Interfacial Energy Level Alignment Research Articles

Self-assembled monolayer engineered interfaces: Energy level alignment tuning through chain length and end-group polarity

We explore the different mechanisms through which self-assembled monolayers can tailor energy level alignment at metal–organic semiconductor interfaces. We show that the large work function variation that can be induced by the self-assembled monolayer on gold has limited ability to tailor the interface energy level alignment of a subsequent organic semiconductor overlayer.

Characteristics of a silicon nanowires/PEDOT:PSS heterojunction and its effect on the solar cell performance.

The interfacial energy-level alignment of a silicon nanowires (SiNWs)/PEDOT:PSS heterojunction is investigated using Kelvin probe force microscopy. The potential difference and electrical distribution in the junction are systematically revealed. When the PEDOT:PSS layer is covered at the bottom of the SiNW array, an abrupt junction is formed at the interface whose characteristics are mainly determined by the uniformly doped Si bulk. When the PEDOT:PSS layer is covered on the top, a hyperabrupt junction localized at the top of the SiNWs forms, and this characteristic depends on the surface properties of the SiNWs. Because the calculation shows that the absorption of light from the SiNWs and the Si bulk are equally important, the bottom-coverage structure leads to better position matching between the depletion and absorption area and therefore shows better photovoltaic performance. The dependence of JSC and VOC on the junction characteristic is discussed.

Reliability Study of Organic Light-emitting Diodes by Continuous-wave and Pulsed Current Stressing

ABSTRACTThe stability of green phosphorescent OLEDs with different structures was evaluated through constant-current stressing. Through the modifications of the ITO anode by different plasma treatments and the hole transport layer (HTL) by incorporating inorganic dopants, we proved that energy level misalignment at the ITO/HTL interface leads to localized joule heating, accelerating defect generation and luminescence decay. Pulsed current stressing was then employed to suppress the joule-heating effect so as to differentiate the thermal and nonthermal factors governing the device degradation. For OLEDs with a large energy barrier at the ITO/HTL interface, the effective lifetime was markedly increased under pulsed operation, whereas in OLEDs with an appropriate interfacial energy level alignment, pulsed stressing with 10% duty cycle only improved the effective half life by ∼15% as compared to continuous-wave stressing, indicating a minor role played by joule heating.

Interaction at the silicon/transition metal oxide heterojunction interface and its effect on the photovoltaic performance.

The interfacial reaction and energy level alignment at the Si/transition metal oxide (TMO, including MoO3-x, V2O5-x, WO3-x) heterojunction are systematically investigated. We confirm that the interfacial reaction appears during the thermal deposition of TMO, with the reaction extent increasing from MoO3-x, to V2O5-x, and to WO3-x. The reaction causes the surface oxidation of silicon for faster electron/hole recombination, and the reduction of TMO for effective hole collection. The photovoltaic performance of the Si/TMO heterojunction devices is affected by the interface reaction. MoO3-x are the best hole selecting materials that induce least surface oxidation but strongest reduction. Compared with H-passivation, methyl group passivation is an effective way to reduce the interface reaction and improve the interfacial energy level alignment for better electron and hole collection.

Cascade energy transfer versus charge separation in ladder-type oligo(p-phenylene)/ZnO hybrid structures for light-emitting applications

Usability of inorganic/organic semiconductor hybrid structures for light-emitting applications can be intrinsically limited by an unfavorable interfacial energy level alignment causing charge separation and nonradiative deactivation. Introducing cascaded energy transfer funneling away the excitation energy from the interface by transfer to a secondary acceptor molecule enables us to overcome this issue. We demonstrate a substantial recovery of the light output along with high inorganic-to-organic exciton conversion rates up to room temperature.

First-principles approach to calculating energy level alignment at aqueous semiconductor interfaces.

A first-principles approach is demonstrated for calculating the relationship between an aqueous semiconductor interface structure and energy level alignment. The physical interface structure is sampled using density functional theory based molecular dynamics, yielding the interface electrostatic dipole. The GW approach from many-body perturbation theory is used to place the electronic band edge energies of the semiconductor relative to the occupied 1b1 energy level in water. The application to the specific cases of nonpolar (101¯0) facets of GaN and ZnO reveals a significant role for the structural motifs at the interface, including the degree of interface water dissociation and the dynamical fluctuations in the interface Zn-O and O-H bond orientations. These effects contribute up to 0.5 eV.

