| Perovskite solar cells(PSCs)have rapid progress with power conversion efficiency(PCE)gradually approaching the theoretical efficiency of single-junction solar cells.However,the stability of perovskite(ABX3)materials has become a major obstacle to its commercialization.Dimension reduction is an effective method to improve the stability of perovskite materials,and the stability usually increases as the dimension decreases.Considering the accompanying adverse effects of quantum confinement,two-dimensional(2D)perovskite is believed as an ideal light-absorbing material with both high efficiency and stability.In this thesis,2D perovskites with three different crystal structures were constructed based on different stabilization mechanisms.In view of the crystal defects,such as defective thin films,disordered orientation,and disorganized phase distribution,observed in the respective systems,corresponding solutions and optimization strategies were provided,aiming to obtain both high-efficiency and stable perovskite solar cells.The specific research work is as follows.(1)Construction of the thermodynamically stable pseudohalogen 2D perovskite(X site adjustment).The effects of methylammonium(MA+),formamidinium(FA+),and Cs+cations as well as I-,Br-,and Cl-anions on the crystallographic,photophysical,and photoelectric properties of the 2D perovskite material were investigated.The smaller MA+ and Cs+ were able to form the 2D A2Pb(SCN)2X2 structure,while the larger FA+ would cause lattice distortion and form the δ phase with poor lightabsorption ability.When replacing I-with Br-and Cl-,the optical bandgap of perovskite increased sharply,limiting their application in solar cells.Among these materials,devices based on MA2Pb(SCN)2I2 and Cs2Pb(SCN)2I2 showed better photovoltaic performance and better stability than their corresponding 3D perovskites.However,the large optical bandgap and poor inter-layer carrier transport limit their device efficiency.Therefore,the pseudohalogen 2D perovskite may not be an ideal lightabsorption material for high-efficiency PSCs.(2)Construction of thermodynamically stable Pb-free Sb-based 2D perovskite(B site adjustment).To solve the problems of poor morphology and uncontrollable halide composition of Sb-based perovskite films,a novel bis(trifluoromethane)lithium sulfonimide(LiTFSI)coordination molecule with multiple O donors was developed.LiTFSI could not only promote heteronucleation,but also form a zero-dimensional(0D)intermediate phase as the spacer molecule among Sb-based pyramidal clusters,which mediated the crystallization and resulted in high-quality and low-defect 2D MA3Sb2I9-xClx films.The slower film formation well stabilized the bandgap of perovskite films with a fixed Cl:I ratio(～7:2).As a result,Sb-based PSCs achieved a record efficiency of 3.34%and retained 90%of its initial efficiency after 1400 hours in the ambient environment.In addition,based on this environmentally friendly and stable Sb-based perovskite material,semi-transparent photovoltaics(ST-PV)with efficiencies of 2.62%-3.06%and average visible light transmittance(AVT)of 42%-23%were achieved,demonstrating the potential of this emerging Sb-based perovskite semiconductor for a wide range of photovoltaic applications.(3)Construction of quasi-2D perovskite with good environmental stability(A site adjustment).To solve the problem of poor interlayer carrier transport caused by random crystal orientation,poor crystal quality and disorganized phase distribution in quasi-2D perovskites,we developed strategies of "vertical-rotation process","rerouting crystallization pathway" and "fully and orderly expanding phase distribution",respectively.(a)A universal approach based on the synergistic effect of NH4Cl and H2O was developed to rotate the crystal orientation of the inorganic layer.The preferential adsorption of NH4Cl on the(202)crystal plane and the subsequent deprotonation of NH4+accelerated by H2O induced the out-of-plane orientation of the inorganic framework,which greatly improved the crystallinity,charge mobility and carrier lifetime of the quasi-2D perovskite films.The(PEA)2(MA)3Pb4I13-based device prepared by the "vertical-rotation process" achieved an efficiency of 17.03%and showed significantly higher stability than that of three-dimensional(3D)devices.(b)Rerouting the crystallization pathway of the quasi-2D perovskite films.By controlling the specific adsorption of NH4IxCl1-x additive on different crystal planes,the dynamic preferred growth of(111)plane was deliberately inhibited,and the(202)planes were exposed as secondary nucleation sites to promote the creation of large grains.As the halogen-regulated deprotonation of NH4+ proceeded,(111)crystal planes gradually recovered its growth dominance,and finally formed a vertically-oriented quasi-2D perovskite film with high uniformity,low defect and desired carrier transport.The resultant device achieved an efficiency of 1 8.5%and a fill factor(FF)as high as 83.4%,which was the highest FF value for quasi-2D PSCs.(c)By adjusting the colloid properties of the precursor solution,the mechanism of phase distribution in quasi-2D perovskite films was explored.The synergistic effect of bulkier NMA cation and NH4Cl was introduced to fully and orderly expand the phase distribution of quasi-2D perovskite films.The aligned phase distribution was beneficial to the energy level matching among multiphases,which overcomed the strong quantum confinement effect,reduced the defect density and promoted the interlayer carrier transport.Therefore,even the quasi-2D PSCs with ultra-low inorganic layer thickness(... |
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