Investigation Of First-principles On Bandgaps Of Ferroelectric Photovoltaic Materials

Due to spontaneous electric polarization in ferroelectric materials,ferroelectric photovoltaic effect is completely different from the conventional p-n junction effect.The output V_oc can be a few orders of magnitude larger than the bandgap of a ferroelectric photovoltaic material.However,traditional ferroelectric materials have very low photocurrent sdue to their large bandgaps.Thus,it is crucial to develop a ferroelectric material with a reduced bandgap.In this work,first principles calculations is used to find the effective method impacting the bandgaps of ferroelectric materials such as BiFeO₃,SnTiO₃ and BiCoO₃,by magnetic ordering,doping,strain,and so on,and investigate its mechanism.The detailed results are as follows:?1?Bandgaps of tetragonal BiFeO₃?t-BiFeO₃?with different magnetic orders are investigated using first-principles calculations.The results show that when the G-type antiferromagnetic order converts to ferromagnetic order,the bandgap of t-BiFeO₃ decreases from 1.530 eV to 1.037 eV and simultaneously the crystal still possesses large polarization?160?C/cm²?.Compared to C-type/G-type t-BiFeO₃,the bandgap narrowing of A-type/FM are originated from the downward shift of Fe 3d e_g antibonding states and the upward shift of O_b2p states due to the ionicity increase of Fe and O_b.This work provides an insight into how to improve the photovoltaic properties by tailoring the bandgap of the t-BiFeO₃ ferroelectric materials.?2?The effects of magnetic ordering and B-site-cation ordering to lower the bandgap of FE-PV are investigated using first-principles calculations.Results show that the most stable structure of tetragonal Bi₂FeCrO₆?t-Bi₂FeCrO₆?is the AS1 structure?Fe/Cr alternate stacking ordering?with C-type antiferromagnetic ordering?defined as AC-t-Bi₂FeCrO₆?.It has a low bandgap?0.780 eV?and high polarization?61?C/cm²?.The electronic structures show that the contributions of the CBM come from the contribution of the Fe 3d states and partly the Bi 6p states,while the contributions of the VBM mainly originate from the Cr 3d states and O 2p states.The results demonstrate that AC-t-Bi₂FeCrO₆ is among the FE-PV materials with the highest application potential.?3?The bandgap tailoring of pseudo-cubic Bi₂FeCrO₆?p-BFCO?with different magnetic orderings was investigated using first-principles calculations.Results show that A-type antiferromagnetic ordering and ferromagnetic?FM?ordering of p-BFCO have a narrowed bandgap?1.120 eV?,indicating that p-BFCO is sensitive to its magnetic ordering.Electronic structure analysis shows that the narrowed bandgap of A-type and FM orderings originate from the downward shift of Fe 3d antibonding states due to the ionicity strengthening of Fe.The polarization of p-BFCO maintains a sufficient value for different magnetic orderings.The bandgap of the C-type ordering is the most stable structure,which can be modulated from 2.167to 1.206 eV by strain based on different substrates,originating from the Bi 6p and Fe 3d antibonding states moving toward the Fermi energy.Meanwhile,the polarization of pc-BFCO increases with increasing strain due to the relative displacement of cations and anions.This work provides an alternative way to lower the bandgap of FE-PV materials.?4?The structure and the optical adsorption property of the tetragonal BiFeO₃ with S doped are investigated by first principles based on density functional theory.The most possible site of S replacing O in BiFeO₃ and the most stable structure are obtained by the calculations of formation energies.The results show that the lattice parameter c and the unit cell volume significantly increase when the S atom replaces the apical O atom?D1?of BiFeO₃.For BiFeO₂S,the rate of c/a is 1.468.The volume of BiFeO₂S is 1.24 times greater than BiFeO₃.In addition,the bandgap decreases to 1.350 eV from 1.530 eV for BiFeO_2.875S_0.125.By the increase of S doping,the bandgap of BiFeO₂S is almost unchanged compared with BiFeO_2.875S_0.125.However,the dispersion of conduction band bottom tends to be aligned than BiFeO_2.875S_0.125,which is benefical to the absorption of visible light.The density of state analysis illustrates that the contributions of the conduction band minimum and the valence band maximum are mainly from the Bi 6pz orbitals and Fe,and Bi 6s orbitals,O 2p_z orbitals and S 2?p_x,p_y?orbitals,respectively.Furthermore,the calculations of the absorption coefficients demonstrate that the substitution of S atoms improves the visible light absorption property of BiFeO₃.?5?The effects of S doping to the bandgap and polarization of the Pb-free SnTiO₃ are investigated.The stable structure of SnTiO_3-xS_x is obtained from the calculation of the two possible S doping sites.It is found that the c axis would be stretched to induce the increase of c/a,and to obtain the narrow bandgap and the large polarization.The bandgap of SnTiO_3-xS_x is nearly same when x is 0.33 and 0.50.However,the SnTiO₂S is not semiconductor but metal when x=1.Furthermore,the density of states analysis demonstrates that the energy of unoccuping state of SnTiO₂S reduced compared with SnTiO₃ when S atom bond with Ti and Sn atom after S doping.The bandgap of SnTiO₂S decreases as the Ti 3d ortitals moves to the lower energy.Bader charge analysis shows that the total ionizability of SnTiO₂S is lower than SnTiO₃,inducing the bandgap decrease of SnTiO₂S.The polarization of SnTiO_3-xS_x increases with the increase of S doping by the Berry phase method.In this work,it is predicted that the narrow bandgap and the large polarization of Pb-free SnTiO₃ can be obtained by the S doping.This provides an effective way to improve the ferroelectric photovoltaic properties of SnTiO₃.?6?By the introduction of sulfur in the perovskite tetragonal BiCoO₃ with C-type antiferromagnetic ordering,the bandgap of BiCoO₂S decreases significantly?about 1.20 eV?while maintaining a large polarization?about 186?C/cm²?that is similar to the value of 179?C/cm² of BiCoO₃.The most noteworthy is that the optical absorption of BiCoO₂S is remarkable higher than those of BiCoO₃ and others FE-PV materials.The decrease of BiCoO₂S bandgap originates from the movement of Co 3d states to a low-energy position due to the reduction of the Co ionicity when the smaller electronegativity of sulfur is introduced into the BiCoO₃ to substitute oxygen.The narrow bandgap and the high optical absorption of BiCoO₂S films grown on different substrates is favorable for FE-PV applications.In addition,the bandgap of BiCoO₂S can be modulated by the doping amount of sulfur,which can help us fabricate the multilayer FE-PV devices based on the different bandgaps from different layers.