【作者】 邓敏；

【导师】 彭强；

【作者基本信息】 四川大学 ， 有机化学， 2022， 博士

【摘要】 现代社会进入了高速发展阶段,人们对能源的需求也随之加大。由于传统化石能源储量的日趋减少以及化石能源使用带来的温室效应和环境污染日益严重,开发清洁可再生能源替代传统化石能源已迫在眉睫。太阳能是一种取之不尽用之不竭的可再生能源,因此受到广泛关注。有机太阳能电池是一种将光能直接转化为电能的光电器件。其具有生产成本低、质量轻、柔韧性好、可以大面积制备等优点,在近年来得到飞速发展。目前,高性能有机太阳能电池主要基于小分子受体材料Y6及其衍生物,其单节光电转换效率已经突破18%。与市售多晶硅无机太阳能电池效率相当,体现出巨大的产业化潜力。新型高性能有机光伏材料的开发是进一步提升有机太阳能电池光电转换效率的有效途径。本论文工作围绕高性能非富勒烯小分子受体材料Y6,系统性地对其分子结构中的各个部分,如中心核、侧链以及末端吸电基团端进行改性,开发了五个系列共十个小分子受体材料,全面研究了Y6体系分子结构与光伏性能之间的关系。具体工作如下:(1)中心核简化工程降低材料合成复杂度。将小分子受体材料Y6的七元稠环缩减成五元稠环,极大地降低了分子的合成难度,引入噻吩为π桥维持共轭结构面积,并在π桥上构建位置异构的烷氧基侧链形成非共价键构象锁,设计合成了小分子受体材料BTP-in-4F和BTP-out-4F。位置异构的烷氧基侧链调节了π桥与中心核和末端吸电基团间的构象。在基于烷氧基朝向末端吸电基团的BTP-out-4F的光伏器件中,获得了13.32%的光电转换效率,而基于PBDB-T:BTP-in-4F光伏器件的光电转换效率小于1%。该项工作表明,通过π桥的引入和恰当的构象锁结构的运用,可以在保持高效率的前提下简化Y6中心核结构,从而实现降低材料合成复杂度的目的。本项工作中还进一步凸显出了侧链位置异构造成的分子构象异构对材料光伏性能的深刻影响。(2)中心核异构工程构建高效固体添加剂。翻转中心核两翼的并噻吩基团,设计合成了具有W构型的Y6同分异构小分子材料i-Y6,并将其作为固体添加剂,提高了基于PM6:Y6光伏器件的性能。中心核的同分异构造成了分子构型的改变,从而影响了材料的堆积方式。i-Y6和Y6具有相同的单元结构和相似的分子尺寸,因此在非晶态时能很好的混合。另一方面,中心核的同分异构造成了分子构型的改变,从而改变了分子的堆积方式。因此,其混合物在溶液加工中的过度形核被抑制,这反而促进了Y6在热退火过程中的结晶。将少量i-Y6作为固体添加剂应用到PM6:Y6共混体系中,有效地优化了共混膜的纳米结构,将其光电转换效率提高至17.43%。该项工作表明,异构化策略是构造新型固体添加剂调节共混膜形貌的一种有效策略。(3)吡咯环侧链工程优化开路电压。基于PM6:Y6的光伏器件开路电压仅为0.83 V,普遍认为其较低开路电压是由于Y6较低的分子轨道能级造成的。为此,通过将Y6吡咯环上的侧链改为癸基环己烷,设计合成了小分子受体材料Cy-4F。癸基环己烷的长链结构和其末端的环己烷位阻单元调整了分子堆积结构,提高了分子轨道能级,从而将器件开路电压提高到了0.94 V。本项工作还进一步将Cy-4F末端吸电基团上的氟原子取代为氯原子,设计合成了小分子受体材料Cy-4Cl。氯原子的引入提高了末端吸电基团的偶极矩,从而获得了更强的光吸收,进一步提高了光伏器件的短路电流密度,在基于PM6:Cy-4Cl的光伏器件获得了13.23%的光电转换效率。该项工作表明,吡咯环侧链工程调节分子堆积是提高开路电压的有效策略。(4)外侧噻吩β位二维侧链工程提高电荷迁移率。在基于PM6:Y6的光伏器件中,PM6提供的空穴迁移率是Y6提供的电子迁移率的3倍以上,电荷传输不平衡是限制其性能进一步提高的重要因素之一。3,4-乙烯二氧噻吩硫烷基(EDOT)是高电荷传输能力基团,被成功应用于高电导率的聚合物电极材料PEDOT:PSS。通过将Y6外侧噻吩β位烷基侧链更换为3,4-乙烯二氧噻吩硫烷基二维侧链,设计合成了小分子受体材料BTP-EDOT-4F。EDOT基团的引入,将材料电子迁移率提高了一倍,使得基于PM6:BTP-EDOT-4F的光电活性层空穴迁移率与电子迁移率比值(μ_h/μ_e)降低到了0.73,极大的改善了不平衡的电荷传输,实现了高达76.40%的填充因子和16.05%的光电转换效率。本项工作还进一步将BTP-EDOT-4F末端基团上的氟原子取代为氯原子,设计合成了小分子受体材料BTP-EDOT-4Cl,也获得了高达76.41%的填充因子和15.87%的光电转换效率。该项工作表明,外侧噻吩β位二维侧链工程可以有效提高Y6体系材料的电子传输能力。(5)端基卤代工程调整分子轨道能级。L8-BO(D-4F)是最近文献报道的高效Y6衍生物,其通过Y6外侧噻吩β位侧链工程实现了材料性能的大幅提高。将其末端吸电基团的氟原子逐步取代为溴原子,设计合成了含有单溴和双溴的小分子受体材料D-3FBr和D-2F2Br。氟原子的位置异构微弱提高了分子轨道能级,在基于PM6:D-3FBr的光伏器件中实现了0.01 V的开路电压提升,其光电转换效率为16.32%。然而基于PM6:D-2F2Br的器件开路电压大幅下降,主要是共混膜的相分离尺寸太大,不利于电荷的传输和收集,阻碍了光伏器件的高效工作。该项工作表明,端基卤代工程是调节小分子受体材料能级的有效途径。本论文通过对Y6体系分子各部分的系统调整,影响了有机太阳能电池的各项器件性能参数,深化了对其构效关系的深入理解,为开发高性能光伏材料提供了理论支持,也为光伏材料分子工程改性储备了多种有效策略。更多还原

【Abstract】 The ever-growing demand for energy of humankind is caused by the rapid development of modern society.With the decreasing reserves of traditional fossil energy and the increase in global warming and pollution caused by the use of fossil energy,it is urgent to develop clean and renewable energy for replacement.Solar energy has received widespread attention due to its characteristic of inexhaustible renewable energy source.Organic solar cells（OSCs）are optoelectronic devices which directly convert photon energy to electricity.They have been developed rapidly in recent years benefiting from the advantages of low cost,light weight,flexibility and suitable for large-area fabrication.To date,the top-performing OSCs are mainly based on the small molecule acceptor material Y6 and its derivatives with the power conversion efficiency（PCE）exceeded 18%in single junction cells.This efficiency is comparable to that of the commercially available polycrystalline silicon solar cells,showing a huge potential for industrialization.The development of novel high-performance