Theoretical Studies On Sulfur Heterocyclic Donors With Charge Transfer Character At Donor/Acceptor Interfaces Toward Organic Photovoltaics

Organic photovoltaics?OPVs?have flourished nowadays by virtue of their potential advantages such as inexpensive manufacturing,flexibility,transparency,light-weight,ease of processing and soon.However,the lower power conversion efficiency?PCE?restricts their production and widespread application,which is attributed to the complex charge transfer process at donor/acceptor?D/A?interface.The theoretical simulation can provide the molecular-level detailsof describing the charge transfer process at D/A interface.Meanwhile,it also can afford theoretical guidelines for the characterization of electrical and optical properties,the evaluation of the photovoltaic performances,and design of promising donor materials through the reasonable structure-property relationships.In this dissertation,we design a series of new molecules via modifying or replacing the building blocks based on the experimentally synthesized thiophene derivative-based donor materials,to screen high-efficacy donors for OPV devices,by means of theoretical simulation.Moreover,charge transfer mechanismat D/A interface is described in depth to improve the design strategies of donor molecules and guide experiments.The studies mainly included thefollowing three parts:1.The OPV constructed from copolymer PDTSBTI?1?and accepter PC71 BM showed a high PCE of 6.41%.Herein,a series of bithiopheneimide?BTI?-based D-A copolymers were investigated based on the reported 1 to screen excellent molecules toward OPV donor materials.Geometry,electronic structures,absorption spectra and intramolecular charge transfer were calculated by density functional theory?DFT?and time-dependent density functional theory?TD-DFT?methods.It is found that the PCE based on the proposed derivative 4 shows a 70 percent improvement by Scharber diagrams compared with its prototype 1.Then,the charge transfer dynamics of 1/PC71 BM and 4/PC71 BMhave been investigated,including the intermolecular charge transfer?inter-CT?and recombination?inter-CR?rates.The theoretical data demonstrate that the ratio kinter-CT/kinter-CR of 4/PC71 BM heterojunction presents about 1 × 105 times higher than that of 1/PC71 BM.These results clearly reveal that the designed donor molecule 4 will be a promising candidate for high-performance OPV device.2.A class of D₁-A-D₂-A-D₁-type small molecule?SM?donors?2-5?was engineered via modifying or replacing the core donor moiety in three building blocks based on the reported DTS?PTTh2?2?1?to screen suitable donor materials for OPVs.The designed2-5 retainedpyridalthiadiazole?PT?as electron with-drawing acceptor groups and hexyl-substituted bithiophene as end-cap units.DFT and TD-DFT calculations were performed to investigate the electronic structures,open circuit voltage,and key parameters closely relevant to the short-circuit current density?including absorption spectrum,electron-hole coherence,energy driving force,charge transfer dynamics,and carrier transport efficiency?.The results manifest that the designed 2-5 show good performance with large open circuit voltage,stable charge transfer and effective charge transport.Surprisingly,the ratios kinter-CT/kinter-CR of 2/PCBM,3/PCBM,and 5/PCBM heterojunctions present over 104 times higher than that of 1/PCBM.Our conclusions indicate that designed PT-based SMs can better the performance of OPVs.3.A combined molecular dynamics?MD?and quantum chemical?QC?simulation method is utilized to investigate charge generation mechanism at TTF/TCNQ?tetrathiafulvalene/tetracyanoquinodimethane?heterojunction.Meanwhile,we discussed whether TTF/TCNQ system is suited to OPV devices.In the basis of all TTF/TCNQ complexes extracted from MD simulation,the amounts of charge transferred from ground states to different excited states and the corresponding energies of charge transfer state are compared and analyzed using QC simulation.Moreover,the electron transfer/recombination rates for these interfacial configurations are also studied.From these data,we have elucidated the underlying reason why TTF/TCNQ heterojunction is inadaptable to OPV application.It is also our eager expectation that the theoretical description of ultrafast charge transfer kinetic in this work,as a promising tool,can be valuable for experimental researchers to understand intrinsic mechanism of heterojunction interfaces in OPV devices and further optimize and design high-efficiency OPV materials.