Energy level alignment at organic/metal interfaces studied by UVphotoemission: breakdown of traditional assumption of a common vacuumlevel at the interface

Electronic structures of model interfaces of organic electroluminescent (EL) devices and porphyrin/metal interfaces were investigated by UV photoemission spectroscopy (UPS). At all the measured interfaces, shift of the vacuum level was observed, showing the formation of an interfacial electric dipole layer. For Alq₃ (tris(8-hydroxyquinolino) aluminum), TPD (N,N'-diphenyl-N,N'-(3-methylphenyl)-1,1-biphenyl-4,4'-diamine), and DP-NTCI (N,N'-diphenyl-1,4,5,8-naphthyltetracarboxylimide)/metal interfaces, interfacial energy diagrams determined by UPS correspond well with the actually observed carrier-injecting character at the interfaces. For ZnTPP (15,10,15,20-zinc-tetraphenylporphyrin), H₂ TPP (5,10,15,20-tetraphenylporphyrin), and H₂T(4-Py)P (5,10,15,20-tetra(4-pyridyl) porphyrin)/metal interfaces, the shifts of the vacuum level as well as the energies of the levels in porphyrins could be expressed as a linear function of work function of the metal substrate. The slope of the linear function depended on the compound. These findings are in contrast to the traditional assumption of common vacuum level at the interfaces, For ZnTPP/metal interfaces, sample exposure to oxygen induced energy level shift in close relation with the change of the substrate work function at oxygen exposure. The present results have clearly demonstrated that direct observation of the interfacial electronic structure by microscopic method such as UPS is necessary for understanding the organic electronic devices such as EL devices and organic solar cells     

Metal-dependent charge transfer and chemical interaction at interfaces between 3,4,9,10-perylenetetracarboxylic bisimidazole and gold,silver and magnesium

Ultraviolet photoelectron spectroscopy (UPS) is used to investigate interfaces between the organic semiconductor 3,4,9,10-perylenetetracarboxylic bisimidazole (PTCBI) and Mg, Ag and Au. The metals span a range of work function and reactivity that leads to the formation of three different types of interfaces. The PTCBI-on-Au interface is abrupt and unreacted, and the relative position of energy levels across the interface precludes charge exchange and occupation of gap states. The lower work function of Ag leads to a metal-to-organic charge transfer and formation of polaron-like states at the PTCBI-on-Ag interface. Finally, the PTCBI-on-Mg interface shows clear evidence of a strong chemical interaction, which alters the electronic structure of the organic molecules at the interface and results in the formation of a different type of gap states. Dipole barriers consistent with the energetic and chemical characteristics of each interface are seen in all three cases. Finally, the three interfaces exhibit nearly identical Fermi level positions with respect to the organic highest occupied and lowest unoccupied molecular orbitals.     

A new photon source for ultraviolet photoelectron spectroscopy of organic and other damage-prone materials

Accurate measurements of the valence electronic structures of organic semiconductors are important for the development and understanding of organic electronic devices, materials, and interfaces. Ultraviolet photoelectron spectroscopy (UPS) is a well-established technique for probing valence electronic structures; however, many organic semiconductors undergo rapid sample degradation upon exposure to traditional laboratory-based vacuum ultraviolet (VUV) photon sources. Here, we report on a novel VUV photon source for UPS measurements that utilizes H Lyman-α emission with a narrow linewidth and a widely tunable intensity, and apply it to a number of organic materials of interest to show its ability to overcome this hurdle of sample degradation. Furthermore, the H Lyman-α source displays no measureable higher energy emission lines, which significantly reduces the background over typical He I discharge sources and allows for the onset of the density of states to be clearly observed over several orders of magnitude.     

