Evidence of Tailoring the Interfacial Chemical Composition in Normal Structure Hybrid Organohalide Perovskites by a Self-Assembled Monolayer

Molecularly Engineered Interfaces in Metal Halide Perovskite Solar Cells

Perovskite solar cells (PSCs) have emerged as a promising candidate for next-generation thin-film photovoltaic technology owing to their excellent optoelectronic properties and cost-effectiveness. To gain the full potential of device performance, an in-depth understanding of the surface/interface science is an urgent need. Here, we present a review of molecularly engineered studies on interface modifications of PSCs. We elaborate a systematic classification of the existing optimization techniques employed in molecularly engineered perovskite and interface materials and analyze the insights underlying the reliability issues and functional behaviors. The achievements allow us to highlight the crucial strengths of molecular design for further tailoring of the interfacial properties, mitigating the nonradiative losses, optimizing the device performance, and retarding the degradation process of PSCs. Finally, the remaining challenges and potential development directions of molecularly engineered interfaces for high-performance and stable PSCs are also proposed.

New High Donor Electrolyte for Lithium-Sulfur Batteries

The unparalleled theoretical specific energy of lithium-sulfur (Li-S) batteries has attracted considerable research interest from within the battery community. However, most of the long cycling results attained thus far relies on using a large amount of electrolyte in the cell, which adversely affects the specific energy of Li-S batteries. This shortcoming originates from the low solubility of polysulfides in the electrolyte. Here, 1,3-dimethyl-2-imidazolidinone (DMI) is reported as a new high donor electrolyte for Li-S batteries. The high solubility of polysulfides in DMI and its activation of a new reaction route, which engages the sulfur radical (S₃ ^•- ), enables the efficient utilization of sulfur as reflected in the specific capacity of 1595 mAh g^-1 under lean electrolyte conditions of 5 μL_electrolyte mg_sulfur ^-1 . Moreover, the addition of LiNO₃ stabilizes the lithium metal interface, thereby elevating the cycling performance to one of the highest known for high donor electrolytes in Li-S cells. These engineered high donor electrolytes are expected to advance Li-S batteries to cover a wide range of practical applications, particularly by incorporating established strategies to realize the reversibility of lithium metal electrodes.

The Potential of a Nanostructured Titanium Oxide Layer with Self-Assembled Monolayers for Biomedical Applications: Surface Properties and Biomechanical Behaviors

This study investigated the surface properties and biomechanical behaviors of a nanostructured titanium oxide (TiO) layer with different self-assembled monolayers (SAMs) of phosphonate on the surface of microscope slides. The surface properties of SAMs were analyzed using scanning electron microscopy, X-ray photoemission spectroscopy, and contact angle goniometry. Biomechanical behaviors were evaluated using nanoindentation with a diamond Berkovich indenter. Analytical results indicated that the homogenous nanostructured TiO surface was formed on the substrate surface after the plasma oxidation treatment. As the TiO surface was immersed with 11-phosphonoundecanoic acid solution (PUA-SAM/TiO), the formation of a uniform SAM can be observed on the sample surface. Moreover, the binding energy of O 1s demonstrated the presence of the bisphosphonate monolayer on the SAMs-coated samples. It was also found that the PUA-SAM/TiO sample not only possessed a higher wettability performance, but also exhibited low surface contact stiffness. A SAM surface with a high wettability and low contact stiffness could potentially promote biocompatibility and prevent the formation of a stress shielding effect. Therefore, the self-assembled technology is a promising approach that can be applied to the surface modification of biomedical implants for facilitating bone healing and osseointegration.

Interfacial Engineering through Chloride-Functionalized Self-Assembled Monolayers for High-Performance Perovskite Solar Cells

The family of organic-inorganic hybrid perovskite (OIHPs) materials is one of the most promising for very high-efficiency photovoltaic solar cell application. In the present work, the effect of a series of self-assembled monolayers placed at the TiO₂-perovskite junction, on the functioning of triple cation perovskite solar cells has been investigated. We show that employing 4-chlorobenzoic acid leads to the marked boosting of the solar cell performances. The starting pristine cell had a power conversion efficiency (PCE) of 20.3% and the chemical engineering permitted to reach a PCE up to 21.35%. Our experimental study completed by density functional theory calculations and modeling show that this progress is due to the reduction of interfacial states, to the improvement of the quality of the OIHP material and to the structural continuity between TiO₂ and the OIHP. Especially, we demonstrate that the interfacial chemical interactions are important to consider in the design of highly efficient devices.

