Synthesis And Photovoltaic Properties Of Non-Fullerene Acceptors With Wide Spectral Response

During the last two decades,organic solar cells?OSCs?have attracted much attention due to the advantages such as lightweight,flexibility and the capability to fabricate thin transparent devices.Fullerene derivatives are the dominated electron acceptors in OSCs because of large electron affinity and isotropic electron transport properties.Nevertheless,fullerene acceptors also have drawbacks,such as weak absorption in the visible and near-infrared regions,limited tunability of energy levels,high cost and poor devices stablility,which limit its development.In recent years,non-fullerene?NF?acceptor materials with broad absorption,high absorption coefficient and easy regulation of energy levels have attracted much attention.In this paper,by finely regulating the chemical structure of ITIC,a series of new NF acceptors were synthesized and studied the effects of some minor changes in chemical structure on the light absorption,aggregation,energy level and photovoltaic performance of materials.Most researches based on ITIC derivatives focus on the improvement of aromatic cores,side chain phenyl groups and terminal groups and there were few studies on the improvement of alkyl chains on side chain phenyl groups.In chapter 2,the oxygen atom was introduced into the side chain and the substitution position of the alkoxy group was changed.The new low bandgap NF acceptros ITOR-IC with para-octyloxy-phenyl substituted and m-ITOR-IC with meta-octyloxy-phenyl substituents were synthesized and characterized.Compared with the controlled SM of ITIC,after the oxygen atom was introduced into the phenyl side chain,new materials demonstrated slightly blue-shifted absorption spectrum,upshifted LUMO energy level and higher aggregation ability in the solid film.Non-fullerene OSCs with ITOR-IC as acceptor and a medium bandgap conjugated polymer J61 as donor demonstrated a PCE of 4.24%,alongside with an open circuit voltage(V_OC)of 0.90 V,a short-circuit current density(J_SC)of 12.26 mA/cm² and a fill factor?FF?of 38.40%.The PCE of the optimal device based on J61:m-ITOR-IC was 4.18%,with a V_OCC of 0.89 V,a J_SCC of 11.96mA/cm²and a FF of 39.30%.Meanwhile,the device from controlled SM of ITIC and J61provided the macimal PCE of 7.60%with a V_OCC of 0.89 V,a J_SCC of 15.66 mA/cm² and a FF of54.38%.The lower J_SCC and FF of the devices from ITOR-IC and m-ITOR-IC might lead to the lower PCEs in contrast with those from the devices from ITIC.The introduction of fluorine can adjust the energy level and band gap,which is beneficial to light collection and increase short circuit current density.However,the study of the influence of fluorine atoms on the photovoltaic performance of ITIC organic SM was almost always introduced into the conjugated main chain.In chapter 3,ITOR4F-IC,m-ITOR4F-IC and m-ITOR8F-IC have been designed and synthesized by introducing fluorine atoms into the phenyl side chain of ITOR-IC and m-ITOR-IC and changed the number of fluorine atoms,respectively.The new materials were exhibited blue shifted absorption spectrum and the more of fluorine atoms,the larger of blue shift value,down-shifted molecular energy levels and greater crystallinitiy and improved photovoltaic performance in comparison with their nonfluorinated counterparts.The best devices fabricated from J61:m-ITOR4F-IC yield a maximum PCE of 5.24%,with a V_OCC of 0.88 V,a J_SCC of 13.01 mA/cm² and a FF of 45.78%.As the number of fluorine atoms increases,the reducement of photovoltaic performance might be due to the narrowing of the absorption spectrum.Due to the rapid development of acceptor materials with broad absorption spectra,the design and synthesis of novel conjugated polymers have also attracted much attention.In chapter 4,benzo[1,2-b:4,5-b]dithiophene?BDT?and thieno[3,2-b]thiophene?TT?were used as building blocks,and two wide band gap polymers of PBDT-TT25 and PBDT-TT36 have synthesized by varing the linker positions between BDT and TT,it exhibited the greatly widened and stronger absorption and the significantly reduced twist angle on the polymer backbone,more linear and planar molecular configuration and thus a 1.86 times increase in SCLC hole mobility was achieved even if elevated HOMO energy level when the linker positions changed from 3,6-to 2,5-positions.Thus,these important improvements led a 1.73times increase in J_SCC and a 22%enhancement in FF and thus a 1.87 times increase in PCE under similar experimental testing condition.