In this talk, I will present a unified interface engineering strategy for perovskite solar devices that couples self‑assembled monolayers (SAMs) at charge‑extraction interfaces with molecular additives within the perovskite bulk. Tailored SAMs provide precise energy‑level alignment, enhanced charge selectivity, and suppressed interfacial recombination, thereby improving efficiency, operational stability, and yield. Beyond single‑junctions, these SAMs enable low‑loss interconnection layers with efficient recombination, improved current matching, and reduced voltage penalties—capabilities that are essential for tandem solar cells.

On this foundation, we report two complementary advances for narrow‑bandgap Sn–Pb perovskites. First, a dipole‑engineered, hole‑transport‑layer‑free architecture is realized by incorporating an intracrystalline dipole‑active additive into the absorber. The additive establishes an internal field for selective hole extraction, regulates crystallization to curb non‑radiative recombination, and mitigates Sn oxidation and related defects. This approach yields efficient HTL‑free solar cells with power conversion efficiencies exceeding 23% and high‑detectivity near‑infrared photodetectors operating around 890 nm, demonstrating that bulk dipole design can substitute conventional hole transport layers while preserving selectivity and stability. Second, a bio‑inspired antioxidant stabilization scheme is introduced, wherein small‑molecule antioxidants function as bulk dopants to enhance oxidation resistance and cleanse grain boundaries, while larger polyphenols assemble at the surface to block oxygen ingress and establish favorable interfacial dipoles. This synergistic bulk/surface strategy reduces voltage losses, improves shelf stability, and also delivers Sn–Pb devices with high performance. 

Finally, we integrate these low‑bandgap Sn–Pb subcells into tandem architectures. SAM‑engineered interconnection layers ensure efficient charge recombination and low resistive losses, enabling high‑quality series connection in both all‑perovskite and organic/perovskite tandems. In the latter, a low‑bandgap organic subcell serves as a benchmark to assess and compare low‑bandgap perovskites, clarifying design rules that link interfacial chemistry, bulk defect control, and optical management to durable, high‑performance tandem photovoltaics.