Author: Li, Yun
Title: Improvement of the performance for perovskite solar cell
Advisors: Leung, Woon-fong Wallace (ME)
Degree: Ph.D.
Year: 2020
Subject: Perovskite solar cells
Hong Kong Polytechnic University -- Dissertations
Department: Department of Mechanical Engineering
Pages: xiv, 145 pages : color illustrations
Language: English
Abstract: Being a low-cost and high-performance photovoltaic device, the hybrid organic-inorganic halide perovskite solar cell (PSC) has attracted extensive attention in the past few years. Significant research progress on PSC is mainly attributed to the properties of the perovskite material, including long-range carrier diffusion length (>1 ┬Ám), broad-wavelength light absorption (up to a wavelength of 800 nm), high charge carrier mobility (25 cm2V-1s-1) and adjustable band gap. There are two different fabricated device architectures, planar heterojunction and mesostructured PSC. Optimizing the quality of the perovskite film and the device configuration is important to improve the PSC performance. Improving film surface coverage and grain size in the film is a cost-effective way for energy harvesting and should be of prime importance. On the other hand, enhancing the charge transport property of the PSC is also a vital aspect that allows the photogenerated electrons to be transported efficiently before recombining with the separated holes. In this study, the methods of developing PSC, characterizing the properties of the developed PSC, and various novel means for achieving high-performance PSC have been investigated. First, an efficient and simple approach has been investigated for using a combination of Dimethyl Sulphoxide (DMSO) to increase crystal size and hydrochloric acid (HCl) to further retard crystallization rate. The combined effect is to improve uniformity and crystallinity of the perovskite film. With this synergistic approach, superior quality perovskite film free from pinholes and with large uniform perovskite crystals over one micron has been obtained. The fabricated device has reached power conversion efficiency (PCE) of 17.8%, providing a 10%-11% improvement in PCE for both the best as well as the average performance. In addition, the incorporation of chloride using HCl formulation has demonstrated to have better stability against degradation from moisture, strong solar irradiation, and high temperature, which is an important finding as stability is one of the key limitations for PSC.
Second, to further improve the quality of the perovskite layer and the charge transport property, home-made, well-controlled, pristine graphene nanofibers were introduced into the perovskite layer of PSC. The introduction of graphene nanofibers into the perovskite layer led to a dramatic increase in the grain size of the perovskite layer to over 2 m, due to improved nucleation and crystallization on the nanofiber surface, which led to much higher FF and Jsc values. Also, the significant increases in Jsc and Voc are attributed to the improved charge-transport properties of the graphene nanofibers with superb charge conductivity introduced into the perovskite layer. This is confirmed independently by the charge transport time using Intensity Modulated Photocurrent Spectroscopy. Under optimized conditions, the device PCE increased from 17.51% without graphene to 19.83% with graphene nanofibers, representing a 13% increase. Third, a method for engineering large, uniform perovskite crystals has been studied. A thin structured electrospun TiO2 nanofiber scaffold has been applied to the dense TiO2 layer. The structured scaffold facilitates nucleation of the perovskite crystals from the nanofibers, especially at the intersections of nanofibers. By orienting the fibers forming polyhedrons and controlling the fiber packing density with uniform pore openings, large uniform crystals with high crystallinity that has good light absorption can be obtained. Further, graphene sheets in roll-up form, to eliminate adverse edge effect, were inserted in the TiO2 nanofibers in a convenient, simple way to enhance charge conductivity of the semiconductor nanofibers. Photogenerated electrons once generated in the perovskite crystals can travel to the TiO2 nanofibers and get injected into the graphene core. Subsequently, they are being transported to the electrode reducing electron-hole recombination thereby improving the current density of the PSC. Crystallizing perovskite in the TiO2 scaffold also eliminates unreacted PbI2 in a two-step crystallization process as compared to the PSC without the scaffold. The optimized PSC device exhibited PCE of 19.30%, which is 11% higher than the device without the nanofiber scaffold with PCE of 17.46%. Fourth, we have developed an efficient and simple method by insertion of an ultrathin graphene nanofibers layer between the TiO2 dense layer and the perovskite layer to reduce the interfacial resistance. The concentration of graphene and the thickness of the graphene nanofiber layer has been optimized. As a result of the improvement of the electron transport at the perovskite-dense layer interface, both the best and average solar cells fabricated reveal 5%-8% increase in PCE as compared to the PSC without the interface layer. The best PCE of the fabricated heterojunction solar cell has reached 18.62%. The results from the four different thrust areas in the present study highlight the significance of improving crystallinity, size and uniformity of crystals in the perovskite film. This was achieved by DMSO intercalating with the organic component, methylammonium iodide (MAI), in a two-step procedure in forming the perovskite, and nucleation and crystallization with the graphene nanofibers and the engineered TiO2 scaffold. The results also highlight the importance of efficient transport of photogenerated electrons reducing recombination by traps at crystal boundaries. Finally, the results also highlight the reduction of interfacial resistance to charge transport and the dense layer/perovskite layer was chosen as a demonstration.
Rights: All rights reserved
Access: open access

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