Formation Of Surface Dipole Research Articles

Modulation of Perovskite Surface Energetics for State‐of‐the‐Art Solar Cells

Perovskite surface energetics, which determines the energy‐level alignment at the interface and thus controls the charge carrier dynamics, is essential to harness the potential of perovskite‐based optoelectronics (e.g., light‐emitting diodes and solar cells). Herein, the recent progress on the modulation of perovskite surface energetics for state‐of‐the‐art solar cells is reviewed. First, the architecture evolution of perovskite solar cells (PSCs) and how the underlying substrates regulate the perovskite surface energetics are discussed, followed by their significant roles on device efficiency and stability. Then, the strategies to tune perovskite surface energetics including intrinsic or extrinsic defect doping, charge transfer doping, surface dipole formation, and surface reconstruction are focused on, which help to boost charge carrier transport. Notably, the state‐of‐the‐art efficiencies have been achieved via effective modulation on perovskite surface energetics. Finally, the insights on the design rules of engineering perovskite surface energetics are provided for the further development of highly efficient and stable PSCs.

Improved Carrier Management via a Multifunctional Modifier for High-Quality Low-Bandgap Sn-Pb Perovskites and Efficient All-Perovskite Tandem Solar Cells.

All-perovskite tandem solar cells (TSCs) hold great promise in terms of ultrahigh efficiency, low manufacturing cost, and flexibility, stepping forward to the next-generation photovoltaics. However, their further development is hampered by the relatively low performance of low-bandgap (LBG) tin (Sn)-lead (Pb) perovskite solar cells (PSCs). Improving the carrier management, including suppressing trap-assisted non-radiative recombination and promoting carrier transfer, is of great significance to enhance the performance of Sn-Pb PSCs. Herein, a carrier management strategy is reported for using cysteine hydrochloride (CysHCl) simultaneously as a bulky passivator and a surface anchoring agent for Sn-Pb perovskite. CysHCl processing effectively reduces trap density and suppresses non-radiative recombination, enabling the growth of high-quality Sn-Pb perovskite with greatly improved carrier diffusion length of >8µm. Furthermore, the electron transfer at the perovskite/C60 interface is accelerated due to the formation of surface dipoles and favorable energy band bending. As a result, these advances enable the demonstration of champion efficiency of 22.15% for CysHCl-processed LBG Sn-Pb PSCs with remarkable enhancement in both open-circuit voltage and fill factor. When paired with a wide-bandgap (WBG) perovskite subcell, a certified 25.7%-efficient all-perovskite monolithic tandem device is further demonstrated.

Surface Passivation and Detrimental Heat-Induced Diffusion Effects in RbF-Treated Cu(In,Ga)Se2 Solar Cell Absorbers.

Alkali postdeposition treatments of Cu(In,Ga)Se2 absorbers with KF, RbF, and CsF have led to remarkable efficiency improvements for chalcopyrite thin film solar cells. However, the effect of such treatments on the electronic properties and defect physics of the chalcopyrite absorber surfaces are not yet fully understood. In this work, we use scanning tunneling spectroscopy and X-ray photoelectron spectroscopy to compare the surface defect electronic properties and chemical composition of RbF-treated and nontreated absorbers. We find that the RbF treatment is effective in passivating electronic defect levels at the surface by preventing surface oxidation. Our X-ray photoelectron spectroscopy (XPS) data points to the presence of chemisorbed Rb on the surface with a bonding configuration similar to that of a RbInSe2 bulk compound. Yet, a quantitative analysis indicates Rb coverage in the submonolayer regime, which is likely causing the surface passivation. Furthermore, ab initio calculations confirm that RbF-treated surfaces are less prone to oxidation (in the form of Ga, In, and Se oxides) than bare chalcopyrite surfaces. In addition, elemental diffusion of Rb along with Na, Cu, and Ga is found to occur when the samples are annealed under ultrahigh vacuum conditions. Magnetic sector secondary ion mass spectrometry measurements indicate that there is a homogeneous spatial distribution of Rb on the surface both before and after annealing, albeit with an increased concentration at the surface after heat treatment. Depth-resolved magnetic sector secondary ion mass spectrometry measurements show that Rb diffusion within the bulk occurs predominantly along grain boundaries. Scanning tunneling and XPS measurements after subsequent annealing steps demonstrate that the Rb accumulation at the surface leads to the formation of metallic Rb phases, involving a significant increase of electronic defect levels and/or surface dipole formation. These results strongly suggest a deterioration of the absorber-window interface because of increased recombination losses after the heat-induced diffusion of Rb toward the interface.

