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Title: The study on high performance inorganic-organic hybrid perovskite solar cells
Authors: Tang, Guanqi
Degree: Ph.D.
Issue Date: 2019
Abstract: Photovoltaic devices (solar cells), as a promising renewable alternative to fossil fuels, can directly convert solar energy into electricity, meet the increasing demand of global energy consumption and then alleviate the environmental pollution. Perovskite solar cells (PSCs), as a star among the photovoltaic family, have attracted tremendous attentions from both academic and industrial fields. The highest certified efficiency so far is boosted to 25.2%, which surpasses the top efficiency of commercial CdTe and CIGS devices. This excellent performance is mainly due to the intriguing optoelectronic properties of perovskite materials, such as high light absorption coefficient, desirable bandgap, long charge diffusion length and high defect tolerance. Thus, the crystal quality of perovskite films is the key issue for the photovoltaic performance of PSCs. Nowadays, the main fabrication method of perovskite thin films is solution-processing (spin-coating) method due to its easy-operation and low cost which enables high potential for commercialization. While the defects in the solution-processed perovskite films prevent the efficiency from reaching the limit value. In this thesis, two works focus on improving the crystallinity of perovskite films by controlling the crystallization process. The employment of lead acetate (PbAc2) as an additive in the perovskite precursor to retard the crystallization process of perovskite films, which could lead to the enlargement of grain size and low defect density. In another work, the MoS2 flakes are introduced as an epitaxial growth template to control the crystal orientation of perovskite film. The obtained highly oriented perovskite films demonstrate low defect density and efficient charge transport. At last, van der Waals interaction is introduced into the grain boundaries of MAPbI3 perovskite by adding 2D PEA2PbI4 to enhance the bendability. The 2D/3D perovskite films demonstrate robust mechanical property and high bending durability since the strain is released by the van der Waals interaction. Retarding the crystallization process of perovskite film is a useful strategy to enlarge the grain size and decrease the grain boundaries, which would lead to the reduction of defect density. We find that PbAc2 can act as a crosslinking agent to make the perovskite intermediate phase more stable by forming strong hydrogen bonding. This stable intermediate phase significantly retards the crystallization process of perovskite films. The SEM characterization illustrates that the grain size is enlarged from 250 nm to 500 nm in average by introducing 3% PbAc2 (molar ratio with respect to PbI2). The defect density of 3% PbAc2 modified perovskite film is lower than the control one by nearly one order of magnitude. Due to the enhanced quality of perovskite films, the efficiency of PSC is increased from 17.25% to 19.07% and the devices stability is improved significantly.
Besides, perovskite films with preferential orientation is highly critical to the high-performance PSCs due to the lower defect density and ideally efficient charge transport crossing the films. However, controlling the crystal orientation is challenging for the solution processing method. Epitaxial growth is a prevalent technology to prepare highly oriented semiconductors, especially two-dimensional materials. Thus, MoS2 is selected as epitaxial growth template for MAPbI3 due to the suitable lattice structure and energy band level. Under transmission electron microscopy (TEM), in-plane coupling between the perovskite and the MoS2 crystal lattices is observed, leading to perovskite films with large grain size, lower trap density and preferential growth orientation along (110) normal to the MoS2 surface. Based on the highly orientated perovskite films, the power conversion efficiency (PCE) of PSCs is boosted from 18.12% to 20.55%. At last, the mechanical property of perovskite films is important for the bendability of FPSCs. It is prevalent that the cracks usually appear along the grain boundaries in the perovskite films to release the strains during bending operation, which leads to the severe degradation of the photovoltaic performance. 2D perovskites with different layers are combined by the van der Waals interaction. Due to the absence of dangling bonds among different layers, the external stress could be release by the slippage between neighboring layers, which enables 2D perovskite materials superior flexibility and deformability. Thus, 2D perovskite can be introduced into the grain boundaries of MAPbI3 layer to release the strains which are normally relaxed by the formation of cracks along the grain boundaries. In our work, 2D PEA2PbI4 is introduced into MAPbI3 films. The TEM characterization confirms that the 2D perovskites mainly distributed at the grain boundaries of MAPbI3 films. The flexible perovskite solar cells (FPSCs) based on MAPbI3 demonstrate PCE of 15.12%. The PCE of FPSCs based on 2D/3D perovskite is boosted to 19.27%. Due to the enhanced mechanical property of perovskite films, the FPSCs based on 2D/3D hybrid perovskite could maintain 94 % of initial PCE after 1000 bending cycles at curvature radius of 3.0 mm. In contrast, the FPSCs base on 3D perovskite retain only 70 % of original PCE. Thus, it is promising and feasible by introducing 2D perovskite at the grain boundaries to enhance the mechanical property of perovskite films. In summary, PbAc2 is employed as an additive to retard the crystallization process of perovskite films to enlarge the grain size and enhance the crystallinity. MoS2 flakes are used as an epitaxial growth template to prepare the perovskite film with highly preferential orientation. These two strategies are successfully used to enhance the photovoltaic performance of PSCs. 2D PEA2PbI4 perovskite is introduced into the grain boundaries of MAPbI3 film to improve the bendability by releasing the strain via van der Waals interaction. The mechanical property of perovskite films and devices are substantially enhanced. These three works pave a way for preparing high performance PSCs based on glass and flexible substrates.
Subjects: Hong Kong Polytechnic University -- Dissertations
Perovskite solar cells
Solar cells -- Materials
Photovoltaic cells -- Materials
Pages: x, 8, 113 pages : color illustrations
Appears in Collections:Thesis

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