Impact of Titanium Dioxide Surface Defects on the Interfacial Composition and Energetics of Evaporated Perovskite Active Layers

This investigation elucidates critical Brønsted and Lewis acid-base interactions at the titanium dioxide (TiO₂) surface that control the interfacial composition and, thus, the energetics of vacuum-processed methylammonium lead iodide (MAPbI₃) perovskite active layers (PALs). In situ photoelectron spectroscopy analysis shows that interfacial growth, chemical composition, and energetics of co-deposited methylammonium iodide (MAI)/PbI₂ thin films are significantly different on bare and (3-aminopropyl)triethoxysilane (APTES)-functionalized TiO₂ surfaces. Mass spectroscopy analysis indicates that MAI dissociates into hydrogen iodide and methylamine in the gas phase and suggests that MAPbI₃ nucleation is preceded by adsorption and coupling of these volatile MAI precursors. Prior to MAPbI₃ nucleation on the bare TiO₂ surface, we suggest that high coverages of methylamine adsorbed to surface defect sites (e.g., undercoordinated Ti atoms and hydroxyls) promote island-like growth of large, PbI₂-rich nuclei that inhibit nucleation and lead to a thick substoichiometric interface layer that impedes charge transport and collection energetics. APTES functional groups passivate TiO₂ surface defects and facilitate more conformal growth of small, PbI₂-rich nuclei that enhance MAPbI₃ nucleation and significantly improve interfacial energetics for charge transport and extraction. This work highlights the considerable influence of TiO₂ surface chemistry on PAL composition and energetics, which are critical factors that impact the performance and long stability of these materials in emerging photovoltaic device technologies.

Pyridine-Terminated Conjugated Organic Molecules as an Interfacial Hole Transfer Bridge for NiOx-Based Perovskite Solar Cells

To engineer the NiO_x/perovskite interface and promote interfacial hole transfer, two pyridine-terminated conjugated small organic molecules (PTZ-1 and PTZ-2) are synthesized to link the NiO_x and perovskite layers for NiO_x-based perovskite solar cells (PSCs). One terminal pyridine group interacts with the NiO_x layer, while the other one coordinates with the Pb atoms of the perovskite layer, erecting an interfacial hole transfer bridge between NiO_x and perovskite. Surface modification of the NiO_x film with the PTZ molecules is able to enhance hole extraction, increase hole mobility and conductivity of NiO_x, reduce defect density, and retard interfacial charge recombination. As a consequence, power conversion efficiency is improved from 12.53 to 16.25 and 17.00% upon surface modifications of NiO_x with PTZ-1 and PTZ-2, respectively. Furthermore, the modified PSCs exhibit almost no hysteresis and show good stability after storage in air (relative humidity of 30-40%) for 500 h without encapsulation.

Halide Perovskites: Is It All about the Interfaces?

Design and modification of interfaces, always a critical issue for semiconductor devices, has become a primary tool to harness the full potential of halide perovskite (HaP)-based optoelectronics, including photovoltaics and light-emitting diodes. In particular, the outstanding improvements in HaP solar cell performance and stability can be primarily ascribed to a careful choice of the interfacial layout in the layer stack. In this review, we describe the unique challenges and opportunities of these approaches (section 1). For this purpose, we first elucidate the basic physical and chemical properties of the exposed HaP thin film and crystal surfaces, including topics such as surface termination, surface reactivity, and electronic structure (section 2). This is followed by discussing experimental results on the energetic alignment processes at the interfaces between the HaP and transport and buffer layers. This section includes understandings reached as well as commonly proposed and applied models, especially the often-questionable validity of vacuum level alignment, the importance of interface dipoles, and band bending as the result of interface formation (section 3). We follow this by elaborating on the impact of the interface formation on device performance, considering effects such as chemical reactions and surface passivation on interface energetics and stability. On the basis of these concepts, we propose a roadmap for the next steps in interfacial design for HaP semiconductors (section 4), emphasizing the importance of achieving control over the interface energetics and chemistry (i.e., reactivity) to allow predictive power for tailored interface optimization.

