Controllable N-type Doping Of Organic Electron Acceptors

Compared to inorganic semiconductors,organic semiconductors（OSCs）have received widespread attention due to their tunable photoelectric properties,good mechanical flexibility,and good solution processing performance.However,the conductivity of OSCs is significantly lower than that of inorganic semiconductors,which limits the further development of OSCs.Doping is an important means to improve the conductivity（σ）,especially the carrier concentration（n）of semiconductors.Through molecular doping,the carrier density and conductivity of OSCs can be significantly improved.Currently,excellent P-type molecular dopants have significantly improved the conductivity of hole transport materials（P-type semiconductors）and have been successfully applied in various organic electronic devices.However,the development of N-type molecular doping in electron transport materials（N-type semiconductors）lags behind,as most N-type molecular dopants are unstable in the air,greatly limiting their application in high-performance devices.In this thesis,four air stable molecular dopants were studied:acridine orange base（AOB）,4-（1,3-dimethyl-2,3-dihydro-1H-benzo[d]imidazol-2-yl）-N,N-dimethylaniline（N-DMBI）,tetrabutylammonium bromide（TBA-Br）and tetra（dimethylamino）ethylene（TDAE）.They conducted efficient and controllable N-type doping of organic semiconductors with high electron affinity,such as[6,6]-phenyl-C₆₁-butyrate methyl ester(PC₆₁BM)and ADA’DA type Y6 based on dithiothiophene[3.2-b]-pyrrolobenzothiadiazole electron deficient nuclei.In the first chapter,the introduction mainly introduces the development of semiconductor doping,the electron transfer mechanism of organic semiconductor doping,the types,the doping mechanism and the doping process of N-type dopants.In the second chapter,PC₆₁BM was doped with AOB,N-DMBI,TBA-Br,and TDAE as N-type dopants.The effects of the four dopants on the conductivity,conductivity activation energy,mobility,energy levels,and morphology of PC₆₁BM films were studied.It was proved that charge transfer occurred between the dopants and PC₆₁BM.Finally,the air stability of the doped blend films was investigated.Firstly,we prepared PC₆₁BM:dopant（AOB,N-DMBI,TBA-Br,or TDAE）blend films using four dopants as electron donors and PC₆₁BM as electron acceptors.The conductivity of PC₆₁BM thin films before and after doping was measured using a four-wire Kelvin method.The results showed that AOB,N-DMBI,TBA-Br,and TDAE could significantly improve the conductivity of PC₆₁BM thin films,respectively increasing the conductivity of PC₆₁BM from 2.34×10^-7 S cm^-1 raised to 5.94×10^-2 S cm^-1、8.65×10^-1 S cm^-1、5.28×10^-2 S cm^-1 and 5.18 S cm^-1.At the same time,the conductivity activation energy（E_a）of PC₆₁BM was decreased from 0.439 e V to 0.227 e V,0.036 e V,0.149 e V,and 0.034 e V,respectively,indicating an effective increase in carrier concentration.The mobility measurement results show that the mobility of PC₆₁BM thin film does not significantly change after adding molecular dopants.Then,the electron paramagnetic resonance（EPR）and photoluminescence（PL）spectra of PC₆₁BM:dopant blend films were measured.The appearance of EPR signals and fluorescence quenching of PL spectra of the blend films demonstrated that four molecular dopants can act as electron donors to transfer electrons to PC₆₁BM,achieving N-type doping of PC₆₁BM.Ultraviolet photoelectron spectroscopy（UPS）measurements show that the Fermi level（E_F）of doped PC₆₁BM shifts upward and the work function（WF）decreases,indicating that doping can reduce the energy level difference between the Fermi level and the lowest unoccupied molecular orbital（LUMO）,reducing the injection or extraction potential barrier of electrons.We have also observed a similar phenomenon through scanning Kelvin probe microscopy（SKPM）,where all four dopants can increase the contact potential of PC₆₁BM thin films.In addition,we investigated the morphological changes of PC₆₁BM before and after doping using atomic force microscopy（AFM）,and the results showed that the addition of dopants did not significantly change the morphology of PC₆₁BM thin films.Finally,the air stability of the conductivity of the blend film was studied.The blend film has N-type semiconductor properties,and the conductivity of all films decreased after exposure to air.In the third chapter,we explored the efficient and controllable N-type doping of Y6 with four N-type dopants:AOB,N-DMBI,TBA-Br,and TDAE.The effects of the four dopants on the conductivity,conductivity activation energy,energy levels,and morphology of Y6 thin films were studied.It was proved that there was an effective charge transfer between the dopants and Y6.Finally,the air stability of the doped blend film was tested.Firstly,Y6 dopant（AOB,N-DMBI,TBA-Br,or TDAE）blend films were prepared using four dopants as electron donors and Y6 as electron acceptors.The electrical conductivity of Y6 thin films before and after doping was measured using four-wire Kelvin method.The results showed that all four dopants could improve the electrical conductivity of Y6 thin films.Using AOB,N-DMBI,TBA-Br,and TDAE as dopants,the conductivity of Y6 was increased from 3.05×10^-7 S cm^-1 raised to 4.51×10^-4 S cm^-1、7.91×10^-3 S cm^-1、7.30×10^-2 S cm^-1 and 2.94×10^-4 S cm^-1.At the same time,the Ea of Y6 was decreased from 0.629 e V to 0.215 e V,0.271 e V,0.063 e V,and0.147 e V,respectively,indicating an increase in free electron concentration.The EPR and PL spectra were then characterized.In the blend film new EPR signals appeared,and the fluorescence quenching of the PL spectrum,indicating a strong electron transfer from the four dopants to the acceptor Y6.The test results of UPS show that the E_F of Y6 moves towards the LUMO direction and the WF decreases after doping,indicating that the addition of dopants reduces the extraction or injection potential barrier of electrons.SKPM characterization indicates that the addition of four dopants increases the contact potential of Y6 doped blend films,and the increased contact potential difference indicates the relative movement of E_F to LUMO,demonstrating the increase in free electron concentration after doping.In addition,the morphology of Y6 thin films before and after doping was studied using AFM.The addition of dopants has little affected on the roughness of Y6 thin films.Finally,the air stability of the conductivity of the blend films was studied,and it was verified that the blend films possess N-type semiconductor properties.It was found that the conductivity of all films decreased after exposure to air.