Synthesis And Properties Of Photovoltaic Donor And Acceptor Materials Based On Benzothiadiazole

The operating principle and research progress of organic solar cells(OSCs)were briefly introduced in this dissertation.And the development and current situation of high-efficiency polymer donors and non-fullerene small molecule acceptors(SMAs)materials were summarized.The frontier orbital(FMO)energy level and the miscibility of the donor/accepter material,the photon-capturing ability,the molecular aggregation behaviors and the morphologies,etc.of the active layer have obviously great effects on power conversion efficiency(PCE)of devices.From the perspective of fine-tuning the absorption spectra,energy levels,aggregation state of donor and acceptor along with the microphase separation of active layer,a series of polymer donors and SMAs containing benzothiadiazole were designed and synthesized.The target molecules were characterized by GPC,¹H NMR,¹³C NMR,MALDI-TOF,and so on.Photophysical and electrochemical properties and morphology of the donor/acceptor materials and blend films were calculated and characterized by density functional theory(DFT)calculation,ultraviolet-visible absorption spectra(UV-Vis),cyclic voltammetry(CV),grazing incidence wide angle X-ray scattering(GIWAXS),atomic force microscope(AFM),transmission electron microscope(TEM),and so on.The photovoltaic performances of the fabricated devices were measured systematically.The main contents are as follows:1.Owing to the enhanced intermolecular interaction,aromatically chlorinated polymers have been vital to the molecular design of?-conjugated polymers for efficient organic solar cells(OSCs).Two structurally similar copolymer donors(Z1 and Z2)based on benzodithiophene(BDT)on alkylbenzene conjugated side chains without and with chlorine atoms were designed and synthesized.Z1 and Z2 exhibit similar absorption spectra at 380-620 nm come along with the similar band-gap(E_g^opt)of 2.01 and 2.02 e V,respectively.Compared with the non-chlorinated polymer Z1,the polymer substitutied by chlorine in the meta-position of the alkoxyphenyl group endows it an enhanced?-?stacking and favorable morphology,which simultaneously the improved J_sc and FF of OSCs,as well as the mobility.When the polymer without functionalized chlorine atom had a PCE of 12.03%,the side chain chlorinated conjugated polymers provided a high PCE of 15.11%with significantly enhanced photocurrent and fill factor.The results demonstrate that the chlorinated side chains could be a promising strategy to design new conjugated polymers toward efficient OSCs.2.Based on the better coplanarity BDT and high mobility BT units,two2D-dimensional polymeric donors(Z3 and Z4)with the same backbone but different side chains were employed to make a detail investigation of their photophysical and photovoltaic performance.The alkylthiophenyl side chains in Z3 endow it appropriate absorption spectrum and energy level,while the extra thienyl incorporated in the side chains of Z4 cause the much stronger?-?interaction,over molecule aggregation and further inferior J_scand FF.Therefore,upon blending with the acceptor Y6,the Z3-based bulk heterojunction organic solar cells(OSCs)achieved a PCE value of 15.12%which is almost 30%higher than the Z4-based devices.The result indicates that the side chain modifying is a fine-tuning strategy of aggregation for 2D dimensional polymeric donors in OSCs.3.Two A₂-D-A₁-D-A₂-type small molecule acceptors(SMAs),DFB-d IDT and BT-d IDT with 2,5-difluorobenzene(DFB)or benzothiadiazole(BT)as electron-withdrawing core(A₁)and a derivative of indanone as A₂ units,were prepared for applications in OSCs.The results indicate that BT unit is more beneficial to forming multiple noncovalent conformational locks of N···S and N···H between BT and IDT unit than DFB in the core,so BT-d IDT showed better molecular coplanarity,higher-lying HOMO energy level,more red-shifted spectrum,superior molar absorption coefficient(1.60×10⁵ M^-1 cm^-1),more complementary absorption spectrum with PBDB-T and better photovoltaic performance than DFB-d IDT.As a result,the BT-d IDT-based OSCs blending with PBDB-T exhibited higher PCE of 10.52%with higher J_sc of 18.59 mA cm^-2than that of the DFB-d IDT-based devices(PCE of 6.71%with J_sc of 15.58 mA cm^-2).These results demonstrate that the A₂-D-A₁-D-A₂-type SMAs incorporated with a suitable electron-deficient core are promising candidates for high performance OSCs.4.Three small molecule acceptors(SMAs),named ZF,ZC and ZB,have been designed and synthesized for organic solar cells(OSCs)by introducing single F,Cl and Br atom on each terminal unit,respectively.The effects of different halogen substituent on molecular aggregation,photophysical and photovoltaic performance have been systematically studied.Owing to their strong electronegativity of halogens,all the acceptors with monohalogenated terminal units possess red-shifted absorption spectra and deeper frontier energy levels compared to the corresponding acceptor without halogen substituents reported in the literature.Among the SMAs,ZB with brominated terminal units was found to show the higher molar absorption coefficient(2.31×10⁵ M^-1 cm^-1),more orderly face-on?-?stacking,higher electron mobility and more favorable morphology when blended with PM6.As a result,the PM6:ZB OSCs yielded a high PCE of up to 15.23% with a high short-circuit current density(J_sc)of 26.38 mA cm^-2,while the corresponding ZF-and ZC-based devices showed the relatively inferior PCEs of 13.36%and 14.71%,respectively.These results demonstrated that the modulation of electron-withdrawing halogen substituents on terminal group provides a promising strategy to design and synthesize efficient SMAs for fabricating high-performance OSCs.5.Ternary organic solar cells(OSCs)were fabricated with polymeric donor material(PM6)and small molecule acceptor(Y6)as the host system and a medium-bandgap(MBG)acceptor(BTIC-Cl)with a high-lying lowest unoccupied molecular orbital(LUMO)energy level as the third component.The ternary blend film PM6:Y6:BTIC-Cl(1:1.2:0.2,wt/wt)was found to show more favorable morphology,more orderly face-on?-?stacking,higher charge mobilities,and more balanced charge transfer.As a result,the optimized ternary OSC with PM6:Y6:BTIC-Cl(1:1.2:0.2)showed an impressive PCE of 16.60%with synchronouly enhanced J_sc of 27.25 mA cm^-2and V_oc of 0.86 V,which is distinctly superior to than those of the binary counterparts based on PM6:Y6(15.50%)and PM6:BTIC-Cl(12.66%).These results provide an insight into the role of MBG small molecule acceptor(SMA)with a high-lying LUMO energy level as the third component in promoting the performance of the ternary OSCs.