Miao Zhou

Performance improvement of perovskite solar cells by employing a CdSe quantum dot/PCBM composite as an electron transport layer

Organic–inorganic hybrid perovskites have recently attracted considerable interest for application in solar cells due to their low cost, high absorption coefficient and high power conversion efficiency (PCE). Herein, we utilize a CdSe quantum dot/PCBM composite as an electron transport layer (ETL) to investigate the structure, stability and PCE of CH3NH3PbI3−xClx perovskite solar cells. It is found that adsorption of the CdSe/PCBM composite reduces the roughness of the perovskite, leading to a high-quality film with a compact morphology. Density functional theory (DFT) based first-principles calculations show that CdSe enhances the chemical stability of CH3NH3PbI3−xClx involving strong atomic orbital hybridization. Interestingly, an inorganic-terminated perovskite surface has much stronger interaction with CdSe compared to the surface with organic CH3NH3 termination, with noticeable interfacial charge redistribution. Experiments on solar cells incorporating the CdSe/PCBM composite as the ETL show enhanced photocurrent and fill factor, which is related to the in-built electric field between CH3NH3PbI3−xClx and CdSe that greatly facilitates the separation of electron and hole pairs. We show an improved PCE of 13.7% with enhanced device stability in a highly humid atmosphere. These joint theoretical–experimental results may provide a new aspect for improving the structural stability and operating performance of optoelectronic devices based on perovskite structures.

Flexible All-Inorganic Perovskite CsPbBr3 Nonvolatile Memory Device

All-inorganic perovskite CsPbX3 (X = Cl, Br, or I) is widely used in a variety of photoelectric devices such as solar cells, light-emitting diodes, lasers, and photodetectors. However, studies to understand the flexible CsPbX3 electrical application are relatively scarce, mainly due to the limitations of the low-temperature fabricating process. In this study, all-inorganic perovskite CsPbBr3 films were successfully fabricated at 75 °C through a two-step method. The highly crystallized films were first employed as a resistive switching layer in the Al/CsPbBr3/PEDOT:PSS/ITO/PET structure for flexible nonvolatile memory application. The resistive switching operations and endurance performance demonstrated the as-prepared flexible resistive random access memory devices possess reproducible and reliable memory characteristics. Electrical reliability and mechanical stability of the nonvolatile device were further tested by the robust current-voltage curves under different bending angles and consecutive flexing cycles. Moreover, a model of the formation and rupture of filaments through the CsPbBr3 layer was proposed to explain the resistive switching effect. It is believed that this study will offer a new setting to understand and design all-inorganic perovskite materials for future stable flexible electronic devices.

Enhanced Stability and Tunable Photoluminescence in Perovskite CsPbX3/ZnS Quantum Dot Heterostructure

All-inorganic perovskite CsPbX3 (X = Cl, Br, I) and related materials are promising candidates for potential solar cells, light emitting diodes, and photodetectors. Here, a novel architecture made of CsPbX3 /ZnS quantum dot heterodimers synthesized via a facile solution-phase process is reported. Microscopic measurements show that CsPbX3 /ZnS heterodimer has high crystalline quality with enhanced chemical stability, as also evidenced by systematic density functional theory based first-principles calculations. Remarkably, depending on the interface structure, ZnS induces either n-type or p-type doping in CsPbX3 and both type-I and type-II heterojunctions can be achieved, leading to rich electronic properties. Photoluminescence measurement results show a strong blue-shift and decrease of recombination lifetime with increasing sulfurization, which is beneficial for charge diffusion in solar cells and photovoltaic applications. These findings are expected to shed light on further understanding and design of novel perovskite heterostructures for stable, tunable optoelectronic devices.

Halogen-Adatom Mediated Phase Transition of Two-Dimensional Molecular Self-Assembly on a Metal Surface

Construction of tunable and robust two-dimensional (2D) molecular arrays with desirable lattices and functionalities over a macroscopic scale relies on spontaneous and reversible noncovalent interactions between suitable molecules as building blocks. Halogen bonding, with active tunability of direction, strength, and length, is ideal for tailoring supramolecular structures. Herein, by combining low-temperature scanning tunneling microscopy and systematic first-principles calculations, we demonstrate novel halogen bonding involving single halogen atoms and phase engineering in 2D molecular self-assembly. On the Au(111) surface, we observed catalyzed dehalogenation of hexabromobenzene (HBB) molecules, during which negatively charged bromine adatoms (Brδ-) were generated and participated in assembly via unique C-Brδ+···Brδ- interaction, drastically different from HBB assembly on a chemically inert graphene substrate. We successfully mapped out different phases of the assembled superstructure, including densely packed hexagonal, tetragonal, dimer chain, and expanded hexagonal lattices at room temperature, 60 °C, 90 °C, and 110 °C, respectively, and the critical role of Brδ- in regulating lattice characteristics was highlighted. Our results show promise for manipulating the interplay between noncovalent interactions and catalytic reactions for future development of molecular nanoelectronics and 2D crystal engineering.

