Gengwu Ji

High-Performance Perovskite Solar Cells Engineered by an Ammonia Modified Graphene Oxide Interfacial Layer

UNLABELLED The introduction of an ammonia modified graphene oxide (GO:NH3) layer into perovskite-based solar cells (PSCs) with a structure of indium-tin oxide (ITO)/poly(3,4-ethylene-dioxythiophene):poly(4-styrenesulfonate) ( PEDOT PSS)-GO: NH3/CH3NH3PbI3-xClx/phenyl C61-butyric acid methyl ester (PCBM)/(solution Bphen) sBphen/Ag improves their performance and perovskite structure stability significantly. The fabricated devices with a champion PCE up to 16.11% are superior in all the performances in comparison with all the reference devices without the GO:NH3 layer. To understand the improved device performances, synchrotron-based grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and UV-visible absorption measurements have been conducted on perovskite films on different substrates. It was found that these improvements should be partially attributed to the improved crystallization and preferred orientation order of peovskite structure, partially to the improved morphology with nearly complete coverage, partially to the enhanced optical absorption caused by the PEDOT PSS-GO:NH3 layer, and partially to the better matched energy-level-alignment at the perovskite interface. Furthermore, the device was shown to be more stable in the ambient condition, which is clearly associated with the improved peovskite structure stability by the GO:NH3 layer observed by the GIXRD measurements. All these achievements will promote more applications of chemically modified graphene oxide interfacial layer in the PSCs as well as other organic multilayer devices.

Visible-light-driven dye degradation using a floriated ZnIn2S4/AgIn5S8 heteromicrosphere catalyst

We report the synthesis of ZnIn2S4/AgIn5S8 (ZIS/AIS) heteromicrospheres with tailorable composition and controllable band gap via partial cation exchange for the first time. Detailed studies of the as-prepared samples were carried out using SEM, TEM, STEM-EDS, XRD and layer-by-layer depth XPS analysis. Based on these results, possible cation exchange was proposed, and a critical concentration of Ag+ in the crystals was identified, below which cation exchange cannot occur. Besides, an enhanced photodegradation mechanism was discussed by analyzing the UV-vis-NIR reflectance spectra and the band structure, illustrating that an appropriate thickness of the AIS layer is beneficial to the generation of active species thus favoring the degradation reaction. Further application of dye degradation demonstrated a remarkable photodegradation rate up to 99.2%. Our results shed light on how the cation exchange occurs between ternary and binary crystals, which may open a new avenue to the field for synthesizing materials at the micro-scale.

On-surface manipulation of atom substitution between cobalt phthalocyanine and the Cu(111) substrate

On-surface fabrication of controllable nanostructures is an appealing topic in the field of molecular electronics. Herein, the adsorption of cobalt phthalocyanine (CoPc) on a Cu(111) surface is investigated utilizing a combination of photoelectron spectroscopy (PES) and density functional theory (DFT). Interestingly, the scenario of atom exchange is discovered at the interface at room temperature (RT), namely the substitution of the cobalt atom in CoPc by a surface Cu adatom. Moreover, thermal annealing enhances the substitution process considerably which is demonstrated to be complete at about 573 K. As revealed by DFT calculations, the driving force for the observed interface transmetalation is most probably provided by the initial strong molecular-substrate interaction between Co atoms and the Cu(111) surface, the external thermodynamic energy gained from thermal sublimation and thermal annealing, and the tendency to form Co–Cu alloy at the interface. While CoPc has been successfully utilized in electrocatalysts for fuel cell applications and CuPc is commonly used as a leading material in organic solar cells, this report of interface transmetalation from CoPc to CuPc in a solid state environment may offer an encouraging approach towards the artificial engineering of organometallic nanostructures and related properties for surface catalysts, molecular electronics and so on.

Graphene oxide as an additive to improve perovskite film crystallization and morphology for high-efficiency solar cells

The quality of a perovskite film has a great impact on its light absorption and carrier transport, which is vital to improve high-efficiency perovskite solar cells (PSCs). Herein, it is demonstrated that graphene oxide (GO) can be used as an effective additive in the precursor solution for the preparation of high-quality solution-processed CH3NH3PbI3 (MAIPbI3) films. It is evidenced by scanning electron microscopy that the size of the grains inside these films not only increases but also becomes more uniform after the introduction of an optimized amount of 1 vol% GO. Moreover, 1 vol% GO also enhances the crystallization of perovskite film with intact preferential out-of-plane orientation as proven by 2-dimensional grazing-incidence X-ray diffraction. As a consequence of the improved film quality, enhanced charge extraction efficiency and optical absorption are demonstrated by photoluminescence (PL) spectroscopy and UV-visible absorption spectroscopy, respectively. Using 1 vol% GO, the fabricated champion heterojunction PSC with a structure of ITO/SnO2/perovskite/spiro-OMeTAD/Au shows a significant power conversion efficiency increase to 17.59% with reduced hysteresis from 16.10% for the champion device based on pristine perovskite. The present study thus proposes a simple approach to make use of GO as an effective and cheap addictive for high-performance PSCs with large-scale production capability.

