Universal Photostability of Non-Fullerene Organic Solar Cells in Tandem Cells

Perovskite-based multi-junction solar cells demonstrate promising potential and offer a solution to overcome theoretical limits of single-junction cells by reducing thermalization losses. Although notable efficiencies are already achieved by emerging perovskite/silicon and all-perovskite tandem devices, significant challenges persist, including the substantial carbon emissions due to energy-intensive silicon wafer production or fundamental stability concerns due to the oxidation of Sn²⁺ to Sn⁴⁺ in narrow-gap perovskites. Solution-processed narrow-gap non-fullerene acceptor (NFA) organic solar cells (OSCs) circumvent these issues and present a compelling choice as rear cells in perovskite-based tandem devices. In our previous work, the benchmark PM6:Y6:PC₆₁BM ternary OSCs maintained approximately 95% of its efficiency after 5000 hours of continuous operation under irradiation with low-energy photons (l = 850 nm), but it degraded rapidly when illuminated with a white light-emitting diode (LED), indicating that the visible spectral region is responsible for device degradation.^[1] In a perovskite-organic tandem solar cell, the wide bandgap perovskite sub-cell serves as a low-pass filter that protects the organic sub-cell against high-energy photons.^[2]While the photostability of the perovskite-organic tandem devices is still limited by the photostability of the wide gap perovskite, NFA based organic solar cells might be a better choice as the narrow gap sub-cell compared to Pb-Sn perovskite solar cells. <br/><br/>In this work, we demonstrate that the favorable photostability is generally found for a wide range of narrow-gap Y-type acceptors (Y18 (E_g = 1.31 eV), CH1007 (E_g = 1.30 eV), mBzS-4F (E_g = 1.25 eV)), that we identified to show great promise in tandem operation by showcasing efficiencies &gt; 23%, among the highest values reported for perovskite-organic tandem solar cells with each of the respective NFAs. On the path to further explore the photostability of NFA based ternary OSCs in more detail, we utilize monochromatic LED sources covering the spectral range from 375-850 nm. Our results reveal that under continuous operation in the maximum-power point with low-energy photons (λ &gt; 590 nm), the devices show excellent long-term stability (&gt; 1000 hours), while higher-energy photons (λ &lt; 530 nm) infer degradation. Combining these wavelength-selective degradation studies with in-situ photoluminescence, Raman spectroscopy and photoluminescence quantum yield investigations, we systematically investigate the distinct contributions of the donor polymer and the Y-type NFA along with possible degradation pathways in both cases. Hereby we also identify the degradation threshold energies of each constituent – an information that is not only crucial for the design of tandem solar cells, but also might provide the lever to design photo-stable single junction organic NFA-based solar cells. <br/><br/><b>References: </b><br/>1. Brinkmann et al. Nature 604, 280 (2022)<br/>2. Brinkmann et al. Nature Reviews Materials 9, 202 (2024)