Organic cation-induced symmetry breaking enables efficient optoelectronic transitions in double halide perovskites

Lead-free double perovskites (A2B+B3+X6) are promising non-toxic alternatives to lead halide perovskites for optoelectronic applications, yet their performance is fundamentally limited by inversion-symmetry-induced parity-forbidden transitions, highlighting the need for systematic exploration of the underlying structure–property relationships and effective symmetry-breaking strategies. Here, based on first-principles calculations, we employ representative Cs2AgInCl6 to systematically investigate how compositional engineering modulates the local distortion of AgCl65− octahedra, transition dipole moments (TDMs), and optical absorption. Theoretical investigations reveal that local distortions effectively lift the parity-forbidden transitions and enhance band-edge absorption. Na or Ga alloying induces only weak distortions and correspondingly almost no enhancement of TDMs, whereas halide substitution exhibits pronounced configuration dependence, with cis arrangements producing substantially larger local distortions and stronger optical activation than trans ones. In contrast, incorporation of organic cations generates pronounced local octahedral distortions through their steric effect and anisotropic hydrogen-bonding interactions, leading to significantly enhanced parity-forbidden transitions and band-edge absorption comparable with MAPbI3. Furthermore, a persistent hydrogen-bond network reinforces the metal–halide framework and improves thermostability. These results establish a robust structure–property relationship between local octahedral distortion and optical activation, highlighting organic cation-induced symmetry breaking as an effective strategy to activate parity-allowed transitions and realize high-performance, stable lead-free DPs for next-generation optoelectronics.