Engineering of Electron-Selective Contact for Perovskite Solar Cells with Efficiency Exceeding 15%

The past 5 years have witnessed the rise of highly efficient organometal halide perovskite-based solar cells. In conventional perovskite solar cells, compact n-type metal oxide film is always required as a blocking layer on the transparent conducting oxide (TCO) substrate for efficient electron-selective contact. In this work, an interface engineering approach is demonstrated to avoid the deposition of compact n-type metal oxide blocking film. Alkali salt solution was used to modify the TCO surface to achieve the optimized interface energy level alignment, resulting in efficient electron-selective contact. A remarkable power conversion efficiency of 15.1% was achieved under AM 1.5 G 100 mW · cm(-2) irradiation without the use of compact n-type metal oxide blocking layers.

Match the Interfacial Energy Levels between Hole Transport Layer and Donor Polymer To Achieve High Solar Cell Performance

The interfacial energy level alignment is shown to play an important role in determining solar cell performance. Replacing hole transport layer poly(3,4-ethylene dioxythiophene)–(polystyrene sulfonic acid) (PEDOT:PSS) with vanadium pentoxide (V2O5) leads to a simultaneous improvement in short-circuit current density (Jsc), open-circuit voltage (Voc) and fill factor (FF) for two donor polymers with deep HOMO energy levels, resulting in a power conversion efficiency (PCE) of 7.03% and 4.14%. This is 18% and 106% increase in PCE over the 5.97% and 2.01% achieved with PEDOT:PSS. V2O5 is shown to increase Jsc and FF by enhancing hole mobility, reducing bimolecular recombination, and facilitating charge collection and to maximize Voc by providing a better ohmic contact. We also demonstrate that PEDOT:PSS still works better for donor polymers with a HOMO energy level around 5.1 eV, such as PTB7.

Organic semiconductor density of states controls the energy level alignment at electrode interfaces.

Minimizing charge carrier injection barriers and extraction losses at interfaces between organic semiconductors and metallic electrodes is critical for optimizing the performance of organic (opto-) electronic devices. Here, we implement a detailed electrostatic model, capable of reproducing the alignment between the electrode Fermi energy and the transport states in the organic semiconductor both qualitatively and quantitatively. Covering the full phenomenological range of interfacial energy level alignment regimes within a single, consistent framework and continuously connecting the limiting cases described by previously proposed models allows us to resolve conflicting views in the literature. Our results highlight the density of states in the organic semiconductor as a key factor. Its shape and, in particular, the energy distribution of electronic states tailing into the fundamental gap is found to determine both the minimum value of practically achievable injection barriers as well as their spatial profile, ranging from abrupt interface dipoles to extended band-bending regions.

Seleno groups control the energy-level alignment between conjugated organic molecules and metals

The charge injection from metallic electrodes into hole transporting layers of organic devices often suffers from deviations from vacuum-level alignment at the interface. Even for weakly interacting cases, Pauli repulsion causes an interface dipole between the metal and conjugated organic molecules (COMs) (so called "push-back" or "cushion" effect), which leads notoriously to an increase of the hole injection barrier. On the other hand, for chalcogenol self assembled monolayers (SAMs) on metal surfaces, chemisorption via the formation of chalcogen-metal bonds is commonly observed. In these cases, the energy-level alignment is governed by chalcogen-derived interface states in the vicinity of the metal Fermi-level. In this work, we present X-ray and ultraviolet photoelectron spectroscopy data that demonstrate that the interfacial energy-level alignment mechanism found for chalcogenol SAMs also applies to seleno-functionalized COMs. This can be exploited to mitigate the push-back effect at metal contacts, notably also when COMs with low ionization energies are employed, permitting exceedingly low hole injection barriers, as shown here for the interfaces of tetraseleno-tetracene with Au(111), Ag(111), and Cu(111).

Barrier height formation in organic blends/metal interfaces: Case of tetrathiafulvalene-tetracyanoquinodimethane/Au(111)

The interface between the tetrathiafulvalene/tetracyanoquinodimethane (TTF-TCNQ) organic blend and the Au(111) metal surface is analyzed by Density Functional Theory calculations, including the effect of the charging energies on the molecule transport gaps. Given the strong donor and acceptor characters of the TTF and TCNQ molecules, respectively, there is a strong intermolecular interaction, with a relatively high charge transfer between the two organic materials, and between the organic layer and the metal surface. We find that the TCNQ LUMO peak is very close to the Fermi level; due to the interaction with the metal surface, the organic molecular levels are broadened, creating an important induced density of interface states (IDIS). We show that the interface energy level alignment is controlled by the charge transfer between TTF, TCNQ, and Au, and by the molecular dipoles created in the molecules because of their deformations when adsorbed on Au(111). A generalization of the Unified-IDIS model, to explain how the interface energy levels alignment is achieved for the case of this blended donor/acceptor organic layer, is presented by introducing matrix equations associated with the Charge Neutrality Levels of both organic materials and with their intermixed screening properties.