Strain-engineered two-dimensional MoS2 as anode material for performance enhancement of Li/Na-ion batteries

Recent years have witnessed a surge of research in two-dimensional (2D) nanostructures for development of new rechargeable Li/Na-ion battery systems. Herein, via first-principles calculations we demonstrate strain-engineered Li/Na adsorption and storage in 2D MoS2 as anode material, aiming to enhance the operating performance of Li/Na-ion batteries. Our results show that tensile strain greatly increases the adsorption of Li/Na atoms on MoS2, and a modest strain of 6% increases Li (Na) adsorption energy by over 70%, which originates from the strain-induced upshift of Mo d states towards Fermi level that interact strongly with Li/Na s states, in analogy with the d-band model in metal catalyst. Significant narrowing of the n-doped semiconducting gap of MoS2 suggests the improved electric conductivity that may benefit charge carrier transport. By mapping out the potential energy surfaces, we show shallow energy barriers of ion diffusion with ~0.2 eV for Li and 0.1 eV for Na. Furthermore, the strain-steered competition between chemical bonding and coulomb repulsion results in high Li/Na storage capability and relatively low average operating voltage. We believe that the fundamental principle underlying the use of strain to enhance performance of renewable ion battery is applicable to other stretchable low-dimensional nanomaterials.

Metastable phase control of two-dimensional transition metal dichalcogenides on metal substrate

Phase control of two-dimensional (2D) transition metal dichalcogenides (TMDs) is important from both scientific and engineering aspects. However, up to now it remains a challenge to stabilize the metastable phase of TMDs under ambient conditions. Herein, via systematic first-principles calculations, we demonstrate that by the appropriate choice of metal substrate as a support for the transferred layer, the metastable phase of MoS2 can be effectively stabilized. By screening 15 widely used metal substrates, we found that Mo(001), W(001) and Hf(0001) surfaces not only stabilize the metastable 1T′ phase against the common 2H phase, but also prevent the structural transformation of 1T′ → 2H by increasing the transition barrier. Remarkably, we reveal the crucial role of charge transfer from the metal surface to Mo d-orbitals of MoS2 that influences the electron occupation of atomic orbitals associated with crystal splitting, which provides an excellent descriptor to determine the stability of the metastable phase. We also propose a novel field-effect transistor made from a single MoS2 layer with a semiconducting 2H phase region connected to two metallic 1T′ phase regions in contact with a metal electrode (Mo, W, Hf), which exhibits an ideal Schottky-barrier-free interface. These findings are generally applicable, offering an attractive and practical approach to engineer the phase transition of 2D TMDs and design novel nanodevices with multi-functionalities.

Two-dimensional Au-1,3,5 triethynylbenzene organometallic lattice: Structure, half-metallicity, and gas sensing

On the basis of first-principles calculations, we investigated the structural and electronic properties of the two-dimensional (2D) Au-1,3,5 triethynylbenzene (Au-TEB) framework, which has been recently synthesized by homocoupling reactions in experiments. Featured by the C-Au-C linkage, the 2D Au-TEB network has a kagome lattice by Au atoms and a hexagonal lattice by organic molecules within the same metal-organic framework (MOF), which exhibits intrinsic half-metallicity with one spin channel metallic and the other spin channel fully insulating with a large energy gap of 2.8 eV. Two branches of kagome bands are located near the Fermi level, with each branch including one flat band and two Dirac bands, which originates from the out-of-plane dxz and dyz orbitals of Au and may lead to many exotic topological quantum phases. We further studied the adsorption of F atoms, Cl atoms, and small gas molecules including O2, CO, NO2, and NH3 on the Au-TEB network, aiming to exploit its potential applications in gas sensors. Detailed analyses on adsorption geometry, energy, molecular orbital interaction, and electronic structure modification suggest the great potential of Au-TEP as a promising alternative for gas sensing. We expect these results to expand the universe of low-dimensional half-metallic MOF structures and shed new light on their practical applications in nanoelectronics/spintronics.