Electron selective contacts based on Al2O3/TiO2/ZnO stacks for crystalline silicon solar cells

In the last years, our research group has focused on the development of Interdigitated Back Contacted (IBC) solar cells based on transition metal oxides. In particular, a very thin Al2O3/TiO2 stack deposited by ALD has demonstrated excellent results. The point of this work is to try to develop a transparent conductive oxide that could be inserted in between the passivating layers and the metal contact in order to improve rear reflector properties. As a good candidate, we focus on Zinc Oxide (ZnO) deposited from DEZ (Ditehylzinc) and water as gas precursors. A deposition rate of 1.76 A/cycle is found for layers deposited at 125 oC indicating a self-limited reaction, i.e. atomic layer deposition process. This film has an optical band gap of 3.26 eV demonstrating good optical properties to work as a back reflector. Unfortunately, contact quality with these layers is very poor. In order to investigate the origin of such a poor performance, we measure ZnO workfunction by Scanning Kelvin Probe resulting in 4.85 eV. This value suggests that the developed ZnO films could work better in a hole selective contact. To demonstrate this point, contacts on MoOx layers were fabricated leading to reasonable contact resistance values of 43 mohmscm. Passivation properties of these contacts will be addressed in the future in order to consider their introduction into the final IBC solar cell.