Electron Transport In Molecules Research Articles

Strategies to improve the mechanical robustness of metal halide perovskite solar cells

We demonstrate the fragility of perovskite solar cells is typically in the small molecule electron transport layer and show strategies for designing more mechanically and operationally robust devices by improving the layers and interfaces.

N-Type Small Molecule Electron Transport Layers with Excellent Surface Energy and Moisture Resistance Siloxane for Non-Fullerene Organic Solar Cells.

Electron transport layers (ETLs) generally contain polar groups for enhancing performance and reducing the work function. Nevertheless, the polar group with high surface energy may cause inferior interfacial compatibility, which challenges the ETLsto balance stability and performance. Here, two conjugated small molecules of ETLs with low surface energy siloxane, namely PDI-Si and PDIN-Si, are synthesized. The siloxane with low surface energy not only enhances the interfacial compatibility between ETLs and active layers but also improves the moisture-proof stability of the device. Impressively, the amine-functionalized PDIN-Si can simultaneously exhibit conspicuous n-type self-doping properties and outstanding moisture-proof stability. The optimization of interfacial contact and morphology enables the PM6:Y6-based OSC with PDIN-Si to achieve a power conversion efficiency (PCE) of 15.87%, which is slightly superior to that of classical ETL PDINO devices (15.27%), and when the PDIN-Si film thickness reaches 28nm, the PCE remains at 13.19% (≈83%), which indicates that PDIN-Si has satisfactory thickness insensitivity to facilitate roll-to-roll processing. Excitingly, after 120 h of storage in an environment with humidity above 45%, the unencapsulated device with PDIN-Si as ETL remains at 75% of the initial PCE value, while the device with PDINO as ETL is only 50%.

N-type small molecule electron transport materials with D-A-D conjugated core for non-fullerene organic solar cells

Two D (donor)-A (acceptor)-D-type small-molecular conjugated electrolytes (SMCEs) electron transport materials (ETMs) with self-doping effects, namely DPPN2TPA and DPPM2TPA, were successfully synthesized by introducing diketopyrrolopyrrole (DPP) as the electron acceptor, triphenylamine (TPA) as the electron donor and quaternary ammonium or imidazolium salts as polar pendants. Impressively, the electron-deficient group DPP is double-doped by the amine-based groups and the electron-rich groups TPA, thus DPPN2TPA and DPPM2TPA exhibit prominent intramolecular charge transfer (ICT) and distinct n-type self-doping property. These characters can increase the electron mobility and ameliorate the work function (WF) of the stable metal Ag to create ohmic contact, making for significant elevated photovoltaic performance of the SMCEs. Especially, DPPM2TPA emerges more prominent optoelectronic property than DPPN2TPA due to the extra interfacial dipoles generated by the tertiary amine of imidazole moieties. The PM6:Y6-based non-fullerene organic solar cells (NOSCs) with DPPN2TPA and DPPM2TPA as ETMs achieve excellent power conversion efficiency (PCE) of 15.93 % and 16.40 %, respectively. It is noticeably superior to the NOSC with classical ETM PDINO (15.35 %). Delightfully, the PCE remained at 14.64 % (≈90 %) when the thickness of DPPM2TPA film reached 37 nm, which manifests that DPPM2TPA has satisfactory thickness insensitivity and can be used to manufacture large-scale devices. Furthermore, DPPM2TPA has conspicuous universality, when DPPM2TPA ETM was applied to the device with PM6: BTP-eC9, a remarkable PCE of 17.31 % was realized. These results demonstrate the potential of DPPM2TPA for industrialized large-scale manufacturing and afford an effective approach for exploring high-performance D-A-D SMCEs NOSCs.

Conformation-dependent charge transport through short peptides.

We report on charge transport across single short peptides using the Mechanically Controlled Break Junction (MCBJ) method. We record thousands of electron transport events across single-molecule junctions and with an unsupervised machine learning algorithm, we identify several classes of traces with multifarious conductance values that may correspond to different peptide conformations. Data analysis shows that very short peptides, which are more rigid, show conductance plateaus at low conductance values of about 10-3G0 and below, with G0 being the conductance quantum, whereas slightly longer, more flexible peptides also show plateaus at higher values. Fully stretched peptide chains exhibit conductance values that are of the same order as that of alkane chains of similar length. The measurements show that in the case of short peptides, different compositions and molecular lengths offer a wide range of junction conformations. Such information is crucial to understand mechanism(s) of charge transport in and across peptide-based biomolecules.

Anti-ohmic single molecule electron transport: is it feasible?

