Pengju Shi, Jiale Zhuang, Dayong Zhang, Chu Li, Stefan Zeiske, Jin Hou, Isaiah W Gilley, Ilhan Yavuz, Ubaid Kazianga, Congqi Li, Yu Zhang, Kefu Huang, Teerapat Itsoponpan, Xin Jiang, Chiung-Han Chen, Chuying Huang, Yi Yang, Xianfu Zhang, Pronoy Nandi, Samantha A Reitz, Antonio Facchetti, Keith P White, Keenan W Wyatt, Michael F Toney, Mercouri G Kanatzidis, Tobin J Marks, Cheng Liu, Bin Chen, Edward H Sargent
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+ centres and does not preferentially address the problem that PbBrx nucleates faster than does PbIx. 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 PbBrx, selectively retarding Br-rich nucleation and synchronizing it with PbIx. 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.