Chen Weijie

Developing new functional applications of metallic glasses in catalysis is an active and pivotal topic for materials science as well as novel environmental catalysis processes. Compared to the crystalline materials with highly ordered atomic packing, metallic glass has a simply disordered atomic structure. Recent reports have demonstrated that the metallic glasses are indeed having many superiorly catalytic properties, yet the understanding of the mechanism is insufficient. In this work, the structural relaxation (α-relaxation) by annealing in an amorphous Fe₇₈Si₉B₁₃ alloy is studied for unraveling the catalytic mechanism at the atomic scale. The volume fractions of the crystalline structures, such as α-Fe, Fe₂Si, and Fe₂B, in the as-received and annealed metallic glasses are fully characterized. It is found that the randomly atomic packing structure with weak atomic bonding in the as-received metallic glass has an efficient electron transfer capability, presenting advanced superiorities in the aspects of production rate of hydroxyl radicals (•OH), dye degradation rate (k), and essential degradation ability (K_SA) for water treatment. The discovery of this critically important work unveils why using metallic glasses as catalysts has higher reactivity than the crystalline materials, and more importantly, it provides new research opportunities into the study of synthetic catalysts.

Compared to highly ordered atomic packing structure of their crystalline counterparts, the excellent catalytic performance of metallic glasses originates from their randomly atomic packing structure, presenting a better electron transfer capability, advanced superiorities in the aspects of production rate of hydroxyl radicals (•OH), dye degradation rate (k), and essential degradation ability (K_SA) for water treatment.

Stable oxide topological insulators (TIs) have been sought for years, but none have been found; whereas heavier (selenides, tellurides) chalcogenides can be TIs. The basic contradiction between topological insulation and thermodynamic stability is pointed out, offering a narrow window of opportunity. The electronic motif is first identified and can achieve topological band inversion in ABO₃ as a lone-pair, electron-rich B atom (e.g., Te, I, Bi) at the octahedral site. Then, twelve ABO₃ compounds are designed in the assumed cubic perovskite structure, which satisfy this electronic motif and are indeed found by density function theory calculations to be TIs. Next, it is illustrated that poorly screened ionic oxides with large inversion energies undergo energy-lowering atomic distortions that destabilize the cubic TI phase and remove band inversion. The coexistence windows of topological band inversion and structure stability can nevertheless be expanded under moderate pressures (15 and 35 GPa, respectively, for BaTeO₃ and RbIO₃). This study traces the principles needed to design stable oxide topological insulators at ambient pressures as a) a search for oxides with small inversion energies; b) design of large inversion-energy oxide TIs that can be stabilized by pressure; and c) a search for covalent oxides where TI-removing atomic displacements can be effectively screened out.

ABO₃ perovskites where B is an electron-rich element would be topological insulators if the cubic structure could be stabilized. But in such band-inverted topological insulators, depopulation of bonding valence states and occupation of antibonding conduction states raises the energy, leading, in some cases, to structural distortions that are poorly shielded in oxides. In the distorted structure the band inversion is reversed.

Most of the high-performance all-polymer solar cells (all-PSCs) reported to date are based on polymer donor and polymer acceptor pairs with largely overlapped light absorption properties, which seriously limits the efficiency of all-PSCs. This study reports the development of a series of random copolymer donors possessing complementary light absorption with the naphthalenediimide-based polymer acceptor P(NDI2HD-T2) for highly efficient all-PSCs. By controlling the molar ratio of the electron-rich benzodithiophene (BDTT) and electron-deficient fluorinated-thienothiophene (TT-F) units, a series of polymer donors with BDTT:TT-F ratios of 1:1 (P1), 3:1 (P2), 5:1 (P3), and 7:1 (P4) are prepared. The synthetic control of polymer composition allows for precise tuning of the light absorption properties of these new polymer donors, enabling optimization of light absorption properties to complement those of the P(NDI2HD-T2) acceptor. Copolymer P1 is found to be the optimal polymer donor for the fullerene-based solar cells due to its high light absorption, whereas the highest power conversion efficiency of 6.81% is achieved for the all-PSCs with P3, which has the most complementary light absorption with P(NDI2HD-T2).

