Fund Project:Project supported by the National Key Research and Development Program of China (Grant No. 2018YFB0703500) and the National Natural Science Foundation of China (Grant No. 11774017)
Received Date:17 December 2020
Accepted Date:21 January 2021
Available Online:27 May 2021
Published Online:05 June 2021
Abstract:Recently, perovskite ferroelectric photovoltaic materials have been studied extensively. Traditional photovoltaic device usually uses the internal electric field formed by PN junction to realize the separation of photogenerated carriers to form the photovoltaic effect, while ferroelectric material, due to the existence of spontaneous polarization, can spontaneously realize the separation of photogenerated electrons and holes without the formation of PN junction, presenting the ferroelectric photovoltaic effect. Chalcogenide perovskite with suitable band gap and visible light absorption is expected to be a new generation of ferroelectric photovoltaic materials. However, its application is limited due to the lack of ferroelectric properties. Hybrid improper ferroelectricity (HIF) in layered perovskites has opened a new way for developing the new ferroelectrics. In contrast to the proper ferroelectricity in which the polarization is the main order parameter as the driving force, the improper ferroelectricity possesses the ferroelectric polarization that becomes a secondary order parameter induced by other orders. In this work, we study the ground state, electronic structure and hybrid improper ferroelectricity of n = 2 Ruddlesden-Popper (RP) Sr3B2Se7 (B = Zr, Hf ) based on the first principles. The total energy calculations and phonon spectrum analysis show that the ground state of Sr3B2Se7 (B = Zr, Hf ) is of A21am polar phase. The hybrid improper ferroelectricity originates from the coupling between two rotation modes of BSe6 octahedron. Electronic structure calculations show that Sr3Zr2Se7 and Sr3Hf2Se7 are semiconductors with direct band-gaps, which are around 1.56 eV and 1.84 eV, respectively. The ferroelectric polarization values calculated by the Berry phase method are around 12.75 μC/cm2 and 9.69 μC/cm2, respectively. The contribution of each atomic layer to the ferroelectric polarization is investigated when the Born effective charge method is used. The results show that the polarization of Sr3B2Se7 (B = Zr, Hf ) mainly comes from the Sr-Se atomic layers. To sum up, Sr3B2Se7 (B = Zr, Hf ) show strong ferroelectric polarization and good visible light absorption characteristics and they are expected to be candidates of a new generation of ferroelectric photovoltaic materials. Keywords:first-principle/ Ruddlesden-Popper/ hybrid improper ferroelectricity/ ferroelectric photovoltaic
表1Sr3B2Se7无畸变顺电相I4/mmm结构经受原子极性位移或不同模式的八面体旋转而形成的所有可能结构, 及其与I4/mmm结构的能量差 Table1.All possible structures derived from the distortionless paraelectric prototype I4/mmm structure of Sr3B2Se7 under atomic polar displacement or different octahedral rotations, and the energy differences between the abovementioned structures and I4/mmm structure.
图 5 (a) Sr3Hf2Se7的A21am铁电相结构的原子位移图; (b)使用BEC方法计算的Sr3Hf2Se7的A21am铁电相结构中每层原子位移对铁电极化的贡献, Sr-Se1表示钙钛矿之间的SrSe原子层, Sr-Se2表示岩盐矿层 Figure5. (a) Cation displacement of the ferroelectric A21am phase of Sr3Hf2Se7; (b) contribution of atomic displacement of each layer to ferroelectric polarization in the A21am ferroelectric phase of Sr3Hf2Se7 calculated by BEC method. Sr-Se1 represents the SrSe atomic layer between perovskites, and SrSe2 represents the rock salt layer