Hitherto, only molecular wires with a regular ohmic behavior in which the electric conductance decreases with the wire length have been synthesized. Implementation of molecular conductors with reversed conductance/length trend (anti-ohmic) might revolutionize the field of molecular electronics, allowing the development of electronic devices with extraordinary properties. It is for this reason that, recently, theoretical efforts have been focused on this topic and different structures have been proposed to show reversed conductance/length behavior on the basis of density functional theory non-equilibrium Green function approach (DFT-NEGF) and topological models. From the previous works, it can be stated that an anti-ohmic molecular wire must display a very small HOMO–LUMO gap and a reversed bond alternation pattern in the case of polyenes and related conjugated systems. In this work, the pursuit of a mechanism by which the anti-ohmic electron transport may arise was carried out by studying the paradigmatic anti-ohmic p-xylylene chain (pX2) at the DFT level in combination with topological models. It has been found that the electron transport in the anti-ohmic regime is favored by a long-range superexchange mechanism, which, contrary to what is expected, is reinforced by the increase in the length of the chain. Moreover, strong links between anti-ohmic character in molecular wires and one-dimensional topological insulator models have been established. Due to the small HOMO–LUMO gap predicted at DFT level, the anti-ohmic character has been put to the proof using a multireference scenario. Preliminary results point out to the presence of different ohmic and anti-ohmic electronic states. In the particular case of pX2 the anti-ohmic states do not correspond to the ground state. These findings require a reconsideration of previous studies on the reversed conductance/length behavior using single reference methodologies.

Transmission channels in the time‐energy uncertainty relation approach to molecular conductance: Symmetry rules for the electron transport in molecules

AbstractThe concept of transmission channel plays a key role in the theory of molecular conductors. They are usually constructed from one‐electron functions (molecular spin orbitals) in the extensively employed LCAO‐MO method. Thus, the electric conductance can be obtained from the transmission matrix, whose diagonalization leads to a set of transmission eigenchannels. In this work, it is shown how using the time‐energy uncertainty relation approach, the electron transport can be also analysed in terms of coupled transmission channels. Here, the transmission channels are constructed in terms of electron deformation orbitals (EDOs), which in turn arise from linear combinations of occupied and unoccupied one‐electron functions. These transmission channels allow linking the electric conduction in molecular junctions with the traditional interpretation in mesoscopic systems, where the electron transport occurs by promotion of electrons from the valence to the conduction bands. Moreover, they lead to an expression of the electric conductance within the Landauer formalism, with the conductance given by the product of the quantum conductance and a transmission function. Transmission channels of junctions formed by polycyclic aromatic chains attached to gold atoms are analysed in detail. Symmetry rules for the MOs involved in these channels are established, giving rise to a straightforward method for the analysis of the transport ability in molecular systems.

Research progress in electron transport layer in perovskite solar cells

Since perovskite solar cells appeared in 2009, its simple preparation process, high photoelectric conversion efficiency and the characteristic of low cost in preparation process let it become the hot spot of both at-home and abroad. Owing to the constant efforts of scientists, the conversion efficiency of perovskite solar cells is more than 20% now. Perovskite solar cells are mainly composed of conductive glass, electron transport layer and hole transport layer, perovskite layer and electrode parts. This paper will briefly introduce the working principle and working process about the electron transport layer of perovskite solar cells. The paper focuses on aspects such as material types (e.g., inorganic electron transport materials, organic small molecule electron transport materials, surface modified electron transport materials and doped electron transport materials), preparation technology of electron transport layer, the effects of electron transport layer on the photovoltaic performance of the devices, and the electron transport layer in the future research.

Advance of Mechanically Controllable Break Junction for Molecular Electronics.

Molecular electronics stands for the ultimate size of functional elements, keeping up with an unstoppable trend over the past few decades. As a vital component of molecular electronics, single molecular junctions have attracted significant attention from research groups all over the world. Due to its pronounced superiority, the mechanically controllable break junctions (MCBJ) technique has been widely applied to characterize the dynamic performance of single molecular junctions. This review presents a system analysis for single-molecule junctions and offers an overview of four test-beds for single-molecule junctions, thus offering more insight into the mechanisms of electron transport. We mainly focus on the development of state-of-the-art mechanically controlled break junctions. The three-terminal gated MCBJ approaches are introduced to manipulate the electron transport of molecules, and MCBJs are combined with characterization techniques. Additionally, applications of MCBJs and remarkable properties of single molecules are addressed. Finally, the challenges and perspective for the mechanically controllable break junctions technique are provided.

