Composition-controlled optical fingerprints and anisotropy of self-trapped excitons in double perovskites
Double perovskites (DPs) offer a compositionally versatile platform for photocatalysis and optoelectronics, but their soft lattices favor strong exciton-phonon coupling and self-trapped excitons (STEs), which produce broad, red-shifted photoluminescence (PL). Here, we combine optical spectroscopy and ab initio calculations to study lead-free, Sb-doped Cs2BInCl6 nanocrystals with B-site alloying from Na to K. Alloying tunes the PL energy from about 2.48 to 2.70 eV, while absorption features near 3.8 eV remain nearly unchanged. Calculations reveal that Sb doping enables bright radiative recombination channels and delocalizes the excitonic wavefunction. We identify STEs responsible for the large PL red shift, associated with an upward shift of dopant-hybridized in-gap states. Na-to-K substitution selectively changes the STE lattice distortion, explaining why PL varies strongly with composition while absorption does not. Theory further predicts pronounced PL polarization anisotropy that reflects the STE wavefunction's spatial structure and depends on B-site composition. Time-resolved anisotropy measurements closely agree with these predictions, providing experimental fingerprints of the excitonic wavefunction and its evolution with alloying. Together, these results disentangle the roles of doping and alloying in lead-free DPs and establish polarization-resolved spectroscopy as a probe of STE wavefunctions.