Highly stable inverted non-fullerene OSCs by surface modification of SnO2 with an easy-accessible material

In order to regulate the energy level of SnO2 films, we currently in this study employed a non-conjugated small molecule 2-(Dimethylamino) ethyl methacrylate (DMAEMA) to modify SnO2 films, which are applied as electron transportation layers in inverted non-fullerene organic solar cells (OSCs). It is demonstrated that an ultra-thin layer of DMAEMA can efficiently reduce the work function of SnO2 due to the formation of surface dipoles. The optimized PCE of OSCs based on photoactive materials PM6:Y6 increased from 13.80% to 15.45% after the SnO2 films being modified with DMAEMA. More importantly, the OSCs with DMAEMEA-modified SnO2 showed an extremely excellent stability, and the unencapsulated devices maintained 95% of the initial PCE after being stored in Nitrogen for 960 h. To the best of our knowledge, the average decay rate of normalized PCE is the smallest among the up-to-date reported studies.

Electronic structure of graphene/MoO3/CuPc interfaces

The interfacial electronic structures of graphene/copper phthalocyanine (CuPc) modified with MoO3 interlayers were investigated using x-ray and ultraviolet photoelectron spectroscopy. Strong p-doping effect has been observed on graphene due to deposition of MoO3 films, causing formation of surface dipole (eD) and shift of graphene Dirac point, and consequently altering the charge transfer at graphene/CuPc interface. The ionization energy of CuPc on the graphene/MoO3 substrate is about 4.97 eV. The interfacial energy level alignments are estimated based on the experimental photoelectron spectroscopy data. Finally, the present experimental data indicate that the height of the electron and hole injection barriers from CuPc to graphene/MoO3 are 1.98 and 0.32 eV, respectively.

Insight into the modification of electrodonor properties of multiwalled carbon nanotubes via oxygen plasma: Surface functionalization versus amorphization

The effect of oxygen plasma treatment on multi-walled carbon nanotubes (MWCNTs) surfaces towards controlled tuning of their electrodonor properties (gauged by the work function) was investigated experimentally (X-ray diffraction, Raman spectroscopy, thermogravimetry, transmission electron microscopy, X-ray photoelectron spectroscopy) and by molecular modeling (Density Functional Theory). The nanotubes were treated with oxygen plasma (0.2 mbar) at varied generator power (20–60 W) and exposure time (0.1–30 min). It was found that the work function changes nonmonotonously upon plasma treatment: after significant increase (from 4.5 eV to 5.9 eV) and passing through maximum the work function decreases and finally reaches a plateau. The experimental results supported by DFT calculations allowed to propose a molecular model explaining the changes in carbon nanotube surface induced by oxygen plasma. Two different oxygen adatom locations: out-of-plane (Csurf–Oadatom) and in-plane (Csurf–Osurf–Csurf) were identified, leading to formation of surface dipoles (formation of negative potential barrier, work function increase) and incorporation of oxygen into the carbon structure (cancellation of the potential barrier, work function decrease), respectively. The critical regions of plasma parameters for oxygen surface decoration and amorphization were identified providing guidelines for rational designing and tuning of MWCNT electrodonor properties.