Conductive atomic force microscopy study of single molecule electron transport through the Azurin-gold nanoparticle system

Transduction of biorecognition events into electrical signals through integration of single redox metalloproteins in bioelectronic nanodevices requires both a reliable electrical contact between the biomolecule and the metallic electrode and an efficient overall conduction mechanism. These conditions have been met in the hybrid system obtained by linking gold nanoparticles on top of Azurin proteins, in turn assembled on gold surfaces. Such an assembling strategy, combined with a conductive atomic force microscopy investigation, has allowed us to put into evidence an unprecedented matching between current and topography features and to attribute the intramolecular charge transport to a non-resonant tunnelling mechanism.

An Orbital Rule for Electron Transport in Molecules

The transfer of electrons in molecules and solids is an essential process both in biological systems and in electronic devices. Devices that take advantage of the unique electronic properties of a single molecule have attracted much attention, and applications of these devices include molecular wire, molecular memory, and molecular diodes. The so-called Landauer formula with Green's function techniques provides a basis for theoretical calculations of coherent electron transport in metal-molecule-metal junctions. We have developed a chemical way of thinking about electron transport in molecules in terms of frontier orbital theory. The phase and amplitude of the HOMO and LUMO of π-conjugated molecules determine the essential properties of their electron transport. By considering a close relationship between Green's function and the molecular orbital, we derived an orbital rule that would help our chemical understanding of the phenomenon. First, the sign of the product of the orbital coefficients at sites r and s in the HOMO should be different from the sign of the product of the orbital coefficients at sites r and s in the LUMO. Second, sites r and s in which the amplitude of the HOMO and LUMO is large should be connected. The derived rule allows us to predict essential electron transport properties, which significantly depend on the route of connection between a molecule and electrodes. Qualitative analyses of the site-dependent electron transport in naphthalene (as shown in the graphics) demonstrate that connections 1-4, 1-5, 2-3, and 2-6 are symmetry-allowed for electron transmission, while connections 1-8 and 2-7 are symmetry-forbidden. On the basis of orbital interaction analysis, we have extended this rule to metal-molecule-metal junctions of dithiol derivatives in which two gold electrodes have direct contacts with a molecule through two Au-S bonds. Recently we confirmed these theoretical predictions experimentally by using nanofabricated mechanically controllable break junctions to measure the single-molecule conductance of naphthalene dithiol derivatives. The measurement of the symmetry-allowed 1,4-naphthalene dithiol shows a single-molecule conductance that exceeds that of the symmetry-forbidden 2,7-naphthalene dithiol by 2 orders of magnitude. Because the HOMO and LUMO levels and the HOMO-LUMO gaps are similar in the derivatives, the difference in the measured molecular conductances arises from the difference in the phase relationship of the frontier orbitals. Thus, the phase, amplitude, and spatial distribution of the frontier orbitals provide a way to rationally control electron transport properties within and between molecules.

Geometric Effects on Conductance in Single Molecule Electron Transport Junctions

AbstractWe studied electron transport properties of a dithiol‐benzene molecule covalently bonded between two gold electrodes by combining ab initio calculations for the central molecule and a green function method to calculate electron transport. Due to the large computational demand, this type of calculations usually involves certain ways of simplification. The simplification commonly used is to fix the contact surface of the electrodes by ignoring the disturbance of the Au contact surface by contacting with the central molecule, i.e. without scattering region relaxation. In this study, we intended to resolve the difference between models with and without the above simplification. The large conductance found in our models without scattering region relaxation is due to the highly symmetric arrangement of the Au contact surface and those layers near the contact. The disturbance of the Au contact surface by the contact of the central molecule is important since the increase of the Au‐S bond and the distortion of the Au atom on the FCC site can lower the transmission coefficient between the two electrodes. In order to obtain better results, the model should include scattering region relaxation. However, when such relaxation is not applicable or demands too much calculation resource, the center molecule of the electronic transport junction should be at least optimized by the calculation level including electronic correlation, i.e. post‐HF methods.

Atomistic Mechanics and Formation Mechanism of Metal−Molecule−Metal Junctions

The present Letter reports a quantitative analysis of contact mechanics in metal-molecule-metal junctions at a single atom/molecule level through investigating their lifetime at cryogenic temperature. We elucidated that the force breaking mechanism of atomic/molecular junctions is stretching speed dependent, attributed to suppression of contact structure relaxation processes at high strain rate conditions. We also provide solid evidence that strain exerted in the preformation stage of molecular junctions poses extra strain energy that accelerates their eventual fracture. Nonetheless, we find that single-molecule junctions subjected to mechanical stretching at 0.6 pm/s can be held for approximately 100 s on average at 77 K and for a much prolonged period when freezing the elongation after forming the molecular junctions by virtue of moderate thermal destabilizations, the fluctuation-free condition of which provides optimal experimental platform for performing reliable measurements of single molecule electron transport properties.

