Formation of Anisotropic Polarons in Antimony Selenide

Abstract

Antimony selenide (Sb₂Se₃) is an attractive candidate for photovoltaics with not yet satisfying efficiency. Beside defects, the polaron formation originated from lattice distortion was proposed to account for the free-carrier trapping and the subsequent carrier dynamics and optoelectronic properties. However, such a mechanism is still lack of the information of structural responses. Here, the evolution pathways of carrier and lattice after photoexcitation are tracked through the pump-probe methods of optical absorption and electron diffraction, which reveal the temporal correlations between the dynamics of both degrees of freedom. Opposite separation changes of atom pairs in the (Sb₄Se₆)_n ribbons are observed within a few picoseconds then followed by the structural distortions induced intermediate state, which last several tens of picoseconds, coinciding with the optical phonon population and the trapping process of carriers, respectively. Together with the analysis of electron diffuse scattering modulated by the Gaussian atomic displacement fields of polaron model, the picture of formation of anisotropic polarons with large size is constructed. The findings reported in this work provide the direct information of carrier and structure for elucidating the polaron scenario in Sb₂Se₃, and probably in other novel optoelectronic materials with anisotropic structure and soft lattice.