  Introducing an isolated intermediate band (IB) into a wide band gap
semiconductor can potentially improve the optical absorption of the material
beyond the Shockley-Queisser limitation for solar cells. Here, we present a
systematic study of the thermodynamic stability, electronic structures, and
optical properties of transition metals (M = Ti, V, and Fe) doped CuAlSe2 for
potential IB thin film solar cells, by adopting the first-principles
calculation based on the hybrid functional method. We found from chemical
potential analysis that for all dopants considered, the stable doped phase only
exits when the Al atom is substituted. More importantly, with this
substitution, the IB feature is determined by $3d$ electronic nature of M^{3+}
ion, and the electronic configuration of 3d^1 can drive a optimum IB that
possesses half-filled character and suitable subbandgap from valence band or
conduction band. We further show that Ti-doped CuAlSe2 is the more promising
candidate for IB materials since the resulted IB in it is half filled and extra
absorption peaks occurs in the optical spectrum accompanied with a largely
enhanced light absorption intensity. The result offers a understanding for IB
induced by transition metals into CuAlSe2 and is significant to fabricate the
related IB materials.