Study on interface engineering of layer-by-layer structure for applications in organic photodetector

The interfacial electronic structure of layer-by-layer 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB)/fullerene C60 was investigated using ultra-violet photoemission spectroscopy (UPS). Photoemission data of period of DCJTB/C60/DCJTB films suggested the formation of surface dipole and interfacial band bending across the interfaces, which greatly facilitates the charge transfer from DCJTB to C60 and from C60 to DCJTB layer as well. When applied this layer-by-layer structure to a near-infrared photodetector, a maximum of photocurrent was achieved by the device with 3 periods of DCJTB/C60 thin films. Finally, the detailed work mechanism of this detector was discussed.

Tuning the work function of indium-tin-oxide electrodes for low-temperature-processed, titanium-oxide-free perovskite solar cells

In this work, low-temperature-processed, titanium-oxide-free perovskite solar cells (PSCs) with power conversion efficiency (12.2%) comparable to that of titanium-oxide-based PSCs were fabricated. The fabricated PSCs exhibited a small hysteresis owing to the two-step preparation method employed for perovskite films. A poly [(9,9-bis(3’-(N,N-dimethylamino) propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN)-tuned indium-tin-oxide (ITO) electrode was employed as a cathode and phenyl-C61-butyric acid methyl ester (PCBM) was used as an electron transport layer. Owing to the formation of surface dipoles, the work function of the ITO electrode could be tuned from −4.75 eV to −4.12 eV, which is more efficient for electron collection. Our proposed method significantly lowers the PSC processing temperature from 500 °C to 100 °C, which is advantageous for commercialization of PSCs and fabrication of flexible PSCs.

(Invited) Rapid In-Situ Carbon and Oxygen Cleaning of In0.53Ga0.47As(001) and Si0.5Ge0.5(110) Surfaces via a H2 RF Downstream Plasma

Channel passivation of SiGe and InGaAs requires removal of dangling bonds and formation of a thermally stable interface. On InGaAs, one solution is the deposition by self-limiting CVD or ALD of a few monolayer of silicon, silicon oxide, or silicon nitride. To form an Si-OH terminated In0.53Ga0.47As(001) surface, two monolayers of Si were deposited at 350°C with self-limiting CVD using Si2Cl6 dosing following by dosing anhydrous HOOH(g). Scanning tunneling microcopy (STM) shows and ordered surface is form while scanning tunneling spectroscopy (STS) shows the surface Fermi level position moves towards midgap due to a surface dipole formation from –OH groups and oxygen bonding to the surface. TMA was dosed at 250°C onto the Si-OH terminated InGaAs(001), and XPS shows the emergence of the Al 2p and C 1s peaks indicative of TMA surface nucleation. The TMA shifts the InGaAs Fermi level back towards the conduction band, consistent with unpinning. MOSCAPs were fabricated to further demonstrate Fermi level unpinning using the Si-OH termination of InGaAs. While this same Si deposition process could be employed on SiGe, a simple annealing process can be employed to form a silicon terminated SiGe surface. A saturation dose of H2O2(g) at 25oC chemisorbs to on Si0.5Ge0.5(110) and Si0.47Ge0.53(001) surfaces leaving the Fermi level on the surface unpinned, and the surface is functionalized with mainly Si-OH, Ge-OH and Si-O-Ge bond. After a subsequent TMA dose at 25oC and annealing at 300oC, XPS and STM verify that a thermally stable and well-ordered monolayer of Al2O3 is formed on SiGe(110) and (001) surfaces with only Al-O-Si bonds and no detectable Ge-O-Si bonds. The H2O2(g) functionalization provides 3 times higher O sites on the surface and 3 times higher TMA nucleation density than H2O(g) at 25°C and 120°C. A variety of strongly bound passivating atoms can induce formation of a nearly pure Si interfacial layer between high-k dielectrics and SiGe. This annealing procedure was employed to form MOSCAP on SiGe(001) using Al2O3 and HfO2 gate oxides after either (NH4)2S or NH3 plasma passivation. For NH3 plasma passivation, angle-resolved XPS showed that the annealing allows formation of nearly Ge-free SiON layer. For higher Ge concentrations, the annealing process is unlikely to form a nearly Ge-free SiON layer so a novel plasma-free chemistry has been develop to deposit SiON using ALD with N2H4(g) as a precursor. Figure 1

