Preparation Of Low Symmetry Perylene-Diimide-Based Acceptors Small Molecules And Study On Photovoltaic Performance

With the deepening of the research on organic solar cells,the design of new materials is increasingly rich,the device preparation method is constantly improved,and the deep theory of batteries is gradually improved.Photosensitive active layer is an important place for the photoelectric conversion of organic solar cells（OSCs）,and the research and development of electron acceptor materials forming the active layer is particularly critical.This paper focuses on the low cost,easy structure of perylene diimide（PDI）based derivatives,through the introduction of different structures,the introduction of the ether chain,and monofluorinated low symmetry benzodithiophene（BDT）bridging series PDI based dimer acceptor materials,and systematically studied the influence of the material structure on the photothermal stability,sunlight absorption capacity,energy level,molecular planarity and aggregation,photovoltaic properties and thin film morphology.The main research contents are described as follows:The chapter one mainly introduces the world’s energy development situation and the current situation of solar energy development and utilization,expounds the basic working principle of OSCs,typical device structure,characterization parameters and the development process of OSCs;then investigates the development and research status of PDI base small molecule acceptor materials,and summarizes the classification of dimer,trimer,multimer and fused ring type;finally,introduces the design idea and research content of the construction of low symmetry PDI base small molecule acceptor materials.The chapter two,the low-symmetry units T-BT and TT-BT coupled to benzothiophene（BT）and thiophene（T）or thio[3,2-b]thiophene（TT）are selected as the connecting nucleus,two acceptors small molecules T-BT-（PDI-HD）₂and TT-BT-（PDI-HD）₂were prepared by Stille coupling reaction with PDI single-bromide（PDI-HDBr）containing 2-hexyl decyl flexible side chain.It is found that after the bridging unit is replaced by T-BT to TT-BT,the maximum absorption peak of the film state absorption peak is 18 nm,the LUMO level decreases from-3.67 e V to-3.69 e V,the extinction coefficient decreases,the molecular planarity increases,the solution aggregation state is enhanced,and the miscibility with the donor material becomes worse.The prepared bulk heterojunction photovoltaic device shows that the optimal mixing ratio of PTB7-Th:T-BT-（PDI-HD）₂is 1:1.5,and the best energy conversion efficiency（PCE）of the device is achieved.With DIO additives,PCE increased to2.88%,open circuit voltage(V_OC)reached 0.81 V,short circuit current density(J_SC)reached7.56 m A·cm^-2,and filling factor（FF）reached 47.33%.After using TT-BT-（PDI-HD）₂,the PCE of the corresponding cell decreased to 2.26%,mainly affected by the reduced V_OCand J_SC,while the reduced LUMO level of the material and the poor solar power capture force may be the main constraints.The chapter three select ether chains of two methoxy（OCH₃）and ethylene glycol mono-methyl ether（Eg）into the outer bay position of PDI to produce PDI-Br-OCH₃and PDI-Br-Eg,coupled with ditin T-BTSn to prepare acceptor small molecules T-BT-PDI-OCH₃and T-BT-PDI-Eg.It is found that compared with the small molecule T-BT-PDI-Eg containing long ether chain Eg,the small molecule T-BT-PDI-OCH₃containing short ether chain has a higher LUMO level,increased extinction coefficient,slightly distorted molecules,weaker aggregation,and better miscibility with the donor material.The optimal efficiency of the device mixed with the donor polymer PTB7-Th reached 2.33%.The related device parameters were V_OC=0.81 V,J_SC=6.62 m A·cm^-2,FF=41.81%.All the parameters were significantly higher than that of T-BT-PDI-Eg with Eg,which is closely related to the increased LUMO level of small molecules with short chain OCH₃,higher electron mobility,high molar extinction coefficient of improved photon absorption,and the film morphology conducive to carrier migration.The chapter four the symmetric thiophene side group modified benzodithiophene（BDT）and monofluoro-substituted thiophene side group BDT（BDTF）were selected as bridging units to construct dimer BDT-（PDI-HD）₂and BDTF-（PDI-HD）₂with PDI-HDBr.It is found that the absorption spectrum of the synthesized material is complementary to the absorption spectrum of the donor material.After the introduction of monofluorine,the light absorption range is slightly narrowed but the extinction coefficient increases significantly,the LUMO level lower,the planarity is improved and the molecular aggregation is enhanced.The photovoltaic device test showed that the optimal device performance of BDTF-（PDI-HD）₂devices（1.08%）was significantly lower than that of BDT-（PDI-HD）₂devices（2.23%）.The corresponding V_OC,J_SCand FF of BDTF-（PDI-HD）₂are all reduced,which may be related to the excessive aggregation caused by the enhanced planarity,which then limits the electron mobility and photocurrent.The above studies show that the low symmetry unit can introduce the adjustable spectral absorption and increase the molecular polarity,which then affects the cell performance,and provides reference for the subsequent material design and optimization.However,the introduction of long ether chains in the bay position and the selection of halogen atoms to build low symmetry molecules still need further verification.