Effects Of Potassium-intercalation On The Geometric And Electronic Structure Of Organic Acceptor Materials PC₆₁BM And ITIC

Introducing reactive metals or their salts as the interfacial materials between the cathode and the active layer can optimize the contact property and then improve the performance of organic electronic devices.These interfacial materials interact strongly with the organic molecules of the active layer and even destroy the molecular structure.Therefore,it is essential to study the detail of the interaction and its effects on the geometric and electronic structure of the organic molecules.In this thesis,I have studied the effects of intercalating potassium atoms into PC61BM and ITIC films with synchrotron radiation photoemission spectroscopy(SRPES)measurements and density functional theory(DFT)calculations.The fullerene-derivative PC61BM and the non-fullerene ITIC are two acceptor materials extensively applied in organic devices such as solar cells and field effect transistors.Thermal stability determines whether an organic film could be prepared by thermal evaporation or not,also determines the upper limit of temperature for post-annealing an organic device.In this thesis,I have conducted some measurements on the thermal stability of four most commonly used photovoltaic acceptor materials PC71BM,IC60BA,besides PC61BM and ITIC.The measurements include thermogravimetric analysis(TGA),differential scanning calorimeter(DSC)and ultraviolet visible(UV-Vis)absorption spectrum.Some samples of the UV-Vis spectrum measurements were prepared by thermal annealing in vacuum.The results exhibit strong chemical reaction between K atoms and PC61BM molecule.K atoms detach the methyl group of PC61BM,provided that the stoichiometry is no less than K4PC61BM.Each PC61BM molecule can at most react with seven or eight K atoms.Most K 4s electrons are transferred to PC61BM and successively occupy the three lowest unoccupied molecular orbitals.A small part of the 4s electrons are transferred to the O atoms of PC61BM after the detachment of the methyl group.ITIC also react strongly with the K atoms.The reaction process is decided by the geometric and electronic structure of ITIC.Each ITIC molecule can at most react with sixteen K atoms.The first six K atoms are attracted by the N and O atoms in the end groups,and these K atoms donate their 4s electrons to the unoccupied orbitals of ITIC.The seventh and eighth K atoms may or may not destroy the molecular strcture of ITIC,depending on the reaction sites.The ninth and the subsequently arrived K atoms surely break some S-C bonds of ITIC.The skeleton of the K-ITIC complexes is always conjugate despite of the breaking of some S-C bonds.In the study of thermal stability,we have revealed the suitable temperature window,260-280?,for preparing PCeiBM films with thermal evaporation in vacuum.For the other materials,intact molecules cannot be evaporated with sufficient sublimation rate.We also obtained the isomerization or decomposition temperatures for PC71BM(285?)and IC60BA(220?).