Role of lone pair electrons in determining the optoelectronic properties of BiCuOSe

The electronic structure of the oxychalcogenides LaCuOSe and BiCuOSe has been studied using O K-edge x-ray emission spectroscopy, x-ray absorption spectroscopy, and density functional theory, in order to examine the effects of the M³⁺ ion configurations. The known distortion of the BiO layers in BiCuOSe compared to the LaO layers in LaCuOCh; the significantly smaller band gap of BiCuOSe (0.9 eV) compared to LaCuOSe (2.8 eV); and similar hole transport properties of the two compounds are explained in terms of the electron lone pairs associated with the Bi d¹⁰s²p⁰ electronic configuration. The Bi $6s$ orbitals are chemically active and form bonding and antibonding states with the oxygen $2p$ orbital. The structural distortion facilitates the interaction between the $6p$ orbital with $6s$ via the antibonding state. For BiCuOSe, the majority of the Bi $6s$ orbital character (i.e., the bonding state) lies below the valence band, with the antibonding state lying below the valence band maximum (VBM). The similar hole transport properties between the two compounds is a consequence of the Bi $6s$ contributing little to the Cu $3d$--Se $4p$ derived VBM. Finally, the band gap narrowing of BiCuOSe compared to LaCuOSe is mostly due to the low energy of the unoccupied Bi $6p$ orbitals along with the upshift of the VBM due to the presence of the O $2p$--Bi $6s$ antibonding states.