Periodic Reporting for period 1 - NEXUS (NEXt generation of sUstainable perovskite-Silicon tandem cells)
Reporting period: 2022-11-01 to 2024-04-30
NEXUS aims to accelerate Europe’s transition to clean energy by developing perovskite-on-silicon tandem photovoltaics, via a new paradigm: a global eco-design approach based on efficiency, cost, sustainability and social aspects. The project’s innovations will make it possible to increase the energy yield per area and lay the foundations for a sustainable and competitive European PV production.
To do so, NEXUS is developing
·efficient perovskite (PVSK) top absorbers made with methods avoiding the use of toxic solvents, stable towards light, heat, humidity and electric fields
·efficient tandem devices (power conversion efficiency (PCE) >33%) that reduce the use of elements like silver (Ag), indium (In), and silicon (Si)
·a module Bill of Material that will ensure high performing (PCE>30%) tandem modules with long-term stability.
NEXUS will also demonstrate
·the durability of the tandem modules in outdoor environment to enable a first bankability report for this technology
·the scalability of the technology with the modeling of GW-scale production (and its impact in terms of raw material use) and with the design of proof-of-concept equipment set
·the economic viability, environmental harmlessness and social acceptance of the technology with the calculation of its lifecycle costing (LCC), environmental and social lifecycle analysis (LCA).
NEXUS is a multidisciplinary team consisting of 13 partners from 7 European countries, 2 associate member countries and 1 widening country; 5 industrial partners & 8 Research & Technology organizations.
To do so, NEXUS is developing
·efficient perovskite (PVSK) top absorbers made with methods avoiding the use of toxic solvents, stable towards light, heat, humidity and electric fields
·efficient tandem devices (power conversion efficiency (PCE) >33%) that reduce the use of elements like silver (Ag), indium (In), and silicon (Si)
·a module Bill of Material that will ensure high performing (PCE>30%) tandem modules with long-term stability.
NEXUS will also demonstrate
·the durability of the tandem modules in outdoor environment to enable a first bankability report for this technology
·the scalability of the technology with the modeling of GW-scale production (and its impact in terms of raw material use) and with the design of proof-of-concept equipment set
·the economic viability, environmental harmlessness and social acceptance of the technology with the calculation of its lifecycle costing (LCC), environmental and social lifecycle analysis (LCA).
NEXUS is a multidisciplinary team consisting of 13 partners from 7 European countries, 2 associate member countries and 1 widening country; 5 industrial partners & 8 Research & Technology organizations.
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.
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.
State-of-the-art PVSK/Si tandem solar cells typically employ solution-processed PVSK layers that use toxic solvents, thick FZ Si wafers and In-based TCO layers. In NEXUS we have demonstrated high efficiency PVSK cells of the desired band gap via vapor phase processing. We have integrated these into sustainable tandem cells by combining them with industrially relevant SHJ cells processed from Cz Si. We have demonstrated the feasibility of eliminating In in the transparent electrodes and CRL in tandem cells and of using reduced-Ag metal paste in the Si cells. Our vapor deposited PVSK top cells exhibit encouraging early stability results. In the 2nd period of the project outdoor operational stability will be compared to in-lab accelerated aging tests. This will enable the prediction of real-world operational lifetimes and contribute to the generation of the first bankability report for the PVSK/Si technology. Key needs to ensure further uptake beyond the life of the project will be identified in the 2nd period.