Interface state and energy level alignment of F4-TCNQ sandwiched between a pentacene film and the ethylene-terminated Si(1 0 0) surface

The electronic states of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) molecules sandwiched between a pentacene (Pn) film and the ethylene-terminated Si(100) surface were investigated using X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS), and their effects on the morphology and the energy level alignment of the Pn film were investigated by atomic force microscopy (AFM) and polarization dependent ultraviolet photoelectron spectroscopy (UPS), respectively. The XPS and NEXAFS spectra show that F4-TCNQ molecules are involved in charge transfer with the surface, and they adsorb with tilt or random orientation. They consist of both neutral and anionic C≡N groups, and the neutral groups are still observed after Pn deposition, indicating the absence of charge transfer between F4-TCNQ and the Pn overlayer. We find that the Pn molecules take a standing-upright orientation on the ethylene/Si(100) surface. F4-TCNQ deposition on the ethylene/Si(100) surface induces some disorder in the Pn overlayer at the Pn thickness lower than 0.60nm. F4-TCNQ deposition leads to an increase in the vacuum level by 1eV, and thus a decrease in the hole injection barrier by 0.6–0.8eV. Thus, the decrease is mainly attributed to the charge transfer between the F4-TCNQ and the ethylene/Si(100) surface.

Modification of PTCDA/Co Interfacial Electronic Structures Using Alq3 Buffer Layer

The interfacial electronic structures and energy level alignment between 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) and Co substrate with a tris-(8-hydroxy-quinoline) aluminum (Alq3) buffer layer have been investigated using synchrotron-based photoemission spectroscopy (PES). It was found that the electronic structures at the PTCDA/Co interface can be modified by inserting an Alq3 buffer layer. As long as equilibrium is established at PTCDA/Alq3/Co interfaces, the electron injection barrier (Δe) at the PTCDA/Co interface decreases by 0.3 eV with an Alq3 buffer layer, which is independent of the thickness of the Alq3 interlayer. This energy level alignment can be satisfactorily explained by substrate to organic overlayer charge transfer through the buffer layer. Our findings have potential applications in spin-valve devices based on n-type organic materials by reducing the interfacial electron injection barrier.

Interface investigation of the alcohol-/water-soluble conjugated polymer PFN as cathode interfacial layer in organic solar cells

In this work, in situ ultraviolet photoelectron spectroscopy measurements were used to investigate the working mechanism of an alcohol-/water-soluble conjugated polymer poly [(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)] (PFN) as the cathode interfacial layer in organic solar cells from the view of interfacial energy level alignment. Fullerene (C60) was chosen as the model acceptor material in contact with PFN as well as two other cathode interfacial layers ZnO and TiO2 in the configuration of an inverted solar cell structure. Significant charge transfer between PFN modified ITO (indium tin oxide) electrode and C60 is observed due to the low work function of PFN. This results in the Fermi level of the substrate pinned very close to the lowest unoccupied molecular orbital of C60 as well as an additional electric field at the cathode/acceptor interface. Both of them facilitate the electron extraction from the acceptor C60 to the ITO cathode, as confirmed by the electrical measurements of the electron-only devices with PFN modification. The better electron extraction originated from the Fermi level pinning and the additional interface electric field are believed to contribute to the efficiency enhancement of the inverted organic solar cells employing PFN as cathode interfacial layer.

Influence of Merocyanine Molecular Dipole Moments on the Valence Levels in Thin Films and the Interface Energy Level Alignment with Au(111)

We study the effect of increasing permanent dipole moment of four dipolar merocyanine dyes on the energy level alignment with Au(111) and the energy levels in thin films with ultraviolet (UV) photoemission spectroscopy. Preceding X-ray diffraction experiments showed that these dipolar molecules form centrosymmetric dimeric units in their bulk crystal structure. The effect of dimerization was also seen in a splitting of the highest occupied level of bilayers and thin films of all four molecules. This split into two components due to intermolecular electronic coupling between two merocyanines is estimated to be 0.2–0.3 eV. Monolayer films on Au(111) did not exhibit such splitting due to a different adsorption structure. A significant ultraviolet radiation sensitivity of the dyes near the metal surface was noticed. Unlike other radiation-sensitive compounds, the reason for molecule damage is related to the secondary electrons produced in the metal substrate during photoelectron spectroscopy measurements and not to the UV radiation itself.

