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 d(10)s(2)p(0) 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.