Xian Cui

High performance carbon nanotube based fiber-shaped supercapacitors using redox additives of polypyrrole and hydroquinone

Fiber-shaped supercapacitors (FSSCs) have attractive applications in flexible devices. Here, we report a high performance carbon nanotube (CNT) based fiber-shaped supercapacitor by adding two redox additives simultaneously: polypyrrole (PPy) to the electrodes and hydroquinone (HQ) to the electrolyte. A core–shell CNT–PPy nanocomposite fiber was prepared by an electrochemical deposition method. In the FSSC with CNT–PPy electrodes, PPy provides pseudocapacitance due to its reversible dropping/de-dropping reactions in polyvinyl alcohol (PVA)/H2SO4 gel electrolyte. The capacitance of the CNT–PPy FSSC reaches 36 F g−1 (127 mF cm−1, 588 mF cm−2, and 17 F cm−3), which is 7 times higher than that of the pure CNT FSSC. By adding HQ to the PVA/H2SO4 gel electrolyte in the CNT–PPy FSSC, the specific capacitance reaches 56 F g−1 (202 mF cm−1, 1168 mF cm−2, and 42 F cm−3), which is 10 times higher than that of the pure CNT FSSC. HQ can enhance ion transfer of the gel electrolyte by the redox reaction of HQ and benzoquinone. PPy and HQ have synergistic effects on the FSSCs. The FSSCs with PPy and HQ also show high stability in the cyclic test for 2000 cycles, and good flexibility under bending, knotting, and tension.

Highly conductive, twistable and bendable polypyrrole–carbon nanotube fiber for efficient supercapacitor electrodes

Carbon nanotubes (CNTs) are a promising candidate for flexible and wearable electronic applications. Here, we report a conductive, twistable and bendable CNT composite fiber with core–shell structure for efficient supercapacitor electrode, where the CNT bundles are coated with a thin layer of polypyrrole (PPy) (denoted as PPy@CNT). The electrochemical properties of the fiber electrode depend on the content and distribution of the PPy. The PPy@CNT fiber exhibits a high specific capacitance of 350 F g−1 and high stability in cyclic testing. The outstanding electrochemical performance originates from the high conductivity of the core–shell structured PPy@CNT composite, which makes charge transfer easily between PPy and CNTs. The PPy@CNT fiber is also stable under bending and twisting.

Enhanced performance of perovskite solar cells by modulating the Lewis acid–base reaction

The Lewis acid-base reaction between PbI2 and solvent molecules is popular in fabricating PbI2 films by a two-step method for making perovskite solar cells. Here, we control the microstructure of PbI2 films through modulating the Lewis acid-base reaction by adding a small amount of N-methyl pyrrolidone into PbI2/DMF solution. PbI2 films with excellent crystallinity and full coverage are fabricated by spin-coating the mixed solution on the substrate, which leads to high quality perovskite layers with low recombination rate and high efficiency for carrier transfer. As a result, the power conversion efficiency of the best perovskite solar cells increases from 13.3% to 17.5%.

Stretchable and compressible strain sensors based on carbon nanotube meshes

Flexible strain sensors have promising applications in healthcare and human movement detection. Herein, we report stretchable and compressible strain sensors based on carbon nanotube meshes (CNTMs) with unique structures consisting of macroscopic grids and microscopic spider-web networks. The stretchable strain sensor shows good reliability for long cyclic tests and can be used for weak stimuli and large motion detection. The compressible strain sensor also shows good reliability after long cyclic tests and can be used to detect large strains induced by walking or running motion. Both the stretchable and compressible CNTM strain sensors are reliable and stable at detecting large stretching and compressing deformation.

Control of the morphology of PbI2 films for efficient perovskite solar cells by strong Lewis base additives

A two-step method is widely used to fabricate highly efficient perovskite solar cells. In a typical two-step method, CH3NH3PbI3 perovskites are fabricated from PbI2 films through an intercalation reaction between PbI2 and CH3NH3I, which usually have a rough surface and residual PbI2 due to a compact PbI2 structure. Here, we introduce some strong Lewis bases into PbI2/DMF solutions in the first step, which is helpful in controlling the morphology of PbI2 films with a mesoporous structure. The mesopores in the PbI2 films provide not only space for accommodating the volume expansion during the reaction between PbI2 and CH3NH3I, but also channels for CH3NH3I solution to diffuse into the PbI2 films, which helps eliminate the residual PbI2 and the dissolution and recrystallization process. As a result, smooth perovskite films without residual PbI2 are easily obtained. The power conversion efficiency, stability and reproducibility of the perovskite solar cells fabricated from the optimized mesoporous PbI2 films improved simultaneously due to the improvement in perovskite films. This work reveals the key roles of solvents (boiling point, Lewis basicity) in controlling the microstructure of the PbI2 films.