Solution processable tungsten polyoxometalate as highly effective cathode interlayer for improved efficiency and stability polymer solar cells

We report for the first time the use of a water-soluble, tungsten polyoxometalate H3PW12O40 (PW12-POM) as an efficient cathode interlayer incorporated into poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM-61) polymer solar cells. The short circuit photocurrent density of the PW12-POM modified device is enhanced by ∼40% the open circuit voltage increases from 0.61V to 0.65V and the fill factor from 0.36 to 0.41, resulting to a power conversion efficiency enhancement of ∼70% (from 1.57% for the reference to 2.7% for the PW12-POM modified device). The improvement is attributed to enhanced electron transfer/extraction at the PW12-POM/Al interface as a result of the favorable interfacial energy level alignment and possible enhancement of the local electric field due to the nanoscale morphology of the PW12-POM layer, as evidenced by AFM measurements. A reduced degradation rate was measured for PW12-POM modified devices stored in dark and measured in ambient conditions. Taking into account the advantageous solution processability of PW12-POM, the large increase in the device efficiency and the improvement of their stability, we manifest that PW12-POM has highly desirable properties in order to be embedded as cathode interlayer in organic photovoltaic cells.

Tuning zinc oxide/organic energy level alignment using mixed triethoxysilane monolayers

Interfacial energy level alignment influences several critical organic optoelectronic device characteristics including charge transfer barriers, turn-on voltage, and open circuit voltage (Voc). Introduction of dipolar molecular monolayers on metal oxide surfaces has allowed improvements in device performance as well as fundamental studies of energy level alignment in these devices. We demonstrate that dipolar mixed monolayers can be covalently bonded to zinc oxide (ZnO) through the triethoxysilane chemical attachment scheme, and that these monolayers tune the work function of the ZnO surface over 0.6 eV. We then employ mixed monolayer-treated ZnO surfaces as the electron-selective contact in inverted bulk heterojunction photovoltaic devices to determine the correlation between the Voc and the work function of the contact. We find the relationship of −0.14 V eV−1 between the Voc and contact work function is consistent with current results and theories of contact influence on Voc.

Engineering of Interfacial Electron Transfer from Donor–Acceptor Type Organic Semiconductor to ZnO Nanorod for Visible-Light Detection

Interfacial electron transfer (IET) plays a key role in photoactive organic/inorganic hybrid nanomaterials and remains elusive with regard to interfacial energy level alignment. In this study, we p...

Excitation Energy Dependent Charge Separation at Hole-Transporting Dye/TiO2 Hetero Interface

Interfacial charge separation in hybrid solar cells depends on both the energetic alignment and electronic coupling between the inorganic and organic semiconducting materials at the hetero interface. In the present work, bilayer solar cells comprising the small molecular semiconducting dye TDCV-TPA (tris-(thienylene-vinylene)-triphenylamine) and dense titanium dioxide (TiO2) films were investigated. The internal quantum efficiency and degree of photoluminescence quenching were found to be excitation energy dependent. The molecular interaction and interfacial energy level alignment was investigated using a combination of UV–vis and photoelectron spectroscopy (PES). Stationary and time-dependent density functional theory calculations were used to assign and distinguish between different experimentally determined molecular energy levels (PES) and electronic transitions (UV–vis). Photoelectron spectroscopy results suggest surface induced interactions of TDCV-TPA with TiO2 involving the peripheral CN-groups of the molecule which would imply a favorable electronic coupling for photoinduced interfacial charge transfer. In an energy level diagram distinguishing between the different electronic transitions in the molecule, the differences in the thermodynamic driving force for electron injection from the excited states were found small. Therefore, it is suggested that the observed higher internal quantum efficiency at shorter wavelength can be rationalized by a more favorable driving force for the regeneration of holes created at the hetero interface at higher excitation energy.

Ionization potential dependent air exposure effect on the MoO3/organic interface energy level alignment

We reported an ionization potential (IP) dependent air exposure effect on the MoO3/organic interface energy level alignment by carrying out in situ ultraviolet photoelectron spectroscopy and synchrotron light based X-ray photoelectron spectroscopy investigations. The electronic structures at MoO3/organic interfaces comprising various π-conjugated small organic molecules with different IP on MoO3 substrate have been systematically investigated. For the molecules with low IP, MoO3/organic interface electronic structures remained almost unchanged after air exposure. In contrast, for the molecules with high IP, the highest occupied molecular orbital (HOMO) leading edge (or hole injection barrier) increases gradually with the increasing molecule IP after air exposure. For the MoO3/copper-hexadecafluorophthalocyanine (F16CuPc, IP: ∼6.58eV) interface, air exposure can induce a significant downward shift of the HOMO level as large as ∼0.80eV.