Investigation On Electron Transport Via Tunable Ferroelectric Polarization And Electron Excitation In Ferroelectric Materials

Organic-inorganic hybrid perovskite solar cells have excellent photovoltaic performance and are attracting attention because their photovoltaic conversion efficiency（PCE）has been rapidly increased from the initial 3.8%to 25.5%.The electron transport layer has been widely studied because it plays a crucial role in the efficiency enhancement of perovskite solar cells.Pb Ti O₃,a ferroelectric material,is introduced as an electron transport layer in a perovskite solar cell,and the separation,extraction and transport of photogenerated carriers are regulated by the built-in electric field generated by the ferroelectric material itself.Ferroelectric materials are widely used in semiconductor components whose performance will be affected in some special environments such as space environment.Space radiation is dominated by protons,the electron excitation induced by proton injection into Pb Ti O₃ and its distribution under the action of built-in electric field are studied by means of time-dependent density functional method（TD-DFT）.In order to further search for ferroelectric materials with excellent electron transport properties,we investigate the optical and electrical properties of R3c-phase Mn Sn O3ferroelectric materials with high electron mobility,while providing a reference for the field of ferroelectric photovoltaics.In this thesis,DFT method is used for computational simulations,combined with experimental preparation and characterization of materials and devices.The main research contents and conclusions are summarized as follows:（I）MAPb I₃ perovskite solar cells with Pb Ti O₃ as the electron transport layer were prepared using spin-coating method.The Pb Ti O₃/MAPb I₃ heterojunction formed a Type-II energy band alignment as characterized by ultraviolet photoelectron spectroscopy（UPS）.The I-V curves of Pb Ti O₃-based solar cells are tested,and it can be concluded that the magnitude of photogenerated current density is significantly changed by polarization regulation.The short-circuit current density can be increased from 12.39 m A cm^-2 at unpolarized to 18.02 m A cm^-2 at forward polarization and reduced to 5.44 m A cm^-2 at reverse polarization.By constructing a Pb Ti O₃/MAPb I₃ heterojunction model and performing local density of states（LDOS）calculations,it is concluded that polarization modulation modifies the energy band structure,which can achieve effective extraction and transport of photogenerated carriers.By combining the lowest triplet state（LTS）simulation of excited states and charge displacement curves（CDC）,the same results are obtained for the transfer of excited electrons in the heterojunction regulated by polarization.（II）The excitation and distribution behaviors of electrons induced by different proton injection velocities for three channels<001>,<011>,and<111>of ferroelectric phase Pb Ti O₃ are investigated by using TD-DFT calculation method.The electron stopping power is influenced by the electron excitation in the material.At low injection proton velocities（<5bohr/fs）the excitation of electrons occurs near the CBM and VBM,mainly from the 2p orbitals of O atoms to the 3d orbitals of Ti atoms.As the proton velocity increases,the excited electrons expand to deep energy levels and the number of excited electrons in each eigenstate increases.The electron states in the deep energy levels have energy level changes even if no electrons are excited.Although the average density of electrons in the<111>channel is larger,the number of excited electrons at low velocities is significantly lower than in the<001>and<011>channels.The injection of protons can capture the electrons,and also produce a depolarized distribution of excited electrons around the apex-angle O atom of the Ti-O octahedron in the ferroelectric polarization direction.（III）Mn Sn O₃ ferroelectric material with R3c phase was synthesized by high temperature and high pressure method.Unlike the conventional ferroelectric materials,its electron effective mass is smaller at 0.18m_e and the band gap is measured about 1.8e V.The electron mobility of Mn Sn O₃ was derived as 29 cm² V^-1 s^-1 by the space charge-limited current（SCLC）technique.Since Mn Sn O₃ is a polar semiconductor with strong coupling of longitudinal-optical（LO）phonons and conduction electrons,the value of the coupling constantαis 2.45,forming a Fr?hlich large polaron,and the theoretical and experimental results of electron mobility are in good agreement.The Raman spectroscopy shows that the Raman shift of the main optical phonon is 416.6 cm^-1,and the corresponding phonon vibration mode is the breathing vibration of the Mn-O octahedron in the polarization direction.Mn Sn O₃ is an antiferromagnetic material with DM interactions and almost no coupling between magnetic atoms and lattice vibrations as determined by electron paramagnetic resonance（EPR）and variable temperature Raman spectroscopy.