*Important notice: This news reports on an unedited version of an accepted paper and is awaiting final editing. Therefore, the paper should not be regarded as conclusive or treated as established information.
By pairing a kesterite bottom cell with a perovskite top cell, researchers have demonstrated a solution-processed tandem architecture that could help expand the efficiency of flexible thin-film solar technologies to above 20%. The study was published in Communications Materials.
Study: Demonstration of overcoming 20% efficiency in kesterite/perovskite tandem solar cells on rigid and flexible substrates. Image Credit: ABCDstock/Shutterstock.com
Kesterite-Perovskite Tandem Potential
Emerging thin-film photovoltaic materials offer promising avenues for advancing sustainable, adaptable solar energy technologies. Among these materials, kesterite compounds, particularly Cu2ZnSn(S,Se)4 (CZTSSe), stand out for their composition of earth-abundant, non-toxic elements, along with favorable optoelectronic properties such as a tunable bandgap, a high absorption coefficient, and mechanical flexibility.
Despite these advantages, the power conversion efficiencies (PCEs) of kesterite-based devices have lagged behind those of more established solar materials, primarily due to complex defect chemistries and less-optimized fabrication methods.
Combining kesterite as a bottom cell with higher bandgap perovskite top cells in tandem architectures can potentially unlock higher efficiencies by better utilizing the solar spectrum while preserving flexibility.
Perovskite solar cells have demonstrated rapid improvements in efficiency through solution processing techniques and have recently been coupled with inorganic thin films, including silicon and CIGS, achieving efficiencies exceeding 34% in rigid devices.
Solution-Based Device Fabrication
The research implemented a solution-processed fabrication route to synthesize high-efficiency kesterite bottom cells on both rigid Mo-coated soda-lime glass (SLG) and flexible molybdenum (Mo) foil substrates.
The flexible substrate preparation involved mechanically flattening Mo foils and depositing a MoO3 interlayer to regulate interactions during chalcogenization. To improve absorber morphology and grain size, sodium doping was introduced via a Na-containing precursor, and silver was alloyed into the kesterite lattice, which has been shown to aid in passivating defects and promoting crystallinity.
Multiple characterization techniques, including scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, current–voltage (J–V) analysis, and optical absorption/transmission measurements, were employed to correlate film properties to photovoltaic performance. The bottom cells incorporated Cu2-xAgxZnSn(S,Se)4 (Ag-alloyed and Na-doped or “Na-ACZTSSe”) absorbers optimized for bandgap tuning around 1.1 eV, suitable for tandem configurations.
For the perovskite top cells, solution-processed Cs0.17FA0.83Pb(I0.90Br0.10)3 thin films with a bandgap of approximately 1.63 eV were used. The cells were fabricated on both rigid glass/ITO and flexible PET/ITO substrates, incorporating charge transport layers tailored to optimize interfaces and charge extraction for each composition.
The tandem devices used a 4-terminal (4T) configuration, in which the top and bottom cells operate independently with optical and electrical isolation, yet are stacked to maximize solar spectrum utilization.
Performance metrics were extracted under standard illumination conditions, and external quantum efficiency (EQE) measurements provided insight into spectral response and filtering effects. Device stability under mechanical bending was tested on flexible substrates by assessing the retention of photovoltaic parameters across multiple bending cycles to evaluate durability for practical use cases.
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Tandem Efficiency Optimization
Sodium doping and silver alloying effectively enhanced the crystallinity and morphology of the kesterite absorber layers on both rigid and flexible substrates. SEM and XRD analyses confirmed well-defined kesterite phase formation without secondary phases or significant defects. This demonstrated sharp Raman peaks indicative of high material quality. The Na-ACZTSSe films exhibited improved grain size and uniformity, contributing to higher carrier collection and reducing detrimental recombination pathways.
Photovoltaic devices fabricated on Mo-coated SLG achieved efficiencies exceeding 16%, whereas devices on Mo foil achieved over 12% efficiency, representing state-of-the-art performance for solution-processed flexible kesterite cells. The performance of kesterite bottom cells was preserved when integrated into tandem architectures, although some efficiency loss occurred due to optical filtering by the perovskite top cells.
Perovskite top cells exhibited high open-circuit voltages (~1.04 V) and fill factors above 78%, resulting in efficiencies around 18% on flexible substrates. The spectral complementarity between the kesterite bottom cell and the perovskite top cell was confirmed by EQE measurements, showing minimal overlap in absorption and enabling efficient utilization of the solar spectrum.
The 4T tandem devices demonstrated overall efficiencies exceeding 22% on rigid substrates and surpassing 20% on flexible substrates, marking the highest reported values for kesterite/perovskite tandems fabricated via solution-based methods.
Mechanical robustness tests on flexible Mo foils confirmed over 90% retention of efficiency after 500 bending cycles with a bending radius of 1 cm, highlighting the kesterite layer’s mechanical resilience. The incorporation of MoO3 interlayers was essential for mitigating mechanical stress-related degradation and ensuring stable electrical contact and interface integrity.
Advancing Flexible Tandem PV
This work establishes an effective, all-solution-processed strategy to fabricate high-performance tandem solar cells integrating kesterite bottom absorbers with perovskite top cells on both rigid and flexible substrates.
By employing sodium doping and silver alloying, the kesterite layers achieved improved crystallinity and photovoltaic characteristics, enabling bottom cells with efficiencies surpassing 16% on rigid glass and over 12% on flexible foils.
The findings provide valuable insights and a blueprint for further optimization, including the eventual fabrication of monolithic two-terminal tandems, paving the way for next-generation solar technologies characterized by high efficiency, mechanical flexibility, and cost-effective manufacturing.
Journal Reference
Gobbo C., Trifiletti V., et al. (2026). Demonstration of overcoming 20% efficiency in kesterite/perovskite tandem solar cells on rigid and flexible substrates. Communications Materials. DOI: 10.1038/s43246-026-01213-x, https://www.nature.com/articles/s43246-026-01213-x