Periodic Reporting for period 2 - SITA (Stable Inorganic TAndem solar cell with superior device efficiency and increased durability)
Periodo di rendicontazione: 2024-03-01 al 2025-08-31
Europe’s climate and energy goals require photovoltaic technologies that deliver higher efficiency while reducing material use, system costs and environmental impact. Tandem solar cells, which combine two complementary photovoltaic technologies in a single device, are a promising route to overcome the efficiency limits of today’s single-junction solar modules. (To highlight the importance of the topic for EU)
The SITA project addressed this challenge by developing innovative two-terminal tandem solar cell concepts based on two technologies with a strong industrial and research base in Europe: Silicon Heterojunction (SHJ) bottom cells and high-bandgap Cu(In,Ga)(Se,S)2 (CIGS) top cells. Two architectures were explored. In the Circuitry 2T concept, the top and bottom sub-modules are voltage-matched and connected in parallel, while in the Bonded 2T concept the silicon bottom cell is bonded in series to the wide-bandgap CIGS top cell on glass, a solution which requires current matching.
These tandem concepts are designed to function without additional cables or power electronics, allowing simple integration into existing photovoltaic systems. Achieving high-efficiency, transparent wide-bandgap CIGS top cells was identified as a key prerequisite for reaching the project’s long-term ambition of tandem devices with efficiencies up to 30%.
The SITA project addressed this challenge by developing innovative two-terminal tandem solar cell concepts based on two technologies with a strong industrial and research base in Europe: Silicon Heterojunction (SHJ) bottom cells and high-bandgap Cu(In,Ga)(Se,S)2 (CIGS) top cells. Two architectures were explored. In the Circuitry 2T concept, the top and bottom sub-modules are voltage-matched and connected in parallel, while in the Bonded 2T concept the silicon bottom cell is bonded in series to the wide-bandgap CIGS top cell on glass, a solution which requires current matching.
These tandem concepts are designed to function without additional cables or power electronics, allowing simple integration into existing photovoltaic systems. Achieving high-efficiency, transparent wide-bandgap CIGS top cells was identified as a key prerequisite for reaching the project’s long-term ambition of tandem devices with efficiencies up to 30%.
SITA carried out coordinated research covering materials development, tandem integration, modelling and sustainability assessment.
Substantial progress was achieved in the development of high-bandgap CIGS top cells. Through optimisation of absorber composition, alloying strategies and back-contact design, a world-record active area efficiency of 16.1% at a bandgap of 1.6 eV was achieved for a sulfide-based solar cell, certified by Fraunhofer ISE (15.5% total area). This result represents a major step forward for transparent top cells suitable for tandem photovoltaics. The best selenium-based solar cell was slightly below at 15.7% active area efficiency (not yet certified).
Figure: Efficiencies of wide-bandgap solar cells obtained by SITA project partners as a function of bandgap energy. Black symbols indicate the state of the art before the start of the SITA project, blue symbols represent results achieved up to the project mid-term, and red symbols show final project results. The red star highlights performance achieved using an industry-compatible process.
Optimized transparent back contacts combining infrared transparency above 80% with low electrical resistance were developed and shown to be stable under the high-temperature conditions required for CIGS processing. Integration techniques for both Circuitry 2T and Bonded 2T tandems were demonstrated, including prototype devices delivered for outdoor testing.
While full-scale prototypes did not yet reach the targeted efficiency due mainly to scalability challenges and remaining limitations in top-cell performance, dedicated test structures and concept studies successfully demonstrated key tandem functionalities. In particular, current matching using reflected (albedo) light was experimentally confirmed and supported by electro-optical modelling. A record output power density of 24.75 mW cm⁻² was demonstrated in a concept study under realistic illumination conditions.
In parallel, life-cycle assessment (LCA) and energy-return analyses were performed to evaluate the environmental and economic performance of the tandem concepts.
Substantial progress was achieved in the development of high-bandgap CIGS top cells. Through optimisation of absorber composition, alloying strategies and back-contact design, a world-record active area efficiency of 16.1% at a bandgap of 1.6 eV was achieved for a sulfide-based solar cell, certified by Fraunhofer ISE (15.5% total area). This result represents a major step forward for transparent top cells suitable for tandem photovoltaics. The best selenium-based solar cell was slightly below at 15.7% active area efficiency (not yet certified).
Figure: Efficiencies of wide-bandgap solar cells obtained by SITA project partners as a function of bandgap energy. Black symbols indicate the state of the art before the start of the SITA project, blue symbols represent results achieved up to the project mid-term, and red symbols show final project results. The red star highlights performance achieved using an industry-compatible process.
Optimized transparent back contacts combining infrared transparency above 80% with low electrical resistance were developed and shown to be stable under the high-temperature conditions required for CIGS processing. Integration techniques for both Circuitry 2T and Bonded 2T tandems were demonstrated, including prototype devices delivered for outdoor testing.
While full-scale prototypes did not yet reach the targeted efficiency due mainly to scalability challenges and remaining limitations in top-cell performance, dedicated test structures and concept studies successfully demonstrated key tandem functionalities. In particular, current matching using reflected (albedo) light was experimentally confirmed and supported by electro-optical modelling. A record output power density of 24.75 mW cm⁻² was demonstrated in a concept study under realistic illumination conditions.
In parallel, life-cycle assessment (LCA) and energy-return analyses were performed to evaluate the environmental and economic performance of the tandem concepts.
The consortium achieved a world-record efficiency of 16% for a chalcopyrite-based wide-bandgap (>1.6 eV) solar cell combining high efficiency with the transparency required for tandem photovoltaic applications. This result represents a significant advance beyond the state of the art for wide-bandgap CIGS technologies and the rapid progress achieved toward the end of the project indicates strong potential for further improvements, particularly for sulfur-based devices, where scalability to industrially relevant areas remains the key next step.
The project also provided the first LCA of silicon–CIGS tandem solar cells, showing that the Circuitry 2T architecture offers environmental advantages over the Bonded 2T concept at similar efficiencies. Sulfur-based CIGS top cells were found to have a small climatic advantage over selenium-based alternatives, due to the lower climatic impact of sulfur as compared to selenium.
If high efficiencies can be achieved at scale, SITA-type tandem modules have the potential to reduce area-related system costs by up to 25% per installed capacity, leading to a significant decrease in the levelised cost of electricity while maintaining competitive material use.
The project also provided the first LCA of silicon–CIGS tandem solar cells, showing that the Circuitry 2T architecture offers environmental advantages over the Bonded 2T concept at similar efficiencies. Sulfur-based CIGS top cells were found to have a small climatic advantage over selenium-based alternatives, due to the lower climatic impact of sulfur as compared to selenium.
If high efficiencies can be achieved at scale, SITA-type tandem modules have the potential to reduce area-related system costs by up to 25% per installed capacity, leading to a significant decrease in the levelised cost of electricity while maintaining competitive material use.