Direct comparative study on the energy level alignments in unoccupied/occupied states of organic semiconductor/electrode interface by constructing in-situ photoemission spectroscopy and Ar gas cluster ion beam sputtering integrated analysis system

Abstract

Through the installation of electron gun and photon detector, an in-situ photoemission and damage-free sputtering integrated analysis system is completely constructed. Therefore, this system enables to accurately characterize the energy level alignments including unoccupied/occupied molecular orbital (LUMO/HOMO) levels at interface region of organic semiconductor/electrode according to depth position. Based on Ultraviolet Photoemission Spectroscopy (UPS), Inverse Photoemission Spectroscopy (IPES), and reflective electron energy loss spectroscopy, the occupied/unoccupied state of in-situ deposited Tris[4-(carbazol-9-yl)phenyl]amine (TCTA) organic semiconductors on Au (ELUMO: 2.51 eV and EHOMO: 1.35 eV) and Ti (ELUMO: 2.19 eV and EHOMO: 1.69 eV) electrodes are investigated, and the variation of energy level alignments according to work function of electrode (Au: 4.81 eV and Ti: 4.19 eV) is clearly verified. Subsequently, under the same analysis condition, the unoccupied/occupied states at bulk region of TCTA/Au structures are characterized using different Ar gas cluster ion beam (Ar GCIB) and Ar ion sputtering processes, respectively. While the Ar ion sputtering process critically distorts both occupied and unoccupied states in UPS/IPES spectra, the Ar GCIB sputtering process does not give rise to damage on them. Therefore, we clearly confirm that the in-situ photoemission spectroscopy in combination with Ar GCIB sputtering allows of investigating accurate energy level alignments at bulk/interface region as well as surface region of organic semiconductor/electrode structure.

Visualizing the Vertical Energetic Landscape in Organic Photovoltaics

“…Specifically, a comparison of UPS surface measurements performed using the 'layer by layer' method with UPS depth profiling using GCIB confirmed an essentially damage-free removal of thermally evaporated organic molecules. 61,62 The combination of UPS with GCIB, however, has not been applied before for solution-processed material systems, such as the active layers of organic solar cells. Herein we demonstrate that ultra-violet photoemission spectroscopy depth profiling using a GCIB etching source allows the accurate determination of the energetic landscape of solution-processed bi-layer and bulk heterojunction active layers.…”

In-Depth Investigation of the Correlation between Organic Semiconductor Orientation and Energy-Level Alignment Using In Situ Photoelectron Spectroscopy

“…Figures 3(a)−(c) 18. The Φ electrode values of PE (4.87 eV), Au (4.89 eV), and MWPE (4.89 eV), which were measured just before the PEN deposition process, were similar.…”

“…On the basis of these results, the energy-level alignments of the PEN/PE, PEN/Au, and PEN/MWPE structures were defined for different PEN deposition times, as shown in 18 The Φ electrode values of PE (4.87 eV), Au (4.89 eV), and MWPE (4.89 eV), which were measured just before the PEN deposition process, were similar. Thus, the differences in the E h values of the different PEN/electrode structures were small.…”

“…The hole-injection barrier (E h ) is determined by the difference between the HOMO level of the OSC layer (E HOMO ) and the electrode work function (Φ electrode ), that is, E h = E HOMO(organic) − Φ electrode . 14,18 In some cases where the OSC layer is too thin, when the valence band structures of OSC and Au electrode appear together, the E h values were determined after subtracting and the valence band of the OSC appear together because the thickness of the OSC is thin, the EB value is determined by considering the valence band structure of Au itself.…”

“…Therefore, the kinetic energy of the Ar atom in the gas cluster is approximately 1000 times lower than that of the accelerated single Ar atoms in Ar ion sputtering, which minimizes sputtering-induced damage to the samples (SI Figures S2(a)−(d)). 18 In order to show the result is reproducible and the small difference discussed herein is meaningful, three-repeat and statistical information for the representative samples is provided (see Table S1 and Figure S3). In addition, atomic force microscopy (AFM), X-ray reflectometry (XRR; Cu Kα: 1.5406 Å), and X-ray diffraction (XRD; Cu Kα: 1.5406 Å) measurements were performed to obtain additional information regarding the dependency of the molecular arrangement of OSCs on the electrode material.…”