Phase-homogeneous mixed-halide perovskites for stable tandem photovoltaics

Mixed-halide wide-bandgap (WBG) perovskites needed in tandem photovoltaics suffer from phase segregation, even at the time of initial film formation: the result of asymmetric nucleation of I-rich and Br-rich phases^1-3. Known homogenization strategies tune Pb²⁺ coordination strength^4-6; however, Pb²⁺-based modulation applies across all Pb²⁺ centres and does not preferentially address the problem that PbBr_x nucleates faster than does PbI_x. Here we introduce a selective coordination principle: we tune local Lewis-base hardness at the donor atom through a molecular dipole, an approach that constrains the polarizability of the oxygen donor's outermost electrons. The harder oxygen donor preferentially coordinates the harder Pb²⁺ of PbBr_x, selectively retarding Br-rich nucleation and synchronizing it with PbI_x. This leads to compositionally homogeneous WBG films, enabling solar cells with bandgaps of 1.62 eV, 1.68 eV and 1.88 eV, each achieving enhanced power conversion efficiency and extended stability (1,500 hours, ≥T₉₀, 1 sun and 65 °C). Perovskite-organic tandem cells fabricated with these WBG films and an infrared-active organic cell deliver certified 27.0% (steady-state 26.4%) efficiency, with T₉₁ (ISOS-L2 at 65 °C) of 1,000 hours.