<p>Efficient wide-bandgap perovskite solar cells have pushed tandem efficiencies to 34.9%, reinforcing their promise for next-generation photovoltaics. However, their commercial adoption is hindered by stability issues of wide-bandgap perovskites, especially under high-temperature maximum power point tracking conditions. Here we report the stabilization of ~1.7-eV wide-bandgap perovskites via intermediate phase evolution, enabling a self-guided crystal-growth mode. A CsI₂Br intermediate phase forms during early stage deposition, directing the oriented growth of polycrystalline films with unique texturing. Atomic-scale scanning transmission electron microscopy reveals that the CsI₂Br <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\((1\bar{2}3)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo>(</mo> <mrow> <mn>1</mn> <mover accent="true"> <mrow> <mn>2</mn> </mrow> <mo>¯</mo> </mover> <mn>3</mn> </mrow> <mo>)</mo> </mrow> </math></EquationSource> </InlineEquation> facet, with a 2.9-Å interplanar spacing, matches the perovskite (200) facet, guiding coherent {100} growth. This results in enhanced crystallinity, with a 2-order-magnitude increase in the (100) diffraction intensity and a reduced full-width at half-maximum from 0.249° to 0.148°, compared with solution-processed films. The resulting solar cells exhibit outstanding thermal and operational stability, maintaining performance under maximum power point tracking for over 3,000 h at room temperature and over 500 h at 110 °C, with a projected lifetime of ~70,000 h. With 21.37% power conversion efficiency and &gt;84% fill factor, this work presents a compelling route towards stable, high-efficiency tandem photovoltaics.</p>