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 phases1-3. Known homogenization strategies tune Pb2+ coordination strength4-6; however, Pb2+-based modulation applies across all Pb2+ centers 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 O-donor’s outermost electrons. The harder O-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 PCE and extended stability (1500 h, ≥T 90 , 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 T 91 (ISOS-L2 at 65 °C) of 1000 h.