Solution of the ZnO/p contact problem in a-Si:H solar cells

This paper addresses the problem of preparing a good p-layer contact with zinc oxide as TCO. Our approach was to deposit pin cells with different p-layer recipes on ZnO coated SnO2:F and on uncoated SnO2:F in one run, in order to obtain a direct comparison of the interface properties of the two TCO materials under the condition of as equal as possible surface morphology. The pin cells prepared on the ZnO surface exhibit a lower fill factor (FF). Our experiments demonstrate that the hydrogen interaction with the ZnO surface plays the most decisive role for the ZnO/p contact. We explain the observed effects using a band diagram of the ZnO/p interface and show that the accumulation layer at the ZnO surface - caused by atomic hydrogen in the plasma - is responsible for the low FF in pin cells. Based on this model the contact problem is solved by introducing a mu c-n-Si intralayer between ZnO and p-layer resulting in an identical high FF on both ZnO and SnO2 substrates.