Organic-organic Heterojunctions Research Articles

A novel all-organic S-scheme heterojunction with rapid interfacial charges migration for efficient photocatalytic H2 production

Poly (triazine imide), as a kind of highly crystalline g-C3N4, exhibits a promising potential for photocatalytic hydrogen production, however, some drawbacks still limit its photocatalytic performance. The strategy of constructing S-scheme heterojunction with different semiconductor photocatalysts enables the effective separation of photogenerated electrons and holes, and the strong oxidative and reductive properties of the original photocatalysts could be retained, which will significantly improve the photocatalytic activity. In this work, we synthesized the organic-organic S-scheme heterojunction between PTI and organic small molecule poly (barbituric acid) (PBA) by hydrogen bond self-assembly method, which results in a significant enhancement of photocatalytic H2 production activity. The H2 production rate of the optimum PBA/PTI-2 sample under visible light irradiation is about 0.92 mmol g–1, which is 5.5 times higher than that of PTI and 14.4 times higher than that of PBA. This excellent photocatalytic performance is attributed to the successful construction of S-scheme heterojunction between PTI and PBA, which effectively accelerates carrier transport and spatial segregation by the formation of a built-in electric field and band bending at the interface. In addition, the S-scheme heterojunction could also reserve the maximum redox capability and enhance the light absorption of the prepared photocatalytic system. This work provides a new strategy and understanding for the design and development of organic-organic S-scheme heterojunction photocatalysts.

Electrochemical formation of photoactive organic heterojunctions. Porphyrin-C60 polymeric photoelectrochemical cells

Fully polymeric donor-acceptor organic-organic heterojunctions were successfully formed by successive electrochemical polymerization steps. C60 holding polymer acted as an electron acceptor layer when it was electrochemically deposited on the top of a porphyrin based conducting polymer with dendrimeric structure. Porphyrin fluorescence emission quenching and energy dependent surface photovoltage analysis demonstrated that the heterojunctions produced photoinduced charge separated states. Also, it was found that after irradiation of heterojunctions the photogenerated electrons traveled to the C60 external surface, while the holes moved to the inner hole transport layer. When the heterojunctions were used in photoelectrochemical cells, the generated light-induced charge separated states were able to electrochemically reduce electron acceptors in aqueous media.

Mapping Energy Levels for Organic Heterojunctions.

An organic semiconductor thin film is a solid-state matter comprising one or more molecules. For applications in electronics and photonics, several distinct functional organic thin films are stacked together to create a variety of devices such as organic light-emitting diodes and organic solar cells. The energy levels at these thin-film junctions dictate various electronic processes such as the charge transport across these junctions, the exciton dissociation rates at donor-acceptor molecular interfaces, and the charge trapping during exciton formation in a host-dopant system. These electronic processes are vital to a device's performance and functionality. To uncover a general scientific principle in governing the interface energy levels, highest occupied molecular orbitals, and vacuum level dipoles, herein a comprehensive experimental research is conducted on several dozens of organic-organic heterojunctions representative of various device applications. It is found that the experimental data map on interface energy levels, after correcting variables such as molecular orientation-dependent ionization energies, consists of three distinct regions depending on interface fundamental physical parameters such as Fermi energy, work function, highest occupied molecular orbitals, and lowest unoccupied molecular orbitals. This general energy map provides a master guide in selection of new materials for fabricating future generations of organic semiconductor devices.

Morphology Change of C60 Islands on Organic Crystals Observed by Atomic Force Microscopy

Organic-organic heterojunctions are nowadays highly regarded materials for light-emitting diodes, field-effect transistors, and photovoltaic cells with the prospect of designing low-cost, flexible, and efficient electronic devices.1-3 However, the key parameter of optimized heterojunctions relies on the choice of the molecular compounds as well as on the morphology of the organic-organic interface,4 which thus requires fundamental studies. In this work, we investigated the deposition of C60 molecules at room temperature on an organic layer compound, the salt bis(benzylammonium)bis(oxalato)cupurate(II), by means of noncontact atomic force microscopy. Three-dimensional molecular islands of C60 having either triangular or hexagonal shapes are formed on the substrate following a "Volmer-Weber" type of growth. We demonstrate the dynamical reshaping of those C60 nanostructures under the local action of the AFM tip at room temperature. The dissipated energy is about 75 meV and can be interpreted as the activation energy required for this migration process.

Tunable Excitonic Processes at Organic Heterojunctions.

Nonradiative Auger recombination and radiative exciplex emissions at organic-organic heterojunctions are found to be mediated by the nature of the majority charge carrier. The Auger recombination is dominant when the heterojunction is electron rich, the exciplex emission is dominant when the heterojunction is hole rich, and both Auger recombination and exciplex emission are present when the heterojunction is charge balanced.

