Researchers from China's Huazhong University of Science and Technology, Optics Valley Laboratory, Taizhou University, Nanjing Tech University (NanjingTech), Southern University of Science and Technology, Henan Normal University, Shandong University and the UK's University of Oxford have developed a robust interconnecting layer strategy for all-perovskite tandem solar cells that addresses key stability bottlenecks associated with conventional designs.
All-perovskite tandems have already surpassed 30% power conversion efficiency (PCE) in double-junction configurations, but long-term operational stability, especially under heat and light, remains a major limitation. A central issue lies in the interconnecting layers, which must simultaneously provide high transparency, efficient charge recombination, and chemical robustness. Standard architectures typically rely on a stack of C60/SnOx/ultrathin Au/PEDOT:PSS. However, PEDOT:PSS introduces parasitic absorption and chemical instability due to its acidic and hygroscopic nature, triggering degradation pathways such as iodine formation and Sn(II) oxidation in tin-lead (Sn-Pb) perovskites. At the same time, the inclusion of ~1 nm Au leads to optical losses via plasmonic absorption and can diffuse into the absorber at temperatures around 65°C, further compromising device stability.
To overcome these limitations, the researchers replaced PEDOT:PSS with an amphiphilic conjugated polyelectrolyte-modified bithiophene-triphenylamine (TPA-based) small molecule as the hole transport layer (HTL) in narrow-band-gap Sn-Pb subcells. This material improves hole extraction, reduces interfacial energy-level mismatch, and supports the formation of high-quality perovskite films, thereby enhancing quasi-Fermi level splitting and minimizing non-radiative losses. As a result, single-junction Sn-Pb devices achieved PCEs of up to 24.0% (MA-free) and 24.2% (MA-containing), alongside significantly improved stability.
In parallel, the team replaced the gold recombination layer with a 5 nm sputtered indium-doped zinc oxide (IZO) film. This ultrathin IZO layer combines high optical transparency with sufficient vertical conductivity, enabling efficient carrier recombination between subcells while eliminating both metal diffusion and parasitic absorption losses. The result is a cleaner optical path and a more stable buried interface.
When integrated into monolithic tandem devices, this redesigned interconnecting structure delivered certified efficiencies of 29.80% for MA-free and 30.19% for MA-containing narrow-band-gap subcells. Device scaling showed strong performance retention, with 1.0 cm² cells reaching 28.7% efficiency and 11.3 cm² mini-modules achieving 25.0%.
Operational stability was also significantly improved. Encapsulated tandem devices retained 90% of their initial efficiency after more than 770 hours, 530 hours, and 220 hours of maximum power point tracking under continuous illumination at 45°C, 65°C, and 85°C, respectively - conditions that typically accelerate degradation in Sn-Pb systems.
These performance gains arise from two coordinated improvements: (1) the TPA-based HTL reduces interfacial chemical reactivity and energy losses while improving hole transport, and (2) the IZO recombination layer eliminates metal-induced degradation and optical penalties. Together, these changes stabilize the buried interfaces - historically one of the weakest points in tandem architectures - while preserving efficient charge flow across the device.
This work presents a pathway beyond PEDOT:PSS-based interlayers and highlights the importance of interfacial engineering in unlocking both high efficiency and long-term reliability in all-perovskite tandem photovoltaics.
