Multifunctional Organic Compounds To Enhance The Optoelectronic Properties And Stability Of Perovskite Cells

Perovskite solar cells(PSCs)are favored by scientists at home and abroad due to their excellent photoelectric properties(broadband light absorption range,long carrier diffusion distance,low defect state density,etc.),and the advantages of low-cost preparation by solution method.After decades of development of PSCs,the photoelectric conversion efficiency has improved by leaps and bounds.At present,the internationally certified efficiency has reached 25.5%,which is comparable to monocrystalline silicon cells and has become a new type of photovoltaic cell with the most industrialization potential.However,before the commercialization of PSCs,the stability problems caused by some key factors must be solved.These main factors include:a large number of defects,ion migration,sensitivity to moisture and unstable doping in the hole transport layer.Therefore,this paper introduces compounds containing multiple functional groups to perform additive engineering and interface engineering on perovskite films to improve the photoelectric performance and stability of PSCs.The following is the main research content of this paper:1.Herein,an innovative Cs+and Ac-codoping strategy is applied to improve the efficiency and stability of FA0.85MA0.15PbI3 PSCs by only introducing Cesium Acetate(CsAc)as dopant.The Cs+ and Ac-ions have been incorporated into the perovskite film,and the doped perovskite films show larger grain size(1.39 um),longer carrier lifetime and lower defect density.It is found that Ac-has a strong interaction with Pb2+ in perovskite,which is benefit for the stability of perovskite via forming stronger Pb-O bonds.The coexistence of Cs+ and Ac-ions in perovskite exhibits an n-type doping effect which upshifted Fermi level of perovskite and is benefit for improving the Voc of PSCs.Finally,a perovskite solar cell with an efficiency of 21.95%was prepared.The doped PSCs retain 97%of their initial PCE values after aging for 55 days in air without encapsulation.More interestingly,during the aging process,the Voc of doped devices reach to a fairly high value of 1.16 V,which is one of the highest Voc for reported iodine and FA-based PSCs.Therefore,CsAc is a very potential additive,which can effectively improve the stability of the device.2.In the FA0.85MA0.15PbI3 perovskite,organic cations(FA+/MA+)are fixed in the inorganic framework through electrostatic interaction.Therefore,it is expected to stabilize the FA+/MA+ ions in the perovskite by introducing more electronegative anions into the inorganic framework.However,the most commonly used bromide ion(Br-)will enlarge the band gap of perovskite.Therefore,the more electronegative anions(such as fluoride ions)are introduced into the grain boundary or surface of the perovskite film.It can greatly increase its electrostatic interaction with organic cations(FA+/MA+)on the grain boundary or on the surface,thereby stabilizing the perovskite phase.In this work,aided by theoretical calculation,we fittingly selected a multifunctional molecule,2,2-difluoropropanediamide(DFPDA).It is hoped that a variety of functional groups can be introduced into the perovskite solar cell at the same time,so that the multiple functional groups can play their respective roles and have a synergistic promotion effect between each other,solve the stability problem while improving the efficiency of the device.Specifically,the carbonyl groups in DFPDA form chemical bonds with Pb2+and passivate under-coordinated Pb2+ defects,at the same time reduces the crystallization rate of the perovskite to obtain a high-quality perovskite film.The amino groups bind with iodide to suppress ion migration and also increase the electron density on the carbonyl groups to further enhance their passivation effect.The F element improves the humidity stability of the film on the surface of the perovskite,and at the same time forms a bridge between the perovskite and the hole transport layer to achieve effective charge transport.The photoelectric conversion efficiency(PCE)of the organic-inorganic hybrid perovskite solar cell prepared based on this method reaches 22.21%.After the unencapsulated device is stored in the air for 60 days,the device with DFPDA additive can maintain 92%of its initial efficiency;thermal stability and light stability have also been significantly improved.3.The replacement of small cations with bulkier organic cations with long alkyl chain or benzene ring to form thin two-dimensional(2D)perovskite passivation layer on three-dimensional(3D)perovskite(2D/3D)has become a promising strategy for improving both efficiency and stability of perovskite solar cells(PSCs).In this work,the surface of the perovskite film was modified by introducing the organic fluoride 4-trifluoromethylphenethylamine hydroiodide(CF3PEAI),a new 2D(CF3-PEA)2FA0,85MA0.15Pb2I7 perovskite top layer formed on the surface of FA0.85MA0.15PbI3 through a facile solvent-assisted interfacial ion exchange process.The 2D(CF3-PEA)2FA0.85MA0.15Pb2I7 capping layer can not only passivate FA0.85MA0.15PbI3 film but also act as super-hydrophobic layer to inhibit water diffusion and significantly enhance the stability.The 2D capping layer can also establish a unique graded band structure at the perovskite/Spiro-OMeTAD interface and form a p-type doping for Spiro-OMeTAD layer which is benefit for efficient charge transport.Optimized PSCs based on 2D/3D heterojunction yield a champion power conversion efficiency(PCE)of 23.1%and improved stability.The device could maintain 84%output for 2400 h aging under environmental conditions without encapsulation,and maintain 81%for 200 h under illumination with encapsulation.This work provides a new paradigm for the use of 2D/3D structures to further improve the performance and stability of perovskite solar cells.