الفهرس 
INTRODUCTION AND LITERATURE SURVEYOnly 14 pages are availabe for public view
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Abstract
In the past few years, metal-halide perovskites have attracted significant attention from the photovoltaic community which leads their efficiencies to rapidly climbed from 3.8% to 23.7%. This evolution has been extraordinary and unprecedented in solar cells history and can be ascribed to the outstanding properties of the perovskite materials. Free-hole perovskite solar cell without precious metals electrode are the chief toward large scale and low cost production of perovskite solar cells, and thus commercialization.
In this study, TiO2/CH3NH3PbI3/C devices were fabricated in air, via two step dipping method, using carbon as a counter electrode with the aid of the facile doctor blade technique, which simplifies the processing and lowers the threshold of fabrication process. The research work presented in this thesis consists of three main parts:
The first part addresses the method to deposit the Carbon paste on the top of perovskite layer as an essential step for PSC fabrication, consequently, three different methods were investigated. The XRD and Raman spectroscopy results confirmed the formation of tetragonal anatase phase of TiO2, with particle of average size 8 nm. In addition, XRD and FTIR results confirmed the formation of hexagonal and tetragonal phases for PbI2 and MAI powders, respectively. As well, XRD pattern of the CH3NH3PbI3 layer, based on FTO glass, identified to the tetragonal perovskite structure (space group: I4cm) with lattice parameters a = 8.85 Å and c = 12.63 Å. The photovoltaic measurements showed that, the way to deposit the carbon paste has a great influence on the power conversion efficiencies and devices reproducibility. In consideration of the same TiO2 layer and carbon paste used, for the three methods, the direct deposition of carbon paste on CH3NH3PbI3 layer, using doctor blading technique, achieved best and non-varying (PCE of 0.95 %) photovoltaic performance, compared with the other methods.
The second part explores the influence of the TiO2 layer on the perovskite film and hence, photovoltaic performance of the PSCs, consequently, five different TiO2 thickness were prepared, using spin coating technique. The structure and microstructure of the different thickness of TiO2 layer were investigated by XRD and UV-vis spectroscopy. The effect of different TiO2 thickness on the structure and photovoltaic performance of the fabricated devices were investigated using XRD and IV measurement, respectively. XRD calculations estimated an average crystallite size of 8, 11, 13, 14 and 17±2 nm for the TiO2 film, coated by one, three, six, nine and twelve layers, respectively. ASF calculations estimated an energy gap of 3.28, 3.26, 3.25, 3.25 and 3.25 eV for those previous films. Accordingly, with the increase in the film thickness, band gap slightly decreases as a consequence of increasing the crystallite size. XRD characterization, for perovskite layer, identified a two-phase pattern: CH3NH3PbI3 and PbI2. Meanwhile, the ratio of PbI2 to CH3NH3PbI3 intensities increased, as increased TiO2 thickness, which was indication for increasing the amount of un-reacted PbI2. The photovoltaic parameters were found to be strongly dependent on the ratio of un-reacted PbI2 as a result of different thickness of TiO2 layer. The investigation showed that, PCE of 2.55% was achieved, employing a TiO2 thickness of 108 nm as an optimum thickness in this study. The performance is attributed to good surface roughness and thicker CH3NH3PbI3 layer accomplished with low amount of un-reacted PbI2.
The third part targets the influence of nano PbI2 powder on PbI2 film and hence CH3NH3PbI3 layer, therefore photovoltaic performance. Consequently, different nano sizes of PbI2 powder were obtained, by different milling time using mechanical milling machine. The structure and microstructure of milled PbI2 powder were investigated using TEM, BET and XRD employing Rietveld method.
The results obtained from Rietveld analysis showed that the milling caused a variation in both the crystallite shape and size. For unmilled powder, the {100} planes had the greater crystallite size of 856 nm compared with {001}, {011} and {012} planes of crystallite sizes 556 nm, 783nm and 688 nm, respectively, which indicated an anisotropic crystallite size. After 9 hours of milling, the mechanical milling leaded to a rapid decrease of the crystallite size to 45 , 38, 37 and 42 nm for {001},{100},{011} and {012}, respectively, which became approximately close together, resulted in a symmetric crystallite shape. TEM analysis for unmilled PbI2, showed a wide size distribution (1319±308 nm) with a significant agglomeration, vice versa for those had milled for 9 hours (13±1.5 nm), which resulted in large surface area, as estimated from BET analysis. XRD and SEM clarified that, the nano sized PbI2 powder in the precursor solution has a significant impact on the morphology of PbI2 films, resulted in an approximately full coverage, smooth and high quality film.
According to the investigation of SEM images for perovskite films, it transpired that, the quality and purity of generated perovskite films were greatly dependent on the morphologies and quality of PbI2 precursor film, in which, a good perovskite film with small particle size distribution and appreciate coverage was corresponded to the high quality PbI2 film. Moreover, XRD for such films displayed only the characteristic peaks of CH3NH3PbI3 and FTO, confirming the complete conversion of PbI2 into perovskite film.
In addition, the photovoltaic measurements revealed that, the enhancement in the quality and purity of the perovskite film had a significant effect on the overall power conversion efficiency. The perovskite device, based on milled PbI2 powder (9 hrs), achieved a PCE of 5.8%, while that based on unmilled PbI2 obtained 2.55%. This enhancement performance is attributed to benefits of milling PbI2 powder which results in a small crystallite size with high surface area and hence a high quality PbI2 film with good coverage.