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Titolo: Characterization and application of Pb-based organometal halide perovskite
Data di pubblicazione: 22-mag-2015
Abstract: In the foreseeable future, the global energy consumption is expected to increase significantly. Solar energy, as an alternative form of energy, has gained popularity as a way to solve the greenhouse gas emission and sustainability problem of fossil fuels. This thesis mainly concerns a novel materials system, namely organometal trihalide perovskite, that is currently receiving considerable attention as light absorber in solar cells, due to the promise to obtain significant improvements in the efficiency of solar cells fabricated with very low cost, scalable techniques. The main idea of this thesis is to study the photophysical properties and the mechanisms affecting the performance of solar cells. In chapter 1, the history of solar cell materials will be reviewed briefly. The synthesis and basic characterization are described in chapter 2. In this chapter, it is explained how the methylammonium iodide was synthesized and purified. Perovskite films were fabricated by three different methods, resulting in very different film morphologies. Perovskite structure was confirmed by X-ray diffraction, and CH3NH3PbI3 shows a tetragonal phase with lattice parameters a=b=8.872 Å and c =12.637 Å. The morphology investigation shows that solution spin-casting method produces needle-shaped crystals, leading to a partial surface coverage and limited conductivity. The two-step from solution method was based on spin-casting PbI2 and CH3NH3I solution gradually, creating the perovskite upon reaction of the two compounds. The result is a film with smaller grains and more uniform coverage. Finally, the vapour assisted method, where a PbI2 film is first obtained by spin-casting from solution, then evaporation of CH3NH3I occurs for several hours in N2 atmosphere. A uniform film was achieved by this method with RMS roughness around 38 nm. The optical properties of the CH3NH3PbI3 are investigated in chapter 3. The optical bandgap of it is 1.64 eV, as extracted from the absorption edge, which is higher than the theoretical 1.55 eV. The absorption coefficient exceeds ~105 cm-1 for incident light wavelength shorter than 500 nm. The transient photoluminescence spectroscopy analysis shows that the lifetime of the excitons could be as high as τ = 80 ns under low excitation conditions. As long as the film is processed in such a way that the mean PL lifetime exceeds several nanoseconds at sun illumination, carrier mobility is sufficiently high to guarantee efficient charge collection in the photovoltaic device. In chapter 4, simple planar solar cells are described, which have been fabricated with compact TiO2 as electron transport layer, covered with perovskite as light harvester; poly (3-hexylthiophene- 2,5-diyl) (P3HT) was spin-casted as hole transport layer; at last, transition metal oxide MoO3 or LiF was evaporated onto P3HT as interfacial modifying material, final electrode was a thin layer of Ag. The relationship between TiO2 morphology and the solar cell performance is discussed. The morphology of compact TiO2 appears to be an important factor to influence the photovoltaic, which still needs further understanding in order to obtain better performing devices. The investigation on perovskite morphology indicates that the vapour assisted two-step deposition technique is useful for preparing perovskite films. We rationalize the crystal growth process with the conjecture that the organic and inorganic components have an efficient reaction by vapour intercalation into the PbI2 film. The resulting film has full surface coverage, microscale grain size and uniform grain structure. The investigation on interfacial modification shows that the solar cell with MoO3 as modification material has excellent performance with a PCE of 7.95%. And the solar cell with LiF as modification material has good performance with a Jsc of 21.73 mA/cm2. Both of the two materials have positive affection to the solar cell. The MoO3 is a proper material for modifying the interface between the electrode and hole transporting layer, which could replace the ITO in a heterojunction solar cell. And the LiF could decrease the work function of the metal contact, which may increase the transporting ability and increase the compatibility of the metal electrode. The investigation implies that the interface engineering is very important to the device science.
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