The Band Gap Tuning And Interfacial Electronic Structure Of Perovskite-ferroelectric Oxides For Photovoltaic Applications

With the rapid development of society and increased energy consumption,it is urgent to develop and utilize solar energy.Photovoltaic technology has been demonstrated to be a promising renewable energy technology to harvest solar energy.Among many photovoltaic materials,perovskite-ferroelectric oxide photovoltaic materials are of particular interest,owing to their high photovoltage,polar structure conducive to the separation of electron and hole,good stability,low price and rich reserves.Theoretically,the bulk photovoltaic effect of ferroelectric materials can achieve great photoelectric conversion efficiency.However,its small photocurrent limits its development.The main factors determining the photocurrent of devices are dependent on the efficient light-matter interaction of photo-electron conversion,including light absorption,charge separation and charge transport.Here,the light absorption depends strictly on a macroscopic static band gap of semiconductors.On the other hand,charge separation and charge transport are sensitive to local electronic structure,Coulomb interaction,and dynamics of photoexcited carriers.Particularly,the prominent charge separation and charge transport properties of ferroelectric oxides are related to their intrinsic ferroelectricity.Among all the possible ferroelectric oxide materials,BiFeO₃ has attracted the attention of researchers due to its relatively small band gap and large polarization.In this dissertation,the band gap tuning and electron reconstruction mechanism of BiFeO₃-based ferroelectric materials are investigated by employing the density functional theory,aiming to enhance the optical absorption,promote charge separation and improve the power conversion efficiency.The main contents include the following aspects:Firstly,a new strategy is demonstrated to effectively tune the band gap of tetragonal BiFeO₃ via superlattice structuring with the ferroelectric BiCrO₃.The(BiCrO₃)_m/(BiFeO₃)_n superlattices are found to exhibit conventional ferroelectric properties but low fundamental band gaps(1.7?2.0eV),which are smaller than either of the parent materials.First-principles calculations reveal that the unexpected band-gap reduction is induced by charge reconstruction due to lattice strain,octahedral distortion,and polarization discontinuity at the BiCrO₃-BiFeO₃ interfaces.Secondly,the sulfur,which has lower electronegativity than oxygen,is chosen to substitute for oxygen in BiFeO₃to tune its band gap.Based on the first-principles calculations,the effects of the substitution of S for O in BiFeO₃ on its electronic structure and photovoltaic properties are investigated.It is found that the BiFeO_3-xS_x has lower electronic and optical band gap than that of pure BiFeO₃.The band gap reduction mainly originates from the decreased charge of Fe due to the introduction of sulfur in BiFeO₃,which causes the Fe 3d unoccupied states to shift toward the low energy level.Especially,the BiFeS₃ has an optimized band gap value of 1.55eV,which is close to the ideal band gap for photovoltaic applications.Meanwhile,the BiFeS₃ has relatively good ferroelectricity and improved electrical transport properties,which are favorable to promote charge separation and inhibit charge recombination.Thirdly,improving the charge separation and charge transport efficiency for ferroelectric materials is of utter importance for optoelectronic applications while it remains a big challenge thus far.Here,a conceptual strategy is proposed to achieve charge separation efficiency close to unity and simultaneously avoid charge recombination during charge transport in a ferroelectric polar-discontinuity superlattice structure,as demonstrated in(BaTiO₃)_m/(BiFeO₃)_n,which is fundamentally different from the exiting mechanisms.The competition of interfacial dipole and ferroelectric polar-discontinuity induces opposite band bending in BiFeO₃ and BaTiO₃ sub-lattices.Consequently,the photoexcited electrons and holes in individual sub-lattices move forwards to the opposite interfaces forming electrically isolated electrons and holes channels,leading to a charge separation efficiency close to unity.Importantly,the spatial isolation of conduction channels in(BaTiO₃)_m/(BiFeO₃)_n enables suppression of charge recombination during charge transport,thus realizing a unique band diagram for spatially orthogonal charge separation and charge transport.Remarkably,(BaTiO₃)_m/(BiFeO₃)_n can maintain high photocurrent and large bandgap simultaneously.Finally,the mechanism of electron reconstruction for perovskite oxide heterostructure is investigated systematically.It is demonstrated that ferroelectric polarization discontinuity plays a critical role in determining the electronic properties of ferroelectric superlattices.The reduction of band gap in superlattices is caused by the large built-in potential,which originates from the strong ferroelectric polarization discontinuity.The coexisted two-dimensional hole gas and two-dimensional electron gas can be created by the large enough built-in potential in superlattice.In addition,it is further unraveled that ferroelectric polarization discontinuity competes with the traditional polar discontinuity,thus leading to various final interface states.In summary,a systematic study based on the density functional theory is carried out to investigate the photovoltaic performance of the BiFeO₃-based ferroelectric materials.On one hand,it is proposed that ordered superlattice structuring and sulfur substitution are effective strategies to tune the band structure of BiFeO₃.On the other hand,a conceptually new band diagram and transport mechanism to realize orthogonal charge separation and transport is proposed.It is found that the ferroelectric polarization discontinuity and its competition with the traditional polar discontinuity play key roles in tuning the band structure and the interfacial electronic properties of superlattices.The presented results will be beneficial to achieve ideal charge separation and charge transport in optoelectronic applications and to design efficient ferroelectric photovoltaic materials.