In this project, the Fellow developed/adopted the Boltzmann transport theory and TDDFT approaches to simulate the charge mobility and explore the photophysics in pristine/defective 2D and 3D materials.
(i) Boltzmann theory forms the basis for describing charge transport in a weak external field. To elucidate the exact nature of charge transport at various levels of disorder, the Fellow went beyond the state of the art and incorporated the impurity scattering into the Boltzmann theory. The Fellow applied the improved semi-classical Boltzmann transport theory to the field-effect devices based on monolayer MoS2 with controlled sulfur vacancies via argon-ion bombardment. The simulated band electron mobility values were corrected to account for the nonlinear dependence of mobile, free, charge carriers with gate voltage. The resulting field-effect mobility showed a power-law dependence with the concentration of sulfur vacancies, much consistent with the experimentally measured results. Such an evolution is ascribed to the combined effect of coulomb impurity scattering and the free versus total charge density renormalization factor.
(ii) Lead iodide methylammonium perovskites (MAPbI3) polycrystalline materials show complex opto-electronic behavior, largely because their three-dimensional semiconducting inorganic framework is strongly perturbed by the organic cations and ubiquitous structural or chemical inhomogeneities. Here, the Fellow took advantage of the newly developed TDDFT-based theoretical formalism that treats electron-hole and electron-nuclei interactions on the same footing to assess the many-body excited states of MAPbI3 perovskites in their pristine state and in the presence of point chemical defects. It was shown that lead and iodine vacancies yield deep trap states that can be healed by dynamic effects, namely rotation of the methylammonium cations in response to point charges, or through slight changes in chemical composition, namely by introducing a tiny amount of chlorine dopants in the defective MAPbI3. The theoretical results are supported by PL experiments on MAPbI3-mClm and pave the way towards the design of defect-free perovskite materials with optoelectronic performance approaching the theoretical limits. The Fellow expected these healing effects to be suppressed in pure inorganic lead-halide perovskites (like, e.g. CsPbI3), which might be the reason for their lower photoconversion quantum yields.
(iii) The state-of-the-art progress suggests the perovskites with formamidinium (FA) cations reach power conversion efficiency exceeding 20%. However, the picture that the dipolar nature of the MA cations heals the defects in MAPbI3 seems invalid in FA-based perovskites due to the much weaker dipoles of FA cations. Meanwhile, the organic cations are reported to be not essential in the lead bromide perovskites. Hence, the puzzles pertaining to the role of organic cations in the organic-inorganic hybrid perovskites still remain. Taking advantage of the TDDFT formalism which treats electron-hole and electron-nuclei interactions on the same footing, the Fellow revealed that the spatial extent of the electronic excitations in defective lead iodide perovskites including MAPbI3 and FAPbI3 is controlled by the size of organic cations. Yet, the role of organic cations is largely weakened as the inorganic lattices shrink, yielding rather similar primary photoexcitations in MAPbBr3 and FAPbBr3 perovskites. These different excited-state properties are ascribed to the varied rigidity in these perovskites.
(iv) The Fellow revealed that the photo-excited charge densities at/near the surfaces of MAPbI3 are sensitively dependent on the orientations of the organic cations. Interestingly, the Fellow noticed that the hydrogen bonding plays a role of stabilizing the surface geometries under solar illumination.
