Silicon modules are the leading technology in the photovoltaic (PV) market, pushed by reasonable cost, great reliability, and good efficiency – allowing market parity. In fact, thanks to a considerable development, Si efficiency at lab scale (27,6%) is approaching the theoretical one (29,4%). Tandem devices are then considered a viable solution to foster the performance of cost-effective solar cells, coupling Si with another material to have two PV junctions. Hence, tandem cells are based on a top semi-transparent cell, absorbing a portion of the solar spectrum and transmitting the leftover to the silicon bottom cell. Simulations indicate ideal tandem efficiency over 40%, currently experimentally exceeding the 30% threshold.

As the absorber material of the top cell for Si-based tandem with 4 terminals, mature PV materials as III-V, CIGS and perovskites have been tested in tandem cells, but their commercial usage is hindered by different issues – including polluting and/or toxic elements, high fabrication cost, energy gap lower than ideal, limited efficiency, or reduced stability over time. Innovative semiconductors with wide band gap (close to 2 eV) and low-cost (thin film approach) are then needed for optimised tandem device on Si with 4 terminals.

Among novel materials proposed as top semiconductor, ternary nitrides Zn-IV-N2 (IV=Sn, Ge, Si) have been attracting interest because of (i) wide and tuneable band gap; (ii) large absorption coefficient; (iii) non-toxic elements; (iv) besides, they can be deposited in thin films by low-cost methods.

In the present project, we investigate the deposition of ZnSnN2 (ZTN) by DC Sputtering and High-Power Impulse Magnetron Sputtering (HiPIMS) – a pulsed technique involving high-power peaks to grow high-quality films. First, depositions from Zn and Sn targets are investigated, both in inert and reactive atmosphere, comparing DC and HiPIMS processes. Later, reactive DC co-sputtering in N2 is performed to deposit ZTN. Thin films are characterised by scanning electron microscopy (SEM) observing columnar grains, X-ray diffraction (XRD) and Raman to detect crystalline phases.

Future development will be aimed to tune the sputtering parameters for larger grains, improved crystallinity, reduced oxygen inclusion and control electrical properties of ZTN.

The Report is available on the Italian site