New protocols to process advanced materials for the renewable energy

In this thesis growth control of organo-lead halide perovskite photoactive semiconductor layers is achieved by development of different synthetic processes. The optoelectronic properties of the as synthesized materials are characterized, and their suitability for fabrication of highly efficient PV devices is evaluated. The hybrid organo-lead halide perovskite is an interesting material for photovoltaic technology due to its optoelectronic properties which allow to reach highly efficient solar cells (from ~4% to > 20%) in less than 8 years.1–3 The chemical-physical properties as well as the morphology of the photoactive layer are highly dependent on the synthetic process used for the deposition, and some of these properties can be easily tuned by varying the chemical composition of the material (e.g. band gap), thereby making this material a versatile component of multijunction architectures.4,5 This Ph.D. project is focused on two different approaches that can improve the efficiency of the PV devices. First, efforts to direct the growth of organo-lead halide perovskite crystals are being discussed and evaluated. Electrical properties in these materials are affected by the orientation of the facets, and the fabrication approach of the p-SC should take in account a finite control of the halide perovskite deposition process. Thus, a layer of well faceted grains was deposited to study the facet dependence of the photo-generated current by means of photoconductive atomic force microscopy. Second, I used nano-imprinting techniques to pattern organo-lead halide perovskites. Interestingly, in silicon based solar cells, the absorbing material is micro-structured to avoid radiation/material losses due to the optical phenomena such as light reflection.6 The same strategy is here proposed to induce the total internal reflection phenomenon at the interface between the methylammonium lead iodide perovskite and the hole transporting material (Spiro-OMeTAD) exploiting the difference in refractive index between the two materials. In the first chapter, a general introduction to the need of developing highly efficient and cost-effective solar cells is provided. The energy crisis and the photovoltaic (PV) technology are briefly described. Then, the physics of a solar cell and the existing PV technologies are reported as well as an historical overview to introduce the perovskite-based solar cells, starting from the dye-sensitized solar cell (DSSC). Hence, the optoelectronic properties of the organo-lead halide perovskite are reported, followed by the description of the deposition methods. The chapter concludes with some general information about the characterization of the solar cell efficiency. In the second chapter,the low pressure vapor assisted solution process (LP-VASP) is reported as useful method to deposit organo-lead halide perovskite: the layer is here characterized. Moreover the synthesis and the deposition of cubic-like methylammonium lead iodide crystals is described. In the third chapter, the fabrication and characterization of solar cell devices are described. In particular the photoactive material is deposited with three different methods: the LP-VASP, the sequential dipping deposition and the combination of both of them. Then the PL and pc-AFM characterization of the layers described in chapter 2 are reported. In the fourth chapter, it is reported the idea to design, develop and apply nanoimprint lithography for patterning of the photoactive material. Silicon masters with suitable nano-structures (pyramids and V-groove) were fabricated and used to transfer the desired pattern on the semiconducting layer.