A Theoretical Study Of Functionalized Non-fullerene Fused-ring Electron Acceptors

Bulk heterojunction(BHJ)organic solar cells(OSCs)have drawn tremendous attention due to their light weight,good flexibility,low cost,rich materials,simple process and adjustable structure.In the past two decades,fullerenes have been used as the main acceptors in the active layer materials of organic photovoltaic(OPV)devices.However,due to their limited photon absorption,difficult structure modification and large voltage loss,the power conversion efficiency(PCE)based on fullerene acceptors is stagnant.Hence,the study of non-fullerene acceptors(NFAs)is becoming more and more important.Especially since 2018,OPV devices based on non-fullerene fused-ring electron acceptors have achieved breakthrough high PCE,which has reached 18.22%to date.Generally,the study of NFAs,on the one hand,mainly focuses on the matching with donor to obtain appropriate energy levels,complementary photon absorption and excellent blend morphology;on the other hand,the modification of acceptors is of vital importance.With reasonable modification strategies,the electronic structure,molecular energy level and UV-Vis absorption spectra of non-fullerene molecules can be effectively improved and regulated.In this paper,two kinds of functionalized non-fullerene fused-ring electron acceptors,i.e.electron-deficient pentacene and Y6 and their derivatives,are mainly selected as the research objects.By means of reliable density functional theory(DFT),time-dependent density functional theory(TDDFT)and Marcus theory,the electronic structure,excited state and charge transport properties of these functionalized non-fullerene molecules are deeply explored and analyzed.The main contents are as follows:(1)Electron-deficient pentacenes have drawn much attention because of its high carrier mobility,simple molecular structure and low cost,which has been generally applied in organic transistors and OSCs.Studies have shown that the lowest unoccupied molecular orbital(LUMO)energy levels of functionalized 6,13-bis(trimethylsilyl alkynyl)pentacenes(P0)are donw-shifted by introducing electron withdrawing groups(EWGs),which lead to the conversion of P0 from electron-rich materials to electron-deficient materials.Therefore,the P0 derivatives can be used as acceptor materials in OSCs.In Chapter 3,we have systemically investigated the effects of different substituents(-CN,-CF₃,-NO₂)and different positions(ortho-position:R1,meta-position:R2)on frontier molecular orbital energies,electron distributions,dipole moment,exciton binding energy and UV-Vis absorption spectra.The calculated results indicate that the molecules substituted with-CN group have much larger dipole moment than the others.For the same EWGs,the dipole moments of the molecules at the R2 position are generally larger than those at the R1 position.Calculation of exciton binding energy for the same EWGs indicates that the molecules at the R2 position are lower than that at the R1 position.Furthermore,the results also show that the exciton binding energies of these functionalized non-fullerene molecules are lower than that of fullerene molecule C60,and they have stronger absorption strength in the both visible and ultraviolet regions compared with C60.In summary,the theoretical calculations demonstrate that electron-deficient pentacene derivatives seem to be potential NFA materials.Moreover,the study on the positions and natures of substituents provides a good idea for the modification of novel NFAs.(2)OSCs based on the non-fullerene fused-ring electron acceptor Y6 have drawn tremendous attention due to the great progress in PCEs.Recently,experimental researchers found that the performance of BTPT-4F(like Y6 but with different side alkyl chains and shorter length of fused-ring skeleton simultaneously)based OSC is much worse than that of Y6 based OSC.However,it is still confusing how the individual change of the fused-ring skeleton length or side alkyl chains of Y6 affects photovoltaic properties,especially molecular arrangements and electron mobility.Therefore,in Chapter 4,we have modelled possible dimer configurations of Y6,BTPT-4F,and two new intermediate NFAs,named BTPTT-4F-C8 and BTPTT-4F-C12,to systematically study the influence of the individual change of fused-ring skeleton or side alkyl chains on molecular packing and electron mobility.The computed results could explain experiment well,and show that the terminal/central alkyl chains and the length of fused-ring all have noticeable influences on molecular planarity and arrangements.The outputs illustrate that extending fused-ring skeleton can increase electron mobility more than engineering side alkyl chains,and removing/increasing terminal/central side alkyl chains seem capable to further improving electron mobility,respectively.The data also demonstrate that high electron coupling and electron mobility mainly depend on the formation of close face-on stacking of end groups and overlapping of partially flat backbones between these molecules.Our discoveries progress the understanding of Y6-based OSC,and thus provide a guideline for future molecular modification of Y6.