Computational Studies Of Donor-acceptor Fused Materials Of Organic Photovoltaics

At present,donor-acceptor molecules are widely used in the active layer of organic solar cells because of their small band gap and wide absorption spectrum.Donor-acceptor molecules can not only be used in the single layer homojunction organic solar,but also can be used as donor or acceptor in bulk heterojunction organic solar cells.In an organic solar cell with a bulk heterojunction structure,donor-acceptor molecule can be used as donor since there are many intramolecular charge transfer path ways in it,and this is beneficial to increase the open-circuit voltage of the organic solar cell.In addition,due to the push-pull effect in the donor-acceptor molecule,a growing number of donor-acceptor molecules are also used as acceptor molecules.Although donor-acceptor molecules are important,there are still many deficiencies in the research of it.For example,how to predict the intramolecular charge transfer path ways of organic optoelectronic molecules fused with multiple donor and acceptors,and how the intensity of the electron-withdrawing group or the electron-donating groups effect on the photoelectric properties of the donor-acceptor molecules.These two aspects remains to be deeply understood,especially the theoretical research in this area is still extremely scarce.Since last century,density functional theory?DFT?and time-dependent density functional theory?TDDFT?have been widely used to study the performance of organic solar cells,and achieved satisfactory results.In this paper,density functional theory and time-dependent density functional theory are used to study several donor-acceptor molecules including organic optoelectronic molecules fused with multiple donor and acceptors and the non-fullerene acceptor Y6 which has been used as acceptor in the organic solar cell with the highest power conversion efficiency.The contents in this work are twofold:?1?It is possible that the power conversion efficiency of organic solar cells with single layer homojunction structure will be further improved with the appearance of donor-acceptor molecules.SubPC-ADP-C60,BDP-ADP-C60 and?PTZ?₂-ADP-C60 are three donor-acceptor molecules with intramolecular charge transfer state synthesized by Bandi et al.,and they can avoid rapid recombination of electron and hole.Based on the experimental study of the charge transfer states of these molecules,we find that the charge transfer states of the three molecules with similar structures are quite different.In the chapter 3,we modeled the corresponding dyads and monomers of these three molecules,and studied the photovoltaic properties such as absorption spectra and intramolecular charge transfer states of these molecules by using density functional theory?DFT?and time-dependent density functional theory?TDDFT?.We find that the method of forming a donor-acceptor molecular by fusing the donor and the acceptor through covalent bond can indeed reduce the band gap and broaden the absorption spectrum of the molecules.In addition,the electron-hole distribution maps of the charge transfer states of the three molecules are calculated and compared with the charge transfer states measured in the experiment.The types of the calculated charge transfer states of the three molecules are more than those measured in the experiment.By comparing the driving forces of each charge transfer state,that is,the HOMO offset and LUMO offset,we can explain the reason why the calculated charge transfer states cannot be observed in the experiment.Through this work,we found a simple method,which is basically applicable in subPC-ADP-C60 and BDP-ADP-C60 molecules.In the future,we will study charge transfer states of other donor-acceptor molecules and improve this method to make it more universal.?2?Y6 is one of the non-fullerene donor-acceptor molecular with A1-D-A2-D-A1structure,and it has been used as the acceptor in the organic solar cell with the highest power conversion efficiency.It is a new organic photvoltaic material with great potential.In the chapter 4,on the basis of Y6 with central part A2 named BT,a series of derivatives of Y6 with different central part A2?BT-O,BT-Se and Benzene?are modeled.The ground state properties of these molecules such as dipole moment,energy levels,and excited state properties such as absorption spectrum range,exciton binding energy have been studied by using density functional theory?DFT?and time-dependent density functional theory?TDDFT?.We also investigated the effect of the change of central part A2 on the photovoltaic properties of the molecule.The calculation results have shown that the existence of central part A2 is essential for the Y6 acceptor,and with the decrease of the electronegativity of the substituted atoms of the central part A2,the trend of energy level,dipole moment,absorption spectrum range,and exciton binding energy are improved.This work may provide an effective strategy for the modification of non-fullerene acceptor based on Y6,that is,we can obtain excellent acceptors of organic solar cell with high power conversion efficiency by changing the intensity of the electron-withdrawing group named A2 of Y6.At the same time,we predict that Y6-Se is a potential acceptor material,because when the dipole moment and exciton binding energy of it are close to the original non-fullerene small molecule acceptor Y6,the band gap of it is lower than that of Y6,the absorption spectrum range of it is wider than that of Y6,and its absorption strength is stronger than that of Y6 in visible region.