| Organic-inorganic hybrid perovskite material is known as"natural photovoltaic materials" owing to its excellent merits in optoelectronic properties,such as high absorption coefficient,ambipolar transport and long charge carrier diffusion length.Since the first report on perovskite solar cells in 2009,the photoelectric conversion efficiency of perovskite solar cells has increased rapidly from 3.8%to 25.7%.At present,perovskite solar cells are considered to be likely to become the next generation of photovoltaic cells to replace silicon-based solar cells.The rapid development of perovskite solar cells is conducive to the excellent properties of perovskite materials.At the same time,it is also closely related to the development of hole and electron transport materials.Nickel oxide(NiOx)is now widely used as the hole transport material in inverted perovskite solar cells owing to its suitable band structure,low cost and good stability.In particular,the fabrication of NiOx films by spin-coating solution with pre-synthesized nanocrystals has attracted much attention from researchers because it does not rely on complicated installations and is suitable for a variety of applications such as flexible devices and large-area devices.However,there are still some practical problems with this convenient solution-processing method.For example,the NiOx films produced by this method exhibit poor homogeneity,low density and rough surface,which affect carrier extraction and the crystallinity of the upper perovskite films.Moreover,the large number of defects on the surface of the NiOx films lead to severe non-radiative recombination,which greatly limits the photovoltaic performance of the devices.In addition,the high valence Ni ions of the NiOx film also induce decomposition of the perovskite crystals at the interface.Therefore,the urgent problem to be solved in the field of NiOx-based perovskite solar cells is to investigate the structural,electrical and optical properties of NiOx films and NiOx/perovskite interfaces,explore the technical methods for the quality improvement of NiOx films and NiOx/perovskite interfaces.In this thesis,the structural,electrical,optical properties and their physical mechanisms of NiOx hole transport layers and NiOx/perovskite interfaces were systematically investigated,and a series of technical methods for the optimization or improvement of the qualities of NiOx hole transport layers and NiOx/perovskite interfaces were proposed or developed.On the basis of this,the effective methods for the improvement of the performances of inverted structure perovskite solar cells with NiOx hole transport layers were explored.The main works in this thesis are shown as follows:(1)The influence of pH of the precursor solution on the size of the NiOx nanoparticles and the performance of their solar cell devices were investigated.By finely tuning the pH of the percursor solution during the preparation,the size distribution of the NiOx nanoparticles were effectively modulated.Based on this,homogeneous dense NiOx films with low surface roughness were successfully fabricated.It has been shown experimentally that the nucleation and growth of the intermediate product Ni(OH)2 nanocrystals can be controlled by tuning the pH of the precursor solution,and then the size distribution and microscopic morphology of the NiOx nanoparticles can be well controlled.The experimental results exhibit that NiOx nanoparticles with a uniform particle size of 5-10 nm and good solution tractability can be obtained when the pH is between 9.5 and 9.8.Based on these particles,we obtained NiOx films with good densities and flat interfaces,which achieved good contact with the perovskite layer and exhibited excellent hole extraction.And the flat interface also improved the quality of the upper layer.The MAPbI3 perovskite solar cell device based on this film achieved a maximum efficiency of 17.39%.(2)The influence of thermal-pressed recrystallizing treatment on the properties of perovskite films and their solar cells.By therrmal-pressed treatment method,the grain sizes,crystallinity and surface planeness of perovskite films prepared on NiOx layers were significantly improved,and their grain boundary defects were greatly reduced.It has been shown experimentally that the pressure can drive fine grains to fuse with each other during the annealing process.The average grain sizes of MAPbI3 films increase from 400 nm to 770 nm as a pressure of 150 MPa and an annealing temperature of 100℃ were applied.Thus,the non-radiative recombination in perovskite films was significantly suppressed and their carrier transport was improved.Most interestingly,the performance of the solar cells based on these films has been significantly improved.The average power conversion efficiency was increased from 16.78%to 18.31%,and a maximum efficiency of 18.75%is obtained.(3)A strategy for the modification of NiOx/perovskite interface by using monolayer organic coupling agent molecule was proposed,and the influence of coupling agent molecule on the properties of NiOx/perovskite interface and their solar cell devices was investigated.The NiOx interface was modified with NDZ311 molecule.It was found that NDZ-311 molecule can chemically bond to the surface of NiOx film to form a monolayer and coordinate with perovskite layer to reduce the interface trap density due to the existence of the phosphate ester group.In addition,NDZ311 monolayer isolates perovskite layer and NiOx spatially,which inhibits the oxidation of I-and deprotonation,leading to distinctly improved device stability.As a result,the photovoltaic performance and stability of the ternary cationic perovskite solar cells were improved.The average power conversion efficiency increased from 18.01%to 19.92%and a maximum efficiency of 20.39%is obtained.(4)We investigated the photovoltaic performance and optical property of semitransparent NiOx-based perovskite solar cells with different active layer thicknesses and improved their bidirectional light transmission by introducing a LaF3 antireflective layer.The maximum light utilization efficiency(LUE=1.93%),the average power conversion efficiency of 9.19%and the average visible light transmission of 21.0%were achieved when the active layer thickness was 106 nm.Further,a LaF3 antireflective layer was fabricated by thermal evaporation on the semitransparent Au electrode.In the presence of a 50 nm LaF3 film,the Au electrode has the lowest reflectivity and the visible light transmission increases from 46.8%to 64.2%.The power conversion efficiency of the devices increased significantly with irradiation of indoor light from Au electrode side.This work demonstrates the potential of NiOx-based semitransparent perovskite solar cells as self-powered smart windows for large stadiums. |
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