Electronic Transport in Single Molecule Junctions: Control of the Molecule-Electrode Coupling through Intramolecular Tunneling Barriers

We report on single molecule electron transport measurements of two oligophenylenevinylene (OPV3) derivatives placed in a nanogap between gold (Au) or lead (Pb) electrodes in a field effect transistor device. Both derivatives contain thiol end groups that allow chemical binding to the electrodes. One derivative has additional methylene groups separating the thiols from the delocalized pi-electron system. The insertion of methylene groups changes the open state conductance by 3-4 orders of magnitude and changes the transport mechanism from a coherent regime with finite zero-bias conductance to sequential tunneling and Coulomb blockade behavior.

Electron transport in molecular junctions

Building an electronic device using individual molecules is one of the ultimate goals in nanotechnology. To achieve this it will be necessary to measure, control and understand electron transport through molecules attached to electrodes. Substantial progress has been made over the past decade and we present here an overview of some of the recent advances. Topics covered include molecular wires, two-terminal switches and diodes, three-terminal transistor-like devices and hybrid devices that use various different signals (light, magnetic fields, and chemical and mechanical signals) to control electron transport in molecules. We also discuss further issues, including molecule-electrode contacts, local heating- and current-induced instabilities, stochastic fluctuations and the development of characterization tools.

Single molecule electron transport junctions: Charging and geometric effects on conductance

A p-benzenedithiolate (BDT) molecule covalently bonded between two gold electrodes has become one of the model systems utilized for investigating molecular transport junctions. The plethora of papers published on the BDT system has led to varying conclusions with respect to both the mechanism and the magnitude of transport. Conductance variations have been attributed to difficulty in calculating charge transfer to the molecule, inability to locate the Fermi energy accurately, geometric dispersion, and stochastic switching. Here we compare results obtained using two transport codes, TRANSIESTA-C and HUCKEL-IV, to show that upon Au-S bond lengthening, the calculated low bias conductance initially increases by up to a factor of 30. This increase in highest occupied molecular orbital (HOMO) mediated conductance is attributed to charging of the terminal sulfur atom and a corresponding decrease in the energy gap between the Fermi level and the HOMO. Addition of a single Au atom to each terminal of the extended BDT molecule is shown to add four molecular states near the Fermi energy, which may explain the varying results reported in the literature.

Theoretical study of molecular conduction: I. Effective Green's function based on perturbation theory

AbstractThere have been many experimental and theoretical studies on molecular conduction, as it is a fundamental parameter in the study of molecular‐scale electronics. We have investigated the features of molecular conduction using a Green's function method, which has often been used to solve problems in quantum transport and is also effective in elucidating electron transport in molecules. We have obtained the novel effective Green's functions, including the first‐order energy corrections, by accommodating the self‐energy of the electrodes as perturbation terms. Although these approximate Green's functions only provide information on the first‐order energy corrections, they can involve the elementary properties of molecular conduction. We propose a scheme for the analysis of the relations between molecular orbitals and their roles in molecular conduction and present analytical calculations for normal and cyclic polyenes. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

Theoretical study of the nonlinear conductance of Di-thiol benzene coupled to Au(1 1 1) surfaces via thiol and thiolate bonds

Di-thiol–benzene (DTB) is one of the most intensively studied systems, both experimentally and theoretically, for electron transport in molecules. Despite this, there persists a gap of three orders of magnitude between the measured and most reliable calculated conductances. In this paper, we present state of the art calculations of the electron transport through DTB coupled to Au(111) surfaces using our newly developed method TranSIESTA. The method is based on density functional theory (DFT) and determines the self-consistent electronic structure of a nanostructure coupled to 3-dimensional electrodes with different electrochemical potentials, using a full atomistic description of both the electrodes and the nanostructure. We find qualitative differences with other more approximative theoretical approaches, although we confirm that the theoretical conductance for a perfectly contacted DTB molecule is several orders of magnitude higher than the value obtained in molecular break junction (MBJ) experiments. We discuss the formation of the molecular contact in the MBJ, and calculate the current–voltage (I–V) characteristics of DTB for geometries which do not include perfect thiolate bonds to one or both of the electrodes.