The Design,Synthesis And Optoelectronic Properties Of Novel Organic Photovoltaic Materials Based On Indeno [1,2-b] Fluorene For Organic Solar Cells

Bulk-heterojunction organic solar cells(OSCs)have attracted extensive attention from academics and industry due to their potential applications in light-weight,flexible,and large-area devices through low-cost solution processing.Very recently,high power conversion efficiencies(PCEs)over 14%have been realized in OSCs due to the innovation in key photovoltaic materials.However,there are still a plenty of urgent challenges to solve in the process from laboratories to industries.Among them,the development of highly efficient active layer materials and excellent interfacial materials are the core issues of solar cell industrialization.As a member of polycyclic aromatic hydrocarbon family,the pentacyclic indeno[1,2-b]fluorene has extended conjugated plane,good charge carrier transporting property and fine stability,which is considered to be a candidate to design high-performance photovoltaic materials.Therefore,a series of novel indenofluorene-based non-fullerene acceptors and polymeric electron transport materials were developed for high-performance solar cells.In chapter 2,four small molecules with wide band gap(IFT-ECA,IFT-M,IFT-TH and IFT-IC),which contain A-D-A structure with the core of indenofluorene,thiophene bridge and different electron-deficient units as end-capping groups,were synthesized and designed,and the effects of flanking groups on their optical properties,electrochemical properties and photovoltaic performance were systematically investigated.The results show that four small molecules display gradient absorptions and Lowest Unoccupied Molecular Orbital(LUMO)energy levels,with the electron-withdrawing ability of end-capping groups increasing.Besides,the photovoltaic devices based on these electron acceptors have different exciton dissociation,charge transport and blend film morphology,thus giving rise to diverse performance.Among them,the best PCE of 7.16%can be achieved in IFT-IC based device,which are due to the relatively higher electron mobility and proper morphology separation.The results indicate the importance of choosing suitable electron-withdrawing groups to construct high-performance non-fullerene acceptors based on A-D-A motif.In chapter 3,by incorporating alkoxy groups into the 3-or 4-position of thiophene in small molecule IFT-IC,which is the best one in chapter 2,two isomeric non-fullerene acceptors(IFTIC-EHO-p and IFTIC-EHO-d),were designed and synthesized.The effects of incorporated position of alkoxy group on their thermal properties,optical properties,electrochemical properties and photovoltaic performance were comparatively investigated.The results indicate that the situations can greatly affect their thermal behaviors and it can also lead to diverse optical absorption,and electronic energy levels due to different conjugative effects between alkoxy group and backbone.When they are used as acceptor materials for OSCs,IFTIC-EHO-p based device can realize a higher PCE of 6.50%,which are mainly relevant to their complementary absorption profile,higher charge mobility and better nano-scale morphology separations.The study provides a guideline for optimizing the photovoltaic performance of acceptor materials through side chain engineering.In chapter 4,in order to overcome the drawbacks of best-performance IFT-IC in chapter 2,such as low electron mobility induced by its’ twisted molecular geometry,three planar small molecular acceptors(TfIF-IC-C8,TfIF-IC-C2C6 and TfIF-IC-C4C8)with different side chains were designed and synthesized by fusing the plane of indenofluorene and thiophene bridge in IFT-IC.Then,a comparative investigation about these small molecular acceptors’ thermal properties,optical absorptions,electrochemical properties were carried out,and their photovoltaic properties based on the devices with PBDB-T as the donor part were evaluated.It shows that TfIF-IC-C8 based device after thermal annealing(TA)can achieve the maximum PCE of 8.57%.Through a series of characterizations,the effects of TA treatment on charge separation,transport,recombination and blend film morphology in OSC devices were studied.TfIF-IC-C8 based blend film after TA treatment might show more ordered molecular stacking with negligible morphology change,which are mainly responsible for the improved charge mobility and higher efficiency in the relevant device.It indicates that variation of side chains plays an important role in fine-tuning aggregation and crystallization of fused polycyclic electron acceptors,and their blend film morphology for high-performance solar cells.In chapter 5,three self-doped,n-type water/alcohol-soluble conjugated polymers(PIF-PTE-N,PIF-PMIDE-N and PIF-PDI-N)with tailored energy levels,which are composed of amine-functionalized indenofluorene and electron-deficient perylenetetracarboxylic acid derivatives,were designed and synthesized.Their optical properties,electrochemical properties,interfacial modification capability and relationship between energy levels and self-doping effects and its’effects on electron transport were studied.It was found that three polymers have tunable absorption,electronic energy levels,and they can effectively decrease work function(WF)of Ag electrode.Besides,they exhibit good electron condutive properties,and PIF-PDI-N shows the strongest doping intensity due to its’ lower-lying HOMO/LUMO energy levels,and the relevant electron mobility is the best one.When three polymers were used as electron transport layers(ETLs)in OSCs with PNTT/PC71BM as active layer,the device based on PIF-PDI-N can give out a high PCE over 9%in the thickness range of 5-50 nm,which provides a wide processing window for making large-area devices by roll-to-roll coating technique.