Interaction of indium oxide nanoparticle film surfaces with ozone, oxygen and water

The interaction of defect‐rich nanocrystalline indium oxide films, which have previously shown to exhibit excellent ozone sensing properties, with O3, O2, and H2O molecules is investigated using ultra‐violet and X‐ray photoelectron spectroscopy. The investigated samples are grown by metalorganic chemical vapor deposition at low temperatures resulting in high oxygen deficiency and high defect density. The ozone‐induced surface oxidation and UV‐induced photoreduction mechanisms of the ozone sensor active material are evaluated with respect to surface stoichiometry and electronic properties including adsorbate features, band bending and surface dipole formation. A strong interaction with ozone and water is found, whereas the interaction with O2 is relatively weak. In all cases the interaction results in the same negatively charged oxygen adsorbate species, which can either be removed by UV light or by annealing resulting in the capability of these films to be used in reversible adsorption induced oxidation and UV/thermal reduction cycles.

Effects of UV-ozone irradiation on copper doped nickel acetate and its applicability to perovskite solar cells.

The effects of UV-ozone (UVO) irradiation on copper-doped nickel acetate and its applicability to perovskite solar cells were investigated. UVO irradiation of copper-doped nickel acetate significantly increased the electrical conductivity (from 4.28 × 10(-4) S cm(-1) to 5.66 × 10(-2) S cm(-1)), which is due to the increased carrier concentration (from 3.53 × 10(13) cm(-3) to 2.41 × 10(16) cm(-3)), and the charge extraction efficiency was enhanced, leading to better compatibility with the hole transport layer. By UVO irradiation, the work function was increased from 4.95 eV to 5.33 eV by the surface dipole formation, which effectively reduced the interface barrier between the hole transport layer and the MAPbI3 light absorbing layer. UVO Irradiation of the underlying layer also allows the MAPbI3 precursors to form better morphology with highly arranged crystallinity. Compared to the cells using non-irradiated copper doped nickel acetate, UVO-irradiated copper-doped nickel acetate devices showed an enhanced open-circuit voltage (3% increase), short circuit current (16% increase), fill factor (5% increase), showing an enhanced power conversion efficiency of 12.2% (21% increase).

Impact of Surface Dipole on Resonant Electron Injection in Scanning Tunneling Spectroscopy

Resonant electron injection and first-principles calculations are utilized to study single-adsorbed selenium (Se) atom on a Si(111)-7×7 surface. Theoretical calculations indicate that a negative dipole of 0.61 eV forms toward the adsorbed Se atom due to electron transfer from the associated Si atoms. The formation of surface dipole modifies the effective tunneling barrier height and causes a shift in the energy of the resonant state formed in the vacuum gap between the tip and the sample surface. The experimental data imply that an outward negative surface dipole of 0.61 eV causes a resonant electron injection bias shift to high voltage of about 0.45 V.

Influence of Surface Adsorption on Work Function Measurements on Gold-Platinum Interface Using Scanning Kelvin Probe Microscopy

Surface potential difference (SPD) on freshly coated gold and platinum electrodes have been found to be much smaller than bulk work functions consideration and to be dependent on time. We show these discrepancies arise due to formation of surface dipoles caused by adsorbed contaminants in ambient environments. The process is reversible by gentle annealing consistent with contaminant hypothesis. Examination of potential changes on individual electrodes suggest that the Pt surface is more sensitive to ambient conditions than the Au surface in accordance with their relative chemical activities. The result has great implication for interpretation of Kelvin probe measurements obtained on practical devices exposed to ambient environments.

