Multiscale Exciton Dynamics In Low Dimensional Organic Semiconductors Heterojunction

Low-dimensional semiconductor materials have emerged rapidly along with the speedy development of nanotechnology in the 20th century.In recent years,the controllable preparation of low-dimensional semiconductor materials,the construction of heterojunctions,and the application of optoelectronic properties have become critical issues that need to be addressed for further development of semiconductor technology,due to the rapid development of the new generations of optoelectronic integration technology.Organic semiconductors not only have the advantages of low cost,simple processing,and diverse structures,but also form by aggregation of weak van der Waals forces between layers which usually exhibit twodimensional transport,and are considered desirable materials for building low-dimensional structures.With the persistent efforts of researchers,a variety of low-dimensional organicorganic(O-O)heterostructures or organic-inorganic(O-I)hybrid structures have been constructed,and their rich interfacial properties provide new opportunities for the discovery and regulation of excited-state new effects.However,compared with conventional organic heterostructures,the more flexible material matching and construction methods of lowdimensional heterojunctions have brought more new phenomena and mechanisms in excitedstate dynamics and device applications.Particularly,the physical mechanisms of exciton generation,transfer,dissociation,and diffusion in low-dimensional organic heterojunctions are still unclear in terms of photophysical mechanisms,and there is a lack of quantitative model interpretation and multiscale regulation of the corresponding exciton dynamics.Therefore,achieving accurate investigation of exciton dynamics processes within low-dimensional organic heterojunctions and elucidating the quantitative relationship between exciton dynamics modulation and device performance in direct correlation are of crucial importance to advance the development of low-dimensional semiconductors.Corresponding work has been carried out to address the above issues in this area,and the specific contents are as follows:(1)In the first study,the photonic energy conversion process between organic molecules and inorganic nanocrystals was revealed,and it was demonstrated that the surface energy modification means can effectively promote the triplet state energy transfer,and the singlelinear state energy transfer and charge transfer processes in low-dimensional O-I hybrids were quantitatively analyzed.With the aid of rubrene and PbS nanocrystals adsorbed with various ratios of Cd2+,the photophysical dynamics of charge,single-state excitons,and triplet excitons during the energy conversion of the hybrid system were completely clarified.With the assistance of ultrafast transient absorption measurement,the charge in rubrene molecules can be transferred to the cation-induced surface state with amazing accuracy on the time scale of 2 ps,leading to the delayed double exciton effect.This discovery opens up a new method to understand the intermediary role of NC surface states.In addition,the systematically studied powerful interactions between the triplet exciton and the surface state resulted in a～38%increase in the maximum photoluminescence lifetime of PbS nanocrystals and a 14%singlelinear state energy transfer efficiency.This work reveals the negligible photophysical dynamics between low-dimensional organic molecules and inorganic nanocrystals,and verifies the role of surface state interaction in organic charge transfer and exciton sensitization,providing theoretical support and guidance for the corresponding near-infrared photodetection applications.(2)In the second work,a large fluorescence amplification was made possible by utilizing the energy funnel effect to get around the limited photoluminescence yield of low-dimensional heterojunctions.The vertical energy funnel channel was constructed in an organic-inorganic heterostructure composed of MoSe2,pentacene and graphene oxide(GO).The channel enabled efficient transfer of photocarriers from GO and pentacene layers into the monolayer MoSe2,achieving up to 3.7 times the fluorescence intensity.Time-resolved fluorescence spectroscopy system was used to reveal the exciton dynamics processes in the operating state of the energy funnel channel.In addition,the fluorescence signal was able to be further enhanced up to 4.7fold due to the weakening of electron-phonon interactions at low temperature conditions.In addition to the effective fluorescence enhancement,a 130-fold increase in photogenerated carrier transfer efficiency can be achieved by effectively tuning the energy shift of the top layer of GO in low-dimensional heterojunctions.This discovery opens up a new path for the future design and development of new optoelectronic devices based on low-dimensional materials by offering a straightforward and adaptable way to alter the luminescence properties in lowdimensional heterojunctions.(3)The third work,a multidimensional O-I heterostructure was built to successfully regulate the diffusion process,carrier transfer rate,and extraction efficiency in the tetracene/MoS2 type II heterostructure.The diffusion mechanism of the correlated onedimensional diffusion model systematically reveals the dependence between the carrier injection and diffusion processes.It is demonstrated that the trap-dependent complexes further restrict the efficient extraction of interlayer carriers at low temperature settings using temperature-dependent time-resolved spectra.The correlation results indicate that the exciton diffusion length in two-dimensional O-I heterostructures is increased,resulting in a breakthrough in the electron transfer rate(9.53 ×109 s-1)and carrier extraction efficiency(60.9%).This work offers crucial instructions for creating optimal band-aligned lowdimensional heterostructures as well as an unique approach to building functioning optoelectronic devices.(4)In the end,with the aid of the modification layer-assisted strategy,the exciton binding energy of the photodetector is effectively reduced and the exciton dissociation,energy transfer and charge extraction processes are facilitated,realizing a significant improvement in the performance of ultrathin self-powered devices.Usually,the advantages of ultrathin selfpowered organic photodetectors are rarely explored due to low optical absorbance and carrier recombination problems.This strategy is not only able to prevent unfavorable charge injection,reduce the dark current,and improve the light absorption capacity of the photosensitive layer.It is also capable of increasing the dielectric constant of the photosensitive layer,enhancing the interlayer energy transfer efficiency and facilitating efficient exciton dissociation.The related results demonstrate that significantly improved device responsiveness(0.45 A/W)and detection rate(1.25 ×1012 Jones)are observed in the ultrathin organic photodetector with modified configuration.This work establishes a clear connection between exciton dissociation,photogenerated exciton utilization and energy transfer channels,and provides important theoretical support for the design and development of efficient ultrathin organic photodetectors.