Solution Processing of Aromatic Imides and Its Thermoelectric and Ferromagnetic Properties

Author:Jiang Qing Lin

Supervisor:ma zuo guang


Degree Year:2019





In recent years,organic conjugated semiconductor materials have been widely used in the fields of electroluminescence,field effect transistors,organic photovoltaic cells,and organic thermoelectrics.Active organic thin film-based devices,such as the most well-known organic light-emitting electronic display,have entered the market with high efficiency,wide viewing angle,high brightness and ultra-lightweight features.This shows that organic conjugated semiconductor materials can play a role in new applications that traditional inorganic semiconductors cannot achieve in the future,and show unique advantages in material preparation and device processing.For example,in the field of organic ferromagnetic materials,synthetic pure organic magnetic materials have been the focus of chemists and materials scientists in the past few decades.An important reason for the development of organic ferromagnetic materials is because they have unique properties different from traditional metal and ionic crystalline magnetic materials.Therefore,it is foreseeable that in the near future,electronic device products based on organic light electromagnetic materials will gradually become the mainstream of human science and technology,showing a broader development prospect.As a typical aromatic conjugated molecule,aromatic imide derivatives have excellent chemical and photothermal stability and excellent carrier transport characteristics,and have been the focus of attention,and have been widely used.However,there are still some scientific problems in the application of aromatic imides,mainly because such materials have large conjugate planes,strong molecular rigidity,and serious molecular aggregation,which makes them poorly soluble and difficult to process solutions,which limits their application.At present,the problem of dissolution of aromatic imide materials can be solved to some extent by grafting side chain groups,etc.,but at the expense of certain properties of the bulk material,such as by modification,after intermolecular reduced carrier mobility,etc.Therefore,the design of a reasonable method can not only solve the dissolution problem of such materials,but also retain the inherent advantages of its rigid molecules.It is a very important and urgent scientific problem in the field of organic optoelectronics.In this paper,we propose a method of high temperature and high pressure of hydrazine hydrate reducing agent for solution processing of insoluble rigid molecules.The prepared solution is processed into a film,which molecular arrangement is tight,and also has good orientation and crystallinity.The stable radical anion can be obtained after oxidation.The application in the field of organic thermoelectric and organic ferromagnetic is explored.In the second chapter,we aim to solve the scientific problem that the macrocyclicπ-conjugated organic molecules are difficult to be solution processed.We innovatively use hydrothermal method to realize the solution processing and prepare a dense and flat film.We use hydrazine hydrate as both a solvent and a reducing agent to dechlorinate and reduce 4Cl-PBI molecules to PBI anion state(most PBI dianion and a small amount of PBI radical anion)at high temperature and pressure to increase dissolution.We prepared a flat and dense film by the method of dropping the obtained solution.The SEM and GIWAXS characterizations show that the PBI molecules are self-assembled after dissolution,with good molecular orientation and good crystallinity.Theπ-πconjugate spacing of the molecules is very small 3.26?,which is closely packed and stacked to form a micro-nano structure.We continue to try this method to perform solution processing of PBI powder which is more conjugated and harder to dissolve.It has also achieved good experimental results,and it is possible to carry out solution processing of such insoluble molecules.Finally,we extend this method to other aromatic imide materials NDI,and even other pigment systems,also achieved good results,indicating that our method of invention is universal,for the solution processing of insoluble molecules in the field of organic semiconductors.And applications offer new opportunities.In the third chapter,we studied the thermoelectric properties of PBI films at different times of oxygen doping.First,we trace the changes in different valence states of PBI through EPR,Raman spectroscopy and XPS spectroscopy.It was found that PBI2-was oxidized to PBI·-and PBI neutral states as the oxidation doping time increased.In terms of thermoelectric performance,the conductivity gradually decreases and the Zebeck coefficient rapidly increases to–3021μV K-1,while the thermal conductivity is basically unchanged.Finally,an excellent ZT value of 0.23 is obtained,which is the current solution for processing n-type organic thermoelectric materials.The highest value.We made a thermoelectric micro device with a maximum output power of 5.14 nW at a temperature difference of 10 K and a load of 55 kΩ.When the PBI2-containing PBI2-,PBI·-,and PBI neutral states contain three chemical valence states,electrons can be transferred from the HOMO of PBI2-and SOMO of PBI·-to the LUMO of neutral state PBI.The n-type transmission mechanism.At the same time,oxygen doping can increase the energy level difference between the Fermi level and the transmission level of the material,which is beneficial to increase the Zebeck coefficient.We use oxygen doping,compared with the doping reported in the literature,it is simple and easy to operate,and does not introduce impurities,high doping efficiency,and finally the reaction is good in material properties.In the fourth chapter,we reported a pure organic ferromagnetic material at room temperature.In the previous work,we found that the PBI molecules are arranged closely and orderly,and after oxidation,they contain a large number of stable free radicals,which is in line with the design principle of organic ferromagnetic materials.This inspired us to explore its application in the field of organic ferromagnetics.In order to ensure the purity of the material and to avoid the interference of ferromagnetic signals by the introduction of metal impurities,we obtained the PDI single crystal material by sublimation.Then,by single crystal analysis,the molecular conjugation spacing was 3.34?.Subsequently,we used a hydrothermal reaction to dissolve the rigid framework PDI crystals in an excess of hydrazine hydrate solvent,and most of the PDI molecules were reduced to soluble dianions.During the subsequent oxidation process,a large amount of PDI·-free radicals were generated and reassembled into nanostructured crystals with a molecular spacing of 3.30?.Magnetic measurements show that this material exhibits macroscopic room temperature ferromagnetism,and X-ray magnetic circular dichroism proves that ferromagnetism is derived from unpairedπelectrons between molecules.Our findings provide new ideas for achieving room temperature ferromagnetic semiconductors and new options for future organic spintronic devices.