The Construction And Stabilization Of All-inorganic Perovskite Solar Cells

The cesium lead-based all-inorganic halide perovskites Cs Pb X₃（X=Br,I）have excellent photophysical properties,such as long carrier diffusion length,large light absorption coefficient,tunable band gap as well as good stability towards optical and thermal stresses.Thus,they have sparked wide research interest as a class of high-performance photosensitive materials.However,the highly efficient all-inorganic perovskite materials（such as Cs Pb I₃ and Cs Pb I₂Br）are facing the challenges of phase instability,their photosensitive black phase（α,βorγphase）can spontaneously change to the more thermodynamically-stable non-photosensitive yellow phase（δphase）at ambient condition.The preparation of stable black phase perovskite and high-quality thin films helps to improve the performance and stability of inorganic perovskite solar cells.In order to overcome the above challenges,we conducted a systematic and in-depth exploration of the preparation of high-quality pure phase perovskite thin films and the stabilization strategy of perovskite phases based on the properties of cesium lead-based halide all-inorganic perovskite materials.High-performance all-inorganic perovskite solar cells were constructed.The research contents and novelties are as follows:（1）We proposed an intermediate phase engineering strategy by using organic salts to improve the inorganic perovskite/metal oxide interface,which promotes the formation of highly efficient inorganic perovskite solar cells.We found that the lack of hydrogen bonding interaction between the all-inorganic perovskite and the commonly used metal oxide transport layer often leads to poor interfacial contact.In view of this,an organic-inorganic hybrid perovskite（OIHP）intermediate phase engineering strategy is proposed:the addition of volatile organic salts to inorganic perovskites,thereby doping the crystal lattice with organic cations（such as methylamine and formamidine ions）to form the OIHP intermediate phase,which facilitates the formation of strong interfacial contacts by hydrogen bonding interaction with metal oxides.Based on this,a champion Cs Pb(I_0.75Br_0.25)₃-based device with a power conversion efficiency of 17.0%and an open-circuit voltage of 1.34 V was realized.（2）The intermediate phase engineering strategy was further applied to the fabrication of black phase Cs Pb I₃perovskite films,and the efficiency of the obtained solar cells was obviously improved.According to the investigation of the phase transformation processes of pure Cs Pb I₃precursor film and Cs Pb I₃ precursor films with different additives（including NH₄I,BAI,MAI,FAI and DMAI）,it was found that the formation of a three-dimensional OIHP intermediate phase is essential for avoiding the generation of the non-photoactive yellow phase and promoting the formation of pureγphase Cs Pb I₃ films.A champion efficiency of 17.70%with a stabilized power output of 17.58%based on the optimized Cs Pb I₃-DMAI device was obtained.（3）Pseudohalide SCN^-was employed for fabrication of two-dimensional（2D）all-inorganic perovskite materials and solar cells with high efficiency.In order to improve the stability of perovskite solar cells,pseudohalide SCN^-was employed to fabricate a series of 2D all-inorganic perovskite materials Cs₂Pb（SCN）₂(I_1-xBr_x)₂（x=0,0.1,0.3,0.5,0.7,1）.They belong to the tetragonal structure.We found that self-trapped photoluminescence in the Cs₂Pb（SCN）₂I₂film is red-shifted with respect to the bandgap peak.They are formed by the transfer of carriers generated by the absorption of photons by the 2D material to the defect area for radiation recombination.The efficiency of solar cells based on Cs₂Pb（SCN）₂I₂ is more than 5%,exceeding the highest reported efficiency of 2D PSCs reported so far.（4）A sub-nanometer compact titanium dioxide electron transport layer was employed for optimizing the efficiency of perovskite solar cells.The thickness of the electron transport layer in perovskite is critical for controlling carrier transport.For organic-inorganic perovskites,we found that a sub-nanometer-thick titanium dioxide layer is sufficient for enhancing the extraction of electrons and reduction of carrier recombination in solar cells.It is shown that an efficiency around 18%can be reliably achieved at the optimized structural constitution of a 230 nm mesoporous Ti O₂ layer and a compact Ti O₂ layer with a wide thickness range（0.5 nm to 30 nm）.