Qingdan Yang

Decomposition of Organometal Halide Perovskite Films on Zinc Oxide Nanoparticles

Solution processed zinc oxide (ZnO) nanoparticles (NPs) with excellent electron transport properties and a low-temperature process is a viable candidate to replace titanium dioxide (TiO2) as electron transport layer to develop high-efficiency perovskite solar cells on flexible substrates. However, the number of reported high-performance perovskite solar cells using ZnO-NPs is still limited. Here we report a detailed investigation on the chemistry and crystal growth of CH3NH3PbI3 perovskite on ZnO-NP thin films. We find that the perovskite films would severely decompose into PbI2 upon thermal annealing on the bare ZnO-NP surface. X-ray photoelectron spectroscopy (XPS) results show that the hydroxide groups on the ZnO-NP surface accelerate the decomposition of the perovskite films. To reduce the decomposition, we introduce a buffer layer in between the ZnO-NPs and perovskite layers. We find that a commonly used buffer layer with small molecule [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) can slow down but cannot completely avoid the decomposition. On the other hand, a polymeric buffer layer using poly(ethylenimine) (PEI) can effectively separate the ZnO-NPs and perovskite, which allows larger crystal formation with thermal annealing. The power conversion efficiencies of perovskite photovoltaic cells are significantly increased from 6.4% to 10.2% by replacing PC61BM with PEI as the buffer layer.

Bimetallic PtPd nanoparticles on Nafion–graphene film as catalyst for ethanol electro-oxidation

A simple electrochemical method is employed to synthesize nanoparticles of bimetallic PtPd alloy on Nafion–graphene film. The alloy nanoparticles exhibit efficient electrocatalytic activity and stability toward ethanol oxidation in alkaline media. With the synergetic effects of the PtPd nanoparticles and the enhanced electron transfer stem from graphene, this anode catalyst has higher current density than the monometallic Pt or Pd catalyst. Moreover, this anode catalyst has a low onset potential at −0.73 V (vs. Ag/AgCl) and a high ratio of forward peak current density to backward peak current density (If/Ib) of 5.45. The results indicate that the catalyst has good tolerance against poisoning by intermediates generated during ethanol electro-oxidation and is effective in cleaving the C–C bond to achieve oxidation of ethanol to CO2.

Arrays of CdSe sensitized ZnO/ZnSe nanocables for efficient solar cells with high open-circuit voltage

Highly ordered arrays of CdSe coated ZnO/ZnSe core–shell nanocables on FTO (SnO2 : F) glass substrates have been synthesized using ZnO nanowires as precursors via in situ successive ion exchanges without any organic ligands involved. While the low open-circuit voltage (VOC) (typically below 0.72 V) is a main factor limiting the power conversion efficiency (PCE) of quantum dot sensitized solar cells (QDSSCs), we design and exploit the arrays of ZnO/ZnSe/CdSe nanocables as efficient photoelectrodes for photoelectrochemical (PEC) solar cells, achieving a PCE of 4.54% and a VOC as high as 0.836 V by using a nanostructured Cu2S counter-electrode under AM 1.5G illumination with an intensity of 100 mW cm−2. The high photovoltage is attributed to the ZnSe layer with a high conduction band edge, which reduces carrier recombination by passivizing the surface of ZnO nanowires and upwardly shifts the conduction band of ZnO in the heterojunction. A VOC up to 0.855 V is achieved for the same cell using a typical platinized FTO (Pt/FTO) counter-electrode. However, the Cu2S counter-electrode, which is demonstrated to have higher catalytic activity, contributes to improvements in the fill factor (FF) and short-circuit current density (JSC) and consequently results in a 55% improvement in PCE.

