Changes in passivation quality of PECV-deposited hydrogenated amorphous silicon layers used in solar cells due to sputter deposition of TCOs

Abstract

The decrease in passivation quality of films of a-Si:H manufactured using plasma-enhanced chemical vapor deposition (PECVD) after exposure to a sputtering process for deposition of a transparent conducting oxide (TCO), which is inevitable for the production of front heterojunction solar cells, is investigated. This so-called sputtering damage is determined in terms of minority carrier lifetimes. We show that sputtering typically causes a decrease of effective carrier lifetimes by a factor of five and that almost complete recovery is possible by short low temperature annealing. The magnitude of this decrease shows no dependence on pressure during sputtering or power applied to the sputtering plasma. However, a strong dependence on a-Si:H film thickness is observed, with thinner films showing a much higher decrease of effective lifetimes. Modeling of lifetimes using the Olibet model is performed. We conclude that a Staebler-Wronskilike defect formation caused by photons in the deep UV range or bombardment by high energy electrons is responsible for the sputter damage.