An approach to device grade amorphous and microcrystalline silicon thin films fabricated at higher deposition rates

We demonstrate the recent developments in the high-rate deposition technique of thin film silicon using a novel approach of plasma enhanced chemical vapor deposition. Hydrogenated amorphous silicon has been developed for thin film solar cells in the past three decades, however the severe photodegradation of the film as well as of the devices prepared at a high deposition rate has hindered the industrialization of this material. The SiH2 structure in the film has been identified as a cause of the photodegradation, and its formation mechanism has been clarified in terms of the electron temperature of the plasma, gaseous phase reactions and surface reactions. We found that the combination of a VHF plasma, proper hydrogen dilution and higher deposition temperature improves the efficiency of solar cells after photodegradation up to 8.2%, which is the highest efficiency under high deposition rate conditions. The high deposition rate technique for microcrystalline silicon has also been developed on the basis of the knowledge that a sufficiently high amount of atomic hydrogen and the reduction in the ion bombardment are the key factors for high quality microcrystalline silicon. A novel deposition technique under high-pressure depletion conditions has been developed in combination with a VHF plasma to obtain a deposition rate as high as 5 nm/s, and further development has been achieved by means of a hollow mesh so that a lot of migrating bright spot appear on the mesh to obtain a deposition rate of 5.8 nm/s with good crystallinity and a defect density as low as 2.6×1016 cm−3, which implies a device grade material. A solar cell with an efficiency of 8.1% has been prepared at a deposition rate of 1.2 nm/s.