Hybrid Interface States and Spin Polarization at Ferromagnetic Metal–Organic Heterojunctions: Interface Engineering for Efficient Spin Injection in Organic Spintronics

Ferromagnetic metal–organic semiconductor (FM‐OSC) hybrid interfaces have been shown to play an important role for spin injection in organic spintronics. Here, 11,11,12,12‐tetracyanonaptho‐2,6‐quinodimethane (TNAP) is introduced as an interfacial layer in Co‐OSCs heterojunctions with an aim to tune the spin injection. The Co/TNAP interface is investigated by use of X‐ray and ultraviolet photoelectron spectroscopy (XPS/UPS), near edge X‐ray absorption fine structure (NEXAFS) and X‐ray magnetic circular dichroism (XMCD). Hybrid interface states (HIS) are observed at Co/TNAP interfaces, resulting from chemical interactions between Co and TNAP. The energy level alignment at the Co/TNAP/OSCs interface is also obtained, and a reduction of the hole injection barrier is demonstrated. XMCD results confirm sizeable spin polarization at the Co/TNAP hybrid interface.     

The electronic structure of Ni-phthalocyanine/metal interfaces studied by X-ray and ultraviolet photoelectron spectroscopy

Nickel phthalocyanine (NiPc) thin films were grown stepwise on polycrystalline gold and silver substrates and the formed interfaces were characterized by X-ray and ultraviolet photoelectron spectroscopies (XPS, UPS). The variation of the XPS core level binding energy with NiPc film thickness yields information about band bending and interface dipoles. The valence band structure of the NiPc thin films was determined by UPS and exhibits four main features at binding energies 1.50 eV, 3.80 eV, 6.60 eV and 8.85 eV, respectively. The NiPc highest occupied molecular orbital (HOMO) cut-off was measured at ∼1.00 eV from the analyzer Fermi level and from the measured work function change of the growing NiPc film a final work function value for NiPc was estimated at 3.90 ± 0.10 eV. The main C1s peak of the NiPc film (∼5.0 nm) consists of two components at 284.8 eV (C–C bonds), 286.2 eV (C–N bonds) reflecting photoemission from multiple carbon sites within the molecule and a satellite at 287.9 eV, whereas the Ni2p and N1s peaks appear at ∼855.9 eV and ∼399.3 eV, respectively and are due to Ni–N bonds. The energy level diagrams of the NiPc/Au and NiPc/Ag interfaces were determined from a combination of the XPS and UPS results, yielding a hole injection barrier of 0.90 ± 0.10 eV for both substrates.     

有机半导体薄膜三极管的研制

采用真空蒸镀法和有机半导体材料酞菁铜,制作Au/CuPc/Al/ CuPc/Au三明治结构的肖特基型栅极有机静电感应三极管.该三极管导电沟道垂直于CuPc薄膜,与采用MOSFET结构的有机薄膜三极管相比导电沟道大幅缩短,有利于克服有机半导体电学性能的缺点.实验结果表明,该三极管驱动电压低,呈不饱和电流-电压特性.其工作特性依赖于栅极电压和梳状铝电极的结构.通过合理设计、制作梳状铝电极,获得了良好的三极管静态、动态特性.     

Small π-conjugated organic molecules based transistor and inverter with Cu electrodes

In absence of metallurgical Ohmic contacts in organic semiconductors, the relative position of metal work function with respect to highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) decides whether a metal electrode is Ohmic or non-Ohmic. Here we report that Cu whose work function is close to HOMO of the small π-conjugated organic molecules: pentacene and copper phthalocyanine (CuPc), can be used to achieve high performance of transistors and inverters. Fermi level is pinned at metal/copper hexadecafluoro phthalocyanine (F₁₆CuPc) interface resulting the barrier for carrier injection from metal to F₁₆CuPc independent of metal work functions. We have fabricated organic field effect transistors and inverters based on pentacene, CuPc and F₁₆CuPc with Cu and Au electrodes and observed that the performance of the devices with Cu electrodes are comparable to the devices with Au electrodes.     