Enhanced Crystalline Phase Purity of CH3NH3PbI3–xClx Film for High-Efficiency Hysteresis-Free Perovskite Solar Cells

Despite rapid successful developments toward promising perovskite solar cells (PSCs) efficiency, they often suffer significant hysteresis effects. Using synchrotron-based grazing incidence X-ray diffraction (GIXRD) with different probing depths by varying the incident angle, we found that the perovskite films consist of dual phases with a parent phase dominant in the interior and a child phase with a smaller (110) interplanar space (d(110)) after rapid thermal annealing (RTA), which is a widely used post treatment to improve the crystallization of solution-processed perovskite films for high-performance planar PSCs. In particular, the child phase composition gradually increases with decreasing depth till it becomes the majority on the surface, which might be one of the key factors related to hysteresis in fabricated PSCs. We further improve the crystalline phase purity of the solution-processed CH3NH3PbI3-xClx perovskite film (referred as g-perovskite) by using a facile gradient thermal annealing (GTA), which shows a uniformly distributed phase structure in pinhole-free morphology with less undercoordinated Pb and I ions determined by synchrotron-based GIXRD, grazing incidence small-angle X-ray scattering, scanning electron microscopy, and X-ray photoelectron spectroscopy. Regardless of device structures (conventional and inverted types), the planar heterojunction PSCs employing CH3NH3PbI3-xClx g-perovskite films exhibit negligible hysteresis with a champion power conversion efficiency of 17.04% for TiO2-based conventional planar PSCs and 14.83% for poly(3,4-ethylenedioxythiophene:poly(styrenesulfonate) (PEDOT:PSS)-based inverted planar PSCs. Our results indicate that the crystalline phase purity in CH3NH3PbI3-xClx perovskite film, especially in the surface region, plays a crucial role in determining the hysteresis effect and device performance.

Fabrication and Characterization of Organic–Inorganic Hybrid Perovskite Devices with External Doping

Owing to its excellent light harvesting, high-charge carrier mobility, and long electronand hole-transport lengths, organic–inorganic lead halide perovskite solar cells have attracted enormous attention recently under the urgent demands of green energy with environmental friendliness. Although various photovoltaic architectures based on alkylammonum lead halides have been fabricated and have achieved impressive power conversion efficiencies (PCEs), there are still several issues that need to be further addressed and solved properly, for example, the requirement of facile fabrication procedure, the chemical stability of perovskite films, and the environmental friendli‐ ness. Herein, we review the recent experimental progress on the external doping of hybrid perovskite devices by organics and metals, which demonstrate the tuning of optical absorption gap and the enhancement of both devices’ stability and perform‐ ance. Doping at varying layers in the perovskite films was discovered to contribute differently to the improvement of the hybrid organic–inorganic electronics. In the end, prospective was also made on the development of hybrid organic–inorganic devices.

Transfer printing of magnetic structures with enhanced performance using a new type of water-soluble sacrificial layer

A transfer printing process is developed for thin film systems using a new type of water-soluble sacrificial layer – uncured polyvinylpyrrolidone (PVP). The bio-compatible PVP has a dual function, acting as both a buffer and a water-soluble sacrificial layer. Compared with the direct deposition of PyCu Giant Magneto-resistance (GMR) sensors onto bio-relevant, thermoplastic and flexible PEEK foils, the sensitivity of sensors transferred onto PEEK from a PVP-buffered Si substrate is raised from 4.1 T−1 to 30.8 T−1 and the GMR ratio is raised from 2% to 12%. This means that comparable values were achieved and sometimes even exceeded on Si substrates. The performance enhancement is proved to be the result of the decoupled arrangement of the substrate material and the tailored mechanical stress state. The transfer process developed is of a general nature and can be applied to other thin film systems and complete electronic components. The transferred sensors are used to count magnetically functionalised containers consisting of Fe-filled carbon nanotubes in a polymeric matrix. A detection rate ≥91% was achieved.

In Situ Observation of Thermal Proton Transport through Graphene Layers

Protons can penetrate through single-layer graphene, but thicker graphene layers (more than 2 layers), which possess more compact electron density, are thought to be unfavorable for penetration by protons at room temperature and elevated temperatures. In this work, we developed an in situ subsecond time-resolved grazing-incidence X-ray diffraction technique, which fully realizes the real-time observation of the thermal proton interaction with the graphene layers at high temperature. By following the evolution of interlayer structure during the protonation process, we demonstrated that thermal protons can transport through multilayer graphene (more than 8 layers) on nickel foil at 900 °C. In comparison, under the same conditions, the multilayer graphenes are impermeable to argon, nitrogen, helium, and their derived ions. Complementary in situ transport measurements simultaneously verify the penetration phenomenon at high temperature. Moreover, the direct transport of protons through graphene is regarded as the dominant contribution to the penetration phenomenon. The thermal activation, weak interlayer interaction between layers, and the affinity of the nickel catalyst may all contribute to the proton transport. We believe that this method could become one of the established approaches for the characterization of the ions intercalated with 2D materials in situ and in real-time.