A series of poly(benzodithiophene-r-fluorinated-thienothiophene) [P(BDTT-r-TT-F)] random copolymers with tunable light absorption characteristics are developed by controlling the ratios of electron-rich BDTT and electron-deficient TT-F units. All-polymer solar cells (all-PSCs) fabricated from these polymer donors and the P(NDI2HD-T2) acceptor achieve efficiencies of up to 6.8% by optimizing the complementary light absorption of the polymer donor and acceptor.

The photovoltaic performance of core/shell quantum dots sensitized solar cells (QDSCs) can be significantly enhanced by optimizing the shell thickness, as demonstrated by Haiguang Zhao, Zhiming M. Wang, Federico Rosei, and co-workers in article number 1701468. By maintaining the core size of the quantum dot, the approach offers new opportunities to fabricate low cost, highly efficient, and long-term stable QDSCs.

Hybrid organic/inorganic perovskite solar cells are invigorating the photovoltaic community due to their remarkable properties and efficiency. However, many perovskite solar cells show an undesirable current–voltage (I–V) hysteresis in their forward and reverse voltage scans, working to the detriment of device characterization and performance. This hysteresis likely arises from slow ion migration in the bulk perovskite active layer to interfaces which may induce charge trapping. It is shown that interfacial chemistry between the perovskite and charge transport layer plays a critical role in ion transport and I–V hysteresis in perovskite-based devices. Specifically, phenylene vinylene polymers containing cationic, zwitterionic, or anionic pendent groups are utilized to fabricate charge transport layers with specific interfacial ionic functionalities. The interfacial-adsorbing boundary induced by the zwitterionic polymer in contact with the perovskite increases the local ion concentration, which is responsible for the observed I–V hysteresis. Moreover, the ion adsorbing properties of this interface are exploited for perovskite-based memristors. This fundamental study of I–V hysteresis in perovskite-based devices introduces a new mechanism for inducing memristor behavior by interfacial ion adsorption.

Ion migration at perovskite interfaces is investigated by varying the interface with cationic, anionic, and zwitterionic functionalities. The zwitterionic polymer interlayer generates an adsorbing boundary at the interface, increasing the local ion concentration, causing current–voltage (I–V) hysteresis in perovskite-based devices. This fundamental study of perovskite I–V hysteresis introduces a new mechanism for device memristor behavior by interfacial ion adsorption.

Fullerene derivatives, which possess extraordinary geometric shapes and high electron affinity, have attracted significant attention for thin film technologies. This study demonstrates an important photovoltaic application using carboxyl-functionalized carbon buckyballs, C60 pyrrolidine tris-acid (CPTA), to fabricate electron transport layers (ETLs) that replace traditional metal oxide-based ETLs in efficient and stable n-i-p-structured planar perovskite solar cells (PSCs). The uniform CPTA film is covalently anchored onto the surface of indium tin oxide (ITO), significantly suppressing hysteresis and enhancing the flexural strength in the CPTA-modified PSCs. Moreover, solution-processable CPTA-based ETLs also enable the fabrication of lightweight flexible PSCs. The maximum-performing device structures composed of ITO/CPTA/CH₃NH₃PbI₃/2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD)/Au yield power conversion efficiencies of more than 18% on glass substrates and up to 17% on flexible substrates. These results indicate that the CPTA layers provide new opportunities for solution-processed organic ETLs by substantially simplifying the procedure for fabricating PSCs for portable applications.

A low-temperature and solution-processed polar C60 pyrrolidine tris-acid (CPTA) electron transport layer (ETL) with conformal morphology, deposited on an indium tin oxide surface through an esterification step, is used to produce hysteresis-free, bendable, and durable perovskite solar cells. Our results suggest that CPTA is a promising candidate to replace metal oxides and shed light on employing these easily fabricated ETLs in other portable photovoltaic technologies.

The all-solution-processed switchable interconnecting layer (ICL) for both inverted and normal tandem organic solar cells (OSCs) is reported for the first time here. The fundamental challenges in the literature arise from mixing multiple functionalities into a single layer. For a widely used ICL composed of an electron transport layer (ETL)/a hole transport layer (HTL), ETL needs not only to efficiently extract electrons from an underneath photoactive layer, but also to fulfill optical, mechanical, chemical and electrical requirements to function as effective tunneling junction ICL with HTL atop. Taking on multiple functionalities for a single ETL makes ETL in ICL highly coupled and difficult to be replaced. This is also the case for HTL. Here, this study demonstrates an all-solution-processed switchable ICL, ETL/recombination layer (RL)/HTL and HTL/RL/ETL, for both normal and inverted tandem OSCs. In switchable ICL, ETL and HTL simply serve as carrier transport layers as they did in single OSCs. Electrical recombination, mechanical protection and chemical separation functionalities are realized by RL alone. This strategy shifts the views of ICL for tandem OSCs from conventionally complicated ETL/HTL tunneling junction ICL, where both ETL and HTL play several different roles, towards simplified ICL where ETL and HTL play a distinct decoupled role, advancing ICL for more adaptable tandem OSCs.

