Study On Suppressing The Redox Reaction At Interface In NiO_x Based Narrow Bandgap Perovskite Solar Cells

As rising star of solar energy collection device,organic-inorganic hybrid perovskite with ABX₃ structure solar cells attracted attention from all around the world because of its excellent photovoltaic properties,low preparation cost,compatibility of multiple preparation techniques and the like.Till now,the power conversion efficiency（PCE）of pure Pb-based perovskite solar cells（PSCs）have rocketed from 3.8%tro 25.7%that is rival the crystal silicon counterpart.According to Schottky-Queisser（S-Q）theory,however,the limit conversion efficiency PCE pure Pb-based PSCs is lower than the narrow bandgap materials due to its unideal bandgap.Owning more ideal bandgap dand thus extended absorption spectrum range,Sn-Pb based narrow band gap perovskite solar cells（NBG PSCs）has a higher theoratical S-Q limit conversion and are expected to achieve higher PCE.More than that,the combination of NBG PSCs and wide bandgap subcell is predicted to surpass S-Q limit efficiency of single cell.In addition,replacing part of Pb²⁺with Sn²⁺can reduce the toxicity of Pb and is more in line with the vision of green development in future.Recently,although the PCE of NBG PSCs with PEDOT:PSS as hole transporting layer（HTL）has achieved 23.6%that is very close to the most reported Pb-based PSCs,the device stability is still a head-scratching problem because the inherent acidity and hydrophibility of PEDOT:PSS would accelerate degradation of upper perovskite layer and thus affecting the stability of devices.Also,device performance would be influenced due to vacancy defects caused by easily oxidized Sn²⁺and easily deprotonated A-site organic cationic amine.NiO_x,an inorganic metallic oxide HTL material,has been favored due to its intial highly physical and chemical stability,deeper valance band maximum thus more matched with perovskite layer,multiple preparation techniques and low cost.However,the application of NiO_x into NBG PSCs would cause much lower performance,which made the relevant research progress develop slowly.Research has revealed that in Pb-based PSCs,due to the existence of highly oxidizing Ni³⁺on the surface of NiO_x thin films,would undergo redox reaction with perovskite to seize the electrons from I^-and protons from A-site organic cations,thus introducing defects at interface,lifting the carrier extraction barrier at the interface and thus resulting in low device performance.Based on the problems of active layer and NiO_xHTL in NBG PSCs,in this thesis,the suppression of redox reaction at the NiO_x/NBG interface was studied to improve the photoelectric conversion efficiency of NBG PSCs devices.This thesis is based on NiO_x-based NBG PSCs,whose structure is ITO/NiO_x/NBG/C60/BCP/Ag,and mainly focuses on the inhibition of defects caused by redox reaction at the NiO_x/NBG interface.The research content of this thesis could be divided into the following three parts:1.Synthesis of NiO_x nanoparticles and performance optimization of control devices.Firstly,NiO_x with uniform particle size and stable dispersion were synthesized,and their properties were characterized in various ways.Then,they were used as HTL for the preparation of NBG PSCs devices,and the preparation process of NiO_x films by spin-coating was optimized.The effects on the performance of（FASnI₃）_0.6（MAPbI₃）_0.4 and FASn_0.5Pb_0.5I₃ NBG PSCs were investigated.Finally,the control device PCE of 17.75%and 17.51%were achieved respectively,which are at the same bench mark level of relevant reported performance at present.In addition,the preparation process parameters of NiO_x films suitable for NBG PSCs control devices were determined,which laid a foundation for the follow-up research work.2.We found that redox reactions can occur between Ni³⁺on the surface of NiO_x and Sn²⁺in NBG,resulting in the loss of protons and electrons of Sn²⁺at the A-site organic cation by Ni³⁺,resulting in vacancy defects of both at the interface,affecting the device performance.A bifacial reduction strategy was proposed to inhibit the reaction at the NiO_x/NBG interface by introducing a reducing regent ascorbic acid（AA）containing both electrons and protons required for redox reaction at the interface.The reduction effect of AA on NiO_x was studied by X-ray photoelectron spectroscopy（XPS）and Fourier infrared spectroscopy（FTIR）,and the redox reaction model at the interface was established.Through a series of optical and electrical characterization methods,the changes of energy band structure and defect state density at the interface of the device were revealed.The experimental results show that the inhibition of redox reaction at the interface reduces the defects caused by Sn²⁺and A-site cation vacancies at the interface,reduces the non-radiative recombination loss,weakens the carrier extraction barrier,and accelerates the extraction speed of holes at the interface.NBG PSC with PCE of 20.48%and NiO_x as HTL is finally realized without affecting its stability.The results provide guaidance for reducing defects and improving device performance by inhibiting the redox reaction at the NiO_x/NBG interface.3.An aprotic material,trimethyl matte iodide（TMSI）,was used for modifying the the NiO_x/NBG interface to suppress the redox reaction.XPS and XRD results show that TMSI can react with perovskite components to form TMS_xFA_1-xPb_0.5Sn_0.5I₃ at the interface.By reducing the supply of A-site cationic protons in the redox reaction at the interface,TMS_xFA_1-xPb_0.5Sn_0.5I₃ can be inhibited,thus reducing the formation of A-site organic cation and Sn²⁺vacancy defects at the interface.The crystallization quality of the film is improved and the non-radiation recombination loss is reduced.Due to this,the transient photocurrent（TPV）and transient photovoltage（TPC）test results of TMSI modified devices show that the carrier lifetime is extended and its extraction at the interface is smoother and faster.Finally,NBG PSCs modified with TMSI achieved PCE of 19.36%without affecting its stability,which provides guidance for suppressing the redox reaction at the NiO_x/NBG interface to reduce defects and improve the performance of NBG PSCs devices.