Does Fermi level alignment hold across organic interfaces? — An investigation using a rotary Kelvin probe

Oxygen effects on the interfacial electronic structure of titanyl phthalocyanine film: p-Type doping, band bending and Fermi level alignment

The alignment of the Fermi level of a metal electrode within the gap of the\nhi ghest occupied (HOMO) and lowest unoccupied orbital (LUMO) of a molecule is\na key quantity in molecular electronics, which can vary the electron\ntransparency of a single molecule junction by orders of magnitude. We present a\nquantitative analysis of the relation between this level alignment (which can\nbe estimated from charging free molecules) and charge transfer for bipyridine\nand biphenyl dithiolate (BPDT) molecules attached to gold leads based on\ndensity functional theory calculations. For both systems the charge\ndistribution is defined by a balance between Pauli repulsion with subsequent\nelectrostatic screening and the filling of the LUMO, where bipyridine loses\nelectrons to the leads and BPDT gains electrons. As a direct consequence the\nFermi level of the metal is found close to the LUMO for bipyridine and close to\nthe HOMO for BPDT.\n

Hybrid (photo-) cathodes consisted of conjugated polymer and hydrogen evolution reaction (HER) co-catalysts are an emerging platform for low-cost solar fuels generation. The electron-accepting Poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′bithiophene)}, known as P(NDI2OD-T2) or N2200, is a promising material to serve as a conjugated polymer cathode or as the electron acceptor for bulk heterojunction photocathodes. Unlike inorganic semiconductor/metal junctions, much less is known about the energetic alignment of the conjugated polymer electrode/metal junctions, hindering rational development of this emerging class of electrodes. In this work, we investigate the electrical doping behavior in N2200 cathode and its Fermi level alignment with gold nanoparticles, which is used here as a model for hydrogen evolution metal cocatalyst. Through UV/visible, Raman and attenuated total-reflectance infrared spectroelectrochemistry, we observed the impact of electrical doping on the vibrational frequencies of neutral, polaron and dianion species in N2200, which suggests electron density changes within the corresponding NDI units. Upon one-electron reduction, C=O stretching frequency of the polaron unit shows a red shift by ~ 68 cm-1, indicating an increased electron density in the C=O bond. Additionally, the C=O stretching frequency of neutral units in the doped N2200 shows a minor red shift of ~ 5 cm-1, suggesting charge transfer from neighboring polaron units. Surface-enhanced Raman spectroscopy measurements of gold nanoparticle-functionalized N2200 electrode revealed that the Au Fermi level only shifts with that of the N2200 upon the polaron formation. This mechanistic study of the electron transfer from doped N2200 to the metal nanoparticles provides insight for the future design of the HER (photo)cathodes – the formal potential of the polymer polaron formation determines the behavior of the catalyst Fermi level and thus modulates the reaction capability.

Band bending is a fundamental issue for discussing organic devices. Band bending with Fermi level alignment between semiconductors and metals are often assumed, although the validity of this scheme in the case of organic semiconductors has been not yet established. In this paper, our recent efforts to examine band bending in organic semiconductors using Kelvin probe method (KPM) are reported. After discussing the applicability of KPM to organic thick film – metal substrate system, the results for C60, TPD, and Alq3 are shown to discuss band bending of the films without intentional doping in ultrahigh vacuum condition. Gradual band bending was observed for C60/metal interfaces although the width of the space charge layer is in the order of 100 nm. In contrast, flat band feature was observed for TPD/metal interfaces probably because of its high purity. These results demonstrate that the frame work of band bending used in inorganic semiconductor interfaces is still valid for organic semiconductors although much thicker films are often necessary to achieve bulk Fermi level alignment. For Alq3/metal interfaces formed in dark condition, we found a new type of band bending where the energy levels change as a linear function of the distance from the interface. The observed location of the vacuum level was far below the Fermi level of the metal substrates, clearly indicating that Fermi level varies place by place in the system. Such electronically non-equilibrium state was quite stable for the order of years. The concept of Fermi level alignment is also discussed in relation to the observed energy diagrams. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoelectron spectroscopy was used to explore changes in Fermi level alignment, within the pi-pi* gap, arising from modifications to the coupling chemistry of conjugated phenylene ethynylene oligomers to the Au surface. Self-assembled monolayers were formed employing either thiol (4,4'-ethynylphenyl-1-benzenethiol or OPE-T) or isocyanide (4,4'-ethynylphenyl-1-benzeneisocyanide or OPE-NC) coupling. The electronic density of states in the valence region of the two systems are nearly identical with the exception of a shift to higher binding energy by about 0.5 eV for OPE-NC. Corresponding shifts appear in C(1s) spectra and in the threshold near E(F). The lack of change in the optical absorption suggests that a rigid shift of the Fermi level within the pi-pi* gap is the major effect of modifying the coupling chemistry. Qualitative consideration of bonding in each case is used to suggest the influence of chemisorption-induced charge transfer as a potential explanation. Connections to other theoretical and experimental work on the effects of varying coupling chemistries are also discussed.

The alignment of the Fermi level of a metal electrode within the gap of the highest occupied and lowest unoccupied orbital of a molecule is a key quantity in molecular electronics. Depending on the type of molecule and the interface structure of the junction, it can vary the electron transparency of a gold/molecule/gold junction by at least one order of magnitude. In this article we will discuss how Fermi level alignment is related to surface structure and bonding configuration on the basis of density functional theory calculations for bipyridine and biphenyl dithiolate between gold leads. We will also relate our findings to quantum-chemical concepts such as electronegativity.

