Research On High Performance Organic–inorganic Perovskite Solar Cells With A Main Focus On The Microstrutures

Nowadays there are serious energy and environmental challenges worldwide.For a sustainable development,it becomes more and more urgent to develop and make use of clean renewable energy.As one of the most important green renewable energy sources,both governments and research institutions paid a great deal of attentions to the development and utilization of solar energy.To efficiently convert solar energy to electricity directly,various solar cells including commercial monocrystalline/polycrystallion silicon solar cells,CdTe and Cu?In,Ga?Se₂?CIGS?based solar cells,dye-sensitized solar cells,quantum dot solar cells,and organic thin film solar cell have made significant achievements.However,they all have their own shortcomings.As a newcomer,in just less than eight years,perovskite solar cells emerged and made quick breakthroughs with their photoelectric conversion efficiency already exceeding 24%,making them very promising for the commercial market.Nevertheless,there are still many problems to be solved before their full commercialization,such as their poor device stability with quick performance degradation after exposure to the atmosphere due to perovskite poor resistance against air and water.In particular,although the low-temperature solution process of perovskite solar cells is ideal for commercial production,which has been developed rapidly,a series of problems still remain challenges to be overcome such as uncontrollable film quality with high density of defects as well as their device hysteresis effects,which largely prevent their commercialization.It is well-known that any material's microstructure is the most important key factor governing its properties as well as its device performance.To solve the problems present in the perovskite films and the fabricated devices based on these films as well as to promote their development and applications,a thorough study of the microstructure of these films is inevitable.It is notable that synchrotron-based grazing incidence X-ray diffraction?GIXRD?is now a powerful tool to investigate thoroughly the microstructure of thin films,which can even study in-situ samples.Thanks to the construction of the Shanghai Synchrotron Radiation Facility?SSRF?,this thesis takes good advantage of GIXRD at the X-ray diffraction beamline of SSRF to focus on the studies of the microstructure of the perovskite films.Experimental devices for in-situ synchrotron based characterization under a variety of controllable sample environments,such as humidity,temperature,and atmosphere with even external tensile force applied,have been developed to meet the challenges to in-situ charaterize perovskite films and device microstructures.The studied subjects include the optimization of the film fabrication processes,the passivation of surface defects,the interfacial engineering of the substrate surface for film growth,and the humid environmental influences.The present studies focus on the changes of the microstrue after different processing.In some cases,the evolution of these changes are even observed in-situ lively.At the micro-scale,the present studies explore the formation and evolution mechanism of the film microstructure.Combined with the conventional film characterizations and the device performance tests,it is able to build up the relationship between the film microstructure and its properties as well as that between the film microstructure and its device performance.With the guide of the gained knowledge,this thesis is able to fabricate successfully high performance perovskite solar cells and some of them yield world-leading power conversion efficiencies.The following summarizes the mian contents of this thesis:1)Different Annealing Processes Lead to Different Perovskite Film Crystalline Phase Purities with different Device Performances and Hysteresis Effects.In this section,GIXRD was used to investigate the different CH₃NH₃PbI_3-xCl_x perovskite film crystalline phase purities after different annealing processes.It was found that the perovskite films?r-perovskite?after widely used rapid thermal annealing?RTA?consist of dual phases with a parent phase and a child phase.In particular,the parent pahse is dominant in the interior whereas the child phase composition gradually increases with decreasing depth till it becomes the majority on the surface,which might be one of the key factors related to hysteresis in fabricated Perovskite Solar Cells?PSCs?.To solve this problem,we significantly improved the crystalline phase purity of solution processed CH₃NH₃PbI_3-xCl_x perovskite film?referred as g-perovskite?by using a facile gradient thermal annealing?GTA?,which shows a pure and uniformly distributed phase structure in pin-hole free morphology.Regardless of device structures?conventional and inverted types?,the planar heterojunction PSCs employing CH₃NH₃PbI_3-xCl_x perovskite films after GTA exhibit negligible hysteresis with significantly enhanced PCE than their counterparts after RTA.Our result indicates that the crystallization phase purity in CH₃NH₃PbI_3-xCl_x perovskite film,especially in the surface region,plays a crucial role in determining hysteresis effect and device performance.2)HI Passivation of the Surface Defects on Perovskite Films and its synchrotron radiation in-situ XRD study.One of the most important factors governing the photovoltaic efficiency and environmental stability of Perovskite Solar Cells?PSCs?is