A Study On Interfacial Structure And Interfacial Electronic Structure Of Perovskite Solar Cells

As a type of safe and clean energy,solar energy is on the focus of research and development in most countries around the world.Especially,there has been have progress by leaps and bounds in photovoltaics with breakthroughs achieved in the photovoltaic conversion efficiency of various new photovoltaic cells.Over the past decade,organic-inorganic hybrid perovskite materials have received key attention from research scientists as a new type of semiconductor material with excellent electro-optical properties such as high light absorption coefficient,long carrier diffusion length,and optimizable bandgap.Although scientists have made continuous efforts with significant progress in the development of high-efficiency perovskite solar cells,there are still many scientific issues to be thoroughly investigated,such as the growth mechanism of high-quality perovskite films,as well as the direct detection and optimization of the interfacial electronic structures.Among them,both the microstructures and electronic structures at interfaces between the perovskite layer and the charge transport layers play important roles in charge transport and charge extraction.There are correlations between these two which also play different roles.Moreover,interface engineering widely-applied to effectively enhance device performance needs to improve both interfacial microstructures and electronic structures.Therefore,interfacial microstructure and electronic structures have always been the focus in the research of perovskite solar cells.However,there is a short of techniques able to directly detect the microstructures of perovskite films at their growth interface.In the same time,the interfacial energy level structures between the perovskite layer and the charge transport layer in high-efficiency perovskite solar cells also need further in-depth experimental investigation.Herein,this paper focuses on two aspects,namely interfacial microstructures and interfacial electronic structures.On one hand,we developed a method to detect the interfacial microstructures of thin films by using synchrotron-based Grazing Incidence X-ray Diffraction（GIXRD）,and then studied the depth-sensitive structural distribution from the film surface down to the interface,On the other hand,organic small molecules were thermally evaporated on perovskite films and the interfacial electronic structures were derived from in situ photoelectron spectroscopy measurements to explore the physical mechanisms for interfacial dipole and band bending between the perovskite films and the hole transport layer.The main findings of this paper are summarized below:1.The widely-used GIXRD,in which X-rays are incident on the sample surface,makes it difficult to separate the diffraction signal from the interface at the bottom side and thus hardly realizes the direct characterization of the microstructures at the perovskite thin film growth interface.Based on the BL14B1 beamline end station of Shanghai Synchrotron Radiation Facility,we developed an experimental method to non-destructively detect the microstructures at the perovskite thin film growth interface by using a flexible substrate:namely the back-incidence synchrotron-based GIXRD obtains the diffraction signal from the growth interface directly by using a suitable grazing incidence angle on the back side of the flexible substrate.On this basis,we investigated MAPb I₃ perovskite thin films:firstly,we verified the phase changes of MAPb I₃ perovskite single-crystal powder at different temperatures from powder diffraction experiments at variable temperature.Then we used both back incidence and frontside incidence GIXRD to investigate the perovskite MAPb I₃ thin films prepared on flexible substrates by one-step anti-solvent process.We found that there is a mixture of tetragonal and cubic phases at room temperature,in which the cubic phase is the majority phase present.A change of phase was identified at different depth from the film surface to the interface in MAPb I₃ films,where cubic phase dominated at the buried interface whereas tetragonal phase dominated on film surface with a mixture of tetragonal and cubic phases in between.It is hypothesized that the phase transition at the interface is difficult due to substrate constraints and therefore cubic phase is kept largely after being cooled down from annealing whereas that the unbound surface completely transforms to tetragonal phase in the same time.These results will contribute to a deeper understanding of the microscopic working mechanism in perovskite solar cells based on MAPb I₃.2.As a high-performance hole transport layer material,small molecule 2,2’,7,7’-tetrakis（N,N-di-p-methoxyphenylamine）-9,9’-spirobifluorene（Spiro-OMe TAD）has been widely used in organic-inorganic hybrid perovskite solar cells.To verify the interfacial energy level matching of Spiro-OMe TAD in all-inorganic perovskite,we prepared the Cs Pb I₂Br film using a one-step process and the interfacial electronic structure between Cs Pb I₂Br and Spiro-OMe TAD was investigated during the evaporation of Spiro-OMe TAD on the Cs Pb I₂Br film by in situ x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy.The results indicate that no detectable interaction was observed at the Spiro-OMe TAD/Cs Pb I₂Br interface,where electrons accumulate due to the formed interfacial state.Thus,an interfacial dipole was formed at the interface with an upward energy band bending at both the Cs Pb I₂Br substrate side and the Spiro-OMe TAD side.The relatively small interfacial hole injection barrier（0.23 e V）and large electron-blocking barrier（1.25 e V）are not only conducive to hole injection and extraction but also can effectively prevent the electron from transferring and reduce the hole-electron recombination.The experimental results show that Spiro-OMe TAD is a suitable hole transport layer for all-inorganic perovskite solar cells based on Cs Pb I₂Br.