We try to understand the fact that fullerene film behaves as n-type semiconductor in electronic devices and establish a model describing the energy level alignment at fullerene/metal interfaces. The C60/Ag(100) system was taken as a prototype and studied with photoemission measurements. The photoemission spectra revealed that the Ag atoms of the substrate diffused far into C60 film and donated electrons to the molecules. So the C60 film became n-type semiconductor with the Ag atoms acting as dopants. The C60/Ag(100) interface should be understood as two sub-interfaces on both sides of the molecular layer directly contacting with the substrate. One sub-interface is Fermi level alignment, and the other is vacuum level alignment.

One- and two-photon photoelectron spectroscopies were used to determine the electronic structure around the Fermi level for self-assembled monolayers of a prototypical molecular wire, 4,4'-(ethynylphenyl)-1-benzenethiol(C 6 H 5 -C≡C-C 6 H 4 -C≡C-C6H5-SH), on Au. One-photon ultraviolet photoelectron spectroscopy indicated a separation between the Fermi level and the peak of the occupied delocalized π levels of 1.9 eV, thus providing a representative value for the hole injection barrier. Two states were identified in two-photon photoelectron spectroscopy measurements corresponding to excitation to the lowest exciton and excitation to an unoccupied final state derived from the e 2 u levels of benzene. The separation between the Fermi level and the corresponding unoccupied π* states is estimated to be 3.2 eV, giving a transport gap of ∼1.9 + 3.2 = 5.1 eV. Occupied states associated with Au-S interactions are observed near the Fermi level for comparison studies on benzenethiol monolayers. Charge transfer associated with the formation of these levels, and their unoccupied counterparts, is suggested to produce the approximately 0.7 eV shift of the Fermi level toward the highest occupied orbitals on the oligomer.

Photodetectors based on two‐dimensional (2D)/ three‐dimensional (3D) semiconductor heterojunction structures are emerging as appealing candidates for high‐sensitivity applications. The performances of these hybrid photodetectors are closely correlated with their current gain mechanism. Carrier recirculation is the most commonly reported mechanism. Recently, a Fermi level alignment mechanism was proposed for 2D graphene/0‐dimensional (0D) quantum dot heterostructures because of the easy Fermi level tunability of the quantum dot. In this article, an interface‐induced gain mechanism using this Fermi level alignment process is proposed and identified based on a 2D graphene/3D GaAs hybrid structure with comparative measurement configurations. Because of the high surface state density of GaAs, the photo‐excited holes tend to become trapped at the graphene/GaAs interface, which can easily lower the interface Fermi level and the Fermi level in graphene via an alignment process. When combined with the high carrier mobility characteristics of graphene, a maximum current gain of 2520 and responsivity of 1321 A W−1 are achieved in the devices. This study clarifies the role of the interface states in the gain characteristics of some 2D/3D hybrid devices, with results that are instructive for optimal device design.

Chapter 3 - Physics of Nanotube/Metal Contacts

Interfacial behaviour of polyaniline as an organic electronic material

We tune the Fermi level alignment between the SnO x electron transport layer (ETL) and Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 and highlight that this parameter is interlinked with current-voltage hysteresis in perovskite solar cells (PSCs). Furthermore, thermally stimulated current measurements reveal that the depth of trap states in the ETL or at the ETL-perovskite interface correlates with Fermi level positions, ultimately linking it to the energy difference between the Fermi level and conduction band minimum. In the presence of deep trap states, charge accumulation and recombination at the interface are promoted, affecting the charge collection efficiency adversely, which increases the hysteresis of PSCs.

The Schottky barrier at the metal-nanotube contact has been a prime issue in the nanoscale devices. Here we use ab initio density-functional calculations to investigate the electronic structure and the Fermi level alignment at the metal-nanotube contacts. Consistent with the common concept of the large small work function of gold aluminum surfaces, the Fermi level of the gold layer is found to be aligned at the valence band edge, while that of the aluminum sits at the conduction band edge of the semiconducting carbon nanotube. However, upon the oxidation, the work function of aluminum surface becomes as large as that of the clean gold surface, causing the Fermi level to be aligned at the valence band edge of the semiconducting nanotube. This suggests that the carrier type of the nanotube field effect transistor could transform from n-type to p-type upon oxygen adsorption on the electrode surface. The oxidation-induced increase of the tunneling barrier is also investigated.

Biased referencing experiments for the XPS analysis of non-conducting materials

Interfacial band offset and band bending of organic semiconductors are critical to understand and improve organic photovoltaic cells. In this study, the energy level alignment of fullerene(C60) / metal-free phthalocyanine (H2Pc) interface which is one of the model interfaces of organic photovoltaic cells has been investigated using UV and X-ray photoemissions. For both ‘H2Pc on C60’ and “C60 on H2Pc' interfaces, 0.3 eV downward energy level shift was observed in XPS at the interface formation. This energy shift is quite steep in contrast to the band bending observed for C60/metal interfaces in our previous study, where thickness of 500nm was required to achieve 0.21eV band bending to get Fermi level alignment between metal electrode and C60. To clarify the origin of the band bending, the effect of the insertion of C60-H2Pc co-deposited layer between C60 and H2Pc layers was also investigated. The result suggested that possible doping of H2Pc to C60 is not main origin of the observed energy shift. We also found that the vacuum level shift at H2Pc/C60 interface is strongly dependent on the deposition sequence of the interface formation.