organic photovoltaic materials is an effective way to further improve the PCE of organic solar cells.This thesis focuses on the high-performance non-fullerene small molecule acceptor material Y6,and systematically modifies various parts of its molecular structure,such as the center core,the side chains and the end groups,to develop ten small molecular acceptors in five systems,in which comprehensively study the relationship between the molecular structure and photovoltaic performance.The specific work is as follows:（1）Center core simplification engineering reduces the complexity of material synthesis.The seven-membered ring structure of the small molecule acceptor material Y6 was disassembled into a five-membered ring,which greatly reduced the synthetic complexity.Thiophene units were introduced asπbridges to maintain the size of the conjugate structure,and alkoxy side chains with positional isomerism were constructed on theπbridges to construct noncovalent conformation locks,thus,small molecular acceptor materials named BTP-in-4F and BTP-out-4F were designed and synthesized.The isomerizedπbridges with alkoxy side chains modulated the conformational isomerization between theπbridges and the center core or the end groups.The PCE of 13.32%was obtained in the photovoltaic devices containing BTP-out-4F with alkoxy groups close to the end groups,while the PCE of PBDB-T:BTP-in-4F based devices were less than1%.This work demonstrated that it is possible to reduce synthetic complexity while maintaining high performances of Y6 derivatives by center core simplification engineering through the introduction ofπbridges and appropriate conformational locks.This work further highlighted the significant influence of isomerization of side chains on the photovoltaic performances.（2）Center core isomerization engineering constructs a high performance solid additive.By inversing the thienothiophene wings of the Y6 molecule,a W-shaped isomer i-Y6 was designed and synthesized as a solid additive to improve the performance of PM6:Y6-based photovoltaic devices.The Y6 and i-Y6molecules mixed well in amorphous phase attributed the identical construction units and the similar molecular sizes.On the other hand,the isomerism of the center core led to the change of molecular configuration,which affected the stacking style of the molecules.Therefore,the over nucleation in the mixture during solution processing was inhibited,which in turn promoted the crystallization of Y6 during the subsequent thermal annealing process.The i-Y6 was found effective to optimize the nanostructure of PM6:Y6 blend system at low additive dosages and enhanced the PCE to 17.43%.This work demonstrated that isomerization is an effective strategy to construct novel solid additives to modulate the morphology of the blended films.（3）Pyrrole side chain engineering optimizes open circuit voltage(V_oc).It is generally believed that the lower V_oc of 0.83 V for PM6:Y6-based photovoltaic devices was ascribed to the lower molecular orbital energy levels of Y6.To address this,a small molecule acceptor Cy-4F was designed and synthesized by changing the side chain on the pyrrole ring of Y6 to decylcyclohexane.The long chain structure of decylcyclohexane and the steric hindrance unit of cyclohexane at its end adjusted the molecular packing and raised the energy levels,which increased the V_oc to 0.94 V.Within this work,a new small molecular acceptor named Cy-4Cl was also designed and synthesized by replacing the fluorine atoms on the end group of Cy-4F with chlorine atoms.The introduction of chlorine atoms increased dipole moment of the end groups,resulting in stronger light absorption,which further improved the short-circuit current density(J_sc),and achieved a PCE of 13.23%based on PM6:Cy-4Cl device.This work demonstrated that pyrrole ring side chain engineering is an effective strategy to regulate molecular packing and thus increase open circuit voltage.