The Interplay of Contact Layers: How the Electron Transport Layer Influences Interfacial Recombination and Hole Extraction in Perovskite Solar Cells

Charge-selective contact layers in perovskite solar cells influence the current density-voltage hysteresis, an effect related to ion migration in the perovskite. As such, fullerene-based electron transport layers (ETLs) suppress hysteresis by reducing the mobile ion concentration. However, the impact of different ETLs on the electronic properties of other constituent device layers remains unclear. In this Kelvin probe force microscopy study, we compared potential distributions of methylammonium lead iodide-based solar cells with two ETLs (planar TiO₂ and C60-functionalized self-assembled monolayer) with different hysteretic behavior. We found significant changes in the potential distribution of the organic hole transport layer spiroMeOTAD, suggesting the formation of a neutral spiroMeOTAD/iodide interface due to a reaction between iodide with p-doped spiroMeOTAD in the TiO₂ cell. Our results show that the ETL affects not only the mobile ion concentration and the recombination at the perovskite/ETL interface but also the resistance and capacitance of the spiroMeOTAD.

Flexible sample cell for real-time GISAXS, GIWAXS and XRR: design and construction

Since the properties of functional materials are highly dependent on their specific structure, and since the structural changes, for example during crystallization, induced by coating and annealing processes are significant, the study of structure and its formation is of interest for fundamental and applied science. However, structure analysis is often limited to ex situ determination of final states due to the lack of specialized sample cells that enable real-time investigations. The lack of such cells is mainly due to their fairly complex design and geometrical restrictions defined by the beamline setups. To overcome this obstacle, an advanced sample cell has been designed and constructed; it combines automated doctor blading, solvent vapor annealing and sample hydration with real-time grazing-incidence wide- and small-angle scattering (GIWAXS/GISAXS) and X-ray reflectivity (XRR). The sample cell has limited spatial requirements and is therefore widely usable at beamlines and laboratory-scale instruments. The cell is fully automatized and remains portable, including the necessary electronics. In addition, the cell can be used by interested scientists in cooperation with the Institute for Crystallography and Structural Physics and is expandable with regard to optical secondary probes. Exemplary research studies are presented, in the form of coating of P3HT:PC₆₁PM thin films, solvent vapor annealing of DRCN5T:PC₇₁BM thin films, and hydration of supported phospholipid multilayers, to demonstrate the capabilities of the in situ cell.

Multiscale Functional Imaging of Interfaces through Atomic Force Microscopy Using Harmonic Mixing

The spatial resolution of atomic force microscopy (AFM) needed to resolve material interfaces is limited by the tip-sample separation ( d) dependence of the force used to record an image. Here, we present a new multiscale functional imaging technique that allows for in situ tunable spatial resolution, which can be applied to a wide range of inhomogeneous materials, devices, and interfaces. Our approach uses a multifrequency method to generate a signal whose d-dependence is controlled by mixing harmonics of the cantilever's oscillation with a modulated force. The spatial resolution of the resulting image is determined by the signal's d-dependence. Our measurements using harmonic mixing (HM) show that we can change the d-dependence of a force signal to improve spatial resolution by up to a factor of two compared to conventional methods. We demonstrate the technique with both Kelvin probe force microscopy (KPFM) and bimodal AFM to show its generality. Bimodal AFM with harmonic mixing actuation separates conservative from dissipative forces and is used to identify the regions of adhesive residue on exfoliated graphene. Our electrostatic measurements with open-loop KPFM demonstrate that multiple force modulations may be applied at once. Further, this method can be applied to any tip-sample force that can be modulated, for example, electrostatic, magnetic, and photoinduced forces, showing its universality. Because HM enables in situ switching between high sensitivity and high spatial resolution with any periodic driving force, we foresee this technique as a critical advancement for multiscale functional imaging.