Cathode Interlayer Materials Based On Isoindigo Derivatives For Organic Solar Cells

Bulk-heterojunction(BHJ)organic/polymer solar cells(OSCs)have drawn enormous attention owing to the unique advantages of synthetic variability,light weight,low cost,large-area roll-to-roll fabrication and the lucrative possibility of integration into flexible devices.Recently,the power conversion efficiencies(PCE)record have been repeatedly refreshed for single-junction OSCs and a milestone PEC value of 11% has been accomplished.A great number of efforts have been devoted to improve PCE and minimize energy loss by rational donor/acceptor materials design,active layer morphology optimization and device engineering.Beyond active layer,the cathode interfacial layer(CIL)located between the cathode and active layer is also an important factor in the realization of high-efficiency OSCs by improving the charge extraction,minimizing series resistance(Rs)and so on.Compared with conventional inorganic CIL such as Ca and LiF,organic water/alcohol soluble CIL are not only much more stable in the presence of oxygen and moisture,but also have good solution processable property and better interfacial contact with organic active layer.Compared with polymer based CIL,the small-molecular CIL have some intrinsic advantages in terms of well-defined structures,high-purity without batch-to-batch variation and easy modification.In this master thesis,we firstly synthesized a series of ionic isoindigo derivatives as cathode interlayer materials,by changing functional groups,we can observe their different photophysical properties.Then,these isoindigo derivatives were modified as cathode interlayers into OSCs devices to find the relationship between the different functional groups incorporated with isoindigo central skeletons and devices performance.1.In chapter II,six isoindigo derivatives for cathode interlayer materials named IID-PyBr,IID-NSB,IIDPh-PyBr,IIDPh-NSB,IIDTh-PyBr and IIDTh-NSB were synthesized by Suzuki coupling reactions and ionic reactions.They were fully characterized by NMR,mass spectra,and element analyses.These compounds can be well dissolved in methanol and ethanol while they are insoluble in other common organic solvents such as chloroform,chlorobenzene,tetrahydrofuran,and toluene,which can avoid intermixing between active layer and interlayer.In addition,we measured their photophysical and electronic properties by UV-vis absorption spectra and cyclic volammetric,which gave these isoindigo materials promise to apply for cathode interlayers into OSCs devices.2.In chapter III,we dissolved these six isoindigo derivatives by methanol as cathode interlayers based on PTB7:PC71BM as active layer and then 100 nm Al was evaporated as a cathode onto cathode interlayer,the conventional OSCs device was accomplished.Compared with the control devices,the OSCs with IID based CIL show simultaneous enhancement of open-circuit voltage,short-circuit current,and fill factor.Systematic optimizations of central conjugated core and side flexible alcohol soluble groups demonstrated that isoindigo based CIL material with thiophene and sulfonate zwitterions substituted groups can efficiently enhance the OSCs performance.The highest power conversion efficiency(PCE)of 9.12%,which is 1.75 times that of the control device without CIL,was achieved for the OSC with isoindigo based CIL.A series of measurements prove that 1.The introduction of thiophene into the IID backbone led to the formation of D-A-D conjugated structure,which can efficiently improve the electron current density.2.As a functional group,sulfonate zwitterions could efficiently connect with cathode and improve the electron extraction.These two important factors could enhance OSCs device PCE.