Preparation And Photovoltaic Properties Of Unilateral Chain Modified Pyran-based Small Molecule Acceptors

With the increasing crisis of energy,organic solar cells have attracted more and more attention.Due to the poor light harvesting ability,hard to proceed molecular tailoring and energy tuning,the fullerene-based acceptors hinder the further development of organic solar cells.Compared to fullerene derivatives,non-fullerene acceptors(NFAs)have attracted much more attention due to their easily modified chemical structure,adjustable energy level and wide absorption spectrum,and they have become the most studied acceptor materials nowadays.The star acceptor molecules generally have a large π conjugated fused-ring unit for electron donor and two strong electron-withdrawing units at each terminal.Our group has been engaged in the research of pyran acceptors for a long time.Based on the previous work,this thesis aims to develop a series of new pyran-based electron-donating core of the electron acceptors construction.Herein,a series of asymmetric and banana-type pyran-based electron-donating units was designed via side-chain engineering on the same side of the pyran blocks,and they have been put into applications of NFAs design.Part Ⅰ Synthesis and Photovoltaic Properties of Asymmetrical Five-membered Mono-pyran based Acceptors.The design of asymmetric acceptors can effectively change the molecular dipole,regulate the intermolecular interactions,and then affect the molecular stacking and crystallization orientation,etc.In this chapter,two acceptor molecules were designed and synthesized with five-membered mono-pyran.Due to the inherent asymmetrical characteristics of the mono-pyran skeleton,the corresponding donor core and the final acceptors are endowed with asymmetrical properties.Then,based on the five-membered fused dipyran,TM-1-1 and TM-1-2 molecules were obtained by using hexylphenyl and ethylhexyl as the side chains.Their UV-vis absorption spectra present strong absorption in the range of 600 nm-800 nm,and the optical band gap is estimated to be 1.41 and 1.39 eV,respectively.The molecular HOMO/LUMO energy levels are calculated to be-5.25/-3.84 eV and-5.40/-4.01 eV,respectively.As there is only one side chain modification site in the conjugated skeleton,the solubility of the obtained acceptors was greatly affected.TM-1-1 only achieved a power conversion efficiency(PCE)of 1.91%.As the replacement of hexylhpenyl with ethylhexyl,the final molecule’s solubility was improved slightly and the PCE was improved to2.86%.PartⅡ Preparation and Photovoltaic Properties of "Banana-type" Dipyran AcceptorsThe literature reported high performances of Y series acceptors are greatly benefited from their unique banana-shaped molecular skeletons.Based on the pyran based studied in the first section,a series of five-member and seven-member dipyran building units were designed and synthesized with unilateral side chain modification.The corresponding absorption spectra of the final acceptors suggested that seven-member dipyran would be redshifted the absorption in 46 nm compared with the five-membered counterpart.By incorporating the π bridges,the conjugated lengths of these molecules are effectively extended and the absorption spectra are successfully extended to 1050 nm.The above acceptors were put into organic solar cells fabrication,and the preliminary results showed that the TM-2-4,a seven-membered fused bipyran acceptor,displayed a better photovoltaic performance with a PCE of 5.91%.