Interfacial Dipole Moment Research Articles

Buried hole-selective interface engineering for high-efficiency tin-lead perovskite solar cells with enhanced interfacial chemical stability

Mixed Sn-Pb perovskites are attracting significant attention due to their narrow bandgap and consequent potential for all-perovskite tandem solar cells. However, the conventional hole transport materials can lead to band misalignment or induce degradation at the buried interface of perovskite. Here we designed a self-assembled material 4-(9H-carbozol-9-yl)phenylboronic acid (4PBA) for the surface modification of the substrate as the hole-selective contact. It incorporates an electron-rich carbazole group and conjugated phenyl group, which contribute to a substantial interfacial dipole moment and tune the substrate’s energy levels for better alignment with the Sn-Pb perovskite energy levels, thereby promoting hole extraction. Meanwhile, enhanced perovskite crystallization and improved contact at bottom of the perovskite minimized defects within perovskite bulk and at the buried interface, suppressing non-radiative recombination. Consequently, Sn-Pb perovskite solar cells using 4PBA achieved efficiencies of up to 23.45%. Remarkably, 4PBA layer provided superior interfacial chemical stability, and effectively mitigated device degradation. Unencapsulated devices retained 93.5% of their initial efficiency after 2000 h of shelf storage.

Dynamically blocking leakage current in molecular tunneling junctions.

Molecular tunneling junctions based on self-assembled monolayers (SAMs) have demonstrated rectifying characteristics at the nanoscale that can hardly be achieved using traditional approaches. However, defects in SAMs result in high leakage when applying bias. The poor performance of molecular diodes compared to silicon or thin-film devices limits their further development. In this study, we show that incorporating "mixed backbones" with flexible-rigid structures into molecular junctions can dynamically block tunneling currents, which is difficult to realize using non-molecular technology. Our idea is achieved by the interaction between interfacial dipole moments and electric field, triggering structured packing in SAMs. Efficient blocking of leakage by more than an order of magnitude leads to a significant enhancement of the rectification ratio to the initial value. The rearrangement of supramolecular structures has also been verified through electrochemistry and electroluminescence measurements. Moreover, the enhanced rectification is extended to various challenging environments, including endurance measurements, bending of electrodes, and rough electrodes, thus demonstrating the feasibility of the dynamic behavior of molecules for practical electronic applications.

Hybrid Metal-Ligand Interfacial Dipole Engineering of Functional Plasmonic Nanostructures for Extraordinary Responses of Optoelectronic Properties.

Programmable manipulation of inorganic-organic interfacial electronic properties of ligand-functionalized plasmonic nanoparticles (NPs) is the key parameter dictating their applications such as catalysis, photovoltaics, and biosensing. Here we report the localized surface plasmon resonance (LSPR) properties of gold triangular nanoprisms (Au TNPs) in solid state that are functionalized with dipolar, conjugated ligands. A library of thiocinnamate ligands with varying surface dipole moments were used to functionalize TNPs, which results in ∼150 nm reversible tunability of LSPR peak wavelength with significant peak broadening (∼230 meV). The highly adjustable chemical system of thiocinnamate ligands is capable of shifting the Au work function down to 2.4 eV versus vacuum, i.e., ∼2.9 eV lower than a clean Au (111) surface, and this work function can be modulated up to 3.3 eV, the largest value reported to date through the formation of organothiolate SAMs on Au. Interestingly, the magnitude of plasmonic responses and work function modulation is NP shape dependent. By combining first-principles calculations and experiments, we have established the mechanism of direct wave function delocalization of electrons residing near the Fermi level into hybrid electronic states that are mostly dictated by the inorganic-organic interfacial dipole moments. We determine that both interfacial dipole and hybrid electronic states, and vinyl conjugation together are the key to achieving such extraordinary changes in the optoelectronic properties of ligand-functionalized, plasmonic NPs. The present study provides a quantitative relationship describing how specifically constructed organic ligands can be used to control the interfacial properties of NPs and thus the plasmonic and electronic responses of these functional plasmonics for a wide range of plasmon-driven applications.

Tunable Schottky contacts in graphene/XAu4Y (X, Y = Se, Te) heterostructures.

