In WP1, partners investigated a range of deposition processes (co-evaporation, sequential evaporation and a hybrid vapor deposition/(clean solvent) solution conversion) and compositions for PVSK absorbers with suitable bandgap (1.67 eV) for combining with Si in tandem solar cells. Co-evaporated PVSK cells achieved PCE close to 20%, while sequentially evaporated and hybrid-processed cells reach PCE of about 18%. First stability results showed that the cells can retain 80% of their performance after 1000h of aging.
Aluminium-doped zinc oxide (AZO) was developed as In-free front transparent conductive oxide (TCO) and yielded similar performances as indium tin oxide (ITO) in devices. Finally, drift-diffusion modeling allowed better understanding of experimental data.
In WP2, the most effective levers for the eco-design of the bottom cell were identified as Si wafer thickness (energy and carbon footprint) and material supply risk (In, Ag). Since the project began, the bottom cells have been made from industry-relevant Cz Si wafers (~160 µm thick), thus dramatically reducing the amount of Si in PVSK/Si cells, typically based on thick (~250 µm) float-zone (FZ) wafers. Three Si-based bottom cell technologies are being explored: silicon heterojunction (SHJ), TOPCon2 and mixed-PERC/TOPCon. To reduce material supply risk, AZO was developed as rear TCO and carrier recombination layer (CRL); Si-based and organic tunnel junctions as In-free CRL. Metal grids based on reduced-Ag screen-printing pastes were tested.
The tandem cells made on Cz Si wafers with fully evaporated top cells reach about 21% PCE, and those with hybrid-processed top cells 25%. Both processes show good covering of the textured Si surface. As a benchmark, a tandem PCE of 30% was achieved using a solution processed top cell.
In WP3, module materials were identified with preliminary tests to verify their ability to be processed at low temperature (<150ºC). They were then tested on Si-based solar cells, to validate the processes and compatibility of the selected materials. Finally, the first functional encapsulated tandem devices were obtained.
To demonstrate the scalability of the technology, a report on the abundance of relevant materials was prepared to assess criticality in a TW-scale production scenario. At this scale, the demand-production-ratio for Ag and In would exceed 20% and the consumption of solvents used for solution-processed PVSK would become challenging. On the economic side, a GW-scale factory was modeled and its total cost-of-ownership calculated, identifying the Si bottom cell as the largest cost contributor.
In WP4, the energy yield (EY) of the technology was modeled with the open-source tool EYcalc whose capabilities were extended to include the degradation of the PVSK and Si sub-cells, enabling accurate modeling of PVSK/Si devices EY in different climates.
WP4 partners also established outdoor measurement infrastructures. Test beds for the electrical performance monitoring of NEXUS tandem devices were installed. Additionally, test benches to assess Pb leaching from PVSK/Si tandem devices in different climates were prepared.
In WP5, the LCC of the technology was assessed: it showed the possibility of achieving a levelised cost of electricity of €ct2/kWh.
To build the environmental (e-) and social (s-) LCAs, eco-design indicators were selected and a reference stack defined. The first results of the e-LCA show a high impact of cell efficiency and degradation and module lifetime. In parallel, data are being gathered from literature and NEXUS partners for the s-LCA.
Finally, to demonstrate the circularity of the technology through recycling, several processes to recover materials (Pb, Ag and In) from the devices are being studied.