Energy level alignment and interface states at α-sexithiophene/Ag interfaces

We present a combined X-ray and ultraviolet photoemission study of the interface between the thiophene based organic semiconductor α-sexithiophene (6T) and silver. Thermally evaporated organic films were deposited in several steps without breaking the vacuum on polycrystalline silver foils. Subsequently, the films were characterized via photoemission spectroscopy. These series of spectra allowed the determination of the energy level alignment at the interface. Our results provide evidence that chemical interaction takes place between flat lying α-6T molecules and the silver surface resulting in interface electronic levels. From the observed well defined molecular orbitals we have analyzed the energy level alignment and have found an interface dipole of −0.7 eV and a hole injection barrier of about 1.8 eV.     

Impact of Ground‐State Charge Transfer and Polarization Energy Change on Energy Band Offsets at Donor/Acceptor Interface in Organic Photovoltaics

The fullerene (C₆₀)/copper phthalocyanine (CuPc) interface is one of the widely used donor/acceptor (DA) interfaces for organic photovoltaics (OPVs), and information on the electronic structure at the interface is essential for fully understanding the energetics of excitons and carriers in OPVs. Here, an investigation into the energy levels at the C₆₀/CuPc interface is made using UV photoelectron, X‐ray photoelectron, and inverse photoemission spectroscopies. The vacuum level and core levels rise with C₆₀ deposition on the CuPc film, which indicates that the interfacial dipole is formed with the negative charge on the C₆₀ side. The interfacial dipole can be formed by the electron transfer from CuPc to C₆₀ in the ground state at the interface, which is indicated by the analysis of the UV–vis–NIR absorption spectrum of the CuPc/C₆₀ blended film. On the other hand, the highest occupied and lowest unoccupied molecular orbitals of CuPc and C₆₀ shift in opposite directions at the interface. This is attributed to the changes of the polarization energies of CuPc and C₆₀ at the interface. The formation of the interfacial dipole and the change of the polarization energy result in the anomalous energy band offsets at the C₆₀/CuPc interface, which are entirely different from those in inorganic p–n junctions.     

Energy Level Bending in Ultrathin Polymer Layers Obtained through Langmuir–Shäfer Deposition

The semiconductor–electrode interface impacts the function and the performance of (opto)electronic devices. For printed organic electronics the electrode surface is not atomically clean leading to weakly interacting interfaces. As a result, solution‐processed organic ultrathin films on electrodes typically form islands due to dewetting. It has therefore been utterly difficult to achieve homogenous ultrathin conjugated polymer films. This has made the investigation of the correct energetics of the conjugated polymer–electrode interface impossible. Also, this has hampered the development of devices including ultrathin conjugated polymer layers. Here, Langmuir–Shäfer‐manufactured homogenous mono‐ and multilayers of semiconducting polymers on metal electrodes are reported and the energy level bending using photoelectron spectroscopy is tracked. The amorphous films display an abrupt energy level bending that does not extend beyond the first monolayer. These findings provide new insights of the energetics of the polymer–electrode interface and opens up for new high‐performing devices based on ultrathin semiconducting polymers.     

Electronic transfer from aluminum into the core of gold nanoparticles capped with conjugated 2-naphthalenethiol

The electronic contact between a bulk metal and metal nanoparticles can be significantly different from that between two bulk metals due to the unique electronic structure in the nanometer size. In this work, the electronic contact between Au nanoparticles and Al is studied by X-ray photoelectron spectroscopy. Al is deposited on a layer of Au nanoparticles capped with conjugated 2-naphthalenethiol (Au-2NT NPs) in high vacuum by e-beam deposition at room temperature. The Au 4f X-ray photoelectron spectrum (XPS) significantly changes after the Al deposition. New XPS bands with higher binding energy appear. The angle dependence of the Au 4f XPS bands indicates that the electron transfer takes place at the contact between Al and Au-2NT NPs. In contrast, the Al deposition hardly changes the Au 4f XPS spectrum for Au nanoparticles capped with saturated 1-dodecanethiol. The effect of the Al deposition on the Au 4f XPS spectrum of Au nanoparticles capped with 2-naphthalenethiol is attributed to the electron transfer from Al through the conjugated 2-naphthalenehiol into the core of Au nanoparticles, as the Fermi energy of Al is higher than Au. This understanding on the contact between metal and metal nanoparticles provides guidance for the development of novel electronic devices.     