Molecular orientation and electronic structure at organic heterojunction interfaces

Due to the highly anisotropic nature of π-conjugated molecules, the molecular orientation in organic thin films can significantly affect light absorption, charge transport, energy level alignment (ELA) and hence device performance. Synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy represents a powerful technique for probing molecular orientation. The aim of this review paper is to provide a balanced assessment on the investigation of molecular orientation at the organic–organic heterojunction (OOH) interface by NEXAFS, as well as the gap-states mediated orientation dependent energy level alignment at OOH interfaces. We highlight recent progress in elucidating molecular orientation at OOH interfaces dominated by various interfacial interactions, gap-states controlled orientation dependent energy level alignments at OOH interfaces, and the manipulations of molecular orientation and ELA in OOH.

Structural Defects Control the Energy Level Alignment at Organic/Organic Interfaces

The dependence of the energy level alignment (ELA) on structural defects at an organic/organic heterojunction (OOH) of perfluoropentacene (PFP)‐on‐diindenoperylene (DIP) was investigated using X‐ray scattering and ultraviolet photoelectron spectroscopy. The density of structural defects near the interface between the PFP and DIP layers was varied by changing the growth temperature of the DIP film. A direct relationship was found between the defect density and the ELA at the OOH; the ELA together with the change in the electrostatic potential (quasi‐interface dipole layer) at the OOH varies systematically with the defect density near the interface. This indicates that a key factor affecting the ELA is the electrostatic potential change across the OOH interface, which is produced by electron transfer from DIP occupied gap states to PFP unoccupied gap states. These gap states originate from the defects and are effectively controlled by adjusting the growth conditions of the organic films. As a result, the ELA at OOH interfaces can be controlled by the density of structural defect, which is important for organic devices employing OOHs, such as organic photovoltaic cells.

Energy Level Realignment in Weakly Interacting Donor–Acceptor Binary Molecular Networks

Understanding the effect of intermolecular and molecule-substrate interactions on molecular electronic states is key to revealing the energy level alignment mechanism at organic-organic heterojunctions or organic-inorganic interfaces. In this paper, we investigate the energy level alignment mechanism in weakly interacting donor-acceptor binary molecular superstructures, comprising copper hexadecafluorophthalocyanine (F16CuPc) intermixed with copper phthalocyanine (CuPc), or manganese phthalocynine (MnPc) on graphite. The molecular electronic structures have been systematically studied by in situ ultraviolet photoelectron spectroscopy (UPS) and low-temperature scanning tunneling microscopy/spectroscopy (LT-STM/STS) experiments and corroborated by density functional theory (DFT) calculations. As demonstrated by the UPS and LT-STM/STS measurements, the observed unusual energy level realignment (i.e., a large downward shift in donor HOMO level and a corresponding small upward shift in acceptor HOMO level) in the CuPc-F16CuPc binary superstructures originates from the balance between intermolecular and molecule-substrate interactions. The enhanced intermolecular interactions through the hydrogen bonding between neighboring CuPc and F16CuPc can stabilize the binary superstructures and modify the local molecular electronic states. The obvious molecular energy level shift was explained by gap-state-mediated interfacial charge transfer.

Role of charge transfer, dipole-dipole interactions, and electrostatics in Fermi-level pinning at a molecular heterojunction on a metal surface

Recently, Niederhausen et al. [Phys. Rev. B 86, 081411(R) (2012)] have reported on the energy level alignment of C${}_{60}$ adsorbed on a bilayer \ensuremath{\alpha}-sexithiophene (6T) film on Ag(111). The possibility of charge transfer from the metal to the C${}_{60}$ through the bilayer 6T as discussed by the authors may have a strong impact on understanding the energy level alignment (ELA) at organic-organic (O-O) heterojunctions grown on electrodes. In this paper, we aim at a comprehensive picture of the ELA at O-O interfaces on a metal. We carry out a detailed investigation of the same pair of materials on Ag(111) as employed previously, however, with varying 6T interlayer thickness. The results allow unambiguous identification of integer charge transfer towards a fraction of the C${}_{60}$ molecules as the mechanism leading to the formation of interface dipoles. Varying the 6T interlayer thickness also reveals the dependence of the observed features on the C${}_{60}$-metal distance. This dependence is quantitatively addressed by electrostatic considerations involving a metal-to-overlayer charge transfer. From this, we demonstrate the important role of dipole-dipole interaction potentials in the molecular layer and electric fields resulting from interface dipole formation for the energy level alignment. These findings provide a deeper understanding of the fundamental mechanisms that establish electronic equilibrium at molecular heterojunctions and will aid the prediction of an accurate energy level alignment at device relevant heterojunctions, e.g. in organic opto-electronic devices.