In Situ Observation of Crystallization of Methylammonium Lead Iodide Perovskite from Microdroplets

It is of great importance to investigate the crystallization of organometallic perovskite from solution for enhancing performance of perovskite solar cells. Here, this study develops a facile method for in situ observation of crystallization and growth of the methylammonium lead iodide (MAPbI3 ) perovskite from microdroplets ejected by an alternating viscous and inertial force jetting method. It is found that there are two crystallization modes when MAPbI3 grows from the CH3 NH3 I (MAI)/PbI2 /N,N-dimethylformamide (DMF) solution: needle precursors and granular perovskites. Generally, needle Lewis adduct of MAPbI3 ·DMF tends to nucleate and grow from the solution due to low solubility of PbI2 . The growth of MAPbI3 ·DMF depends on both the concentration of MAI and temperature. It tends to form large perovskite domains on substrates at high temperature. The MAPbI3 ·DMF coverts to nanocrystalline perovskite due to lattice shrinkage when DMF molecules escape from the Lewis adduct. Granular perovskite can also directly nucleate from the solution at high concentration of MAI due to compositional segregation.

Enhanced performance of perovskite solar cells by strengthening a self-embedded solvent annealing effect in perovskite precursor films

The solvent embedded in the intermediate phase is widely observed in the fabrication of perovskite films. The perovskite precursor films obtained from Lewis adducts through molecular exchange contain some residue solvent. It has an intrinsic solvent annealing effect during the annealing process. Here, we pre-deposit a protective layer on the perovskite precursor films to retard the escape of solvent during the annealing process. The restricted solvent strengthens the solvent annealing effect during the formation of perovskite films. As a result, the perovskite quality, including grain size and crystallization, is improved significantly, which leads to efficient charge transportation, low recombination rate, and enhancement of the photovoltaic performance of the corresponding perovskite solar cells.

Elucidating the Key Role of a Lewis Base Solvent in the Formation of Perovskite Films Fabricated from the Lewis Adduct Approach

High-quality perovskite films can be fabricated from Lewis acid-base adducts through molecule exchange. Substantial work is needed to fully understand the formation mechanism of the perovskite films, which helps to further improve their quality. Here, we study the formation of CH3NH3PbI3 perovskite films by introducing some dimethylacetamide into the PbI2/N,N-dimethylformamide solution. We reveal that there are three key processes during the formation of perovskite films through the Lewis acid-base adduct approach: molecule intercalation of solvent into the PbI2 lattice, molecule exchange between the solvent and CH3NH3I, and dissolution-recrystallization of the perovskite grains during annealing. The Lewis base solvents play multiple functions in the above processes. The properties of the solvent, including Lewis basicity and boiling point, play key roles in forming smooth perovskite films with large grains. We also provide some rules for choosing Lewis base additives to prepare high-quality perovskite films through the Lewis adduct approach.

High annealing temperature induced rapid grain coarsening for efficient perovskite solar cells

Thermal annealing plays multiple roles in fabricating high quality perovskite films. Generally, it might result in large perovskite grains by elevating annealing temperature, but might also lead to decomposition of perovskite. Here, we study the effects of annealing temperature on the coarsening of perovskite grains in a temperature range from 100 to 250 °C, and find that the coarsening rate of the perovskite grain increase significantly with the annealing temperature. Compared with the perovskite films annealed at 100 °C, high quality perovskite films with large columnar grains are obtained by annealing perovskite precursor films at 250 °C for only 10 s. As a result, the power conversion efficiency of best solar cell increased from 12.35% to 16.35% due to its low recombination rate and high efficient charge transportation in solar cells.

Crystallization of CH3NH3PbI3−xBrx perovskite from micro-droplets of lead acetate precursor solution

The physical properties of organic–inorganic hybrid perovskite materials are greatly influenced by their composition, crystallinity, and morphology. Here, we investigate the growth of CH3NH3PbI3−xBrx perovskite from micro-droplets of a methylammonium iodide/lead acetate/dimethylformamide precursor solution added with different concentrations of PbBr2, which helps to control the morphology and crystallinity of the perovskite film. We find that both perovskite and its intermediate containing an acetate group growing from the micro-droplets. By adding some PbBr2, there is a tendency to form cubic perovskite crystals due to partial substitution of iodine with bromide. There is composition segregation when the crystals grow freely from the droplets. Smooth and uniform CH3NH3PbI3−xBrx perovskite films with a tunable band gap are thus fabricated from the precursor solution added with PbBr2.