| The power conversion efficiency (PCE) of the solar cell based on organic-inorganic hybrid perovskite materials increased from 3.8% to 22.1% sinceits first discovery in 2009.Because of its high optical absorption coefficient, low cost and easy preparation method, such material has attracted more and more attention.Although great progress in improving device PCE has been made, the basic formation mechanism ofthe perovskite material is still not clearly understood.In this thesis,spectroscopic techniques such as photoelectron spectroscopy (XPS and UPS), X-ray absorption spectroscopy (XAS) have been applied to study the electronic structure of several lead halide perovskite compounds, and its hole transport properities upon depositing a layer of organic thin film. The main research content includes the following three parts:(1) Formation mechanismof organic-inorganic hybrid perovskite CH3NH3PbI3 andCH3NH3PbI3-xClx.The chemical species during perovskite formationistracked usingin situXPS on successive depositions of thermally evaporated methylammonium iodide (CH3NH3I) on a lead diiodide (PbI2) film or a lead chloride (PbCl2) film. This deposition method mimics the "two-step" synthesis method commonly used in device fabrication.We find that several competing processes occur during the formation of CH3NH3PbI3and CH3NH3PbI3-xClx. Our most important finding is that CH3NH3I can dissociate into CH3I and NH3, and the former molecule can be incorporated into the resulting perovskite. These findings provide important insight on how synthesis methods can affect the performance of CH3NH3PbI3(or CH3NH3PbI3_xClx) solar cells.(2) Electronic structure and hole transport atCH3NH3PbI3-xClx/NPB (N,N’-Bis(1-naphthalenyl)-N,N’-bisphenyl-(1,1’-biph-enyl)-4,4’-diamine) interferce.CH3NH3PbI3-xClxfilms prepared by different methods (spin coating, co-evaporation, layer by layer deposition), arestudied usingin situltraviolet photoelectron spectroscopy (UPS). We find all kinds of CH3NH3PbI3-xClx perovskite produce similar curve profiles inthecore levels spectra as well as band structure, while NPB could act as an appropriate hole transport layer only for film made by spin-coating.Large hole transport barrier forms in the other two films.(3) Theorientation of the organic cations in organic-inorganic hybrid perovskite. A comparative study of the electronic structure of methylammonium (CH3NH3) in CH3NH3PbI3 thin film synthesized using either one-or two-step deposition protocols is performed using angle-resolved soft X-ray XAS at the C K-edge. We find that our XAS measurements can be accurately related to the ground state unoccupied density of statesusing a simple crystal field model. We further find that that films made by the one-step deposition protocol exhibit angle-dependent features, indicating long-range alignment of the CH3NH3 molecules, although the angle-dependency decreases as the film thickness increases. No angle-dependency was observed in the films made via the two-step deposition method.Formamidinium lead iodide (HC(NH2)2PbI3) thinfilms made with different annealing temperatures (60℃ and 160℃) show different crystal structures, but the cations exhibit the same alignment behavior. |