Modification of metal/semiconductor junctions by self-assembled monolayer organic films

Two new metal/molecule/semiconductor contacts, Au/n-Si/TDA/Au and Au/p-Si/ODM/Au, were fabricated to understand effect of organic compounds, tridecylamine and octadecylmercaptan self-assembled monolayer (SAM) films, on electrical charge transport properties of the metal/semiconductor junctions. The morphology of the organic monolayers deposited on Si substrates was investigated by atomic force microscopy. The molecular coverage of ODM deposited on p-Si is poorer than that of TDA on n-Si substrate. The ideality factors of the p-Si/ODM and n-Si/TDA diodes were found to be 1.66 and 1.48, respectively. The electrical results show that the tridecylamine monolayer passivated junction has a lower ideality factor. The ideality factor indicates clear dependence on two different type functional groups R-SH (Thiol) and R-NH₂ (Amin) groups and it increases with different functional groups of organic molecule. The barrier height φ_b value of the n-Si/TDA diode is smaller than that of p-Si/ODM diode, as a result of chain length of the SAM organic molecules. The interface state density D_it values of the diodes were determined using conductance technique. The n-Si/TDA diode has the smaller interface state density according to p-Si/ODM diode.We have evaluated that the organic molecules control the electronic parameters of metal/semiconductor diodes and thus, organic modification helps to get one step closer towards to new organic assisted silicon based microelectronic devices.     

用HREELS和UPS研究多孔硅的电子结构

使用高分辨电子能量损失谱(HREELS)和紫外光电子能谱(UPS)研究了新腐蚀的多孔硅(PS)样品的电子结构。实验结果表明,从HREELS谱中能量损失阚值测得的PS的能隙移到2．9eV,与文献报道的光激发谱(PLE)结果相近。UPS结果表明PS的费米能级到价带顶的距离不同于单晶Si。结合HREELS和UPS结果可以初步得出PS与Si界面的能带排列。     

CuPc buffer layer role in OLED performance: a study of the interfacial band energies

We present a UV photoemission (UPS) and topographic study of the indium–tin-oxide (ITO) anode used in organic light emitting diodes (OLEDs). The performance of these devices has been shown to be improved by introducing a thin layer of copper phthalocyanine (CuPc) between the anode and the hole-transport layer (HTL). While the device current at constant driving voltage increases with increasing CuPc thickness, the efficiency is optimized at 12–18 nm. In this article we address the issue of the charge (hole) injection process at the anode interface and demonstrate the importance of directly measuring the vacuum levels in quantitative study of the energy levels of OLED interfaces. As a result of this insight, many calculations relating to the relative energies of the bands at OLED interfaces may have to be revised and corrected. The interface morphologies were also studied using AFM to determine any changes with film growth.     

Electronic structure and current injection in zinc phthalocyanine doped with tetrafluorotetracyanoquinodimethane: Interface versus bulk effects

The p-type doping of zinc phthalocyanine (ZnPc) with the highly electronegative tetrafluorotetracyanoquinodimethane (F₄-TCNQ) is investigated via direct and inverse photoemission spectroscopy and in situ current–voltage (I–V) measurement. The electron affinity of F₄-TCNQ and the ionization energy of ZnPc are found to be energetically compatible with an electron transfer between the highest occupied molecular orbital (HOMO) of the host and the lowest unoccupied molecular orbital of the dopant. The Fermi level is near mid-gap in undoped ZnPc, and drops to 0.42 and 0.18 eV above the HOMO in the 0.3% and 3% doped films, respectively, consistent with efficient p-doping. The dependence of the Au/ZnPc:0.3%F₄-TCNQ/Au I–V characteristics on the thickness of the organic film is analyzed in terms of injection-limited versus space-charge-limited current. The analysis demonstrates that the large doping-induced increase in hole current is primarily due to improved carrier injection via tunneling through the narrow interface space charge layer.     

