Resistance and passivation of metal contacts using n-type amorphous Si for Si solar cells

A low fill factor remains one of the critical issues for successful implementation of amorphous Si layers in back-contact solar cells. In this work, the metal-phosphorous doped hydrogenated amorphous silicon (a-Si:H) contact is studied in terms of contact resistance while maintaining a high passivation level of the crystalline silicon bulk material after metal deposition and during long-term solar cell operation. On top of these contacting and passivation-preservation requirements, the metal back-surface reflection has to be large in order to reflect as much light as possible to generate high output current densities. Two different contact metals, Al and Ti, with Ti being combined in a stack with either Al, Pd/Ag or Cu, are investigated. For these two metals with comparable metal work function, the material choice shows only a minimal effect on the contact resistance value. The main parameters for obtaining a low-resistive, ohmic contact lie in the tuning of the n+ a-Si:H layer thickness and the application of a thermal annealing step. Contact resistance values down to 10 mΩ cm2 are obtained on an intrinsic/n+ a-Si:H layer stack with a remaining effective lifetime of several milliseconds after metallization and anneal. It is shown that a thin Ti (5 nm) layer is needed in order to obtain a thermally stable contact that guarantees a reliable long-term solar cell operation. The optical disadvantage of having Ti at the backside of a Si solar cell can be compensated by combining this very thin Ti layer with Cu. This results in an improvement of the back-reflectance compared to a direct Al contact.