A Study Of Active Layer Optimization And Interface Regulation For High-efficiency Perovskite Solar Cells

Since the 21st century,global issues such as the greenhouse effect and energy crisis have greatly enhanced people’s attention paid to the new green energy sources and solar energy has been widely applied due to its characteristics including cleanness,renewability,and abundance.While solar cells use the photovoltaic effect to directly convert solar energy into electric energy,perovskite solar cells among the study of all kinds of photovoltaic cells have appeared as one of the fastest developed,the most dynamic,and the most popular research fields in recent years,due to the unique advantages of perovskites including the high light absorption coefficient,high carrier mobility,low exciton binding energy,and adjustable band gap.The photoelectric conversion efficiency of perovskite solar cells has also soared from 3.8%in the first report by Miyasaka in 2009 to 25.7%certified in 2021,showing bright development prospects.However,the perovskite film prepared by the low temperature solution method usually contain a wide variety of defects,especially at the film surface,crystal boundaries,and interface which disrupte the devices performance and stability.As it is well-known,a highly crystalized active layer with good interfacial contacts is the key in preparing high-performance solar cells and defect passivation is one of the effective methods to push the photoelectric conversion efficiency of perovskite solar cells closer to their theoretical efficiency of 31%with also improved device stability to a certain extent in the same time.To improve film high quality in spin coating,anti-solvent is widely applied to help achieving smooth high-quality films.Due to the rapid volatilization of the solvent leading to instantaneous supersaturation of the precursor solution and thus accelerating perovskite nucleation,however,perovskite crystalline grains become smaller.It was shown that the introduction of appropriate small polymer molecules in the anti-solvent can not only ameliorate the surface hydrophobicity of the film but also enhance the crystallization quality of the film.On the other hand,perovskites are multi-ionic crystals with low defect formation energies,therefore,adding an appropriate amount of additives into the perovskite precursor can regulate their components.In particular,the additives with lone electron pair can interact with Pb I₂to form Lewis adducts,which can not only regulate perovskite nucleation and crystallization but also passivate the crystal boundary with fewer non-radiative recombination traps.In the meanwhile,as perovskite nucleation occurs on hetero-substrates,their crystal growth is dominated by hetero-nucleation.Thus,it is essential to make use of interface treatments to adjust the substrate surface energy controlling the perovskite growth dynamics with reduced interface defects.Herein,to develop high efficient and stable perovskite solar cells,this thesis mainly adopts anti-solvent engineering,additive engineering,and interface engineering on one hand to control the crystal growth improving crystallization and preferential crystal orientations with fewer defect states,and on the other hand to promote interface energy level matching.In addition,with the help of the advanced synchrotron-based techniques at Shanghai light source,the structure-efficiency relationship between the microstructures of perovskite thin films and the performance of the corresponding perovskite photovoltaic cells has been studied thoroughly in this thesis.The main research results of this thesis are as follows:1.Anti-solvent engineering to enhance perovskite film surface crystallization for perovskite solar cells.We adopted one-step solution spin coating to prepare perovskite films for solar cells with a structure of ITO/Sn O₂/FAMA（PTB7）/Spiro-OMe TAD/Au.Additive Poly{4,8-bis[（2-ethylhexyl）oxy]benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl-alt-3-fluoro-2-[（2-ethylhexyl）carbonyl]thieno[3,4-b]thiophene-4,6-diyl}（PTB7）was introduced into the anti-solvent to regulate the crystallization kinetic process during the film growth.The crystallization degree of perovskite films at different depths were characterized by depth-sensitive grazing incidence X-ray diffraction（GIXRD）.It was found that PTB7 mainly improved the crystallinity on the film surface,as the Lewis base coordination bonds formed between under-coordinated Pb atoms on surface and O/S atoms in PTB7 could effectively passivate defects on the surface and at the crystal boundary,which is conductive to reducing non-radiative recombination.Meanwhile,PTB7 contains a large number of alkyl chains improving the hydrophobicity of the film surface.Finally,a highest photoelectric conversion efficiency（PCE）of 21.41%for the fabricated devices was achieved.An device retained over 90%of its initial PCE after 350 h under light irradiation of 100?m W?cm^-2by a white LED lamp at the maximum power point at room temperature in a nitrogen-filled glove box which demonstrate its excellent stability.2.Additive engineering to enhance perovskite film multiple preferentialorientations for stable high-efficient perovskite solar cells.We introduced tartaric acid（TA,tartaric acid）into the perovskite precursor solution and deposited perovskite films by low-temperature one-step solution process.The optimized doping ratio of 0.5 wt%was found to lead to dense perovskite films formed with grain size increased significantly.The analysis of the 2D-GIXRD data revealed that TA can regulate crystal growth leading to perovskite films with enhanced multiple preferential orientations and higher crystallinity,less defects favoring efficient charge transport along multiple directions.The prepared solar cell structure is ITO/Sn O₂/Cs FAMA/Spiro-OMe TAD/Au.Photoelectron spectroscopy measurement of perovskite films revealed that TA can also regulate the energy level alignment at the corresponding interfaces in the device,prompting charge extraction and transportation at these interfaces.Finally,devices prepared using TA additive present a champion photoelectric conversion efficiency to 21.82%from 19.70%.The device efficiency of a solar cell using TA additive remained over 92%of its initial value after 1200 h aging at room temperature in dark ambient air environment or over89.6%of its initial value after 560 h under full-sun illumination in the nitrogen environment.Therefore,the present work demonstrates that the use of TA as additive in perovskite precursor is a facile and versatile approach to fabricate high quality perovskite films with enhanced multiple preferential orientations for stable high-efficiency perovskite solar cells.3.Interface engineering for nickel oxidized（Ni O_X）base inverted structureperovskite solar cells.As an inorganic p-type semiconductor,Ni O_Xis currently one of the most influential hole transport layer materials with high carrier mobility,outstanding optical light transmittance,super chemical stability,and low manufacturing cost.However,the efficiency of Ni O_Xbased inverted structure perovskite solar cells lags behind that of those Formal-structured cells,mainly due to the surface defects of Ni O_X（such as-OH^-/-NO₃^-dangling bonds）leading to adverse chemical reactions with perovskite especially at the Ni O_X/perovskite interface and severely limiting the performance of cells.Thus,we introduced 4,4’-cyclohexylidenebis[N,N-bis（p-tolyl）-aniline]（TAPC）for the regulation of the Ni O_X/perovskite interface.On one hand,it can improve the crystallization of the perovskite films and passivate the defects on the Ni O_Xsurface,thus reducing the interface charge non-radiative recombination.On the other hand,the interface HOMO energy level between the hole transport layer and the active layer can be improved prompting carrier extraction and transportation,reducing the loss of interface voltage.Finally,the prepared perovskite solar cells without methamine（MA）molecule with an inverted p-i-n structure of ITO/Ni O_X/TAPC/Cs FA/PCBM/BCP/Cu achieved a champion photoelectric conversion efficiency of 21.04%with an open-circuit voltage enhanced from 1.04 e V to 1.11 e V.