Applications Of Multi-functional Organic Small Molecules In Perovskite Solar Cell And Related Device Physics Study

In order to alleviate the global energy shortage and environmental pollution,researchers in science and industry are trying hard to explore new renewable energy.As clean energy,sunlight irradiation can be utilized by solar cells and has been more and more important in the sustainable development.In the past 20 years,the installation of solar cell modules has doubled every 2-3 years,and its cost has decreased 300 times in the past 40 years.This is due to the continuous exploration and innovation of materials,device structures and technologies by researchers.At this stage,solar cells have developed to the third generation,thin-film solar cells,and the power conversion efficiency（PCE）of devices has gradually increased to more than25%.As a new photovoltaic technology in recent years,the performance of organic-inorganic hybrid perovskite solar cells has improved rapidly.Advantages including not only high PCE that reached the commercial standard,but also the simple,low cost fabrication process and application scenario adaptability,have made it a popular photovoltaic.This paper is based on the development of notable and efficient perovskite solar cells,and tries to provide feasible and effective solutions to the common problems existing in perovskite photovoltaic devices.As we all know,the golden triangle of efficiency,cost and stability is an important index to evaluate all kinds of photovoltaic devices,including perovskite solar cells.At present,the record PCE of perovskite solar cells based on formamidine lead iodine（FAPb I₃）perovskite light absorbing layer is more than 25%,which is completely comparable to the efficiency of traditional inorganic semiconductor photovoltaic devices such as Si,Ge and Ga As.Additionally,perovskite photovoltaic devices are far lower than inorganic cells in manufacturing complexity and cost.However,as an ionic crystal,the intrinsic characteristics of perovskite materials（low crystallization activation energy）lead to poor water,oxygen,light and thermal stability of devices.Therefore,it is urgent to develop more strategies that can effectively increase the resistance of perovskite materials against water,oxygen and suppress light and thermal degradation.In addition,the high-efficiency perovskite light absorbing layer is usually based on lead.With the demand for safety and environment friendly production for lives,the problem of the typical perovskite components containing heavy metal,lead,has attracted more and more attention.Therefore,the preparation of high-efficiency,stable and low toxicity perovskite solar cell is a priority at this stage.It is worth mentioning that in the development of low toxicity perovskite,using tin,the same host element of lead,to completely or partially replace lead is a widely approved approach for further study.Among them,tin-lead binary perovskite has been comparable to lead based perovskite in efficiency,and even has greater theoretical efficiency than lead based perovskite devices（its band gap is closer to the ideal band gap of S-Q theory）.However,tin substitution inevitably introduces a new problem of Sn²⁺oxidation,and appropriate countermeasures need to be developed.Interface and doping engineering are important approaches for researchers to develop efficient,stable and low toxicity perovskite solar cells.To solve problems including the structural defects,water and oxygen erosion,Sn²⁺oxidation and other problems in the perovskite layer,we developed passivate,water and oxygen isolation and antioxidant materials for surface and interface’encapsulation’or doping to modify perovskite.In this paper,we reasonably designed the molecular structure of surface interface treatment or bulk doping materials,and explored the application of multifunctional molecules in efficient,stable,and low-toxic perovskite solar cells.The perovskite light absorbing layer used in these three parts of the paper is based on the principle of gradually reducing the content of toxic lead in perovskite,which aims to improve the photovoltaic performance,the stability of devices and reduce the toxicity of perovskite.At the same time,for each part,we try to reveal the mapping relationship between the material properties involved in the perovskite modification process and the carrier dynamics process of the device from the perspective of device physics.This paper is divided into the following three parts,which correspond to the relevant discussions in Chapter 2,3 and 4.In the first part of the work,we take methylamine lead iodine（MAPb I₃）perovskite as the light absorbing layer and introduce a self-assembled monolayer of 4-chlorobenzoic acid（4-Cl BA）on the perovskite surface for interface modification.The results show that the carboxyl of 4-Cl BA can effectively passivate the uncoordinated Pb²⁺on the perovskite surface and inhibit the non radiative energy loss caused by the defects.At the same time,4-Cl BA introduces an interface dipole between perovskite and hole transport layer,which effectively increases hole extraction and suppresses the loss of interface carrier recombination.Finally,the hydrophobic 4-Cl BA groups form a protective layer on the surface of perovskite,reduce the hydrophilicity of perovskite and improve the resistance of perovskite layer to water and oxygen corrosion.Finally,the PCE of the champion device is near 21%,and the storage stability T₈₀ life of the device is improved from 100 hours to 700 hours.In the second part,we studied the mixed binary perovskite soalr cell with Tin-lead ratio of 3:7 in order to reduce the heavy metal lead in perovskite and prepare lower toxic perovskite soalr cell.To solve the problems including the structural defects during solution-fabrication process of perovskite and Tin component oxidation et al.,two-dimensional（2D）perovskite was in-situ grown on the surface of three-dimensional（3D）tin-lead binary perovskite by using the reductive large cation ligand tyramine ion.The introduction of tyramine based two-dimensional perovskite not only passivates the structural defects on the surface of perovskite through coordination,but also suppresses the phase separation problem caused by the migration of halogen ions and enhances the optical stability of perovskite.Due to the reducibility of organic cations,and the 2D/3D perovskite heterojunction,the oxidation of Sn²⁺in perovskite components was effectively inhibited and avoided the p doping and decomposition caused by the oxidation of Sn²⁺.The tyrimine based 2D perovskite enhance the hydrophobicity of the system.The PCE of tin-lead binary perovskite solar cells reach 18.3%,and the T₈₀ life of encapsulated devices was over 1000 hours.In the third part,we further reduced the Lead content in perovskite layer and prepared Lead lean perovskite solar cells with Tin-Lead ratio of 6:4.The main problems in this system are the low quality of perovskite polycrystal caused by the unbalanced crystallization rate of Tin-Lead components,the high density of structural defects and severe Sn²⁺oxidation.Therefore,we initially doped three reducing natural polyphenol products of apigenin,quercetin and resveratrol into perovskite precursor.The results show that apigenin and quercetin can effectively form chelates with Sn²⁺,so as to regulate the crystallization kinetics of perovskite.At the same time,apigenin shows obvious Sn²⁺selective chelation between Pb²⁺and Sn²⁺.The doping of apigenin effectively improved the crystallinity,homogeneity and grain size of the final perovskite film.The structural defects in the film are reduced.In addition,the three kinds of natural polyphenols molecules effectively inhibited the oxidation of Sn²⁺in the crystallization and subsequent device aging process of Sn-Pb perovskite films.Finally,device based on the apigenin doped FA_0.7Pb_0.3Sn_0.5Pb_0.5I₃ devices yield a PCE of 21.1%with an enhanced stability.Our results also show that these flavonoids with specific chelation provide a new research avenue to solve the problem of inconsistent crystallization rate of perovskite Tin-Lead components.