Design Of Dithieno [3,2-b:2’,3’-d] Pyrrol Extended Cores Based Asymmetric Electron Acceptors And Their High-efficiency Organic Solar Cells

The remarkable development of non-fullerene acceptors in the past six years has promoted the power conversion efficiency（PCE）of organic solar cells（OSCs）to an unprecedented level.However,most of the research work so far is basically carried out by trial and error,and the structure-property relationship is still poorly understood.In addition,the PCEs of fused-ring electron acceptors（FREAs）such as Y6 have exceeded 18%,while their commercial application is severely limited by complicated synthesis steps,expensive material costs and unresolved device stability.Taking these into account,we have developed series of dithieno[3,2-b:2’,3’-d]pyrrol（DTP）derived asymmetric cores by adopting thieno[2’’,3’’:5’,6’]-s-indaceno[2’,1’:4,5]-thieno[3,2-b]thieno[2,3-d]pyrrole（IPT）as the basic backbone in this thesis,which mainly focuses on the synergistic effect of electron-donating core and end group,the modulation of device stability via side chains optimization,the screening of frameworks of unfused acceptors,the matching of side chain and end group and so on.And a library of fused-ring/unfused acceptors are explored through the core optimization.Simultaneously,the structure-property relationship is systematically studied,and some key issues such as synthesis cost and device stability are also been preliminarily analyzed.Firstly,selecting the optimal asymmetric core IPT as the basic framework,we develop a novel heptacyclic asymmetric core DTPPSe with stronger electron-donating ability by flanking DTP and selenophenol to benzene via cyclopentadiene bridges.In order to obtain narrow optical band gap(Eg^opt)acceptors with suitable energy levels,end group engineering is applied to design and synthesize three DTPPSe-cored asymmetric FREAs（i.e.,DTPPSe-IC,DTPPSe-2F and DTPPSe-4F）.Compared with the non-fluorinated DTPPSe-IC,fluorinated DTPPSe-2F and DTPPSe-4F exhibit red-shifted absorption and significantly deeper energy levels.When blended with PBDB-T,the blend film based on PBDB-T:DTPPSe-2F shows a more excellent phase separation morphology,a fibrous charge transport network with clear boundaries,and a tighterπ-πstacking distance.The resultant PBDB-T:DTPPSe-2F devices deliver the highest PCE of 13.76%and low energy loss（Eloss）of 0.56 e V.This work proves that monofluorinated end group can effectively modulate absorption and energy levels,thereby obtaining high-performance narrow band gap acceptors.Next,N-functionalized two-dimensional（2D）side chains are firstly introduced into the design of asymmetric FREAs to further optimize IPT asymmetric core,and three acceptors（IPT2F-Th,IPT2F-Ph and IPT2F-TT）are synthesized.Due to the steric hindrance between 2D side chains and adjacent hexylbenzene,diverse dihedral angles can be observed between different 2D side chains and asymmetric backbone.This deflection limits the conjugation of 2D side chains with planar backbone,endowing three acceptors similar energy levels and solution absorption.However,different 2D side chains have a huge impact on the intermolecular packing and the morphology of blend film.Among them,the PBDB-T:IPT2F-TT based blend shows higher carrier mobility,ordered fibrous phase separation and more suitableπ-πstacking,which contributes the corresponding device better PCE of 14.02%and fill factor（FF）of 75.06%.Outstanding thermal stability and light soaking stability are also observed in optimal PBDB-T:IPT2F-TT based cells with PCE over 12%maintained after thermal or light aging for 100 h.This work demonstrates that hexylthieno[3,2:b]thiophene is superior to the other two side chains,and N-functionalized 2D side chain is an effective strategy to simultaneously improve photovoltaic performance and device stability.Subsequently,rapid increase in efficiency highlights the contradiction between PCE and materials cost,which prompts us to tailor the asymmetric core IPT.Utilizing the most classic frameworks currently and DTP as the basic building block,three unfused acceptors DBT-4F（A-D1-D-D2-A type）,DBD-4F（A-D1-D-D1-A type）and DBTD-4F（A-D1-A1-D1-A type）are designed.Theoretical calculation shows that the asymmetric DBT-4F has a more coplanar backbone than the other two symmetric unfused acceptors,which is conducive to intermolecularπ-πstacking and charge transport.As a result,the PBDB-T:DBT-4F based devices exhibit more effective photon collection,higher and more balanced charge transport and less charge recombination,contributing to the highest PCE of 12.14%.The relatively lower PCEs of DBD-4F and DBTD-4F based devices are mainly reflected in their lower JSC and FF,which may be due to a large range of phase separation regions with unclear boundaries distributed in their blends.The successful DBT-4F illustrates that DTP building block can be used for the design of high-performance unfused acceptors,and the asymmetric A-D1-D-D2-A framework is more suitable for DTP-based unfused acceptors.Finally,the fluorine atoms in end groups of DBT-4F are replaced with chemically cheaper chlorine atoms to further enhance intramolecular charge transfer（ICT）and narrow Eg^opts.By modulating the alkoxy side chains on central benzene,three unfused acceptors namely DBT-EH,DBT-BO and DBT-HD are synthesized to balance the solubility and crystallinity changes brought about by chlorinated end groups.It is found that the regulation of central side chains influences Eg^opts of unfused acceptors,and diverse dihedral angles of 21-25°between the middle benzene and flanked units are also formed.Morphology studies reveal that 2-hexyldecyl-substituted DBT-HD blends better with PBDB-T,and fibrous phase separation morphology and excellentπ-πstacking can be observed in their blends.Therefore,the corresponding PBDB-T:DBT-HD based device yields a high PCE of 13.57%and a high FF of73.39%.This work obtains the best chlorinated unfused acceptor DBT-HD at the time,indicating asymmetric unfused core DBT has the potential to develop cost-effective unfused acceptors through rational molecular optimization strategies.