Comparison study of metal nanoparticles grown on polarity‐patterned ferroelectrics by scanning probe microscopy

We report the surface distribution of metal (Ag, Au) nanoparticles (NPs) grown on polarity‐patterned ferroelectrics by photochemical reaction. Single crystalline periodically polarity‐patterned LiNbO3 (PPLN) was used as a ferroelectric substrate. The NPs were grown by the ultraviolet light exposure of the PPLN in the aqueous solutions including metals. Dimension and surface distribution of the NPs were measured, and the polarity of the patterned regions was identified by scanning probe microscopy. For both of Ag and Au, the NPs formed on +z polarity regions are larger and denser than those on –z polarity regions. In particular, the largest Ag‐NPs were grown along the polarity boundary regions of the PPLN, whereas Au‐NPs were not specifically grown on the boundary regions. Also, the size and density of the NPs increased with longer ultraviolet exposure time. We will discuss the difference of the surface distribution of the NPs depending on the polarity of ferroelectrics in terms of variation of the surface band structures and the electric field distribution induced by polarity bound charges, surface dipole formation, and defects. This study suggests the possibility of controlling the size and position of metal NPs on the surfaces by polarity patterning of the ferroelectrics. Copyright © 2011 John Wiley & Sons, Ltd.

Profiles of Work Function Shifts and Collective Charge Transfer in Submonolayer Metal–Organic Films

AbstractVacuum level shifts Δ(d) at metal–organic (m–O) interfaces indicate the formation of surface dipoles for film thickness d ≤ dML up to a monolayer (ML). Shifts or profiles Δ(θ) of submonolayer films are nonlinear in the coverage θ = d/dML ≤ 1, which points to cooperative interactions between adsorbed molecules. Adsorption with weak nonspecific bonding is modeled as charge transfer (CT) between molecules M and localized surface states S of the metal. The dipole μ0 of ions S−M+ or S+M− gives upper bounds for the vacuum level shift ϕ0 and dipole–dipole repulsion V0 at θ = 1. Partial CT ρ(θ) < 1 is found self consistently and accounts for published profiles Δ(θ) of representative planar and nonplanar molecules with dML∼ 4 and ∼10 Å. Initial adsorption at θ ∼ 0 has considerable ionic character, ρ(0) ∼ 1/2, that decreases to ρ(1) ∼ 1/10 at θ = 1. Planar molecules with small μ0 and V0 have slightly nonlinear profiles while molecules with large μ0 and V0 have highly nonlinear Δ(θ). Collective CT is a phenomenological model for m–O interfaces with nonspecific bonding. The CT model is contrasted to fixed dipoles on the surface, to calculations of Δ(1) and to simulations of sub‐ML films.

Surface chemical analysis and ab initio investigations of CsI coated C fiber cathodes for high power microwave sources

CsI coated C fiber cathodes are promising electron emitters utilized in field emission applications. Ab initio calculations, in conjunction with experimental investigations on CsI-spray coated C fiber cathodes, were performed in order to better understand the origin of the low turn-on E-field obtained, as compared to uncoated C fibers. One possible mechanism for lowering the turn-on E-field is surface dipole layers reducing the work function. Ab initio modeling revealed that surface monolayers of Cs, CsI, Cs2O, and CsO are all capable of producing low work function C fiber cathodes (1 eV<Φ<1.5 eV), yielding a reduction in the turn-on E-field by as much as ten times, when compared to the bare fiber. Although a CsI-containing aqueous solution is spray deposited on the C fiber surface, energy dispersive x-ray spectroscopy and scanning auger microscopy measurements show coabsorption of Cs and I into the fiber interior and Cs and O on the fiber surface, with no surface I. It is therefore proposed that a cesium oxide (CsxOy) surface coating is responsible, at least in part, for the low turn E-field and superior emission characteristics of this type of fiber cathode. This CsxOy layer could be formed during preconditioning heating. CsxOy surface layers cannot only lower the fiber work function by the formation of surface dipoles (if they are thin enough) but may also enhance surface emission through their ability to emit secondary electrons due to a process of grazing electron impact. These multiple electron emission processes may explain the reported 10–100 fold reduction in the turn-on E-field of coated C fibers.

