Stability Investigation Of Organic Semiconductor Materials In Ionic States Based On In-situ Spectroelectrochemistry

Organic semiconductor（OS）materials have been widely applied in photoelectric fields including organic photovoltaics,field effect transistors and secondary batteries due to the advantages of light weight,rich structures and flexibility.During the device operation,OS materials are acting as charged states with ionic features caused either by charges injected from metal electrodes or generated via photo-induced exciton separation.However,OS materials are unstable resulting from traps caused by structure defects,impurities and oxygen and water.The position of traps usually lies in the energy gap,which can capture electrons or holes,leading to the low carrier mobility and stability.The carrier mobility can regard as the interchange of carries between adjacent molecules,which is similar to the oxidation or reduction of molecules.The above behavior is similar to electrochemical reaction,in which the electron transfer progress happens between the electrodes and materials,resulting in the formation of ionic states and following reactions.Consequently,it is reasonable to analyze the stability of ionic states in OS materials through electrochemical methods to deeply understand the aging mechanism.Based on the above problems,in situ electrochemical techniques are chosen to study the stability of OS materials involving（1）Ionic stability and“electric aging”mechanism of active-layer materials in organic photovoltaics;（2）The ionic stability of n-type organic capacitor and electrochromic materials,and the key point is the influence of trace water and oxygen（～20ppm）in the solvent and the method to improve the stability;（3）Effects of stability of ionic radicals during electrochemical polymerization（EP）on product structures,film morphology and doping/dedoping processes.The main contents are summarized as follows:1.In the second chapter,ionic stability and“electric aging”mechanism of active-layer materials in organic photovoltaics are investigated.Non-fullerene acceptors with the core of benzothiadiazole thiophene and the side group of cyano ninone are chosen to test the in situ spectroelectrochemistry.The results reveal that the intensity of intramolecular charge transfer（ICT）peak decrease with the negative potential loading,resulting from the reduction of cyano ninones to radical anions,leading to the disappearance of intramolecular charge transfer process.During the oxidation process,part of the anions are oxidized to neutral states,and the unrecovered part leads to the electrochemical aging through the formation of side products.The structure of side products is the formation of C-C from C=C connected with the core and the side group.A potential at around the onset reduction of materials is used in order to simulate the device operation process.In situ spectroelectrochemistry results of three spin-coating films with cyclic voltammetry and chronoamperometry methods show that the ratio of the irreversible part caused by the formation of by-products is almost 10%after 200 cycles and three reaction processes were observed.Besides,by comparing halogen substituted acceptors,we find that the aging rates of Y6（F-TPBT-IC）,Y8（Br-TPBT-IC）and Y7（Cl-TPBT-IC）increase successively.The above differences result from the successively decreased LUMO levels of three molecules.The lower LUMO level of the molecule makes it easier to be reduced and form side products.2.In the third chapter,the ionic stability of n-type organic capacitor and electrochromic materials is investigated.Three n-type materials with different LUMO levels,Bth Cz,AQCz and PBICz EP films are chosen to test the cyclic stability.The results show that the cyclic stability of three materials is related to the relative position between their LUMO levels and that of electron traps formed by water and oxygen（-3.6～-3.8 e V）.Materials with LUMO levels lower（PBICz）than that of electron traps exhibit high stability.While electrode materials with LUMO levels higher than（Bth Cz）or similar to（AQCz）that of electron traps exhibit low stability,and the larger the difference is,the lower the stability is.In order to increase the stability,an electron trap filler with high HOMO level（-2.98 e V）,bis（pentamethylcyclopentadiene）cobalt（II）（DMC）,is added in the electrolyte to deactivate traps spontaneously by donating electrons to traps.The optimized concentration of DMC is 5×10^-4mol L^-1.After the addition of DMC,the charge retention of Bth Cz and AQCz EP films increases by 68.6%and 26.3%,respectively.The in situ spectroelectrochemistry reveals that DMC acts as a“dewater agent.”After deactivating electron traps,DMC forms an oxidation state of Co（III）without undesirable side reactions with electrode materials.3.In the fourth chapter,9-subsitituted hydrogen,phenyl,and ethyl are chosen to study the ionic stability effects during EP on product structures,film morphology and doping/dedoping process.The three monomers exhibit a two-step electron transfer progress,in which the polymerization is invalid in low potential range from 0 V to 0.90 V（vs.Ag/Ag⁺）because of the formation of stable inner-carbazole radical cations.While in high potential range from 0 V to1.25 V（vs.Ag/Ag⁺）,the two outer carbazoles are oxidized simultaneously and couple through3,3-position to generate tricarbazole polymers.In situ techniques are introduced to analyze the polymerization mechanism.The results show a faster deposition of poly-H-3Cz with aggregated morphology and the inclination to form long-chain polymers,and a slower deposition of poly-P-3Cz and poly-E-3Cz with smooth morphology and the inclination to form short-chain polymers.The differences are attributed to the low stability of P-3Cz and E-3Cz radical cations than that of H-3Cz.Besides,during the oxidation of the inner carbazole,doped counter anions（PF₆^–）in P-3Cz is not fully dedoped due to the high steric resistance of phenyl,resulting in the further doping of n-Bu₄N⁺at lower potential.The cyclic tests of three EP films reveal that during the doping process,3,3-dicarbazole and inner carbazole are oxidized in succession,and the films exhibit“dipolaron”absorption on UV-vis spectra;vice versa.After30 cycles,poly-H-3Cz EP film shows the best stability,followed by poly-E-3Cz and poly-P-3Cz EP films.