Hybrid and nanostructured materials for photovoltaic and gas sensing applications: preparation and properties

The world-wide increasing demand for energy is one of the most important issues of the era we are living in. Owing to the limited availability of fossil fuels and the strict necessity to reduce the pollutant emissions, great attention has been devoted towards clean and renewable energy resources. Solar energy conversion, in particular, is considered one of the most promising alternatives to face the energy-related challenges, as sunlight is likely the most abundant clean source of energy capable to satisfy the need for energy on global scale with minimum detrimental impact on the environment. Therefore, the development of efficient, cost effective and reliable photovoltaic devices is one of the key purposes of the scientific research in these years. In this scenario, in the last two decades great interest has been addressed to hybrid and dye-sensitized solar cells, that can be manufactured more inexpensively with respect the traditional photovoltaic devices based on silicon and compound semiconductors. A couple of year ago, the emerging of hybrid metal halide perovskites MAPbX3 (X = I, Br, Cl) as sensitizers in nanostructured solar cells has represented a breakthrough in this field, leading to the achievement of impressive energy conversion efficiencies and opening the way to the realization of novel device architectures. Anyway, despite the huge interest instantaneously arisen about this new class of materials and its application in more and more efficient photovoltaic devices, the origin of the observed outstanding performances has still to be identified and, more generally, a broad spectrum of hybrid perovskite properties is not accurately understood yet. The first part of the thesis reports the activity concerning hybrid perovskites. MAPbI3 perovskite films on glass substrates were obtained following either a single step solution process, but using different solvents (N,N-Dimethylformamide or γ-butyrolactone), or a two-step dipping procedure. The structural, morphological and optical analysis highlighted that the preparation route mainly affects the morphology, while the crystalline structure and the bandgap are substantially unchanged. The influence of baking time was investigated in MAPbI3 films prepared in different atmospheres from both stoichiometric and MAI-rich precursor solutions, pointing out that the highest light absorbance is achieved by preparing the films in inert atmosphere from stoichiometric precursor solution. Preliminary experiments were performed aiming at sensitizing by MAPbI3 mesoporous ZnO nanosheets to be used for photoanode fabrication, showing that sensitization was likely achieved, even if the process must still be optimized. Mixed MAPbI3-xBrx films with Br content varying in the whole 0 ≤ x ≤ 3 range were synthesized by properly mixing MAPbI3 and MAPbBr3 precursor solution. Structural analysis confirmed that a solid solution can be formed in the whole 0 ≤ x ≤ 3 range. The optical characterization confirmed the possibility to tune the bandgap by varying the Br molar fraction and pointed out that the samples were characterized by a low composition disorder, with a maximum Urbach energy of ≈ 85 meV for x = 1.89. The effect of baking parameters was investigated in MAPbI2Br samples prepared from single precursor solutions having different stoichiometries, finding that only nearly-stoichiometric solutions result in perovskites whose composition depends to a minor extent on the annealing procedures, which makes the synthetic process more reproducible and more promising for large scale solar cell production. The structural and optical characterizations of mixed MAPbI3-xClx perovskites demonstrated that, regardless of the components ratio in the precursor solution, Cl incorporation in an iodide-based structure is possible only at relatively low concentration levels. However, even if the material band gap remains substantially unchanged, the Cl doping dramatically improves the charge transport within the perovskite layer, explaining the outstanding performances of meso-superstructured solar cells based on this material. The second part of the thesis describes the activity concerning zinc oxide (ZnO) nanostructures. Single-crystal highly porous ZnO nanobelts were prepared by thermally decomposing ZnS(en)0.5 hybrid parent nanostructures synthesized through a solvothermal route on Zn substrates. The investigation at the nanoscale of the ZnS(en)0.5 → ZnS → ZnO conversion pointed out that hybrid decomposition of ZnS(en)0.5 results in porous ZnS nanobelts, that are gradually transformed into ZnO by an exchange reaction between oxygen and sulfur. Pores form in the whole nanostructures due to the strong lattice contraction associated with the ZnS → ZnO transformation. Control of the ZnO nanobelts distributions was achieved by patterning the Zn metallization on alumina substrates, allowing the fabrication of two contacts structures used for the electrical characterization. The comparison between cathodoluminescence spectra and electrical measurements suggested the presence of a residual sulfur doping that was confirmed by means of EDX analysis. ZnO mesoporous nanosheets and nanotetrapods were used as active layer in gas sensing devices, that were characterized in different atmospheres by means of impedance spectroscopy. The impedance spectra of both nanostructures in the presence of ethanol (CH3CH2OH) and carbon monoxide (CO) at different temperatures and gas concentrations were described by the same equivalent circuit. The different behavior of the sensors response observed for dry carbon monoxide and ethanol suggested a relevant influence of the absorbed water molecules on the conduction in these systems.