Lead-lean and MA-free perovskite solar cells with an efficiency over 20%

Tin-lead perovskite solar cells (Sn-Pb PSCs) with low band gap (1.2–1.4 eV) are expected to achieve the maximum-power conversion efficiency (PCE) of single-junction devices given by the Shockley-Queisser limit. However, over 40 mol % Pb²⁺ is necessary to suppress the oxidation of Sn²⁺, which causes serious p-type self-doping. Here, we propose a new galvanic displacement reaction (GDR) method, through using lead powder as lead source and reductant simultaneously to resolve the above-mentioned issue. Lead powder could fully reduce Sn⁴⁺, but not Sn²⁺, in precursor, meanwhile suppressing the formation of iodide in film. Finally, Sn-Pb PSCs with low lead content and highest efficiency for MA-free-based devices (PCE: 18.34% for 8.5 mol % Pb²⁺, 20.01% for 18.7 mol % Pb²⁺) were realized. The unencapsulated devices retained unchanged or 81% of the original efficiency after storing for 2,352 h or tracking at maximum-power point (MPP) for 700 h in N₂ atmosphere (O₂ ≤ 50 ppm).