Design And Synthesis Of Fluorinated Organic Semiconductor Materials And Study For Their Phontovoltaic Properties

The fluorinated organic semiconductor material has excellent chemical properties because of the existence of fluorine atoms.Fluorine is the most electronegative element.Its van der waals radius is slightly larger than the hydrogen atom and is the smallest electronegative group.The chemical bonds formed by fluorine and other elements are very strong.The 2s and 2p orbits of fluorine are perfectly overlapped with the corresponding track of the C atom,resulting in low polarization.Therefore,the strong electronegativity of fluorine atom can reduce the highest occupied molecular orbital(HOMO)energy level of the donor,thereby increasing the open-circuit voltage(VOC)of the organic solar cells(OSCs).In addition,the fluorine atom can form non-covalent interaction with other atoms,such as F…F,F…H and F…S,which is beneficial to improve the charge transfer performance and crystallinity of organic semiconductors.Moreover,the strong chemical bonds and low polarizability of F-C bonds can lead to good stability,hydrophobicity,and inoxidizability,which is conducive to improving the stability of OSCs.Therefore,the application of fluorinated organic semiconductor materials to the active layer of OSCs can not only improve the power conversion efficiency(PCE)of OSCs,but also improve the stability of the devices.The main research content and results in this dissertation are as follows:(1)We firstly systematically study the synergistic effects of central core size and terminal fluorination on fused ring electron acceptors(FREAs).From the results of theoretical analysis and spectral absorption,we infer that the intramolecular charge transfer(ICT)effect is not only related to the amount of charge transfer,but also may be related to the distance between the positive and negative charge center.Extending central cores can effectively improve molecular coplanarity and reduce energy loss(Eloss).In addition,the extension of the central cores can not only increase the crystallization of the molecule and reduce the π-π stacking distance,but also enhance the face-on orientation of the molecule,which is conducive to charge transport.Compared with the extension of the central core,the terminal fluorination has a greater impact on molecular absorption,energy level,crystallization,morphology and vibrational relaxation.With the further expansion of the length of the central core,the degree of changes in molecular absorption,crystallization and vibrational relaxation gradually decrease.When a certain length is reached,the change will reach saturation.Therefore,6TIC-4F and 8TIC-4F display strong crystallization and similar absorption,and devices based on PTB7-Th:8TIC-4F has the minimal Eloss of 0.51 eV,a moderate PCE of 10.4%and a high VOC of 0.75 eV.However,the charge separation driving force of the blend film is less,and the phase separation is relatively large,which limits the further improvement of its performance.In contrast,6TIC-4F features more appropriate energy level,absorption,crystallization,and phase separation,resulting a high PCE of 11.61%,which is one of the highest values based on FREAs with over 1000 nm absorption.This study offers deep understanding of relationship between molecular structure and performance and provide more guidelines for future high-performance FREAs design.(2)Based on the above chapter studies,we learned that fluorine atoms have a great impact on the planarity of materials and crystalline orientation.Then,a new multifunctional liquid crystal(CL)molecular BDTPF4-C6 based on tetrafluorobenzene unit was developed to construct a high-efficiency and stable ternary OSCs in this chapter.The results indicate that the BDTPF4-C6 is better compatible with the PM6 and is mainly located in the PM6 phase compared with Y6.The BDTPF4-C6 enhances the crystallization of PM6 and reduces the crystallization difference between PM6 and Y6.Therefore,the device based on PM6:Y6:BDTPF4-C6 has higher and more balanced charge transport than PM6:Y6.In addition,a double Foster resonance energy transfer occurs from BDTPF4-C6 to PM6/Y6,increasing more charges in ternary device.In addition,due to the introduction of BDTPF4-C6,the exciton diffusion length increases and the effective exciton dissociation is promoted.Therefore,compared with the binary device of PM6:Y6,the PCE of the PM6:Y6:BDTPF4-C6 ternary device increases from 15.66%to 17.28%due to its more complementary light absorption,higher exciton dissociation efficiency,weaker trap-assisted recombination and more balanced charge transport.Moreover,the device based on PM6:BTP-eC9:BDTPF4-C6 achieved a high PCE of 18.30%.In this chapter,a new and effective strategy is proposed to precisely adjust the crystallization differences between the donor and the acceptor to realize more balanced charge transport,which improves the device performance and stability of OSCs.(3)On the basis of the above two works,we designed a third component BTPPF4-C6 with the same intermediate core of the BTP-eC9 to regulate the active layer of PM6:BTP-eC9:BTPPF4-C6 also contains two tetrafluorobenzene units with strong crystallization.This chapter aims to add the BTPPF4-C6 into PM6:BTP-eC9 binary system to adjust the phase size,enhance exciton diffusion length,and increase efficiency of exciton dissociation.Through the differential scanning calorimeter(DSC)test,we found that the BTPPF4-C6 is highly crystalline and its stacking mode is reversed in parallel stacking mode by single crystal analysis.The planarity is very good,which is conducive to charge transport.Absorption spectrum also shows that it can complement the absorption of donor and acceptor,which is conducive to increasing short-circuit current(Jsc).Therefore,the PCE of the PM6:BTP-eC9:BTPPF4-C6 ternary device was significantly improved.In addition,our studies confirmed that the BTPPF4-C6 could optimize the morphology of the active layer,resulting in improved fill factor(FF).Therefore,the PCE of the ternary device reached 18.21%.