New insights on the substantially reduced bandgap of bismuth layered perovskite oxide thin films

Abstract

Oxide perovskites are promising candidates for future solar energy conversion applications due to their intriguing properties, e.g. above bandgap photovoltage, which can be beneficial to overcome fundamental limitations of well-established semiconductor materials. Yet, their inherent wide bandgap remains a hurdle. This work revisited the long-standing concerns about the bandgap reduction in bismuth layered perovskite oxides through a combination of theoretical and experimental approaches. Ab initio calculations suggest that the reduced bandgap in cobalt-doped bismuth titanate can be attributed to the increase of density-of-states in the vicinity of the valence and conduction band edge. More importantly, comparison of the energy landscape for different cobalt substitution sites and its correlation with electronic bandgap shows that substituting Ti with cobalt equally in both inner and outer perovskite blocks with respect to the vicinity of Bi₂O₂ layer, is the most energetically favoured configuration with a significant 45% reduction of bandgap compared to the undoped bismuth layered oxide. The experimental replication of the bandgap tuning in cobalt modified Bi_3.25La_0.75Ti₃O₁₂ thin films prepared under optimal processing conditions strongly underpins the validity of our DFT prediction. Such compositional and structural engineering enabled a much narrower bandgap (∼1.7 eV) followed with an approximately 8-fold enhancement of the photocurrent density compared to the parental structure. This work will facilitate the design and synthesis route for tailoring the conventional wide bandgap ferroelectric oxides to be applied in photovoltaic and optoelectronic devices.