Researchers from City University of Hong Kong and Chinese Academy of Sciences recently demonstrated a bio‑inspired antioxidant strategy that enables record performance in narrow‑bandgap tin-lead (Sn-Pb) perovskite solar cells, achieving a champion power conversion efficiency (PCE) of 23.46% and enabling monolithic all‑perovskite tandem devices with 29.95% efficiency (29.44% certified). By precisely engineering both the perovskite bulk and its surface with complementary organic molecules, the team addresses the long‑standing problem of Sn2+ oxidation that has limited the quality and stability of Sn-Pb absorbers.
The work targets one of the key bottlenecks in all‑perovskite tandem architectures: the development of high‑performance, narrow‑bandgap Sn–Pb bottom cells. In these materials, the divalent tin ions (Sn2+) are prone to oxidation, which leads to precursor degradation, impurity formation and poor device performance. To mitigate this, the researchers introduce a dual‑molecule, bio‑inspired antioxidant approach that separates bulk stabilization from interface passivation, using gallic acid (GA) and tannic acid (TA) in distinct, complementary roles.
GA, a small antioxidant molecule, is incorporated as a dopant into the perovskite precursor, where it preferentially localizes at grain boundaries throughout the bulk film. At these boundaries, GA imparts oxidation resistance, helping to maintain Sn in its divalent state and suppressing the formation of excess SnI2 impurity phases that typically arise from Sn2+oxidation and iodide loss. This leads to improved film quality, with cleaner grain boundaries and reduced defect density, which in turn supports more efficient charge transport and reduced non‑radiative recombination within the active layer.
TA plays a complementary role at the perovskite surface. Owing to its larger molecular framework, TA tends to assemble into a robust passivation layer at the film surface when applied as a post‑treatment. This layer serves two crucial functions: it physically hinders oxygen intrusion, protecting the underlying perovskite from external oxidative attack, and it chemically passivates under‑coordinated surface sites that would otherwise act as trap states. In addition, the TA layer establishes a dipole at the perovskite interface, which facilitates interfacial charge transfer by improving band alignment and lowering barriers for carrier extraction.
Together, the GA and TA treatments act synergistically to enhance oxidative stability against both intrinsic and extrinsic stimuli. Intrinsically, they help stabilize the perovskite during precursor conversion and film formation, limiting degradation pathways linked to Sn2+ oxidation. Extrinsically, they protect the finished film from neutral oxygen and superoxide species that can form during operation and further damage the perovskite lattice. This dual protection strategy yields Sn-Pb perovskite films with significantly improved structural and electronic quality, enabling devices with a champion PCE of 23.46%.
When implemented in monolithic all‑perovskite tandem architectures, these stabilized narrow‑bandgap Sn-Pb bottom cells support an impressive tandem efficiency of 29.95%, with an independently certified value of 29.44%. These results demonstrate that bio‑inspired antioxidant engineering at both the bulk and interface levels can be a powerful route to overcoming the intrinsic instability of Sn-Pb perovskites, paving the way for highly efficient, narrow‑bandgap sub‑cells and pushing the practical performance of all‑perovskite tandems toward their theoretical limits.
