Preparation And Photovoltaic Performance Of Novel Perovskite Solar Sells By Chemical Vapor Deposition

In recent years, the new emerging organic-inorganic hybrid perovskite solar cells have attracted great attention due to their high efficiency and low cost. Basically, CH3NH3PbI3 is used as the light absorbers of the novel perovskite solar cells with high absorption coefficient, suitable direct band gap, excellent charge transport property, and high defect tolerance, which has been regarded as one of the most promising photovoltaic materials for solar energy conversion. To date, the perovskite absorbing films are dominantly prepared by the vacuum thermal evaporation method and the non-vacuum solution route. However, the vacuum-based technology requires expensive equipment and complicated process. On the other side, it is difficult to control the liquid-phase reaction in solution process, and always results in a rough film surface and incomplete coverage, which would seriously affect the photovoltaic performance of their devices. Therefore, it is urgent to develop a simple film preparation technology to promote the development of perovskite solar cells and commercial applications.In this thesis, the low-cost, scalable, and easily manipulated chemical vapor deposition (CVD) method is innovatively used for the preparation of CH3NH3PbI3 absorbers. Meanwhile, systematic characterizations on perovskite films are proceede for optimizing the preparation technology and achieving high-efficient photovoltaic devices (PVs). This thesis mainly includes the following aspects:Firstly, a facile low-pressure chemical vapor deposition (LPCVD) technology is developed to fabricate CH3NH3PbI3 films, which can effectively reduce the over-rapid intercalating reaction rate and easily overcome this blocking issue during the solution process. As a result, the prepared uniform perovskite films exhibit good crystallization, strong absorption, and long carrier diffusion length. More strikingly, CH3NH3PbI3 absorbers by LPCVD demonstrate excellent moisture-resistant feature even under laser illumination and high-temperature conditions. At the same time, high efficiency of 12.73% is successfully achieved under fully open-air conditions.Secondly, we present a simpler in situ tubular chemical vapor deposition (ITCVD) method to fabricate large-area perovskite films (4cm×4cm), and a high efficiency of 12.2% is also successfully achieved based on our first batch of planar type PSCs. Meanwhile the novel roll-over phenomenon in J-V curves of PSCs is first investigated in this study, and Impedance spectroscopy, capacitance-voltage measurements and XPS measurements demonstrate that the observed "roll-over" in the J-V curve is due to the presence of PbI2/CH3NH3PbI3 heterojunction in PSCs, which can help us to further understand the fundamental working mechanism of PSCs.Finally, a novel CH3NH3Cl-assisted two-step sequential chemical vapor deposition method (STCVD) is developed to fabricate perovskite films, and the open question about the exact role of Cl during the G-S growth process is first disclosed. As a result, it was found that CH3NH3Cl can react with Pbl2 films quickly, and refine the grain size to become a seed crystal template. With the assistance of CH3NH3Cl, crack-free and highly uniform perovskite layers with ultra-large grains were obtained. As such, the charge recombination in grain boundaries was tremendously reduced and the PV performance was significantly enhanced. Comparing with common CVD, the device Voc via STCVD was boosted from 0.915 V to 1.001 V, and the best efficiency was also greatly improved from 10.29% to 13.76%. Notably, the as-prepared device without encapsulation exhibited a good stability in an air environment. They still have an efficiency of 4.15% even undergoing upon exposure to the ambient conditions for 48 days. Therefore, the desired controllable fabrication procedure for efficient and stable PSCs has been elucidated in this work, and our research sheds light on understanding the exact role of Cl in the G-S growth process of perovskite layers, which will expected to provide theoretical and experimental bases and a new study approach.