  In this study, we demonstrate that introducing of less electronegative
R-elements (R = La or Pr) into the Bi-O charge reservoir layer of BiCuSeO
enhances the chemical bond ionicity and leads to an increase of both, the
charge carrier concentration and the effective mass. Although the charge
carrier mobility decreases upon Bi$^{3+}$ to R$^{3+}$ substitution, the
electronic transport properties are significantly improved in a broad
temperature range from 100 K to 800 K. In particular, the electrical
resistivity decreases by two times, while the Seebeck coefficient drops from
323 $\mu$V K$^{-1}$ to 238 $\mu$V K$^{-1}$ at 800 K. Thus, a power factor of
nearly 3 $\mu$W cm$^{-1}$ K$^{-2}$ is achieved for Bi$_{0.92}$La$_{0.08}$CuSeO
sample at 800 K. Meanwhile, a noticeable decrease of the lattice thermal
conductivity is observed for the doped samples, which can be attributed to the
enhanced point defect scattering mostly originated from atomic mass
fluctuations between R and Bi. Ultimately, a maximum zT value of nearly 0.34 at
800 K is obtained for the Bi$_{0.92}$La$_{0.08}$CuSeO sample, which is ~30%
higher than that of pristine BiCuSeO.
