The Quantum Chemistry Study On The Effective Conversion Of Donors To Acceptors In Heterojunction Organic Photovoltaic Cells

In recent years,organic photovoltaic(OPV)devices are favored due to their advantages such as simple process,low cost and environmental friendliness.Naturally,donors and acceptors,who compose the structure of OPV devices,have become hotspots.However,the acceptors are easy to be oxidized during the synthesis,so the development lags far behind that of the donors.In order to promote the development of acceptors and further improve the energy conversion efficiencies(PCEs)of OPV devices,scientists are trying to obtain new acceptors by simply modifying the existing donors and acceptors,which have been achieving certain effects.On the basis of this idea,a series of novel DPP-based acceptors were obtained by simple modification of donors and acceptors and their acceptor properties were characterized by quantum chemistry methods.The specific contents are as follows:1.In order to improve the PCEs of OPV devices Roncali and coworkers synthesized 2,5-Diethylhexyl-3,6-bis(5-(be-nzofuran-2-yl)thi-ophen-2-yl)-pyrrolo[3,4-c]pyrrole-1,4-dithione(BFT-SS)by substituting the two oxygen atoms of 2,5-Bis(2-ethyl-hexyl)-3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-pyrrol[3,4-c]pyrrole-1,4-dione(BFT)using two sulphur atoms.Unfortunately due to the very low the lowest unoccupied molecular orbital(LUMO)energy level the PCEs are less than 0.001%when server as donors.So BFT-SS and its rotamer BFT-SS' were boldly assumed to be acceptors and 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophen-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carbo xylate-2-6-diyl]](PTB7-Th)was selected to be the donor to prove the assumption.Moreover,the contrastive analysis of the parent compounds and analogues were carried out by quantitative chemical methods to explore the reason why BFT-SS and the isomer BFTSS' could be acceptors,namely,the cause of the conservation for donors to acceptors.The results show that S in thioketo groups,which contributes to LUMO larger than the highest occupied molecular orbital(HOMO),could twist the molecule and reduce the LUMO energy level resulting in the low charge injection barriers and small reorganization energy(?)values.Therefore,BFT-SS and BFT-SS'can be acceptors to some extent.2.It is very necessary to explore the mechanism for widely and reasonably promoting the conversion of donors to acceptors.The monomer of terthiophene(poly[{2,5-bis(2-hexyldecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]p yrrole-1,4-diyl}-alt-{[2,2':5'2"-terthiophene}])(PDDP3T)and poly(3,6-bis(5-hexylde-cyl-2thenyl)-2,5-dihydro-2,5-dipyrrolo[3,4]pyrrolo-1,4-dionethiazole}(PDPP2TzT)were chosen as the donor and acceptor,respectively,to explore the mechanism of the conversion for donors to acceptors by density functional theory(DFT)and time-dependent density functional theory(TD-DFT).Then a series of small molecule acceptors(SMAs)were designed according to this mechanism.The results show that the mechanism of the conversion for donors to acceptors is that the introduction of electron-withdrawing groups has an effect on the electron density,the intermolecular interaction and the LUMO energy level.Besides,the designed small molecule acceptors have good accepted properties except b10.3.In order to optimize the performance of the acceptors obtained by donors reasonably,a variety of SMAs based on benzothiadiazole(BT)were designed by the transformation of federation groups,acceptor units and substituents.The effects of each part on the properties were discussed by the frontier molecular orbitals(FMOs),energy levels,energy gaps(Eg)and absorption spectra,and will be further proved by the neural network model.Also the PCEs will be predicted by the neural network.