Synthesis Of Fluorinated D-A Conjugated Polymer Donor Materials And Its Photovoltaic Properties

Economic development is inseparable from the demand for petrochemical energy,and the application of a large amount of fossil fuels has irreversibly harmed people's living environment.Therefore,it is imminent to seek clean energy(such as wind energy and solar energy).As a direct use of solar energy devices,the most widely used today are still crystalline silicon and inorganic semiconductor materials.However,organic materials are rarely applied for commercial solar cells.At present,most theoretical study is focus on D-A-type polymer and small molecule materials,which can effectively reduce the band gap,broaden the absorption.On this basis,the introduction of strong electron-withdrawing group F can further enhance the optical properties,so that the device short-circuit current and open circuit voltage will be greatly improved.In this paper,we have successfully constructed a series of F-containing D-A conjugated polymers and anlogues.The intermediates and the final chemical structure are confirmed by the nuclear magnetic resonance spectroscopy(NMR).Meanwhile,the subsequent characterization of the material involves the testing of the thermal stability,optical properties,electrochemical properties and photovoltaic properties.Finally,the density functional theory(DFT)is applied to explore planar structure properties and dipole moment.(1)We have systematically studied physical and chemical properties as well as the photovoltaic properties of P1-P6 that are constructed by benzodithiophene,dithienosilole,dithienopyrrole derivatives and thiophene-thiophene derivatives.Ultraviolet–visible spectroscopy(UV-Vis)indicates that these six polymers have good absorption properties;Thermogravimetric Analysis(TGA)shows that these polymers have excellent stability to meet the most basic solar cell requirements;Cyclic Voltammograms(CV)displays Highest Occupied Molecular Orbital(HOMO)level and Lowest Unoccupied Molecular Orbital(LUMO)level of the polymer,matching the fullerene acceptor.It is also confirmed by the density functional theory calculation.Using P1 and P4 as an example,theoretical calculation show that introducting F on the acceptor units improves the planarity of the molecular backbones when the thiophene side chains exist.For P2,P5,P3 and P6 where the polymer has no thiophene side-chain,the F on the acceptor unit do not promote the planarity of the molecular skeleton.The reduction in steric hindrance is due to intermolecular separation by alkyl side chains away from the polymer molecule backbone,which can promote the degree of polymer aggregation.As a result,P1 shows strong aggregation in solution and ?-? chain stacking,planar molecular framework and low molecular steric hindrance.Finally,the energy conversion efficiency of P1 is close to 7.1%.(2)In order to study the effect of molecular planarity and dipole moment on photovoltaic performance.F as well as ester groups and alkyl side chains of varying lengths are introduced into the thienothiophene with no change to the D monomer.The change of dipole moment caused by the introduction of its side chain has been studied by DFT.Using QP-A and QP-D as an example,the introducting F in the acceptor unit improves the planarity of molecular.Meanwhile,the dipole moment of molecule gets enhanced.The incorporation F for QP-B and QP-C do not promote the planarity relative to corresponding QP-E and QP-F.However,the dipole moment of QP-B and QP-C still get improved.Therefore,both planarity and dipole moment play a synergistic role in improving the photovoltaic performance.If the dipole moment is improved under the premise of ensuring the planarity,the internal charge transfer can be strengthened,thus the short-circuit current will be greatly improved.If not like this,the short-circuit current still get improved,but it is not as obvious as the former.Finally,the QP-A with a highly planar molecular skeleton and a large dipole moment display excellent power conversion efficiency 4%.(3)D1 [5,5'-bis(trimethyltin)-2,2'-bithiophene] D2 [3,4'-difluoro-5,5'-bis(trimethyltin)-2,2'-bithiophene] as a donor unit,five polymers 4-P1(D1: D2 = 1: 0),4-P2(D1: D2 = 7: 3),4-P3(D1: D2 = 5: 5),4-P4(D1: D2 = 3: 7),4-P5(D1: D2 = 0: 1)are synthesized by random copolymerization.UV-vis absorption,cyclic voltammgrams,TGA,Gel Permeation Chromatography(GPC)are used to characterize these five polymers,which display excellent absorption and thermal stability.Through the device test,it is successfully verified that the simultaneous introduction of F at D and A is an effective method to improve device efficiency.Simultaneously,it is found that the photophysical properties of 4-P2(the F on D unit accounted for 21% of the entire molecule F)have greatly been improved.Finally,the 4-P2 shows 5.57% conversion efficiency.