Solution-processed conductive interconnecting layer for highly-efficient and long-term stable monolithic perovskite tandem solar cells

Although stacking of multiple subcells with complementary bandgaps via an interconnecting layer (ICL) to form multi-junction tandem devices appears to be one of the most promising strategies to increase the power conversion efficiency (PCE) of organic-inorganic hybrid perovskite solar cells (PeSCs), the realization of high efficiency and long-term stable tandem devices based on solution-processed ICL remains highly challenging. In this study, a promising solution-processed ICL consisting of cross-linked p-doped hole transport layer (HTL) and n-doped fullerene electron transport layer for efficient and stable monolithic perovskite tandem solar cells is demonstrated. For the first time, high-performance photo-cross-linkable p-doped HTL material based on poly-[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) is developed by incorporating 1,2-bis[4-(azido-methyl)phenyl]-1,2-diphenylethene (TPE-MN3) and molybdenum tris-[1-(trifluoroethanoyl)-2-(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfdCOCF3)3) as the cross-linking agent and p-type dopant, respectively. This HTL exerts multiple positive impact on the device characteristics, including good solvent resistance, appropriate energy level matching, good electrical conductivity, and high compatibility with different perovskite systems. With these desirable characteristics, the series-connected tandem solar cells afford a PCE up to 18.69%, which represents the highest efficiency reported to date for monolithic all-perovskite tandem cells. Additionally, owing to high conductivity of the interfacial layers, a promising PCE up to 16.20% is attained even when the active area is extended to 1.2 cm2. More encouragingly, the encapsulated tandem cells exhibit remarkable long-term stability, maintaining ≈ 91% of its initial PCE after 9300 h (≈387 days) of air exposure. This work represents an important step forward toward the realization of large-area solution-processed perovskite tandem solar cells with high efficiency and long-term stability.