Study On Exciton Dissociation In Non-fullerene Organic Solar Cells

Organic solar cells have obtained great attention owing to the advantage of the large-area flexible device fabrication by low-cost solution processing method.The efficiencies of organic solar cells have made a huge breakthrough,ascribing to the development of new photovoltaic materials,the improvement of device preparation processes and so on.It is still an important challenge for further improving the power conversion efficiency of organic photovoltaic devices.However,compared with inorganic materials,the weak interaction of Van der Waals force and hydrogen bond dominate in organic materials,leading to the low dielectric constant,high electron-phonon coupling coefficient and large exciton binding energy.Therefore,the charge generation needs to overcome the large exciton binding energy in organic solar cells,which causes large energy loss in organic photovoltaic device and affects the device performance seriously.Many non-fullerene organic solar cells can achieve efficient exciton dissociation and obtain excellent device performance with the low energy loss.Nevertheless,the complex photo-physical processes and the working mechanisms involved in exciton dissociation are still unclear Hence,it is important to understand the physical mechanism of exciton dissociation in-depth,elaborate the physical mechanism of exciton dissociation and quantify the relationship between exciton dissociation and the device performance parameters.Based on the above key scientific issues,we systematically study the kinetic processes of donor/acceptor exciton dissociation in the non-fullerene organic solar cells.The specific work is as follows:(1)The dissociation of donor exciton based on singlet fission material.We choose the singlet fission material TIPS-pentacene as donor and the non-fullerene material IT-4F as acceptor to fabricate the organic photovoltaic blend.The dynamics of singlet exciton and triplet exciton in the blend are systematically studied.Singlet fission is an exothermic process in TIPS-pentacene film,which facilitates efficient triplet exciton generation in the blend.The lowest unoccupied molecular orbital(LUMO)of IT-4F acceptor material is lower than the triplet energy level of TIPS-pentacene,which guarantees the dissociation of triplet exciton.The crystallinity of the blend film is revealed by the steady-state absorption spectra and grazing incidence wide-angle X-ray scattering measurements.Combined with the triplet exciton dynamics in the transient absorption spectra,it is found that effective singlet fission occurs in the blend film,and the dissociation of the multiple exciton state dissociation at the interface between donor and acceptor,(1(*TIPS-pentacene)+TIPS-pentacene?1(TT)?3(TIPS-pentacene)+Then,the electron-hole pair suffers geminate recombination,(3(TIPS-pentacene)++e-?3*TIPS-pentacene)generating triplet exciton.Besides,singelt fission process can also generate triplet exciton.The optical and electrical properties of TIPS-pentacene films demonstrate efficient carrier transportation in the crystallinity films.This work elucidates the complex photophysical processes and competitive relationship between singlet fission and exciton dissociation in singlet fission organic solar cell with bulk heterojunction structure,which provides experimental guidance for the realization of high-performance singlet fission organic photovoltaic devices with the bulk heterojunction structure.(2)The dissociation of non-fullerene acceptor exction.We ultilized ultrafast spectroscopies to systematically study the hole transfer processes from five acceptor materials(m-ITIC,ITIC,IHIC,IEICO-4F,IEICO)to the copolymer donor J91.By means of transient absorption spectroscopies with the selective acceptor excitation,the relationship between HOMO energy offset(0.05-0.42 eV)and the time scale of hole transfer(1-1000 ps)is studied.The results show that ultrafast hole transfer can be achieved in the system with low HOMO energy offset,which is consistent with the result of free hole transfer at the interface between donor and acceptor.This suggest the dirven force of acceptor exciton dissociation is low.In addition,combined the charge transfer efficiencies with the photovoltaic device performance,it is found that the hole transfer efficiency is a key parameter affecting the device performance.The capacitance-frequency spectra and the density functional theory calculation confirm intra-and inter-molecular delocalized exciton of the non-fullerene acceptor.Time-resolved photoluminescence spectra demonstrate that the hole transfer originates from the dissociation of weakly bound exciton rather than the Frenkel exciton.We propose that the dissociation of weakly bound acceptor exciton dominates the hole transfer process in the "acceptor-donor-acceptor" non-fullerene organic solar cells.This work provides a new perspective for designing novel acceptor molecules,understanding acceptor exciton dissociation mechanism and the optimizing device performance.(3)The inter-molecular delocalized exciton dissociation of the non-fullerene acceptor.In order to further reveal the effect of intra-molecular charge transfer exciton and inter-molecular delocalized exciton on the hole transfer processes,we studied the delocalization and dissociation of acceptor exciton based on the PM6:Y6 high-performance system.The ultrafast transient absorption spectra of the PM6:Y6 blend solution show that there is no effective hole transfer between donor and acceptor.However,inter-molecular delocalized excitons were observed in the transient absorption spectra in PM6:Y6 blend film,owing to the enhancement of inter-molecular interaction.Compared with Y6 film,the number of inter-molecular delocalized exciton was significantly reduced,indicating that inter-molecular delocalized exciton dissociate into charges effectively at the interface between donor and acceptor.Besides,we ultilized temperature-dependent steady-state photoluminescence spectra and capacitance-frequency spectra to study the effect of molecular packing on the acceptor exciton delocalization in Y6,IEICO-4F,ITIC,PCBM films,revealing that intra-molecular charge transfer exciton is tightly bound and the inter-molecular delocalized exciton is highly delocalized in the acceptor films.This work elaborates the influence of intra-and inter-molecular exciton delocalization in the hole transfer process,which has important significance for studying the mechanism of exciton dissociation in non-fullerene organic solar cells.