Observation of Electrical Switching, Reverse Rectification and Hysteresis in Nanostructured Organic–Organic Heterojunction

Nanostructured organic-organic (O-O) heterojunction was fabricated by using the thin films of a hole transporting material, copper phthalocyanine (CuPc) and an electron transporting material, copper hexadecafluoro-phthalocyanine (F16CuPc). The nanostructured thin films were characterized by optical absorption spectra, FESEM, AFM, X-ray diffraction, etc. Grain size of CuPc and F16CuPc on the substrate surface was different. XRD analysis shows that the crystallinity of the double layer films/heterojunction decreases as compared to the single layer film. The heterojunction sandwich structure ITO/F16CuPc/CuPc/Al, in the present study has shown a good diode like current-voltage (I-V) characteristics with reverse rectifying characteristics. In addition, electrical switching and hysteresis phenomena have also been observed in both sides of the voltage polarities. Interestingly, the single layer sandwich structure of the type ITO/CuPc/Al and ITO/F16CuPc/Al did not show any noticeable electrical switching and hysteresis in I-V characteristics as compared to double layer heterostructure. The reverse rectification has been explained on the basis of band bending due to the accumulation of charge carriers near the junction and the electrical switching has been explained considering the charge carriers trapping and detrapping at the O-O interface.

Dual Channel Operation Upon n‐Channel Percolation in a Pentacene‐C60 Ambipolar Organic Thin Film Transistor

Thickness resolved measurements of ambipolar thin-film transistor characteristics track the charging of an organic-organic heterojunction. Combined with structural investigation methods such as AFM and GIXS, this leads to a better understanding of the physics in state of the art devices such as organic solar cells, organic light emitting diodes and light emitting TFTs.

Charge depletion in organic heterojunction

Until now two types of organic-organic heterojunction (OHJ) have been observed in P-N junctions formed between undoped-organic semiconductors. Charge-transfers across OHJs are either negligible or showing electron transfer from P-type to N-type materials, leading to charges accumulation near the interface. Here, we observed that junction of 4,4′,4′′-tris(2-methylphenyl-phenylamino)triphenylamine (m-MTDATA)/bathocuproine (BCP) show the third-behavior. Electrons in BCP (N-type) transfer to m-MTDATA (P-type), leading to depletion of mobile majority carriers near the junction. While “depletion junctions” are typical in inorganic semiconductors, there are no reports in undoped-OHJ. Formation mechanism of depletion OHJs and fundamental differences between inorganic and organic HJs are discussed.

The role of gap states in the energy level alignment at the organic–organic heterojunction interfaces

The interface properties of organic-organic heterojunctions (OOHs), such as interface energy level alignment (ELA), interfacial charge transfer, interface nanostructuring, molecular orientation and so on, play an essential role in determining the device performance for some technologically important organic electronic devices, encompassing organic solar cells, bipolar organic field-effect-transistors, and organic light-emitting-diodes. The aim of this article is to provide a balanced assessment on the understanding of the ELA at the small-molecule based OOH interfaces with well-defined molecular orientation, with particular emphasis on the role of gap states in organic thin films. A generalized picture of gap states determined ELA at the OOH interfaces is provided and their implications in relevant organic electronic devices have been discussed.

Effect of Gap States on the Orientation-Dependent Energy Level Alignment at the DIP/F16CuPc Donor–Acceptor Heterojunction Interfaces

The interface properties of organic–organic heterojunctions (OOHs) between diindenoperylene (DIP) and copper hexadecafluorophthalocyanine (F16CuPc), including interface morphology, molecular orientation, and energy level alignment, have been investigated systematically by atomic force microscopy, in situ ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and synchrotron-based near-edge X-ray adsorption fine structure (NEXAFS) measurements. As revealed by NEXAFS measurements, DIP molecules adopt the standing orientation on the standing F16CuPc on the SiO2 substrate, while they have the lying-down configuration on the flat-lying F16CuPc on the highly oriented pyrolytic graphite substrate. From the UPS and XPS studies, there is a large interfacial charge transfer and a band-bending-like behavior in the standing DIP/F16CuPc OOHs, while the vacuum level is almost aligned at the lying-down DIP/F16CuPc OOH interface. We propose that the defects-induced gap states have a crucial...

Molecular-scale investigation of C60/p-sexiphenyl organic heterojunction interface

In situ low-temperature scanning tunneling microscopy (LT-STM) and ultraviolet photoelectron spectroscopy (UPS) experiments have been carried out to investigate the interface properties at the C(60)∕p-sexiphenyl (6P) organic-organic heterojunction interface, including the interfacial energy level alignment and the supramolecular packing structures. As revealed by UPS measurements, the vacuum level is almost aligned at the C(60)∕6P interface, suggesting that the interface is dominated by weak intermolecular interactions, such as van der Waals and π-π interactions. In situ LT-STM experiments also indicate the formation of a molecularly sharp C(60)∕6P interface with hexagonally-close-packed C(60) layers nucleated atop 6P layer on graphite.

