Tailoring Of Quasi-two-dimensional Perovskite Organic Ligands And Their Device Performance

In recent years,organic-inorganic hybrid perovskite solar cells(PSCs)have attracted extensive attention from researchers and industry professionals due to their excellent photoelectric conversion performance and low-cost solution processing,a and are considered to be an outstanding representative of the new generation of thin-film solar cell technology,its highest certified power conversion efficiency(PCE)has reached 25.7%,which is comparable to the efficiency record of monocrystalline silicon solar cells(26.1%).However,the organic components(methylammonium MA~+or formamidinium FA~+)in the traditional three-dimensional(3D)perovskite structure will decompose under the condition of moisture,oxygen,and strong light,resulting in the degradation of the perovskite crystal structure,making it lose its photoelectric conversion ability.This device attenuation phenomenon seriously restricts the development of perovskite photovoltaic devices.Two-dimensional(2D)perovskites can fundamentally improve the stability of perovskite films and devices,mainly due to the introduction of organic ligands that endow perovskites with a more stable crystal structure that can effectively inhibit ion migration.However,the introduction of traditional organic ligands results in strong quantum confinement effect and dielectric confinement effect,which increases the exciton binding energy and reduces the dissociation and transport efficiency of photogenerated carriers,which is the root problem for the reduced efficiency of 2D PSCs.In this thesis,aiming to improve the photogenerated carrier transport efficiency of2D perovskite,two novel organic ligands containing sulfur were designed to synthesize related 2D perovskite single crystals,and then 2D perovskite films and solar cells were fabricated.The research showed that the introduction of new organic ligands can endow perovskite films with favorable crystallinity and optical properties,and then the carrier transport properties and the performance of the final device were significantly improved.The specific research conclusions are as follows:1.Based on the design principles of 2D perovskite organic ligands,a new type of organic amine,cystamine,was selected as the spacer of 2D perovskite.First,a cystamine-based perovskite single crystal was synthesized,and the crystal structure analysis showed that the cystamine molecule can effectively insert into the 3D perovskite lattice,thereby templating the 2D perovskite structure to form a 2DDJ perovskite.After that,the single crystal was directly added to the perovskite precursor solution and a quasi-2D perovskite thin film was prepared.The thin film showed good crystallinity and optical absorption ability.Finally,the n=5 quasi-2D PSC based on cystamine obtained the champion PCE of 13.5%.This was mainly due to the existence of disulfide bonds in the cystamine molecule,the shortest distance between adjacent disulfide bonds is 3.47?,indicating that there is an obvious strong interaction between the ligands,stabilizing the crystal structure.On the other hand,the existence of disulfide bonds increases the charge density of the organic layers,thereby,the charge transport between adjacent inorganic layers was enhanced.2.Another multifunctional organic amine,L-cysteine(Cys),was chosen as the organic spacer for 2D perovskite.Such organic amine has large dipole moment of 2.45D,much larger than that of butylamine(1.22 D),which is beneficial to realize carriers transport across inorganic layers.Also,there is a strong S…S interaction between the sulfhydryl groups of the ligands,and it is speculated that a disulfide bond may be formed,which realizes the transformation of the structure from the RP to the DJ phase,thereby making the 2D perovskite structure more stable.In addition,the hydroiodide of Cys is a room temperature molten salt,which weakens the inhomogeneous phase distribution of the film due to the solubility difference between organic and inorganic components in the traditional 2D perovskite precursor solution,resulting in a high phase purity of the obtained 2D perovskite film.Finally,the n=4 quasi-2D PSC based on Cys obtained a PCE of 14.2%,the PCE of butylamine control device was 12.5%,this is mainly due to improvements in film quality,including high film crystallinity and orientation and low nonradiative recombination.