Theoretical Study On Charge Transfer And Its Dynamics Of Organic Photovoltaic Heterojunction Interface

Organic solar cells?OPV?have been widely studied for their many advantages.As an important part in OPV,the active layer composed of electron donors and acceptors is crucial to the performance of the device.Many experiments have proved that the modification of active layer,include donor/acceptor interface configuration,molecular engineering of materials and electric field can effectively adjust the performance of devices,include open circuit voltage(V_OC),short circuit current density(J_SC),etc.In order to explore the intrinsic mechanism of the effects of donor/acceptor interface configuration,donor halogenation,electric field,and fusion thienyl into backbone of acceptor on the optoelectronic properties of OPV.In this thesis,the complex models composed of donor and acceptor molecules were constructed,based on the density functional theory?DFT?and time-dependent DFT methods,combined with optimally tuned range separation parameters and solid polarization effects,the effects on the geometries,electronic structures and excitation properties of monomers and complexes were investigated.According to Marcus theory,the effects on the charge transfer?CT?,exciton dissociation?ED?and charge recombination?CR?processes at the interface are analyzed.The main work of this thesis includes:To investigate the molecular configuration effects of donor/acceptor heterojunction interface on the interfacial electronic processes,the Face-on,Edge-on and End-on configurations of p-SIDT?FBTTh₂?₂/C₆₀ complex were selected as the model systems.The results show that most of the excited states for donor exhibit intramolecular CT character,and the similarity of the excitation characters?CT and local excitation?and energies among three different configurations indicates that the electronic structure and excitation properties are insensitive to the interfacial molecular configurations.Whereas,the rates of ED and CR processes heavily depend on it These results underline the importance of controlling interfacial molecular configuration,and then the morphology at heterojunction interface in OPV.The model systems of F_n ZnPc?n=0,4,8,16?/C₆₀ and Cl_n SubPc?n=0,6?/C₆₀complexes were selected to study the effects of donor halogenation on the interfacial properties of complexes.Small differences in molecular orbital?MO?energy gaps,excitation and spectral properties of the monomers F_n ZnPc?n=0,4,8,16?and Cl_n SubPc?n=0,6?indicates that halogenation cannot effectively tune electronic and optical gap.However,the significant reduced MO energies indicate that halogenation causes remarkable influence on the energy level alignment at interfaces.Halogenation also enhances intermolecular binding energies between C₆₀ and donors,and increases the CT excitation energies of complexes,which are favorable for improving V_OC.In addition,for F_n ZnPc/C₆₀0 and Cl_n SubPc/C₆₀complexes,the CR rates dramatically decrease?several orders?with increasing of halogen atoms(except for F₁₆ZnPc/C₆₀),which means that the CR process can be suppressed by halogenation.The special case of F₁₆ZnPc/C₆₀ underlines the importance of fluorination levels in molecular design of donor.This study theoretically explains the halogenation effects of donors in OPV,which is helpful for the molecular design of donor materials.To explore the effects of electric field on the electronic processes of the OPV heterojunction interface,the pentacene/C₆₀ complexes with Face-on and Edge-on configurations were constructed as model systems.The effects of electric field on MO energies,indicating its effectiveness for modifying interface electronic structures and energy level alignments.The analysis of excitation properties underlines that,the increasing of electric field that is perpendicular to interface can increase CT energetic loss,reduce the lowest CT excitation energy and the V_OC.The rates of CT,ED and CR support that introducing electric field can effectively modulate the kinetics of electron processes,in particular,the CR process can be dramatically suppressed by applying electric filed pointing from acceptor to donor with suitable strength.The mechanism of fusion thienyl into backbone of NFAs was theoretically studied by using the heterojunctions of non-fullerene acceptors?NFAs,BTPT-4F and BTPTT-4F?and donor P2F-EHp.It was found that,fusing thienyl into backbone elevate the highest occupied molecular orbital?HOMO?energy,reduce the gap between the HOMO and the lowest unoccupied MO,lessen dipole moment,enhance the light absorbance and generate red-shift of absorption bands.The datas of P2F-EHp/BTPT-4F and P2F-EHp/BTPTT-4F complexes indicate that,fusing thienyl into backbone of acceptor modifies the excitation character of donor/acceptor complexes,increases CT excitation energy and the number of local excitation and CT hybridized low-lying excited states,amplifies the CT rate,reduces the ED rate,and dramatically suppress CR rate,leading to the better performance of BTPTT-4F.However,the main reasons of the worse performance of BTPT-4F can be attributed to it's larger dipole moment and faster CR rate.