Cause of the fill factor loss of a-Si:H p-i-n devices with ZnO:Al front electrode: Blocking contact vs. Defect density

The true cause of the low fill factor (FF) observed in amorphous silicon (a-Si:H) p-i-n solar cells deposited onto ZnO:Al (AZO) remains unclear. Some authors explain this phenomenon as a consequence of an AZO/p-a-SiC:H blocking contact, while others sustain that the growth of a more defective a-Si:H p-i interface is actually the cause. This work tries to find out evidences that support either of these hypotheses by analyzing a series of a-Si:H devices prepared on Asahi-U™ type glass/SnO₂:F (FTO) coated with different AZO thickness. A detailed study of the diode response at forward bias showed the existence of a light-dependent AZO/p barrier contact, which provided an answer to the sudden worsening of the FF in the transition from bare FTO to AZO coated FTO. On the other hand, the AZO thickness dependence of the device collection voltage indicated a progressive increase of the defect density in the a-Si:H structure, which led to a reduction of the open circuit voltage. These results suggested that both mechanisms could simultaneously affect the device FF. In fact, we propose that both approaches could be valid and they do not have to exclude each other. The uncertainty in the control of AZO/p interface properties could be responsible of the apparent controversy.