Study On The Microstructure And Multi-scale Excited State Dynamics Of Insulator Diluted Organic Photovoltaic Cells

Organic photovoltaic(OPV)cells as the photovoltaic device with broad application prospects have achieved rapid development due to their advantages of light-weight,low-cost,flexible and printing production.Currently,the highest power conversation efficiency(PCE)of OPV device has exceeded 18%due to the emergence of high-efficiency non-fullerene acceptor materials.However,the photovoltaic efficiency of the OPV device is extremely sensitive to the thickness of the active layer.The increase of thickness can increase the complexity of the morphology structure and trap state density,and thereby suppress charge transport and improve the charge recombination.Therefore,the thickness of the high-efficiency OPV device is limited to around 100 nm,which can not meet the requirements of commercial printing production.In order to improve the thickness tolerance of OPV devices and prepare the high-efficiency thickfilm OPV device,the low-cost and simple insulating polymer dilution concept was adopted in this paper to optimize the molecular stacking and trap distribution of the active layer with large thickness,which improves the performance of thick-film OPV devices and achieves the preparation of high-performance OPV devices with multi-scene applications.In addition,through the synchrotron radiation,electrical measurement and ultrafast spectroscopy technology,the molecular-scale morphology,trap state density and excited state dynamics in the thick active layer were explored.The corresponding physical mechanism and correlation were quantitatively analyzed.The detailed research works are as follows:(1)In OPV cells,the excited state dynamics,such as exciton behaviors,charge transfer process,etc.,play a key role in determining the generation of photogenerated carriers and device performance.However,most of the related research works mainly focus on the ideal bulk-heterojunction(BHJ)morphology,and do not consider the morphological structure and trap distribution at the donor-acceptor interface.Especially in the thick active layer,increasing the active layer thickness can elevate the complexity of the morphology and difficulty of optimization.Therefore,we focused on a set of high-efficiency photovoltaic systems to study the molecular packing structure,interfacial trap state density and excited state dynamics in each BHJ film by means of electrical measurements and ultrafast spectroscopy.The experimental results show that higher molecular order and ideal interfacial contact between donor and acceptor in the amorphous domain can effectively reduce the trap density in the photovoltaic system,leading to a longer exciton lifetime.The reduced trap density can accelerate the electron transfer process.On the contrary,the participation of thermal energy ensures efficient hole transfer,and the hole transfer rates of the BHJ photovoltaic systems selected in this work are maintained at 0.55 ps-1,independent of the trap density.In addition,we introduced the discotic liquid crystal(DLC)material with edge-on molecular arrangement into high-efficiency OPV systems,and study the effect of the molecular arrangement in excited state dynamics.The measurements of grazing incidence wide-angle X-ray scattering technology and transient absorption spectra indicate that the three-dimensional molecular arrangement can effectively suppress the formation of triplet excitons caused by bimolecular recombination in the active layer.This part of the work focuses on exploring the effects of different morphologies,crystal structures,and trap density on the dynamics of various excited states,and constructs quantitative correlation images between molecular order,interfacial trap states,and behavior of excited states,which contributes to improving the performance of thick-film OPV devices and provides a theoretical basis for the research of the physical mechanism of thick-film OPV devices.(2)In the second part of this paper,we adopted the insulating polymer polypropylene(PP)dilution strategy into the high-efficiency non-fullerene photovoltaic system PM6:Y6.Based on the quantitative correlation between morphology,trap state and excited state behaviors,the morphology and trap states in the corresponding thick-film OPV devices are regulated,which optimizes the charge transfer,transport and recombination processes.The measurements of grazing incidence wide-angle X-ray scattering technology and space-charge-limited currents at different temperatures demonstrate that PP can improve the molecular order and intermolecular interactions in the thick-film active layer,and thereby reduce the trap density and energy disorder.Therefore,compared to the control device,the thick-film diluted OPV devices exhibit higher photovoltaic parameters and thickness tolerance.At the active layer thickness of around 300 nm,the PCE of the diluted OPV device is 15.5%,which is higher than that of the thin-film devices.This research demonstrates that the insulator dilution strategy can effectively suppress the generation of trap states and provides a new understanding for commercial printing production of OPV cells.(3)In the third part of this research work,we introduced the insulating material polypropylene(PP)as the third material in OPV devices(PBDB-T:PC7,BM and PTB7:PC71BM),and achieved high-efficiency thick-film organic solar cell devices.It is found that PP significantly improves the crystallinity of the donor materials,and inhibits the aggregation of fullerene acceptors.The device with PP exhibits accelerated charge transport and exciton dissociation.Additionally,the reduction of aggregate size can improve the dissociation of excitons.Therefore,PP can effectively solve the challenges in thick-film OPV cells,such as low mobility and obvious recombination phenomenon.Based on the above research results,we used PP to fabricate high-efficiency thick-film organic solar cell devices and increase the optimal thickness to about 300 nm,which contributes to the development of the roll-to-roll mass production process of OPV.As a material with low cost,wide source and good universality,the application of PP in thick-film OPV cells promotes the commercialization process of the OPV industry,further broadens the application prospects of insulating materials,and is a great way to prepare high-efficiency thick-film organic photovoltaic devices.(4)The absorption spectra of OPV materials match well with 11-polymer OPV system PBDB-T:N2200 with 5%insulating polymer polystyrene(PS)the spectra of indoor light sources.Therefore,OPV devices are regarded as one of the best candidates that drive lowpower electronic devices in indoor environments.However,the indoor OPV device is sensitive to the trap states due to the low light intensity of indoor light sources.In the l ast research work,we diluted the a and fabricated high-efficiency ultra-thick indoor OPV devices.The 5%PSdiluted photovoltaic device exhibits improved crystallinity,lower trap density and accelerated charge transfer process than the binary control device.Therefore,the 1-μm thick diluted indoor photovoltaic device exhibits a high fill factor(FF)of 67.01%and PCE of 17.06%under the illumination of 1200 lux LED light source.This research work broadens the application prospects of OPV devices,contributes to the improvement of the thick-film indoor OPV device performance,and promotes the commercialization process of OPV cells.