Effect Of Organophosphorus Ligands On The Performance Of Methylammonium-free Perovskite Solar Cells

Perovskite solar cells（PSCs）have attracted much attention due to their high power conversion efficiency（PCE）,facile fabrication,and low production cost.In more than ten years of development,its certified photoelectric conversion efficiency has reached25.73%,which is comparable to the current mainstream crystalline silicon solar cells.However,the long-term stability of PSCs is still a stumbling block hindering the further industrial development of PSCs.Among various perovskite structures,formamidine cesium perovskite FA_xCs_1–xPb I₃（0<x<1）shows excellent photoelectric properties because a small amount of Cs⁺can prevent FAPb I₃ changing from photoactiveα-phase to inactiveδ-phase,thus stabilizingαphase FAPb I₃.At the same time,FA_xCs_1–xPb I₃ perovskite has strong thermal stability,because their structures do not include fragile methylamine（MA）which is easy to volatilize at low temperature.However,it is difficult to obtain FA_xCs_1–xPb I₃ perovskite films with high crystal quality because the rapid aggregation of mixed cations makes nucleation and crystal growth difficult to control,resulting in films with rough surfaces,small grain sizes,and high defect densities.In view of this,this paper regulates the nucleation and crystallization process of perovskite in the FACs system through additive and component engineering to improve the film quality of the photosensitive layer to optimize the performance and stability of PSCs.At the same time,we also innovatively give a solution to the problem of lead protection,and effectively reduce the leakage of Pb²⁺by using the component engineering strategy,which provides a comprehensive reference for the commercial application of perovskite solar cells.The main research contents of this paper are as follows:（1）DOPO,a flame retardant,is introduced into the precursor to adjust the crystallization of perovskite and passivate the defects of perovskite film.Due to the strong coordination and intermolecular interaction between phosphophenanthrene and Pb²⁺,the crystallinity and stability of perovskite films are greatly improved.The efficiency of FA_0.85Cs_0.15Pb I₃ PSCs with an active area of 0.255 cm² was significantly improved from17.95%to 21.13%with negligible hysteresis.This additive strategy using multifunctional molecules has demonstrated the practical application prospects of PSCs with high efficiency and stability.（2）We introduce ethylenediaminetetramethylene phosphate（EDTMP）as a dopant in the precursor solution to improve the stability and efficiency of PSCs and form a Pb²⁺leak-proof barrier in harsh weather.Due to the multiple interactions between EDTMP and perovskite molecules,the addition of EDTMP can act as a crystal growth control agent and passivator for perovskite films,thereby slowing down the crystallization rate of the films and obtaining high-quality perovskite films.At the same time,we gained insight into the effect of doping modifiers on tuning the interfacial energy level alignment as well as charge transport and charge recombination under photoexcitation.Finally,the PCE of FA_0.85Cs_0.15Pb I₃ PSCs with an active area of 0.255 cm² increases from 19.28%to 22.36%.Meanwhile,the 5×5 cm module with an effective area of 19.32 cm² also achieved the best PCE of 19.16%.In addition,the unencapsulated EDTMP-modified PSCs exhibited better air and thermal stability.More importantly,because EDTMP can have a strong coupling with Pb²⁺in aqueous solution and act as a metal chelator,the prepared device exhibits excellent in-situ absorption capacity of lead ions.（3）Besides a series of optimization strategies for the perovskite photosensitive layer,the fast charge transfer at the interface between the photoactive layer and the charge transport layer is also very important.Various defects and energy barriers at the interface will lead to the non-radiative recombination of charge carriers during the charge transfer process,resulting in a decrease in charge collection efficiency,and ultimately hindering the improvement of the stability of PCEs and PSCs.Here,we report a dual-interface modification strategy by using different phosphonic acid molecules to modify the interface between the Sn O₂ layer and the perovskite layer,and the interface between the perovskite layer and the hole transport layer,respectively.Before depositing perovskite layer,acidic diphenyl phosphonite chloride（DPC）with anti-ultraviolet performance was used to modify the surface of Sn O₂,so as to reduce the corrosion of the perovskite film by OH^-in Sn O₂ and adjust the growth process of perovskite crystal.Methyldiphenylphosphine oxide（MPO）,which can coordinate with Pb²⁺,is used in the post-treatment of the perovskite film surface to passivate the defects at the film surface and grain boundaries.The effects of organic interfacial modifiers on charge transport and charge recombination were comprehensively studied by photoluminescence spectroscopy and electrical measurements.The best FA_0.85Cs_0.15Pb I₃ device with an active area of 0.255cm² achieved a PCE of 23.37%.Furthermore,the best PCE of 20.29%was obtained for the device with an active area of 1 cm².More interestingly,due to the strong UV-absorbing ability of DPC,the final device modified with double interface can still maintain 91.4%of the initial efficiency after 1000 hours in air,and 87.8%after 500 hours in AM1.5G light.