Study On Photovoltaic Performance Improvement Of Plane Heterojunction Perovskite Solar Cells By Low-dimensional Carbon-based Materials

Due to its low cost,excellent photoelectric characteristics and high photoelectric conversion efficiency,perovskite solar cell（PSCs）has become the next generation photovoltaic technology that is most likely to replace silicon solar cell after the first and second generation solar cells,such as crystalline silicon solar cell,silicon-based thin film solar cell and polymer solar cell.In addition,initial photoelectric conversion efficiency of 3.8%for PSCs has been improved to 25.5%,making it the most promising third-generation solar photovoltaic cell.The selection,preparation and optimization of the carrier layer and the perovskite absorber layer are the key steps to prepare high efficient PSCs.For the PSCs with normal structure,the electron transport layer not only accelerates the carrier transport,but also affects the crystallization of the perovskite absorption layer and further affects the film quality.For the PSCs with inverted structure,the conductivity and surface morphology of the hole transport layer are directly related to the chemical nucleation of the perovskite film above,which is an indispensable factor to achieve high photovoltaic performance of the device.The morphology of the absorption layer,grain size,film thickness and other factors have important influence on the photoelectric conversion efficiency and stability of PSCs.In order to further improve the overall photovoltaic performance of PSCs,we need to find new materials to optimize the carrier transport layer and absorption layer of the cell.Among many materials,carbon-based materials have the characteristics of high conductivity,low cost,excellent stability and so on,which has been widely applied in the field of optoelectronic devices.In this thesis,the PSCs are selected as the research object,and low dimensional carbon materials are used to optimize the performance of electron transport layer,hole transport layer,perovskite absorption layer and related interface.Their influences on the photovoltaic performance of PSCs have been revealed,and the relevant physical mechanism has been clarified.It is mainly divided into the following three parts:1.Inhibition of carrier recombination at heterogeneous interface and improvement of charge separation efficiency are effective ways to improve the performance of PSCs.Interfacial modification between anode and electron transport layer or the construction of multi-layer electron transport layer are effective ways to achieve efficient charge extraction and collection.Combining the advantages of these two,the energy loss of PSCs can be further reduced and the photoelectric conversion efficiency can be improved.Herein,we design a sandwich structure of Sn O₂-CQDS-Sn O₂（S-C-S）electron transport layer,i.e.an ultra-thin layer of carbon quantum dots（CQDs）with adjustable bandgap is inserted between the ultra-thin Sn O₂ bottom layer and the ultra-thin Sn O₂ top layer.The ultra-thin Sn O₂ layer at the bottom can effectively passivate the defects on the FTO and reduce the carrier recombination at the interface between the FTO and the electron transport layer.The CQDs layer enhances the optical transmission performance of the electron transport layer,speeds up the electron transport process,and improves the ability of hole blocking.The device with S-C-S electron transport layer achieves a photoelectric conversion efficiency of up to 20.78%,which eliminates the hysteresis phenomenon to the greatest extent.The research results provide a new idea for the design of new electronic transport materials for solar cells and lay the foundation for further realizing higher photoelectric conversion efficiency of PSCs.2.The performance of inverted PSCs is highly dependent on the surface characteristics and hole extraction ability of the hole transport layer.To improve the conductivity and hole transport ability of the hole transport layer and optimize the surface morphology of the hole transport layer is an effective method to achieve high efficiency and stability of the PSCs.Therefore,we propose a simple and feasible strategy,that is,to add CQDs to PEDOT:PSS.The effective phase separation of PEDOT chain and PSS chain was achieved under the action of Coulomb force after doping CQDs with both significantly positive and negative functional groups,which not only formed a more uniform and compact hole transport layer,but also further optimized the electrical conductivity and hole extraction effect.In addition,the perovskite crystals on the CQDs-doped PEDOT:PSS hole transport layer show a preferred orientation along the（001）crystal direction,which is conducive to the hole transport at interfaces.The average photoelectric conversion efficiency of the inverted PSCs prepared by CQDs doped PEDOT:PSS is 19.33%,and the hysteresis phenomenon is negligible.This method can be well compatible with printing technology to realize mass production of PSCs with high efficiency and adjustable crystal orientation.3.Perovskite thin films with good surface morphology and large grain size are ideal materials for obtaining high-performance optoelectronic devices.At the same time,optimizing charge transport behavior is a necessary condition for further improving the optoelectronic performance of devices.Herein,we adopt a simple method to improve the crystallization of perovskite.The carbon based material of g-C₃N₄/CQDs composite is introduced into the perovskite precursor solution to provide enough space for the growth of perovskite in the form of a framework.The exisitence of g-C₃N₄/CQDs can delay the crystallization rate of perovskite to obtain the perovskite film with large grain size and low defect state density.At the same time,g-C₃N₄/CQDs reduces the energy barrier in the form of micro heterojunction to improve the charge transfer,and effectively solves the carrier recombination occurring on the grain surface,thus realizing the improvement of the performance of PSCs.The efficiency of the optimized PSCs reaches 20.49%,and it has good stability in atmospheric environment.This study provides new materials,new ideas and new methods for effectively controlling the crystallization process and optimizing the charge transport behavior.