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.
