Theoretical Investigations On Electron-phonon Couplings And Thermoelectric Properties Of Novel Nanomaterials

The energy conversion efficiency of organic-inorganic hybrid CH₃NH₃PbI₃perovskite based solar cells has been rising rapidly in recent years.This can partly be attributed to the high mobility and long diffusion length of charge carriers in CH₃NH₃PbI₃.Meanwhile,due to its ultralow thermal conductivity and high Seebeck coefficient,excellent thermoelectric property of CH₃NH₃PbI₃ is expected.By combining first-principles band structure calculations and Boltzmann transport theory,we studied the effects of longitudinal acoustic phonon and ionized impurity scatterings on the charge transport and thermoelectric properties of cubic and tetragonal CH₃NH₃PbI₃.The intrinsic upper-limit carrier mobilities of²500 cm² V^-1 s^-1 and⁷000 cm² V^-1 s^-1 are predicted for electrons and holes,respectively,when only the longitudinal acoustic phonon scattering is considered.This has been confirmed by experiment.However,the ionized impurity scattering will dominate when the ionized impurity density is larger than 10¹⁸ cm^-3.The room-temperature thermoelectric figure of merit zT of tetragonal CH₃NH₃PbI₃ is predicted to approach unity when the hole density reaches 6×10¹⁸ cm^-3,while the highest zT attained is 1.26 for tetragonal CH₃NH₃PbI₃ at the phase transition temperature of 330 K.Based on band structure calculations,we further point out that p-doping of CH₃NH₃PbI₃ can be realized by controlling the density of CH₃NH₃⁺defects,without degrading its charge transport property.Our calculation indicates that CH₃NH₃PbI₃ perovskite combines the advantages of the low thermal conductivity of organic materials and the high electrical conductivity of inorganic materials for potential applications in optothermoelectric devices and thermoelectric cooling devices at low-temperature range.With unique electronic properties,two-dimensional nanomaterials also show great potentials in improving the thermoelectric efficiency.Recently,the high performance of layered SnSe single crystal as thermoelectric materials has invoked enormous research interests in group IV-V laminar materials and their two-dimensional counterparts within the thermoelectricity community.GeAs₂ possesses complex in-plane atomic structure as SnSe.Following the demonstration of stability of monolayer GeAs₂,we systematically studied the charge carrier scatterings from all the phonon modes by employing the density functional perturbation theory and Wannier interpolation method.For monolayer GeAs₂ with polar chemical bonds,the optical and acoustic phonon scatterings are found to be equally important.The electron mobility of monolayer GeAs₂ is calculated to be 224 cm² V^-1 s^-11 in a direction at 300 K,which is comparable to that of monolayer MoS₂(²00 cm² V^-1 s^-1).The band structure of monolayer GeAs₂features flat band and multi-valley degeneracy,resulting in high thermoelectric power factor for both electrons and holes,which is comparable to single-crystal SnSe.For monolayer GeAs₂ as a p-type semiconductor,the optimal thermoelectric figure of merit is 1.8 at 900 K,while it is 2.1 at 900 K as an n-type semiconductor.Our theoretical studies have laid the foundation for further exploring the potential of group IV-V two-dimensional nanomaterials in thermoelectrics.