High hole drift mobility in a-Si:H deposited at high growth rates for solar cell application

Time-of-flight measurements on hydrogenated amorphous silicon deposited with a remote expanding thermal plasma at growth rates up to 12 nm/s have revealed a 7 to 10 times larger hole mobility than for films deposited with conventional rf-PECVD. The electron mobility on the other hand is up to 3 times less. Based on a determination of the density of states by post-transit photo-current analysis we suggest a comparable defect density at mid-gap as for films deposited with rf-PECVD. These material properties have been obtained at a substrate temperature of 400°C, which is needed to obtain solar grade material at these growth rates. Possible causes of these particular material properties, which may have application in thin film solar cells, are discussed. Furthermore we show that the high substrate temperature is still a drawback in solar cell preparation when using the standard p-i-n configuration.