Design,Synthesis And Photovoltaic Properties Of Organic And Polymer Electron Acceptors

As a potential photovoltaic technology,organic solar cells(OSCs)have broad application prospects due to their advantages of solution processability,large-area processing,light weight,flexibility and semitransparency.In the past few years,the development of non-fullerene electron acceptor materials has led to significant breakthroughs in the power conversion efficiency of OSCs.On the other hand,the problems of stability,environmentally friendly processing,low cost are still the bottleneck limiting the commercial application of OSCs.The development of novel non-fullerene acceptors is an effective way to tackle these issues.Research topic of this thesis is to develop novel electron acceptors in OSCs.Different from the traditional A-D-A’-D-A type small molecule electron acceptor and imide-based polymer acceptor materials,we develop three types of new organic/polymer electron acceptors,including alcohol-processable small molecule acceptors based on p-π*conjugated triarylborane,all-fused-ring electron acceptors with high stability,boron-nitrogen coordination bond(B←N)based polymer acceptors containing Se atoms.The specific research results are summarized as follows:1.alcohol-processable small molecular acceptors based on triarylboraneOrganic small molecule acceptors tend to be soluble in halogenated or aromatic solvents,but these solvents can be harmful to the environment and human body.Conversely,they are generally insoluble in eco-friendly solvents,such as alcohols,limiting the large-scale manufacturing of organic solar cells.In response to the problem of environment-friendly solvent processing of organic small molecule acceptor materials,we developed a p-π*conjugated small molecule acceptor BDT based on triarylborane.BDT shows unique alcohol solubility even in the absence of polar side chains,which is favorable for alcohol-processed OSCs.In BDT,the p-π*conjugated triarylborane is used as core,and two electron-withdrawing 2-(3-ethyl-4-oxothiazolidin-2-ylidene)malononitrile units are serve as the end-capping groups to link the core by bithiophene bridges.BDT exhibits low-lying ELUMO/EHOMO of-3.61 eV/-5.73 eV,an electron mobility of 1.37 × 10-5 cm2 V-1 s-1 and the absorption onset of 574 nm,which meet the requirements of electron acceptor materials.The introduction of boron atoms in BDT increases the dipole moment of the conjugate skeleton and enhances the flexibility of the conjugate backbone,so that BDT is easily soluble in polar solvents such as n-hexanol,even in the absence of polar side chains.Using BDT as electron acceptor and n-hexanol as processing solvent,we successfully fabricated OSC device with a power conversion efficiency(PCE)of 1.03%.In addition,we designed and synthesized a p-π*conjugated small molecular acceptor B-H with triarylborane as the core unit,bithiophene as the bridging units and 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile as the end-capping groups.Then,by introducing butyl or butoxy groups to the bridging thiophene units to regulate the opto-electronic properties,the small molecule electron acceptors B-b and B-bo were developed.As a result,the attachment of butyl groups to the bithiophene bridges brings about a slightly twisted backbone,which in turn promotes good solubility and homogeneous donor/acceptor blend morphology,whereas the introduction of butoxy groups leads to improved planarity,favorable stacking in the film state and a red shift absorption spectrum.OSC devices based on these molecules exhibit encouraging photovoltaic performances with power conversion efficiencies reaching up to 4.07%.2.All-fused-ring small molecule acceptors with high stabilityFor the chemical structures of typical organic small molecular electron acceptors,a fused-ring electron-rich core and two strong electron-accepting terminals are covalently linked by two vinyl units.The vinyl linkers may be broken under illumination and basic conditions,leading to OSC devices stability problem.To tackle these issues,we propose to connect the electron-deficient terminal groups and the central unit in all-fused way to develop fully fused-ring small molecule acceptor material and improve the stability of OSCs.The all-fused-ring molecule is composed of one electron deficient benzotriazole core,two electron-rich thienopyrrole bridging units,and two electron-deficient malononitrile-functionalized end-cappers.Using different alkyl side chains,we synthesized three all-fused-ring small molecule acceptors FM1,FM2 and FM3.Because of the all-fused-ring skeleton,these small molecule acceptors show superior photostability and chemical stability.Among them,FM2 exhibits a low-lying ELUMO/EHOMO of-3.89 eV/-5.77 eV,a high electron mobility of 6.0 × 10-4 cm2 V-1 s-1,an optical bandgap of 1.50 eV,and a maximum absorption wavelength of 769 nm.FM2 has suitable crystallinity and can be blended with the polymer electron donor D18 to form a good phase separation morphology.The PCE of the organic solar cell device based on D18:FM2 can reach 10.82%.Most importantly,the intrinsic stability of FM2 leads to its excellent OSC device stability.After illumination for 16 h，the device of FM2 retained 91%of its initial PCE value while the control device of Y6 kept only 54%of its initial PCE value.On the basis of the above work,by introducing two or four fluorine substituents into the terminal groups of FM2,we designed and synthesized the small molecule acceptors FM2-2F and FM2-4F,respectively.The effect of end-group fluorination on the optoelectronic properties and device properties of small molecule acceptor materials were systematically studied.The results show that the introduction of fluorine atoms of these small molecule acceptors can reduce the LUMO/HOMO energy levels,redshift the absorption spectra,increase the molar absorption coefficient,enhance the intermolecular/intramolecular interaction,and thus improve the charge transport.The Jsc and FF of the corresponding organic solar cell devices are increased,while the VOC is decreased.3.Regulating the optoelectronic properties of polymer acceptors containing the boron-nitrogen coordination bond(B←N)by introducing selenium atomsPolymer electron acceptors containing B←N have excellent photoelectric properties,and have broad application prospects in OSCs.In order to finely tune optoelectronic properties of polymer electron acceptors containing B<-N,by replacing the S atoms with Se atoms,we have synthesized three new polymer acceptors containing B←N,P-Se,P-2Se and P-3Se,which contain one,two and three Se atoms in the repeating units,respectively.The replacement of S atoms with Se atoms results in slightly downshifted ELUMO and slightly upshifted EHOMO as well as redshifted absorption spectra.Among the three polymer acceptors,P-2Se shows the strong crystallinity in thin film and high electron mobility.All-PSC devices based on the three polymer acceptors containing B←N exhibit a PCE of 5.37%with VOC higher than 1.0 V.