Research Of Multi-Scale Excited State Dynamics And Stability In Non-Fullerene Organic Solar Cells

Organic solar cells have received extensive attention due to their unique advantages such as flexibility,light weight,semitransparency and ease of solution processing.In recent years,non-fullerene organic solar cells have made remarkable progress,and their highest power conversion efficiency has exceeded 18%for single-junction devices.The progress has significantly accelerated the process of commercialization.However,to realize the practical application of organic photovoltaic devices,a key challenge is to obtain very high device efficiency when the photoactive layer is sufficiently stable.At the present stage,the photovoltaic efficiency of non-fullerene organic solar cells is still lower than that of perovskite solar cells,and the long-term stability of the actual devices is far from reaching the standards for commercial applications.Therefore,in order to further improve the power conversion efficiency and stability of organic solar cells,in this paper,we propose a new multi-scale excited state dynamics model that can reveal the potential evolution of device performance.Constructing the essential correlation between the ultrafast photophysical mechanism and the ultraslow stability process.Taking time and space as the main research direction,performing in-depth exploration and discussion on the behaviors of exciton diffusion,hole transfer and kinetic aggregation of non-fullerene acceptors.The detailed research content is as follows:(1)The application of fluorination and chlorination strategies has yielded a large number of new organic functional materials,and has made great progress in improving device efficiency of organic solar cells.However,in these high-performance organic photovoltaic systems,the underlying mechanism of their influence on photocarrier dynamics is still unclear.Relying on the 2×2 photovoltaic matrix composed of PBDB-T,PBDB-T-2C1,ITIC and IT4F,we systematically investigated the interplay between chlorinated donors and fluorinated acceptors.The experimental results indicate that the chlorination of polymer donor can enhance the diffusion and relaxation rate of excitons and promote the extraction of positive polarons.The IT4F-based photovoltaic systems exhibit more efficient charge transfer and a larger number of long-lived polarons,which is very conducive to the generation of charge carriers.The synergistic effects of fluorination and chlorination can improve the delocalization of singlet excitons,reduce the binding energy of excitons and the Coulomb capture radius,thereby increasing the efficiency of charge separation and decreasing the possibility of bimolecular recombination.In addition,the simultaneous application of fluorination and chlorination can optimize molecular packing and nanoscale phase separation,thus promoting exciton diffusion and dissociation as well as charge carrier transport.This work mainly explores the effects of fluorination and chlorination strategies on these fundamental photophysical mechanisms,which not only provides a novel perspective for developing the high-performance organic materials,but also exhibits important value for further enhancing the power conversion efficiency of organic photovoltaic devices.(2)The synergistic application of the respective advantages of fullerene derivatives and non-fullerene acceptors based on a ternary strategy is an effective method to simultaneously improve device efficiency and stability.In this work,we incorporated fullerene derivative ICBA into the state-of-the-art PBDB-T-2F:BTP-4Cl system to prepare the high-performance ternary organic solar cells.The experimental results of contact angle measurements confirm the better compatibility between ICBA and BTP-4C1,which indicates that a small amount of amorphous ICBA is mainly distributed in BTP-4Cl to form the well-mixed acceptor phase.This significantly reduces the exciton decay loss due to excessive phase separation and enhances the morphology stability of ternary blends.Adding appropriate amount of ICBA can not only induce the long-range Forster resonance energy transfer to BTP-4Cl,but also improve the ultrafast hole transfer rate from BTP-4Cl to PBDB-T-2F,which promotes more efficient charge carrier generation in organic photovoltaic devices.Based on the advantages of enhanced light-harvesting property,promoted charge transfer efficiency,more balanced charge transport and more stable bulk-heterojunction morphology,the ternary organic solar cells exhibit an average power conversion efficiency over 16.5%and the superior photostability.The results show that fullerene derivatives used as the dual-functional additives have great potential to simultaneously strengthen device efficiency and photostability.(3)Long-range exciton diffusion on the picosecond time scale is a key step in the working mechanisms of organic photovoltaic devices.However,it is still a severe challenge to control this process in a wide temporal scale in ternary organic solar cells.In this work,we have incorporated the spherical fullerene derivative PC71BM as the third component into two representative PM6:IT4F and PM6:BTP-4C1 systems to fabricate ternary organic solar cells,which yields the simultaneously enhanced device efficiency and thermal stability.Based on these results,we systematically reveal how the exciton diffusion behaviors can be significantly improved and maintained from ultrafast to ultraslow temporal scale.The improvement of exciton diffusion can be attributed to two independent factors:(1)the dual Forster resonance energy transfer promotes the directed exciton diffusion and quenches high-energy excited states rapidly,thereby stabilizing the "energy donor".(2)The more optimized multi-length scale morphology induces a large number of donor/acceptor interfacial region to shorten the actual distance of exciton diffusion and improves the stability of nanomorphology in ternary blends.Finally,the diffusion length of IT4F singlet excitons induced by dual FRET effects is more than 10 nm calculated by employing exciton-exciton annihilation strategy.This work not only provides a feasible method for enhancing device efficiency and stability from the perspective of regulating exciton diffusion,but also enriches the working mechanisms of fullerene acceptors,thereby resulting in more choices and possibilities for the future development of organic photovoltaic devices based on non-fullerene acceptors.(4)The degradation of metastable morphology is a challenging obstacle for the practical application of organic photovoltaic devices at this stage.In this work,we have employed versatile alloy states to suppress the kinetic aggregation behaviors of Y-series non-fullerene acceptors during the aging process,yielding highly efficient and stable ternary organic solar cells.The self-stable polymer acceptor PDI-2T and small molecule donor DRCN5T were selected as the third component,respectively.The alloy-like model was separately applied to the host donor and acceptor materials of the state-of-the-art PM6:BTP-4Cl binary system,resulting in the simultaneously improved device efficiency and storage stability.In these two ternary systems,two separate working mechanisms can reveal the stabilizing function of versatile alloy states:(1)the acceptor alloys enhance the conformational rigidity of BTP-4Cl molecule to suppress their intramolecular vibration for rapid relaxation of highly excited states to stabilize BTP-4Cl acceptor.(2)The donor alloys optimize the fibril network microstructure of PM6 polymer and inhibit the unfavorable dynamic diffusion and aggregation of BTP-4Cl molecules.Furthermore,on the basis of the excellent stability of bulk heterojunction morphology,the non-radiative defect trapping coefficients have been substantially decreased,and there are no long-lived trapped charge species.The experimental results emphasize a new protection mechanism that can enhance the long-term stability of ternary organic solar cells by constructing versatile alloy states.