Electronic structure of phthalocyanine derivative-protected π-junction Au nanoparticles

Phthalocyanine derivative-protected Au nanoparticles (H₂Pc-AuNPs) can be processed without post-treatment into electroconductive electrodes that exhibit high electrical conductivity. However, the reason for the high electrical conductivity of AuNP films is still unclear. To better understand the carrier transport mechanism in H₂Pc-AuNP films, we performed ultraviolet photoelectron spectroscopy, inverse photoelectron spectroscopy and X-ray photoelectron spectroscopy to probe the electronic structure of 2,3,9,10,16,17,23,24-octakis [2-(dimethylamino)ethylthio] phthalocyanine and 2,3,11,12,20,21,29,30-octakis[2-(dimethylamino)ethylthio]naphthalocyanine as ligands for AuNPs. We demonstrated that the ligand molecules are chemisorbed onto AuNP’s surface through formation of Au–S bonds. It can be expected that strong interaction between π-orbitals of the molecules and Au orbitals.     

ZnO薄膜对于CdTe太阳电池的影响*

通过直流磁控溅射法在ITO薄膜上沉积的ZnO薄膜可以作为CdTe太阳电池的高阻层。通过XRD,可见-红外可见光谱仪和四探针法分析了制备薄膜的结构,光学和电学性质。通过紫外光电子能谱和X射线光电子能谱深度刻蚀法分析了ITO/ZnO和ZnO/CdS薄膜的界面性质。结果表明：ZnO 作为高阻层有良好的光学和电学性质。ZnO 薄膜降低了ITO和CdS之间的势垒。 制备出来电池有ZnO（没有ZnO）的能量转换效率和量子效率是12.77% (8.9%) 和 >90% (79%)。 进一步,通过AMPS-1D模拟分析了ZnO薄膜厚度对于CdTe太阳电池的影响。     

Plasmonic Internal Photoemission for Accurate Device In Situ Measurement of Metal‐Organic Semiconductor Injection Barriers

Current injection in organic semiconductors remains difficult to predict due in large part to the challenge of characterizing the contact energy barrier and interface density of states directly in organic electronic devices. Here, resonant coupling to surface plasmon polariton modes of a metal contact is demonstrated as a means to carry out internal photoemission (IPE) accurately in disordered organic semiconductor devices and enable direct measurement of the contact injection barrier by isolating true IPE from spurious sub‐gap organic photoconductivity. The substantial increase in sensitivity afforded by resonant coupling enables measurement in the low‐field injection regime where deviation from the standard Fowler prediction is explained quantitatively by the existence of a broad distribution of interface states. This technique is broadly applicable to metals and surface treatments commonly used in organic light emitting diodes, thin film transistors, and photovoltaics, and should therefore provide a quantitative basis to understand and model current injection in these devices over their entire operational lifetime.     

Electronic structures and chemical reactions at the interface between Li and regioregular poly (3-hexylthiophene)

Interfaces between metals and π-conjugated polymers play an important role in the organic electronic and optoelectronic devices such as polymer-based light-emitting diodes (PLEDs) and photovoltaic devices. In this study, synchrotron radiation photoemission spectroscopy (SRPES) and X-ray photoelectron spectroscopy (XPS) have been applied to in situ investigate the chemical reactions and electronic structure during the interface formation of Li on the regioregular poly(3-hexylthiophene) (rr-P3HT) thin films. Upon Li adsorption onto P3HT at 300 K, Li dopes electrons into P3HT, inducing the occurrence of the P3HT band bending. Moreover, Li can diffuse into the subsurface and react with both S and C atoms in the thiophene rings, leading to the formation of Li₂S and Li–C complex. Compared to the interface of Ca/P3HT, the diffusion/reaction depth of Li is much larger at the Li/P3HT interface. Through the investigation of the evolution of core level and valence band spectra together with secondary electron cutoff an energy level alignment diagram at the Li/rr-P3HT interface is derived.