Processing of thin film silicon solar cells under ionic bombardment control

We present a novel processing technique using VHF glow-discharge plasma under high-gaseous pressure together with silane depletion conditions for fabricating thin film micro crystalline silicon (muc-Si) at high growth rate over 30 Angstrom/s. Ionic bombardment which deteriorates the structural and electrical quality of (muc-Si) at high growth rates can be reduced by increasing the pressure and excitation frequency, and atomic hydrogen density which is required for crystal formation can be increased under depletion conditions. We demonstrate the advantage of this novel method, high-pressure depletion (HPD) method in the respects of structural properties measured by Raman, XRD and TEM as well as electrical properties such as photo- and dark-conductivity and defect density. The disadvantage of this method, on the other hand, is dust formation due to high deposition pressure. We have attempted to overcome this dust formation by using a very narrow electrode gap, pulsed discharge and modification of gas flow. A tentative understanding of the mechanism of this dust formation is discussed. Finally, we demonstrate a successful application of the HPD method to muc-Si based solar cells with an efficiency over 8.5% fabricated at temperatures lower than 180 degreesC.