Chemical hardness engineering boosts perovskite tandem efficiency to 30.3%

All-perovskite tandem solar cells are promising candidates for next-generation photovoltaics, as they harvest sunlight more efficiently than single-junction devices and can be fabricated through low-temperature solution processing. However, their performance is often limited by asynchronous crystallization in multicomponent perovskite films, in which different parts of the system crystallize at different times. This leads to compositional and structural unevenness, which reduces device efficiency and stability.

Now, a research team led by Prof. Ge Ziyi and Prof. Liu Chang at the Ningbo Institute of Materials Technology and Engineering (NIMTE) of the Chinese Academy of Sciences has developed a chemical-hardness-guided strategy to control crystallization in all-perovskite tandem solar cells—achieving a certified power conversion efficiency of 30.3% in rigid devices and 28.0% in flexible tandems.

The study is published in Nature Nanotechnology.

Guiding crystallization with targeted additives

To achieve this result, the researchers developed an additive design strategy based on hard-soft acid-base (HSAB) theory. They identified difluoro(oxalato)borate (DFOB⁻) for wide-bandgap perovskites and tetrafluoroborate (BF₄^⁻) for narrow-bandgap perovskites as effective additives. These additives selectively coordinate with precursor species to synchronize nucleation and crystal growth, thereby suppressing uneven vertical phase distribution and improving film uniformity.

Structural and optical analyses further revealed that the strategy promotes homogeneous nucleation and uniform crystal growth, while preventing halide redistribution that typically causes defects and stress accumulation.

Efficiency gains and device stability

As a result, the efficiency of wide-bandgap perovskite solar cells increased from 18.5% to 20.1%, while narrow-bandgap devices improved from 21.6% to 23.3%. When integrated into monolithic two-terminal tandem architectures, the optimized rigid device achieved a peak efficiency of 30.3%, with an open-circuit voltage of 2.16 V and a fill factor of 85.2%.

The devices also demonstrated strong operational stability. For example, the optimized rigid device retained 92% of its initial efficiency after 1,000 hours of maximum power point tracking. Flexible tandems, on the other hand, reached 28.2% efficiency (certified at 28.0%) and maintained 95.2% of their initial efficiency after 10,000 bending cycles.

This study establishes a general chemical principle for regulating crystallization in compositionally complex perovskite systems. The findings provide a pathway to simultaneously improve efficiency and durability in both rigid and flexible devices, thereby advancing the development of lightweight, scalable photovoltaic technologies.

Provided by Chinese Academy of Sciences