Graphene-based (G-based) heterostructures have recently attracted considerable research interest in the field of two-dimensional nanodevices owing to their superior properties compared with those of separate monolayers. In this study, the electronic properties and Schottky barrier heights (SBHs) of G/XAu4Y (X, Y = Se, Te) heterostructures were systematically analyzed through first-principles calculations. G/SeAu4Se, G/SeAu4Te, and G/TeAu4Se are n-type Schottky contacts with Φn = 0.40, 0.38, and 0.55 eV respectively, whereas G/TeAu4Te is a p-type Schottky contact with Φp = 0.39 eV. In G-based heterostructures consisting of SeAu4Te that has a 0.22-Debye intrinsic dipole moment, the intrinsic dipole moments in different directions enhance or weaken the interfacial dipole moments corresponding to the charge transfer at the interface, resulting in different Φn values of G/SeAu4Te and G/TeAu4Se. Furthermore, vertical strain and external electric field, which influence charge transfer, are applied to G/XAu4Y heterostructures to modulate their SBHs. Taking G/TeAu4Te as an example, the p-type contact transforms into an almost ohmic contact with decreasing vertical strain or positive external electric field. The findings of this study can provide insights into the fundamental properties of G/XAu4Y for further research.

Highly Efficient Nonfullerene Organic Solar Cells with Nickel Oxide Hole‐Transporting Layer: Using Dipole‐Induced Energy‐Level Modification

To date, poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is the most commonly used hole‐transporting material (HTM) for nonfullerene acceptor (NFA) organic solar cell (OSCs) due to its high hole conductivity and reproducibility. However, considering the significant breakthroughs made by novel donor and NFA combinations such as PM6 and Y6, the development of HTMs which can outperform PEDOT:PSS is very limited. Here, for the first time, high‐performance NFA OSCs based on simple and solution‐processed nickel oxide HTMs with high power conversion efficiency (PCE) and stability are reported. Two different kinds of hydroquinone sulfonic acid (HSA) salts, which have a permanent dipole moment, are added to NiO solution. Due to the interfacial dipole moment generated by these salts and effective work function modulation of NiO, efficient hole transportation at the interface between hole‐transporting layer and bulk heterojunction (BHJ) layer by minimizing the energetic barrier is promoted. As a result, using PM6 and the Y6‐BO‐4F‐based BHJ layer, the PCE of 14.19% with very high open‐circuit voltage of 0.86 V is achieved, which is comparable with that of PEDOT:PSS‐based OSCs. For 1200 h under ambient atmosphere, OSCs based on NiO containing HSA salts demonstrate enhanced ambient stability compared to that of PEDOT:PSS.

A Multifunctional Self‐Assembled Monolayer for Highly Luminescent Pure‐Blue Quasi‐2D Perovskite Light‐Emitting Diodes (Advanced Optical Materials 20/2022)

Highly luminescent pure-blue perovskite light-emitting diodes (PeLEDs) are realized with (2-(3,6-dichloro-9H-carbazol-9-yl)ethyl)phosphonic acid (36ClCzEPA) as multifunctional hole-injection layer. With the synergetic effects of reduced hole injection barrier by strong interfacial dipole moment and interfacial passivation through the Pb–Cl electronic coupling, the PeLEDs reach a maximum external quantum efficiency of 4.80% at electroluminescence emission of 470 nm. For further details see article number 2201313 by BongSoo Kim, Jin Young Kim, and co-workers.

Inorganic–Organic Interfacial Electronic Effects in Ligand-Passivated WO3–x Nanoplatelets Induce Tunable Plasmonic Properties for Smart Windows