An all-solution-processed switchable interconnecting layer (ICL) for tandem organic solar cells (OSCs) is demonstrated the first time with hole transporting layer (HTL)/recombination layer (RL)/electron transporting layer (ETL) and its counterpart ETL/RL/HTL for inverted and normal structure configuration respectively. This three-layered switchable ICL controls the complexity of fabricating tandem OSCs to be as simple as single OSCs.

Under the presence of an externally applied electric field, hybrid and all-inorganic perovskite materials reveal both ionic and electronic conductivity. In article number 1602283, Pablo Docampo, Rubén D. Costa, and co-workers investigate this internal ion migration and rearrangement of different ionic species, namely bromide and methyl ammonium cations respectively, within perovskite nanoparticles in thin-film devices and find it to resemble the well-known signatures of the ionic motion in light-emitting electrochemical cells.

Perovskite structured CsPbX₃ (X = Cl, Br or I) quantum dots (QDs) have attracted great attention in the past few years for appealing application potentials in photovoltaic and optoelectronic devices. In this report, the CsPbX₃ QDs are shown to perform as a new probe for metal ions with high sensitivity, high selectivity and instant response by the quenching or enhancing of the photoluminescence (PL). Through experimental and calculation efforts, the probing mechanisms are investigated. A wide probing window for Cu²⁺ and Yb³⁺ ions ranging from 2 × 10⁻⁹ to 2 × 10⁻⁶m is exhibited for CsPbBr₃ QDs. In practice, the CsPbBr₃ QDs are successfully applied for fast probing Cu²⁺ ions in edible oils and vehicle lubricating oils with the precision consistent to the values measured by inductively coupled plasma (ICP). Thus, it provides a promising powerful tool in detecting certain metal ions in biological and industrial organic solution systems.

Perovskite CsPbX₃ (X = Cl, Br, or I) quantum dots are shown to perform as a new probe for certain metal ions with high sensitivity, selectivity, wide probing window, and instant response by quenching or enhancing of the photoluminescence. In practice, it is successfully applied for probing Cu²⁺ ions in various edible oils and lubricating oils.

Organic–inorganic hybrid perovskite solar cells are attracting the attention of researchers owing to the high level of performance they exhibit in photovoltaic device applications. However, the attainment of an even higher level of performance is hindered by their anomalous current–voltage (I–V) hysteresis behavior. Even though experimental and theoretical studies have suggested that the perovskite materials may have a ferroelectric nature, it is still far from being fully understood. In this study, the origin of the hysteresis behavior in CH₃NH₃PbI₃ perovskite thin films is investigated. The behavior of ferroelectricity using piezoresponse force microscopy is first examined. Then, by comparing the scan-rate-dependent nano/macroscopic I–V curves, it is found that ion migration assisted by the grain boundaries is a dominant origin of I–V hysteresis from a macroscopic viewpoint. Consequently, the observations suggest that, even though ferroelectricity exists in the CH₃NH₃PbI₃ perovskite materials, ion migration primarily contributes to the macroscopic I–V hysteresis. The presented results can provide fundamental guidelines to the resolution of hysteresis issues in organic–inorganic hybrid perovskite materials.

This study reports on the origin of the hysteresis in CH₃NH₃PbI₃ perovskite thin films. The presence of the ferroelectricity is demonstrated, and further, ion migration assisted by the grain boundaries is demonstrated as a dominant origin of macroscopic I–V hysteresis. Consequently, the observations suggest that although ferroelectricity exists, ion migration primarily contributes to the macroscopic I–V hysteresis.

Understanding and controlling defect generation and movement in organic-inorganic lead halide perovskite materials will not only lead to more stable devices but also new device concepts. In article number 1700527, Wei D. Lu and co-workers reveal the formation, migration and annihilation of iodine vacancies in CH₃NH₃PbI₃ films under an electric field and/or light illumination. An electric-write and light-erase memory device is demonstrated.