（4）Two-dimensional side chain engineering on theβ-position of outermost thiophene improves charge transport mobility.In PM6:Y6-based photovoltaic devices,the hole mobility of PM6 was over 3 times of the electron mobility of Y6,and the imbalance of charge transport was one of the important factors limiting the further improvement of its performance.3,4-Ethylenedioxythiophene（EDOT）,a high charge-transporting group,has been successfully applied to the highly conductive polymer electrode material PEDOT:PSS.The small molecular acceptor BTP-EDOT-4F was designed and synthesized by replacing the alkyl side chain on theβ-position of outermost thiophene unit of Y6 with EDOT thioalkyl.The introduction of two-dimensional EDOT group doubled the electron mobility of the material,reduced the ratio of hole mobility to electron mobility in active layer based on PM6:BTP-EDOT-4F to 0.73,which greatly improved the balance of charge transport.Hence,a fill factor（FF）of 76.40%and a PCE of 16.05%were achieved.In this work,BTP-EDOT-4Cl was also designed and synthesized by further replacing the fluorine atom on the end group of BTP-EDOT-4F with chlorine atom.A FF as high as 76.41%and a PCE of 15.87%were reached in respective devices.This work demonstrated that two-dimensional side chain engineering on theβ-position of outermost thiophene can effectively improve the electron transport ability of Y6 system materials.（5）End group halogenation engineering adjusts molecular orbital energy level.L8-BO（D-4F）is a highly efficient Y6 derivative reported in recent literatures.It achieved a substantial improvement in material properties by suitable side chain engineering on the thiophene unit of Y6.The brominated and dibrominated small molecule acceptor materials D-3FBr and D-2F2Br were designed and synthesized by gradually replacing the fluorine atom at its end groups with the bromine atom.The position isomerism of fluorine atom in the end group slightly increased the molecular orbital energy levels,achieving 0.01 V increase in V_oc in the PM6:D-3FBr-based devices,which led to a PCE of 16.32%.However,the V_oc of PM6:D-2F2Br-based devices dropped significantly caused by the excessive phase separation size of the blend film,which was not conducive to the transport and collection of charges,hindering the efficiency of the photovoltaic devices.This work demonstrated that end-group halogenation engineering was an effective way to regulate the energy levels of the molecular orbitals of small molecule acceptor materials.In this thesis,each part of the Y6 system were systematically adjusted,which comprehensively affected the device performance parameters of OSCs,deepened the understanding of the relationship between structure and performance,provided theoretical support for development of high-performance photovoltaic materials,and reserved a variety of effective strategies for molecular engineering modification on photovoltaic materials.更多还原