Transition-metal oxide (TMO) nanocrystals (NCs), displaying localized surface plasmon resonance (LSPR) properties, are an emerging class of nanomaterials due to their high stability, high earth abundance, and wide range of spectral responses covering the near-to-far infrared region of the solar spectrum. Although surface passivating ligands are ubiquitous to colloidal NC-based research, the role of ligands, specifically the impact of their chemical structure on the dielectric and LSPR properties of TMO NC films, has not been investigated in detail. Here, we report for the first time the chemical effects at the metal–ligand (inorganic–organic) interfaces influencing the optical constants and LSPR properties of thin films comprising highly oxygen-deficient, sub-stoichiometric, LSPR-active tungsten oxide (WO3–x) nanoplatelets (NPLs). We studied ligands with two different types of binding head groups, aromatic conjugation, and short and long hydrocarbon chains. Using density functional theory calculations, we determine that the changes in the interfacial dipole moments and polarizability control the permittivity at the interface, resulting in the alteration of dielectric and LSPR properties of ligand-passivated NPL in thin nanocrystalline films. The photochromic properties of WO3–x NPL passivated with different ligands in thin films have also been investigated to highlight the impact of interfacial permittivity caused by the chemical structures of passivating ligands. Taken together, this study provides a fundamental understanding of emerging properties at the metal–ligand interface that could be further optimized for energy efficiency in smart windows.

Tailoring electric dipole of hole-transporting material p-dopants for perovskite solar cells

•Efficient doping of PTAA via regulatory molecular dipole •Surface doping of perovskite via oriented dipole •Trap states restrain via the oriented dipole •High efficiency of modules based on PTAA Li-TFSI/t-BP are the most widely employed p-dopants for hole-transporting materials (HTMs) within the state-of-the-art perovskite solar cells (PSCs). The hygroscopicity and migration of these dopants, however, lead to devices with limited stability. To solve this problem, we report here on a diphenyl iodide cation and pentafluorophenyl boric acid anion-based dopant (DIC-PBA) with an oriented interfacial dipole moment as an alternative to Li-TFSI/t-BP. Theoretical and experimental data reveal that DIC-PBA exhibits deep doping of poly[bis(4-phenyl)(2,4,6-triMethylphenyl)aMine] (PTAA) and also creates p-doping of perovskite surface, which originates from ionic interactions-derived dipole arrangement that yields fast interfacial charge transport. The improved intrinsic stability of PSCs originates from the inhibition of dipole moment degeneration on the perovskite surface. Devices prepared with DIC-PBA yielded high efficiency of 22.86%, and the modules (aperture area: 33.2 cm2) efficiency reached 19.13%. Importantly, the storage stability also significantly improved exceeding to 90% after aging 1,200 h under air ambient. Li-TFSI/t-BP are the most widely employed p-dopants for hole-transporting materials (HTMs) within the state-of-the-art perovskite solar cells (PSCs). The hygroscopicity and migration of these dopants, however, lead to devices with limited stability. To solve this problem, we report here on a diphenyl iodide cation and pentafluorophenyl boric acid anion-based dopant (DIC-PBA) with an oriented interfacial dipole moment as an alternative to Li-TFSI/t-BP. Theoretical and experimental data reveal that DIC-PBA exhibits deep doping of poly[bis(4-phenyl)(2,4,6-triMethylphenyl)aMine] (PTAA) and also creates p-doping of perovskite surface, which originates from ionic interactions-derived dipole arrangement that yields fast interfacial charge transport. The improved intrinsic stability of PSCs originates from the inhibition of dipole moment degeneration on the perovskite surface. Devices prepared with DIC-PBA yielded high efficiency of 22.86%, and the modules (aperture area: 33.2 cm2) efficiency reached 19.13%. Importantly, the storage stability also significantly improved exceeding to 90% after aging 1,200 h under air ambient.

Construction of Charge Transport Channels at the NiOx/Perovskite Interface through Moderate Dipoles toward Highly Efficient Inverted Solar Cells.

NiOx-based perovskite solar cells (PSCs) have attracted much attention because of their low fabrication temperature, suppressed hysteresis, and superior stability. However, the poor interfacial contacts between NiOx and perovskite layers always limit the progress of PSCs. Here, we applied 2-thiophenemethylamine (TPMA) as charge transport channels at the interface between NiOx and perovskite layers. The introduction of TPMA provides moderate dipole moment pointing to the perovskite side and effectively promotes the charge transportation. Meanwhile, TPMA anchorage also passivates the defect states at the surfaces of both NiOx and MAPbI3, which compensates the voltage loss due to the change in NiOx work function induced by the dipole. Thus, the device performance has been significantly enhanced in both electrochemical properties and power conversion efficiency. Our work has demonstrated a new way of improving current and voltage in the NiOx-based PSCs simultaneously through a moderate interfacial dipole moment toward highly efficient PSCs.

