared at low temperature (200 °C) by rf plasma-enhanced chemical vapor deposition using gas mixtures of SiH4, H2, and BF3. These diagrams depict the regimes of accumulated thickness and H2-dilution ratio R=[H2]/[SiH4] within which p-type amorphous Si:H [a-Si:H], mixed-phase Si:H [(a + μc)-Si:H], and single-phase microcrystalline Si:H [μc-Si:H] films are obtained in depositions on R=0 a-Si:H surfaces. The performance of n–i–p solar cells incorporating p-layers deposited under the same conditions as those used in the phase diagram development has been correlated with the deduced p-layer characteristics. This correlation demonstrates that Voc is maximized when the highest possible R value is used for the p-layer while ensuring single-phase amorphous film growth throughout its thickness.