Study On Defect Passivation In N-i-p Type Perovskite Solar Cells Based On Interface Modification And Solvent Engineering

Being widely studied,organic-inorganic hybrid perovskite solar cells（PSCs）have shown remarkable efficiency growth in the last decade and now reach an amazing 25.7%.It not only comes from the excellent properties of hybrid perovskite,such as tunable optical band gap,long carrier diffusion length and high light absorption coefficient,but also depends on the progress of technology like new fabrication approaches,solvent engineering,interface engineering and so on.Such high photoelectric conversion efficiency（PCE）also indicates that PSCs have great potential to compete with conventional crystalline silicon solar cells in the future market.Avoiding the process of sintering electron transport layer（ETL）under high-temperature,planar n-i-p type PSCs can be prepared by low-temperature process,which is conducive to the cost reduction.In addition,the efficiency of n-i-p type PSCs is generally higher than that of p-i-n devices by 1-2%.These two advantages attract much attention in perovskite community.Due to the excellent properties,Sn O₂ ETL improves the performance of n-i-p type PSCs to a new level,exceeding the PSCs based on traditional Ti O₂ ETL and suggesting the greater application potential of Sn O₂.However,Sn O₂ ETL based n-i-p type PSCs still face several problems in the optimization process,the most prominent of which is the serious defects in the device.These defects generally exist at the interface between the bottom ETL and perovskite layer as well as in the perovskite film.There are mainly ion mismatch,adsorption groups and lattice mismatch at the interface,while a large number of grain boundaries,cracks and pinholes in the perovskite film.They can not only affect the carrier transport and perovskite film growth,but also accelerate the penetration of water and oxygen to decompose the devices.Therefore,in order to improve the efficiency and stability of PSCs,it is an important task to take necessary measures to passivate and inhibit these defects.Taking Sn O₂ based n-i-p type PSCs as the research object,this paper focuses on the defects at Sn O₂/perovskite interface as well as in perovskite film.The small molecule based interface modification and solvent engineering were utilized to passivate and inhibit the defects.The specific research content includes the following three aspects:1.Optimization of the perovskite material for Sn O₂ based n-i-p type PSCs.Focusing on domestic and foreign studies,in order to improve the bench mark of device performance,we replaced the crucial perovskite layer in PSCs from the traditional MAPb I_3-xCl_x to（FAPb I₃）_0.93（MAPb Br₃）_0.07 with better properties.The properties of film formation and light absorption in different perovskite were investigated by a series of characterization methods.The results show that（FAPb I₃）_0.93（MAPb Br₃）_0.07 perovskite films have better crystal quality,which is conducive to the cell stability.More importantly,the narrower band gap gives（FAPb I₃）_0.93（MAPb Br₃）_0.07 perovskite a wider and stronger light absorption,which effectively enhances the short circuit current density of the devices.Finally,we achieved an average device efficiency of 19.29%based on（FAPb I₃）_0.93（MAPb Br₃）_0.07 perovskite,and the unencapsulated devices exhibited superior stability.Therefore,we determined the specific research system and laid the foundation for the subsequent work.2.The multifunctional organic conjugated small molecule 3-thiophenboric acid（TBA）was utilized to passivate defects at Sn O₂/perovskite interface and its roles are not just limited to defect passivation.The interaction mechanism of TBA at the interface was studied by combining density functional theory（DFT）and experimental methods like X-ray photoelectron spectroscopy（XPS）,etc.Carrier dynamics,interfacial band structure,defect states and morphology were systematically demonstrated by a series of optical and electrical characterization methods to explore the roles of TBA.The results show that the energy level arrangement between Sn O₂ and perovskite can be optimized due to the dipole effect of TBA.Defects at both sides of interface like ion mismatch and adsorbed groups are effectively passivated by the functional groups of TBA,inhibiting the non-radiative recombination.Besides,TBA provides a non-wetting substrate for perovskite nucleation,which improves the poor morphology caused by lattice mismatch.As a result,we achieved a champion efficiency of 21.80%,much higher than 19.74%of control one.In addition,the optimized devices show excellent stability under long-term storage,high humidity and continuous illumination.Our results provide a novel strategy to improve the efficiency and stability of PSCs synergistically by multi-functional interface modification materials.3.In order to reduce the defects in perovskite film,we focused on the solvent system of perovskite precursor and prepared perovskite films by replacing traditional Lewis base ligand dimethyl sulfoxide（DMSO）with tetramethylurea（TMU）.The interactions between different ligands and solutes in intermediate phase were characterized by Fourier transform infrared spectroscopy（FTIR）.The effects of TMU on perovskite film formation,defects and film stability were systematically revealed by a series of optical and electrical methods.The results show that TMU has a stronger coordination ability than DMSO,so the intermediate phase is more stable.The perovskite film forming from TMU has large grain size,low grain boundary density and few cracks and pinholes,which reducs the defects.In addition,the carrier transport in the film is enhanced due to the reduction of grain boundary density and optimization of crystal orientation.Based on these,we achieved a high PCE of 21.05%.At the same time,the reduction of grain boundaries,cracks and pinholes inhibits the penetration of external water and oxygen.Thus,the optimized devices exhibits excellent stability when aging at～50%RH.These results provide a novel strategy for solvent engineering to reduce defects of perovskite film and further improve the efficiency and stability of PSCs.