Interface polarization in heterovalent core-shell nanocrystals.

The potential profile and the energy level offset of core-shell heterostructured nanocrystals (h-NCs) determine the photophysical properties and the charge transport characteristics of h-NC solids. However, limited material choices for heavy metal-free III-V-II-VI h-NCs pose challenges in comprehensive control of the potential profile. Herein, we present an approach to such a control by steering dipole densities at the interface of III-V-II-VI h-NCs. The controllable heterovalency at the interface is responsible for interfacial dipole densities that result in the vacuum-level shift, providing an additional knob for the control of optical and electrical characteristics of h-NCs. The synthesis of h-NCs with atomic precision allows us to correlate interfacial dipole moments with the NCs' photochemical stability and optoelectronic performance.

Inhibiting octahedral tilting for stable CsPbI2Br solar cells

AbstractThe intrinsic phase instability of CsPbI2Br perovskite hinders its development and application in solar cells. Herein, we adopt 1‐butyl‐3‐methylimidazolium hexafluorophosphate (BMIMPF6), a hydrophobic ionic liquid (IL), to improve the phase stability of CsPbI2Br. Density functional theory calculation reveals that the formation energy of CsPbI2Br with BMIMPF6 is reduced, thereby restraining the [PbX6]4− octahedral tilting. The reduced structural distortion and relaxed lattice strain result in improved phase stability of CsPbI2Br. In addition, the interfacial dipole moment increases after introducing BMIMPF6, which facilitates the charge transfer. Consequently, the CsPbI2Br solar cells with BMIMPF6 deliver a power conversion efficiency (PCE) of 16.2% along with excellent stability. The unencapsulated devices with BMIMPF6 maintain 98.9%, 88.6%, and 99.5% of the initial PCEs after 1200 h storage in N2, 1000 h storage in air (30% relative humidity), and 200 thermal cycles (25–100°C), respectively. image

Simultaneous Enhanced Efficiency and Stability of Perovskite Solar Cells Using Adhesive Fluorinated Polymer Interfacial Material.

For enhancing the performance and long-term stability of perovskite solar cell (PSC) devices, interfacial engineering between the perovskite and hole-transporting material (HTM) is important. We developed a fluorinated conjugated polymer PFPT3 and used it as an interfacial layer between the perovskite and HTM layers in normal-type PSCs. Interaction of perovskite and PFPT3 via Pb-F bonding effectively induces an interfacial dipole moment, which resulted in energy-level bending; this was favorable for charge transfer and hole extraction at the interface. The PSC device achieved an increased efficiency of 22.00% with an open-circuit voltage of 1.13 V, short-circuit current density of 24.34 mA/cm2, and fill factor of 0.80 from a reverse scan and showed an averaged power conversion efficiency of 21.59%, which was averaged from forward and reverse scans. Furthermore, the device with PFPT3 showed much improved stability under an 85% RH condition because hydrophobic PFPT3 reduced water permeation into the perovskite layer, and more importantly, the enhanced contact adhesion at the PFPT3-mediated perovskite/HTM interface suppressed surface delamination and retarded water intrusion. The fluorinated conjugated polymeric interfacial material is effective for improving not only the efficiency but also the stability of the PSC devices.

Tuning the Interfacial Dipole Moment of Spacer Cations for Charge Extraction in Efficient and Ultrastable Perovskite Solar Cells

The two-dimensional (2D)/three-dimensional (3D) heterojunction perovskite solar cell (PSC) has recently been recognized as a promising photovoltaic structure for achieving high efficiency and long-...

Evolution of Electronic Structures of Polar Phthalocyanine-Substrate Interfaces.