the quality of their perovskite films.The study in this section demonstrates that CH₃NH₃PbI₃ perovskite films after anti-solvent washing are far from perfect on the surface,where the crystallization degree is quite low with complex multi-phases associated with numerous defects and notable chemical inhomogeneity.Herein,we report a novel anti-solvent washing treatment simply using Hydrogen Iodide?HI?additive in chlorobenzene?CB?which can enormously improve CH₃NH₃PbI₃ films surface with high crystallization phase purity and chemical homogeneity,leading to excellent structural stability under humidity and even tensile force observed by the synchrotron radiation in-situ XRD study.Based on such high quality films with defect-lesssurface,fabricatedplanarPSCswithanarchitectureof ITO/TiO₂/CH₃NH₃PbI₃/Spiro-OMeTAD/MoO_x/Au present a champion PCE of 19.94%and improved stability under humidity.Moreover,fabricated flexible planar PSCs on PET/ITO substrates with a champion PCE of 17.32%are shown to be more robust under tensile force than their control devices.The present study thus not only provides more insight into the facile defects passivation process on the surface achieved via balancing halide ions,but also provides a practical but efficient treatment which can be widely used to fabricate high quality CH₃NH₃PbI₃ films for environmentally stable high performance device applications.3)Interfacial Modification of the Substrate Surface for Perovskite Film Growth.The losses of performance and stability of low-temperature processed planar PSCs comes mainly from charge recombination at the interface between the perovskite films and their imperfect substrate surface.In this section,we modified the charge transport layers in PSCs to manipulate the interfacial morphologies and electronic structures.GIXRD was used to study the interfacial and the perovskite film microstructures in combination with the device test results for a better understanding of the micro-mechanism governing the device performance.Interfacial Modification of the Hole Transport Layer:the introduction of an ammonia modified graphene oxide?GO:NH₃?layer between the hole transport layer and perovskite was found to improve the crystallization and preferred orientation order of peovskite structure with a better matched energy-level-alignment at the perovskite interface.The fabricated devices with a champion PCE up to 16.11%are superior in all the performances in comparison with all the reference devices without the GO:NH₃ layer.The stablity of the fabricated PSCs were also significantly enhanced by the presence of the GO:NH₃ layer.All these achievements will promote more applications of chemically modified graphene oxide interfacial layer in the PSCs as well as other organic multilayer devices.4)Synchrotron Radiation In-situ Study of Perovskite Formation and Degradation Mechanisms.?1?An in-situ Study of Annealing Induced Perovskite Formation Mechanisms.Post annealing is a vital step for the formation of perovskite films.In this section,we made use of situ real-time synchrotron-based GIXRD to monitor a step-by-step gradual structure transformation from distinct mainly organic-inorganic hybrid materials into highly ordered CH₃NH₃PbI₃ crystal during annealing.Especially,a re-crystallization process of perovskite crystal was observed for the first time during such an annealing procedure:trace amount of pristine perovskite structure forms during spin coating becomes more disordered or even amorphous with increasing temperature,and then re-appears and finally becomes the dominant structure upon further annealing.This observed re-crystallization would help to enhance the perovskite crystallization and preferential orientations.The present study not only illustrates clearly the decisive roles of post-annealing in the formation of solution-processed perovskite to better understand its formation mechanism,but also demonstrates the crucial dependences of device performance on the perovskite microstructure in PSCs.?2?In-situ Study of Perovskite Degradation Mechanisms in a Humid Environment.Humid environment plays a vital role affecting the performance stability of PSCs.Therefore,in-situ monitoring the micro-structural evolution of perovskite films in real time will help to reveal the micro-mechanism for the device performance decay induced by humidity.In this section,a device providing a controllable humid environment was developed at X-ray diffraction beamline of SSRF,which was used to monitor in-situ the perovskite film micro-structural evolution in real time in a humid environment by using GIXRD.After a perovskite film being exposed to the environment with a relative humidity of 60?2%,besides the gradual decrease of perovskite components and its crystallinity,a new perovskite intermediate phase structure was observed in the early stage of the exposure for the first time,which should be attributed to the multiphase structure transformed from the gradual degradation of the perovskite crystalline.In the meanwhile,further characterization of the films using conventional techniques indicated that humidity reduced perovskite crystallinity with a worse film morphology.The tests of PSCs demonstrated that their performances dropped due to the humidity.The present in-situ synchrotron based work demonstrated well the close relationship between the device performance and the perovskite film microstructure,which help for a better understanding of the degradation mechanism for the organic metal halide perovskites.