A recyclable carbon nanoparticle-based fluorescent probe for highly selective and sensitive detection of mercapto biomolecules

Carbon nanoparticles (CNPs) with strong blue emission are synthesized using a microwave-assisted hydrothermal method. The fluorescence of the CNPs can be completely quenched by Hg2+ through an effective electron or energy transfer process due to the synergetic strong electrostatic interaction and metal-ligand coordination. Based on this, a system containing Hg2+-quenched CNPs (CNP-Hg2+) is designed to be a sensitive and selective turn-on fluorescent probe towards cysteine (a type of mercapto biomolecule) with a detection limit of 15 nM. The fluorescence of CNP-Hg2+ aqueous solution can be repeatedly turned on and off for over 10 times by alternative addition of cysteine and Hg2+, respectively. After 10 cycles, the fluorescence intensity could be recovered to as high as 85% of the original value of CNPs. Remarkably, the sensing process is able to be observed by the naked eye under UV irradiation. Furthermore, the sensing is specific to biothiols and the sensor is able to work in living cells.

Phase Conversion from Hexagonal CuSySe1–y to Cubic Cu2–xSySe1–y: Composition Variation, Morphology Evolution, Optical Tuning, and Solar Cell Applications

In this work, we report a simple and low-temperature approach for the controllable synthesis of ternary Cu-S-Se alloys featuring tunable crystal structures, compositions, morphologies, and optical properties. Hexagonal CuS(y)Se(1-y) nanoplates and face centered cubic (fcc) Cu(2-x)S(y)Se(1-y) single-crystal-like stacked nanoplate assemblies are synthesized, and their phase conversion mechanism is well investigated. It is found that both copper content and chalcogen composition (S/Se atomic ratio) of the Cu-S-Se alloys are tunable during the phase conversion process. Formation of the unique single-crystal-like stacked nanoplate assemblies is resulted from oriented stacking coupled with the Ostwald ripening effect. Remarkably, optical tuning for continuous red shifts of both the band-gap absorption and the near-infrared localized surface plasmon resonance are achieved. Furthermore, the novel Cu-S-Se alloys are utilized for the first time as highly efficient counter electrodes (CEs) in quantum dot sensitized solar cells (QDSSCs), showing outstanding electrocatalytic activity for polysulfide electrolyte regeneration and yielding a 135% enhancement in power conversion efficiency (PCE) as compared to the noble metal Pt counter electrode.

Spectroscopic study on the impact of methylammonium iodide loading time on the electronic properties in perovskite thin films

Solution processed metal–organic halide perovskite photovoltaic devices have recently drawn tremendous attention due to their simplicity of fabrication and high efficiency. Despite numerous reports on optimizing perovskite films with different fabrication approaches, there is limited understanding on the correlation between sensitive processing conditions and the microstructural and electronic properties of perovskite films. Here we combine several opto-electrical spectroscopy techniques to investigate the methylammonium iodide (MAI) loading time effect on the doping density profile and uncoordinated ions in resulting CH3NH3PbI3 perovskite thin films. We find that even in a very short period of different loading times within two minutes, there is a significant impact on the device power conversion efficiency (PCE) from 2% to over 15%. It is found that the doping density profile is inhomogeneous across the perovskite film with too short MAI loading time, resulting in an S-shape in the current density–voltage (J–V) characteristics. On the other hand, devices with too long loading time have excess uncoordinated ions attributed to the J–V hysteresis. By using combined spectroscopy techniques to pinpoint the electronic properties in perovskite films, this work would shed light on the understanding of the controversial origins of the reported S-shape and hysteresis in perovskite photovoltaic cells.

Graphene oxide as an efficient hole-transporting material for high-performance perovskite solar cells with enhanced stability

In recently developed high-efficiency metal organometal halide perovskite solar cells (PVSCs), electron and hole transporting materials have shown key roles in determining the growth of perovskite crystals, as well as the performance and stability of the device. However, interlayer materials which can facilitate both high efficiency and stability at low cost are still limited. Here, we demonstrate that, by controlling the thickness of solution-processed graphene oxide (GO), one can achieve a balance of high work function and conductivity. Using GO with the optimized thickness as a hole-transporting material (HTM) in PVSCs, a high power conversion efficiency (PCE) of 16.5% with no hysteresis has been achieved with excellent light-soaking photocurrent stability in comparison with a commonly used organic-based HTM. Under high humidity and continuous light soaking, the encapsulated perovskite devices retained >80% of their initial efficiency for >2000 h. Detailed studies on the GO binding energy, charge transfer efficiency with perovskite, and crystal morphology shed light on the origin of the observed improvement in photovoltaic performance. Benefiting from the merits of low temperature, solution processability and low cost, the proposed GO fabrication methods could aid scalable production of PVSCs with high PCE and excellent stability.