The electronic structures and core-level spectra of chlorogallium phthalocyanine (ClGaPc) molecules of different thicknesses (submonolayer to multilayer) adsorbed on a polycrystalline Au substrate and a highly oriented pyrolytic graphite (HOPG) substrate, before and after thermal annealing, were investigated using photoelectron spectroscopic techniques for better understanding the charge-transfer properties. The energy-level diagrams (ELDs) of the ClGaPc thin films are found to evolve with film thickness, substrate nature, and thermal annealing. The interfacial dipole moment in the active Au substrate and the molecular dipole moment in the inactive HOPG substrate mainly dictate the ELD. Annealed monolayer films on both the substrates seem to adopt a similar well-ordered Cl-up orientated molecular organization, which is quite interesting, as it certainly indicates a substrate-nature-independent energy minimum configuration. The strong interaction of the active Au substrate gives rise to additional charge transfer and state transfer (of Ga) as evident from the formation of a former lowest unoccupied molecular orbital (F-LUMO) level in the highest occupied molecular orbital (HOMO) region and a low binding energy peak in the Ga 2p3/2 core level. The presence of strong F-LUMO and molecular-dipole-related HOMOd levels in the predicted monolayer of well-ordered Cl-up oriented molecules on the Au and HOPG substrates, respectively, creates the optimum energy-level alignment (ELA) for both the systems, while the opposite shift of the vacuum levels in two different substrates makes the ionization potential (IP) for such a monolayer either minimum (on the Au substrate) or maximum (on the HOPG substrate), which is useful information for tuning the charge injection across the interface in organic semiconductor-based devices.

Precisely Controlled Two-Dimensional Rhombic Copolymer Micelles for Sensitive Flexible Tunneling Devices

Nanoscale two-dimensional (2D) organic materials have attracted significant interest on account of their unique properties, which result from their ultrathin and flat morphology. Supramolecular 2D ...

Reduced hysteresis in perovskite solar cells using metal oxide/organic hybrid hole transport layer with generated interfacial dipoles

Herein, we report a facile method to reduce the J–V hysteresis using nickel oxide(NiOx)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) double hole transport layer (DHTL) fabricated p-i-n perovskite solar cells. The device using the NiOx/PEDOT:PSS DHTL shows a reduced hysteresis with an average hysteresis index of 0.020. This is smaller than a single NiOx device (0.142) or single PEDOT:PSS (0.093). First-principles calculations reveal that O and S in PEDOT:PSS couple with the Ni in NiOx, and PSS forms hydrogen bonds with O on NiOx. This coupling generates an interfacial dipole moment directed toward the PEDOT: PSS layer, evidenced by dark current-voltage plots. Electrochemical impedance spectroscopy (EIS) indicates that these dipoles lead to kinetically-favorable charge extraction and contribute to the suppressed hysteresis. Kelvin probe force microscopy indicates that the generated dipoles dispel the accumulated iodine ion vacancies at the hole transport layers side that enhanced charge collection and thereby decreased the hysteresis. Intensity modulated photocurrent spectroscopy demonstrates that the device with the NiOx/PEDOT:PSS DHTL shows faster charge transport rate ktrans than that of device with single PEDOT:PSS or NiOx, and further demonstrates that a more efficient outward charge flow in PSCs with the NiOx/PEDOT:PSS DHTL reduces hysteresis.

Adsorption of Polarized Molecules for Interfacial Band Engineering of Doped TiO2Thin Films

Owing to their chemical and mechanical stability, metal-oxides have emerged as potential alternatives for conventional pure-metal and organic molecule-based solid-state electronic devices. Traditionally, band engineering of these metal-oxides has been performed to improve the efficiency of solar cells and transistors. However, recent advancements in the field of oxide-based electronic devices demand reversible band structure engineering for applications in next-generation adaptive electronics and memory devices. Therefore, this work aims to reversibly engineer the surface band structure of doped metal-oxides using stable organic ligands with weak dipoles. Para-substituted benzoic acid (BZA) ligands with positive and negative dipole moments were adsorbed in situ on the surface of TiO2:Ni2+ thin film to modify the interfacial dipole moment, and the valence band structure was probed using surface-sensitive ultraviolet photoelectron spectroscopy (UPS). UPS, paired with density functional theory (DFT) simulations, demonstrate the ability to selectively tune interfacial electronic/chemical landscapes with ligand-dependent dipole moment. The unique ability to reversibly tune the band bending at the organic–inorganic interface of doped metal-oxide semiconductors using molecular dipoles is expected to play a key role in the development of metal-oxide-based adaptive electronics that outperform the conventional polymer-based and Si-based devices.

