ELECTRONIC STRUCTURE AND OPTICAL PROPERTIES OF LEAD/TIN HALIDE PEROVSKITE FOR SOLAR CELLS APPLICATION

• Main Author: Sukma Handayani, Yolla
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/58027
• Institution: Institut Teknologi Bandung

Metal halide perovskite materials for solar cell applications have received great attention because their solar cells can produce high power conversion efficiencies, which can reach more than 25% within one decade of their development period since the first report. One of the best performing metal halide perovskites is methylammonium lead halide (MAPbI3/CH3NH3PbI3). MAPbI3 has good characteristics in terms of its high absorption coefficient, long charge-carrying half-life, direct bandgap structure, and low-cost fabrication process. Another thing that making this material interesting is the possibility of varying its compositions. In general, this material has an ABX3 structure, where B is a metal cation such as Pb2+ and Sn2+, X is an anion of halide elements and A can be an organic or inorganic anion. However, several reports mentioned that not all possible compositions can produce solar cells with good performance. Apart from being a light absorber, such as MAPbI3 organic-inorganic perovskite, inorganic perovskite such as Cs2SnI6 shows different functions, namely as a hole conductive material (hole transport material). This dissertation research aims to understand the influence of the anion and cation compositions in this perovskite on their electronic-optical properties and their performance as an active layer in solar cells. Based on the present research results, namely the investigation of the electronic and optical structural properties of the organic and inorganic halide ABX3 perovskite (A = MA+ (methylammonium), FA+ (formamidinium), IM+ (imidazolium), IP+( isopropylammonium); B = Pb2+; and X = I-) through the measurements of UV-Vis (ultraviolet-visible spectroscopy), XRD (x-ray diffraction), and UPS (ultraviolet photoelektron spectroscopy), it is found that not all A cations can form a perovskite crystal structure. The UV-Vis and XRD measurement results show that only MAPI (or MAPbI3) and MAPBI (or MAPbBrI2) samples can form perovskite crystals. Meanwhile, FAPI, IMPI, and IPPI samples do not form perovskite crystals. This is confirmed by the UPS spectrum measurement results, which show that the band-edges of the FAPI (CH(NH2)2PbI3), IMPI (C3H5N2PbI3), and IPPI (C3H10NPbI3) samples are almost the same as the band-edges of their organic precursor molecules (FAI / formamidinium iodide, IMI / imidazolium iodide, and IPI / isoprophylammonium iodide). This is contrary to the UPS spectrum of MAPbI3 that shows a significant band edge shift, which can be associated with the formation of the valence band. The results are in agreement with the computation results on its electronic structure by using the DFT (density functional theory) method, which shows the formation of the valence and conduction bands in MAPbI3 perovskite. Regarding the function of this perovskite as a hole transport material, the synthesis and characterization of cesium halide-based inorganic perovskite have been carried out. The synthesis process was performed by a wet chemical method with the product in the form of nanoparticles. The results of X-ray diffraction (XRD) measurements show that the formed perovskite is Cs2SnI6, as already mentioned in some literature where the Cs2SnI6 form is more stable and easier to form than the CsSnI3. In order to understand why this CsSnI3 form is more appropriate as a hole-transport material, computational studies on their electronic structures have been also carried out. The computational study of this perovskite was initiated from CsPbX3 (X = F, Cl, Br, I). The bandgap energy was found to be strongly influenced by the halide anions in the perovskite structure. The bandgap value decreases with the anion change of the F- anion to the I- anion. This well corresponds to the previous results, where the halide anion is very important in the formation of the valence band. The Cs2SnI6 computation results show a different character of electronic structure, which is indicated by the presence of a sub-bandgap level near the topmost valence band level. The PDOS (projected density of states) curve also indicates that the valence band is dominated by the halide p orbital, while the conduction band is formed by the p-halide orbital which is hybridized with the Sn (5s) orbital. The results provide an insight into why the Cs2SnI6 can act as a hole transport material in solar cells.