Dominance of broken bonds and nonbonding electrons at the nanoscale

Although they exist ubiquitously in human bodies and our surroundings, the impact of nonbonding lone electrons and lone electron pairs has long been underestimated. Recent progress demonstrates that: (i) in addition to the shorter and stronger bonds between under-coordinated atoms that initiate the size trends of the otherwise constant bulk properties when a substance turns into the nanoscale, the presence of lone electrons near to broken bonds generates fascinating phenomena that bulk materials do not demonstrate; (ii) the lone electron pairs and the lone pair-induced dipoles associated with C, N, O, and F tetrahedral coordination bonding form functional groups in biological, organic, and inorganic specimens. By taking examples of surface vacancy, atomic chain end and terrace edge states, catalytic enhancement, conducting-insulating transitions of metal clusters, defect magnetism, Coulomb repulsion at nanoscale contacts, Cu(3)C(2)H(2) and Cu(3)O(2) surface dipole formation, lone pair neutralized interface stress, etc, this article will focus on the development and applications of theory regarding the energetics and dynamics of nonbonding electrons, aiming to raise the awareness of their revolutionary impact to the society. Discussion will also extend to the prospective impacts of nonbonding electrons on mysteries such as catalytic enhancement and catalysts design, the density anomalies of ice and negative thermal expansion, high critical temperature superconductivity induced by B, C, N, O, and F, the molecular structures and functionalities of CF(4) in anti-coagulation of synthetic blood, NO signaling, and enzyme telomeres, etc. Meanwhile, an emphasis is placed on the necessity and effectiveness of understanding the properties of substances from the perspective of bond and nonbond formation, dissociation, relaxation and vibration, and the associated energetics and dynamics of charge repopulation, polarization, densification, and localization. Finding and grasping the factors controlling the nonbonding states and making them of use in functional materials design and identifying their limitations will form, in the near future, a subject area of "nonbonding electronics and energetics", which could be even more challenging, fascinating, promising, and rewarding than dealing with core or valence electrons alone.

Evidence for surface dipole modifications in In2O3-based transparent conductors

Surface dipole modifications were identified for contamination-free In2O3-based transparent conducting oxides by ultraviolet photoelectron spectroscopy on both thin film and bulk ceramic specimens. In particular, heating in air was found to result in an increase in ionization potential and work function. The formation of surface dipoles may be related to the unique structure (crystal, defect) of bixbyite-based materials. These findings have important ramifications for the tuning of work functions in In2O3-based transparent conductors.

Cesium ion sputtering with oxygen flooding: Experimental SIMS study of work function change

We investigated the work function (WF) change of a silicon surface being under cesium ion bombardment and simultaneous oxygen flooding with various oxygen pressures at the sample surface. It was found that WF of Cs + ion sputtered Si decreases under oxygen flooding. This decrease provides an essential grow of secondary ion yields of some negative ions, sputtered from Si. At the same time Si − ion yield decreases approximately in two times. In the paper we have discussed possible explanations of our experimental data: we considered a surface composition change, formation of surface dipoles and work function change caused by oxygen adsorption, and their relationships between each other.

Importance of Van Der Waals Interaction for Organic Molecule-Metal Junctions: Adsorption of Thiophene on Cu(110) as a Prototype

We report ab initio calculations for the interface energetics of a weakly adsorbed organic molecule on a metal surface, which serves as a model interface relevant for organic electronics. The studied thiophene ring is found to be physisorbed on the Cu(110) surface with an adsorption energy of -0.50 eV. Nonlocal correlations, i.e., van der Waals interactions, are solely responsible for the binding in this weakly interacting system, and the choice of the proper exchange-correlation function is crucially important. The adsorption of thiophene lowers the metal work function due to the formation of surface dipoles while no sizable charge transfer is found.