Mechanism of the Fermi level pinning at organic donor–acceptor heterojunction interfaces

We investigate the energy level alignment and the Fermi level pinning mechanism at organic donor–acceptor heterojunctions interfaces by using the model organic–organic heterojunctions (OOHs) with well-defined molecular orientation of the standing copper (II) phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) films on the standing copper-hexadecafluoro-phthalocyanine (F16CuPc) thin films on SiO2. We identify two distinct regions for the energy level alignment by in situ ultraviolet photoelectron spectroscopy investigation. In region (I) where the work function (WF) of the underlying substrate is larger than the ionization potential (IP) of the top organic layers, the substrate Fermi level is pinned at the leading edge of the HOMO peak accompanied by a decreasing of the WF; in region (II) where the WF is smaller than the IP of the top organic layers, a downward shift of both the HOMO and vacuum level is observed. In connection with the defect induced gap states, we provide a detailed explanation for this thickness dependent energy level alignment and Fermi level pinning mechanism at the organic donor–acceptor OOH interface.

Orientation-controlled charge transfer at CuPc/F16CuPc interfaces

Molecular orientation-controlled charge transfer has been observed at the organic-organic heterojunction interfaces of copper-hexadecafluoro-phthalocyanine (F16CuPc) or copper(II) phthalocyanine (CuPc) on both standing-up and lying-down CuPc or F16CuPc thin films. In situ synchrotron-based photoemission spectroscopy reveals that the charge transfer at the standing F16CuPc/CuPc or CuPc/F16CuPc interface is much larger than that at the lying F16CuPc/CuPc or CuPc/F16CuPc interface. This can be explained by the orientation-dependent ionization potentials of well-ordered organic thin films, which place the highest-occupied molecular orbital of the standing CuPc film much closer to the lowest-unoccupied molecular orbital of the standing F16CuPc film, facilitating stronger charge transfer as compared to that at the lying OOH interfaces. Our results suggest the possibility of manipulating interfacial electronic structures of organic heterojunctions by controlling the molecular orientation, in particular for applications in ambipolar organic field transistors and organic photovoltaics.

Molecular Orientation Dependent Energy Level Alignment at Organic−Organic Heterojunction Interfaces

Molecular orientation dependent energy level alignments at organic−organic heterojunction (OOH) interfaces have been investigated with synchrotron based high-resolution photoemission spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) measurements. Model systems of the lying-down 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) films on both standing-up and lying-down copper hexadecafluorophthalocyanine (F16CuPc) and copper(II) phthalocyanine (CuPc) thin films have been used to illustrate the molecular orientation dependent interface properties. The formation of different interface dipoles at the heterojunction interfaces is strongly influenced by the orientation dependent ionization potentials of the underlying F16CuPc or CuPc thin films. This is attributed to the intrinsic surface dipoles induced in the standing-up F16CuPc (CuPc) film due to the polar intermolecular C−F (C−H) bonds formed at the interface. In situ NEXAFS measurements reveal that the room-temperature deposition of PT...

Charge transfer at n-doped organic-organic heterojunctions

The impact of n-doping on the electronic structure of two organic-organic heterojunctions (OOH) is investigated with ultraviolet photoemission spectroscopy. The two OOHs have a type II staggered gap configuration corresponding to donor-acceptor pairs applicable to photovoltaic cells. Both molecular top acceptor layers have large electron affinity, which leads to electron transfer from the donor layer. The result is a significant interface dipole (Δ∼0.4–0.6eV) between the two molecular films, at both undoped and doped interfaces, consistent with a stronger interaction between molecular constituents than in wide, largely overlapping, band gap OOHs.

Substrate-dependent electronic structure of an organic heterojunction

This work focuses on organic-organic heterojunctions (OOHs) formed between two small-molecule, low band-gap, semiconductors, tris{2,5-bis(3,5-bis-trifluoromethyl-phenyl)-thieno}$[3,4\text{\ensuremath{-}}b,$$h,n]\text{\ensuremath{-}}1,4,5,$$8,9,12$ $\ensuremath{-}\text{hexaaza}\ensuremath{-}\text{triphenylene}$ (THAP), and copper phthalocyanine (CuPc). The organic layers are deposited on various substrates, and the energy level alignment between them is investigated by ultraviolet photoemission spectroscopy. The electronic structure of the OOH is found to depend on the work function of the organic underlayer predeposited on the different substrates. The vacuum level offset between THAP and CuPc, which consists of the sum of the interface dipole and the molecular level shift, varies from $0.26\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1.37\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The interface dipole between the two organic films linearly changes with the work function of the organic underlayer.