Doping and interfacial modification of the P3HT/ZnO photovoltaic interface: Insights from density functional theory

In this work, we examine how the properties of the poly-3-hexylthiophene(P3HT)/ZnO solar cell are affected by interfacial modification with phenyl-C61-butyric acid (PCBA) and doping with Sr. Using density functional theory calculations on ideal models of the P3HT/ZnO(101¯0) interface, we characterize the equilibrium structures of the pure and modified interfaces, evaluate their ideal work of separation, and examine the energy level alignments and electronic structure. We find that doping with Sr and introducing PCBA have little impact on interfacial distances and do not strengthen interfacial adhesion. Rather, the individual and cumulative impacts of Sr-doping and PCBA are observed in changes to the energy level alignments, including the open circuit voltage and the energy offset that drives exciton separation. Sr dopants and PCBA cause these energetic shifts through differing effects on the electrostatic potential offset and interfacial dipole moments.

Improved organic solar cell by incorporating silver nanoparticles embedded polyaniline as buffer layer

The role of silver nanoparticles (AgNP) in polyaniline (PANI) as a buffer layer for ITO/AgNP-PANI/PANI/Al solar cell was investigated. It is observed that AgNP-PANI buffer layer significantly improves the electrical parameters such as diode-ideality factor, series-resistance, energy-barrier height, and shunt-resistance as a growing function of AgNP concentration. On-the-other hand oppose to the dark current-voltage response, 0.5% concentration of AgNP in buffer layer shows the most optimum photovoltaic response and cause to increase the power conversion efficiency (PCE) nearly 5 times compared to same solar cell without buffer layer. Such improvements in electrical parameters can be interpreted as the reduction in interfacial trap states as well as enhancement in interfacial dipole-moment by AgNP embedded buffer layer for given photovoltaic device. While, the observed optimum photovoltaic behavior at 0.5% AgNP concentration is may be due to the trade-offs between gains and losses for optical absorption enhancement, self-absorption heating and interface recombination losses respectively. It is also observed that the AgNP embedded PANI buffer layer approach is an effective solution to lower the photovoltaic degradation and hence improves the stability of the photovoltaic devices.

Attaching titania clusters of various size to reduced graphene oxide and its impact on the conceivable photocatalytic behavior of the junctions—a DFT/D + U and TD DFTB modeling

DFT/D + U and density functional based tight binding (DFTB) molecular modeling was used to investigate the role of the structural, electronic and optical properties of reduced graphene oxide surface (r-GO), hybridized with hydrated TiO2 moieties of various size, ranging from small molecular Ti2O4 clusters into extended Ti43O86 rutile type nanocrystals of ~5 nm diameter. The calculated adhesion energies, varying from −5.048 eV (r-GO|Ti2O4), −12.159 eV (r-GO|Ti5O10), −18.499 eV (r-GO|Ti15O30) to −42.484 eV (r-GO|Ti43O86), indicate high stability of these composites. It was shown that electronic interactions at the r-GO|(1 1 0)TiO2 interface give rise to net charge flow from the r-GO substrate towards the TiO2 moieties, analyzed in terms of the partial charge density 3D plots and an interfacial dipole moment formation. The DOS structure of the composites was calculated by means of the time dependent DFTB approach, and the position and composition of the VB and CB edges, along with the presence of weak mid-gap 2p C states originating from the intact graphene-like patches in the r-GO substrate were discussed in detail in the context of conceivable photocatalytic activity of the composites. The constructed band alignment diagram implies formation of the staggered type II scheme, with the electric field offset that is sensitive to the titania cluster size. In the case of the nano-reticular TiO2, where only a fraction of the Ti atoms is engaged in the Ti–O–C linkers formation, recombination of the photogenerated charges is inhibited owing to favorable spatial separation effect. For small molecular TiO2 clusters with all Ti cations anchored to the r-GO layer fast cross-relaxation quenches the beneficial interfacial charge separation effect, since the strong hybridization of the oxygen and carbon states provides a convenient pathway for the efficient electronic coupling between the CB edge states of r-GO and the VB edge states of the TiO2 moieties. A phenomenological model of the molecular r-GO|Ti2O4 and the reticular r-GO|Ti43O86 composites was constructed in account for different photocatalytic behavior of both junctions.