Influence of transparent conductive oxides on passivation of a-Si: H/c-Si heterojunctions as studied by atomic layer deposited Al-doped ZnO

In silicon heterojunction solar cells, the main opportunities for efficiency gain lie in improvements of the front-contact layers. Therefore, the effect of transparent conductive oxides (TCOs) on the a-Si: H passivation performance has been investigated for Al-doped zinc oxide (ZnO:Al) layers made by atomic layer deposition (ALD). It is shown that the ALD process, as opposed to sputtering, does not impair the chemical passivation. However, the field-effect passivation is reduced by the ZnO:Al. The resulting decrease in low injection-level lifetime can be tuned by changing the ZnO:Al doping level (carrier density = 7 x 10(19)-7 x 10(20) cm(-3)), which is explained by a change in the TCO workfunction. Additionally, it is shown that a similar to 10-15 nm ALD ZnO:Al layer is sufficient to mitigate damage to the a-Si: H by subsequent sputtering, which is correlated to ALD film closure at this thickness.