Highly Selective On-Surface [2 + 2] Cycloaddition Induced by Hierarchical Metal-Organic Hybrids.

On-surface synthesis of phenylenes is a promising strategy to form extended π-conjugated frameworks but normally lacks selectivity in achieving uniform products. Herein we demonstrate that the debromination reaction of 2,3-dibromophenazine (DBPZ) on Au(111) and Ag(111) surfaces can vary significantly considering the involvement of metal-organic hybrids (MOHs). On Au(111), [2 + 2] and [2 + 2 + 2] cycloadditions facilitate instantaneously upon the debromination occurring, while on Ag(111), several MOHs have been observed under sequential thermal annealing, leading to finally the uniform [2 + 2] cycloaddition product exclusively. By means of scanning tunneling microscopy (STM) and bond-resolved atomic force microscopy (BR-AFM), we have unambiguously depicted the chemical structure of related reaction intermediates and unraveled the undocumented role of hierarchical evolution of MOHs in steering the chemical selectivity.

Influence of Molecular Configurations on the Desulfonylation Reactions on Metal Surfaces.

On-surface synthesis is a powerful methodology for the fabrication of low-dimensional functional materials. The precursor molecules usually anchor on different metal surfaces via similar configurations. The activation energies are therefore solely determined by the chemical activity of the respective metal surfaces. Here, we studied the influence of the detailed adsorption configuration on the activation energy on different metal surfaces. We systematically studied the desulfonylation homocoupling for a molecular precursor on Au(111) and Ag(111) and found that the activation energy is lower on inert Au(111) than on Ag(111). Combining scanning tunneling microscopy observations, synchrotron radiation photoemission spectroscopy measurements, and density functional theory calculations, we elucidate that the phenomenon arises from different molecule-substrate interactions. The molecular precursors anchor on Au(111) via Au-S interactions, which lead to weakening of the phenyl-S bonds. On the other hand, the molecular precursors anchor on Ag(111) via Ag-O interactions, resulting in the lifting of the S atoms. As a consequence, the activation barrier of the desulfonylation reactions is higher on Ag(111), although silver is generally more chemically active than gold. Our study not only reports a new type of on-surface chemical reaction but also clarifies the influence of detailed adsorption configurations on specific on-surface chemical reactions.

Efficient Perovskite Solar Cells Based on Tin Oxide Nanocrystals with Difunctional Modification.

Tin oxide (SnO2 ) nanocrystals-based electron transport layer (ETL) has been widely used in perovskite solar cells due to its high charge mobility and suitable energy band alignment with perovskite, but the high surface trap density of SnO2 nanocrystals harms the electron transfer and collection within device. Here, an effective method to achieve a low trap density and high electron mobility ETL based on SnO2 nanocrystals by devising a difunctional additive of potassium trifluoroacetate (KTFA) is proposed. KTFA is added to the SnO2 nanocrystals solution, in which trifluoroacetate ions could effectively passivate the oxygen vacancies (OV ) in SnO2 nanocrystals through binding of TFA- and Sn4+ , thus reducing the traps of SnO2 nanocrystals to boost the electrons collection in the solar cell. Furthermore, the conduction band of SnO2 nanocrystals is shifted up by surface modification to close to that of perovskite, which facilitates electrons transfer because of the decreased energy barrier between ETL and perovskite layer. Benefiting from the decreased trap density and energy barrier, the perovskite solar cells exhibit a power conversion efficiency of 21.73% with negligible hysteresis.

Multiple-Function Surface Engineering of SnO2 Nanoparticles to Achieve Efficient Perovskite Solar Cells.

The mismatched energy-level alignment and interface defects of the SnO2 nanoparticles' electron transport layer (ETL) and perovskite layer worsen the efficiency of the perovskite solar cell. Herein, we devise a multiple-function surface engineering of SnO2 nanoparticles. TBA+ ions improve the dispersion and stability of colloidal T-SnO2 nanoparticles and act as a bridge between the ETL and perovskite layer through the electrostatic interaction with anions, thus suppressing the charge recombination and reducing the energy loss. I- ions passivate oxygen vacancies of SnO2 nanoparticles but also halide vacancies of the perovskite layer. Furthermore, the conduction band edge of T-SnO2 is enhanced to match the energy alignment with the perovskite, which reduces the energy offset for electron transfer. As a result, the champion solar cell based on T-SnO2 presented a power conversion efficiency of 21.71% with a VOC of 1.15 V and negligible hysteresis, which are much higher than those of the reference device.

Constructing and Transferring Two-Dimensional Tessellation Kagome Lattices via Chemical Reactions on Cu(111) Surface.

Two-dimensional (2D) tessellation of organic species acquired increased interests recently because of their potential applications in physics, biology, and chemistry. 2D tessellations have been successfully constructed on surfaces via various intermolecular interactions. However, the transformation between 2D tessellation lattices has been rarely reported. Herein, we successfully fabricated two types of Kagome lattices on Cu(111). The former phase exhibits (3,6,3,6) Kagome lattices, which are stabilized via the intermolecular hydrogen bond interactions. The latter phase is formed through direct chemical transferring from the former one maintaining almost the same Kagome lattices, except for that the unit cell rotates for 4°. Detailed scanning tunneling microscopy and density functional calculation studies reveal that the chemical transformation is achieved by the formation of the N-Cu-N metal-organic bonds via dehydrogenation reactions of the amines.

Gradient-Band Alignment Homojunction Perovskite Quantum Dot Solar Cells.

The inorganic perovskite CsPbI3 that exists in the form of a quantum dot (QD) shows a stable cubic structure, attracting much attention for its application in solar cells. However, too many grain boundaries in the perovskite QD (PQD) layer block the transport of carriers, resulting in the potential loss of solar cells. Herein, we devise a gradient-band alignment (GBA) homojunction, which is constructed from three layers of PQDs with different band-gaps to form a gradient energy alignment. The GBA structure facilitated the charge extraction and increased the carrier diffusion length of the PQD layer because of the additional driving force for the electrons. In addition, the homojunction made from the same substance could minimize the lattice mismatch of the active layer. As a result, the champion solar cell based on the GBA homojunction layer achieved a high open voltage VOC of 1.25 V and a power conversion efficiency (PCE) of 13.2%.