The Study Of Ring-fused Perylene Diimide Based Acceptors For Organic Solar Cells

Organic solar cells（OSCs）have attracted extensive attentions due to their advantages of light weight,solution processing ability and flexible devices.However,only few non-fullerene acceptors can achieve high efficiency.Moreover,the relationship between the chemical structure of materials and the performance of device is unrevealed,which greatly limits the design of high performance acceptor.In response to the above challenges,this thesis designed and synthesized a series of ring-fused perylene diimide（PDI）derivatives,and prepared them into organic solar cells,obtained the highest photoelectric conversion efficiency（PCE）of PDI-based acceptors（12.56%）that has been reported so far.Moreover,the molecular configurations,optical,electrical and photovoltaic properties of the acceptors were studied in detail,revealed the relationship between the chemical structure of PDI-based acceptors and device performance.The results and innovations are summarized as follows:Firstly,in order to suppress the strongπ-πstacking of PDI unit and improve the light absorption performance of molecules,a kind of multi-stage twisted propeller-like small molecule acceptor was designed and synthesized with an aromatic pentaphenyl core and four PDI wings.On this basis,the properties of acceptor is further improved by ring-fused and heteroatom substitution.The results show that the core ring-fused strategy is beneficial to enhance the light absorption of PDI-based acceptor.Moreover,the ring-fused strategy reduced the film-forming recombination energy and enhances the intermolecularπ-πstacking,thus optimizing the morphology of the active layer,using ITO/Zn O/PTB7-Th:Acceptor/Mo O₃/Al device structure,and ultimately improving the photovoltaic performance.Introducing fluorine or selenium atoms into the PDI unit has influence on the absorption spectrum,energy level and PCE of the acceptor.The introduction of selenium atoms will increase the molecular fused rings,enhance the molecule stacking,and then increase the charge mobility.The introduction of F atoms can reduce the LUMO and HOMO energy levels of the acceptors increase the energy level difference between the donor and the acceptors,improve the exciton dissociation efficiency,reduce exciton recombination,and obtained the highest photovoltaic performance.Secondly,on the basis of the three-dimensional structure can suppress the excessive one-dimensionalπ-πstacking of PDI molecules,in order to further inhibit the recombination energy of PDI molecules during the film formation process,a series of non-planar rigid ring-fused PDI-based acceptors were designed and synthesized.The influence of cis-trans isomerism on the performance of these acceptors was investigated.In the absorption spectrum,these PDI-based acceptors have no obvious stacking peak,which proves that the non-planar structure can effectively suppress theπ-πstacking of PDI-based acceptors.The absorption spectra of the solution state and the film are similar,indicating that the molecular configuration in two states is similar,which is attributed to the rigid ring-fused structure.Trans structure acceptor have wider absorption spectra and higher the molar extinction coefficient,after being blended with the donor material,it can absorb more photons,which is beneficial to increase the short-circuit current density of the device,thereby improving the photovoltaic performance.In addition,the influence of halogen atom substitution on these non-planar ring-fused PDI-based acceptors was also studied.Among them,tetrafluoro-substitution acceptor has the highest extinction coefficient,indicating that the introduction of fluorine atoms is beneficial to enhance the photon absorption capacity of the molecules,increase the short-circuit current density,using ITO/Zn O/PTB7-Th:Acceptor/Mo O₃/Al device structure,and ultimately improve the photovoltaic performance.Then,in order to further enhance the intramolecular charge transfer and optimize the light absorption and energy level,the PDI unit and the electron-rich IDT were ring-fused to form a donor-acceptor-donor（DAD）type core,and then combined with the electron-deficient fluorinated indanone to obtained a series of A-DA’D-A acceptors.With the different connection positions of the PDI unit and the cis-trans isomerism of the IDT backbone of the acceptors,the influence of the dual cis-trans isomerism acceptors on the material properties and device performance was investigated.The stacking peaks in the absorption spectra of these cis-trans isomers are obviously different,indicating that different cis-trans isomers have different molecular packing forms.Studies have shown that the absorption of trans-structure molecules is more red-shifted,and the red-shift of the absorption spectrum after film formation is greater,which can use more sunlight.Among them,the film absorption spectrum of the dual-trans-structure tt-PDFC covers 725 nm,and the extinction coefficient reaches 4.32×10⁵ M^-1 cm^-1,which is good for absorbing sunlight and achieving higher photocurrent.The tt-PDFC molecule achieves a photoelectric conversion efficiency of 12.56%with the device structure of ITO/PEDOT:PSS/PM6:tt-PDFC/Zn O/Ag,the highest efficiency of perylene imide acceptor materials reported so far.Moreover,its fill factor is as high as 81.3%,which is suitable for the preparation of thick film devices.When the film thickness reaches 570 nm,the photoelectric conversion efficiency still reaches 9.66%.Finally,on the basis of previous chapter,the influence of the chemical structure of the A-DA’D-A type PDI-based acceptors on the photovoltaic performance was studied.The difference on optical,electrchemical and optoelectronic properties were investiegated with different end groups,side chains and conjugated skeletons of acceptors.Firstly,the research on the end groups of the tt-PDFC molecule shows that the weak electron withdrawing end groups increase the LUMO energy level of the molecule,thereby increasing the open circuit voltage(V_oc)of the device.However,the ensuing band gap increase is not conducive to the increase of the photocurrent of the device.Secondly,the side chain modification of this type of acceptors was studied,and the results showed that the large sterically hindered phenyl side chain is not conducive to the molecules stacking and hinders the transfer of charges.Thirdly,expand the conjugation length of the molecular skeleton.The absorption spectrum of the modified molecule FPDTFC-Ph shows a significant red shift,so the short-circuit current density of the device has been significantly improved,but the open circuit voltage and fill factor have both decreased.However,both the open circuit voltage and the fill factor have dropped significantly,and finally the PCE is decreased.The above research results provide a reference for further improvement of A-DA’D-A acceptor that contained PDI unit.