The detrimental effect of excess mobile ions in planar CH3NH3PbI3 perovskite solar cells

The origin of the impact of mobile ions in perovskite solar cells (PVSCs) has recently become a hot topic of debate. Here, we investigate systematically the structural effect and various recombination pathways in PVSCs with different ion concentrations. By probing the transient ionic current in PVSCs, we extract mobile ion concentrations in a range of 1016 cm−3 to 1017 cm−3 depending on the processing conditions during a two-step process. The PVSC with the lowest ion concentration has both the highest efficiency over 15% and shelf-life over 1300 hours. Interestingly, in contrast to the commonly adopted models in the literature, we find that the crystal size and the bimolecular and trap-assisted recombination are not responsible for the large difference in photovoltaic performance. Instead, by using transient photocurrent and steady-state photoluminescence approaches, we find that the large reduction of short-circuit current (Jsc) in mobile ion populated devices is ascribed to the slow decay in photocurrent and the increasing amount of non-radiative recombination. In addition, we also find that the excess mobile ions trigger the deformation of perovskite to PbI2, which severely reduces the device lifetime. The results provide valuable information on the understanding of the role of excess mobile ions in the degradation mechanism of PVSCs.

Core/Sheath Organic Nanocable Constructed with a Master–Slave Molecular Pair for Optically Switched Memories

Here, we propose a new approach to solve this problem by distributing the above functions to two optoelectronically coupled molecules. Briefl y, a molecule with good photochromic switching properties is used as a “master” molecule. Via optoelectronic coupling, the master molecule will control a “slave” molecule that has no photochromic properties but has good electrical and/or optical properties. The essential optoelectronic coupling is achieved here with a core/sheath nanocable structure, which provides intimate contact between the “master” and the “slave” molecules. Through this design, molecules with more specialized functions can be developed individually, thus meeting the need for developing all-round materials. Here, we demonstrate the above concept with a core/sheath nanocable constructed with two small molecular materials with matched energy levels to enable optoelectronic coupling. The photochromic master molecule used here is a diarylethene derivative, 1-[2-methyl-5-phenyl-3-thienyl]-2-[2-methyl-5-( p -(methyl)phenyl)3-thienyl]-hexaflne ( MPT–MMPT–HFCP ), which has been widely studied for its excellent photochromic properties. [ 1–7 , 14 ] Coronene is used as the slave molecule for the following reasons: 1) it has a large aromatic system and is a good holetransporting semiconductor, with electrical properties that have potential tunability upon combined use with MPT-MMPT-HFCP ; 2) its photoluminescence (PL) spectrum has extensive overlap with the absorption spectrum of MPT-MMPT-HFCP , which may induce effi cient intermolecular energy transfer for the emission modulation; and 3) the π – π interaction between polycyclic aromatic molecules would provide driving force for self-assembling

The effects of oxygen on controlling the number of carbon layers in the chemical vapor deposition of graphene on a nickel substrate

While oxygen is typically considered undesirable during the chemical vapor deposition (CVD) of graphene on metal substrates, we demonstrate that suitable amounts of oxygen in the CVD system can in fact improve the uniformity and thickness control of the graphene film. The role of oxygen on the CVD of graphene on a nickel substrate using a propylene precursor was investigated with various surface analytical techniques. It was found that the number of carbon layers in the deposited graphene sample decreases as the concentration of oxygen increases. In particular, single-layer graphene can be easily obtained with an oxygen/propylene ratio of 1/9. In the presence of oxygen, a thin layer of nickel oxide will form on the substrate. The oxide layer decreases the concentration of carbon atoms dissolved in the nickel substrate and results in graphene samples with a decreasing number of carbon layers.