ITIC is the milestone of non-fullerene small molecule acceptors used in organic solar cells. We have studied the electronic states and molecular orientation of ITIC film using photoelectron spectroscopy and X-ray absorption spectroscopy. The negative integer charge transfer energy level is determined to be 4.00±0.05 eV below the vacuum level, and the ionization potential is 5.75±0.10 eV. The molecules predominantly adopt the face-on orientation on inert substrates as long as the surfaces of the substrates are not too rough. These results provide physical understanding of the high performance of ITIC-based solar cells, also afford implications to design more advanced photovoltaic small molecules.
We have studied the
reaction of PC61BM ([6,6]-phenyl-C61-butyric
acid methyl ester) film with K atoms using photoemission
spectroscopy measurements and density functional theory calculations.
It is found that the molecular structure of PC61BM keeps
intact until the intercalation stage of K3PC61BM. This is because that the C60 cage of the molecule
attracts the three first intercalated K atoms (per molecule) through
the electron transfer from the K 4s states to the LUMO and LUMO+1
orbitals. The fourth intercalated K atom then detaches the methyl
group and bonds with the two O atoms of the molecule. Additional K
atoms can still bond with the molecule until the LUMO+2 orbital is
filled, and the highest stoichiometry of K-intercalated PC61BM is K7–8PC61BM. The results indicate
that the electrode interface of PC61BM-based devices is
far from understood.
We have studied the electronic states at Ca/PC61BM interface using photoemission spectroscopy. It is found that the state of unoccupied molecular orbitals of the top molecular layer (TML) becomes occupied by the electrons transferred from the Ca atoms. The work function of the heavily doped TML of PC61BM film is smaller than that of metal Ca, and thus the contact between the TML and metal Ca is Ohmic. A transition layer (TL) of several molecular layers forms beneath the TML due to the diffusion of the Ca atoms. The TL is conductive and aligns its Fermi level with the negative integer charge transfer level of the interior PC61BM. The built-in electric field in the TL facilitates the electron transport from the interior of the PC61BM film to the TML.
The electronic structure of PC61BM film on clean Ag(100) and Ag(111) surfaces has been measured with photoemission spectroscopy. The interfaces are chemisorption. It is necessary to combine the two sub-interfaces model and the integer charge transfer model for thoroughly understanding the interfacial electronic structure. The electron injection barrier can then be determined without inverse photoemission measurements, and the PC61BM/Ag(111) system is found to be ohmic contact for electron transport. The analysis method should be applicable to many other organic/metal interfaces.
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