Bottom Passivation of Sn–Pb Perovskites Using Ethylenediamine–Phosphonic Acids for Efficient HTL-Free Solar Cells

Tin-lead (Sn-Pb) perovskite solar cells are promising for tandem applications but face challenges with Sn²⁺ oxidation and defect-induced recombination, particularly with conventional hole transport layers (HTLs). We introduce ethylenediamine tetramethylphosphonic acid (EDTMP) as a novel interfacial modifier for Sn-Pb PSCs. EDTMP offers dual passivation capabilities through phosphonic acid anchoring to FTO and diamine interaction with perovskite defects. This strategy significantly reduces nonradiative recombination, extends carrier lifetimes, and improves film quality. Devices incorporating EDTMP achieved a remarkable power conversion efficiency exceeding 22% and excellent operational stability. Our findings provide a pathway toward simplified, scalable, and stable high-efficiency perovskite photovoltaics.