Precise Regulation Of Two-dimensional/Three-dimensional Perovskite Bulk Heterojunction

Organic and inorganic halide perovskite solar cells based on three-dimensional（3D）perovskite crystal structure have excellent photovoltaic performance,and their power conversion efficiency is improved to 25.5%.However,3D perovskite materialis easy decompose within oxygen and water,resulting in its device low stability.The inert large-volume spacer cations of two-dimensional（2D）perovskite structure not only interact strongly with[Pb I₆]^4-improving its structural stability,but also act in a barrier layer obstructing water and oxygen,which makes the 2D perovskite with excellent environmental stability.However,wide band gap and quantum well of 2D perovskite limits its photovoltaic performance.The 2D/3D perovskite heterojunction can combine the stability of 2D perovskite,and retain the photoelectric properties of 3D perovskite.However,The 2D perovskite phase structure in the traditional 2D/3D bilayer perovskite heterojunction is difficult to accurately control,which does not match the energy band between the electron transport layer and 2D perovskite.Therefore,it is difficult to be applied to structure of the invert devices.Metastable quasi-2D perovskite forming in the mixture 2D/3D perovskite heterojunction is the most serious challenge to the effectiveness of this strategy.In order to better balance the stability and high efficiency of perovskite solar cells,pure phase layer flake 2D perovskite were inserted in the 3D perovskite crystal boundary to construct 2D/3D bulk heterostructure as a promising strategy.Broad band gap of 2D perovskite phase restain lateral carrier transmission within 3D perovskite,which not only promotes the charge carrier dynamics,also improves the stability of device operation.Conventional 2D/3D bulk heterojunction was prepared by in situ method that a small amount of inert cationic organic amine insteaded of the small volume of organic amine in 3D perovskite precursor solution,which inevitably spontaneous forms of the value of n unpredictable quasi-2D perovskite.Different n values 2D perovskite phase in heterojunction causes the differences of carrier transport modes,which caused the degradation of its device performance.Therefore,we proposed a precrystallization strategy that pure phase structure 2D perovskite single crystals with rigidπconjugated organic ligand（PYA）₂Pb Cl₄and（PYA）₂Pb I₄were mixed into Cs_0.17FA_0.83Pb(I_0.87Br_0.13)₃iodine-based perovskite precursor.The phase transition of（PYA）₂Pb Cl₄to quasi-2D iodine-based perovskite was inhibited due to the lattice mismatch of Cl/I.The pure phase structure is retained at the grain boundary of 3D perovskite,forming bulk heterojunction with type I band arrangement,which inhibits the transverse charge transfer and improves the carrier lifetime.On the contrary,the phase transformation of（PYA）₂Pb I₄is difficult to suppress.quasi-2D perovskite with different n values causes the messy phase structure at the heterojunction,forming a heterojunction structure with chaotic type II band arrangement.Secondly,due to the difference of raw materials’batch,environmental sensitivity and solubility of precursor components in polar aprotic solvent,it is difficult to achieve accurate stoichiometric control of perovskite components in traditional precursor solutions.The difference of perovskite films surface properties is an important factor of low repeatability of the performance of its battery devices.However,the perovskite films prepared by the precursor based on dissolution of perovskite single crystal can inherit the advantages of high crystallinity,accurate stoichiometric ratio,low well density and good environmental stability and phase stability of the parent crystal of perovskite.Therefore,combined with the purity study of 2D phase in the perovskite heterojunction,we proposed the solution strategy of double single crystal.First of all,the CS_0.1FA_0.9Pb(I_0.9Br_0.1)₃three-dimensional perovskite single crystal and（3-TMA）₂Pb Cl₄two-dimensional perovskite single crystal were preparated.Then,the（3-TMA）₂Pb Cl₄two-dimensional perovskite crystal was introduced into the precursor prepared by dissolved 3D perovskite crystal to architecture 2D/3D bulk heterojunction structure.which combines the advantages of 3D perovskite and 2D perovskite,preparing high efficient and stable perovskite solar cells.The results show that the stoichiometric ratio deviation,crystallinity and defects of the films prepared based on 3D crystal precursor solution are reduced,and its device efficiency repeatability are significantly improved.With the introduction of two-dimensional（3-TMA）₂Pb Cl₄,the 2D perovskite phase is located at the grain boundary of 3D perovskite.The carrier transport at the heterojunction structure tends to be type I,which passivates the defects at the grain boundary.The efficiency of the device is improved to 22.3%,and the environmental stability of the heterojunction film is also greatly improved.