Xizu Wang

Identification of catalytic sites for oxygen reduction and oxygen evolution in N-doped graphene materials: Development of highly efficient metal-free bifunctional electrocatalyst.

Doping of graphene with nitrogen imparted bifunctional electrocatalytic activities for efficient energy conversion and storage. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are critical to renewable energy conversion and storage technologies. Heteroatom-doped carbon nanomaterials have been reported to be efficient metal-free electrocatalysts for ORR in fuel cells for energy conversion, as well as ORR and OER in metal-air batteries for energy storage. We reported that metal-free three-dimensional (3D) graphene nanoribbon networks (N-GRW) doped with nitrogen exhibited superb bifunctional electrocatalytic activities for both ORR and OER, with an excellent stability in alkaline electrolytes (for example, KOH). For the first time, it was experimentally demonstrated that the electron-donating quaternary N sites were responsible for ORR, whereas the electron-withdrawing pyridinic N moieties in N-GRW served as active sites for OER. The unique 3D nanoarchitecture provided a high density of the ORR and OER active sites and facilitated the electrolyte and electron transports. As a result, the as-prepared N-GRW holds great potential as a low-cost, highly efficient air cathode in rechargeable metal-air batteries. Rechargeable zinc-air batteries with the N-GRW air electrode in a two-electrode configuration exhibited an open-circuit voltage of 1.46 V, a specific capacity of 873 mAh g−1, and a peak power density of 65 mW cm−2, which could be continuously charged and discharged with an excellent cycling stability. Our work should open up new avenues for the development of various carbon-based metal-free bifunctional electrocatalysts of practical significance.

Cesium Carbonate Functionalized Graphene Quantum Dots as Stable Electron-Selective Layer for Improvement of Inverted Polymer Solar Cells

Solution processable inverted bulk heterojunction (BHJ) polymer solar cells (PSCs) are promising alternatives to conventional silicon solar cells because of their low cost roll-to-roll production and flexible device applications. In this work, we demonstrated that Cs2CO3 functionalized graphene quantum dots (GQDs-Cs2CO3) could be used as efficient electron-selective layers in inverted PSCs. Compared with Cs2CO3 buffered devices, the GQDs-Cs2CO3 buffered devices show 56% improvement in power conversion efficiency, as well as 200% enhancement in stability, due to the better electron-extraction, suppression of leakage current, and inhibition of Cs(+) ion diffusion at the buffer/polymer interface by GQDs-Cs2CO3. This work provides a thermal-annealing-free, solution-processable method for fabricating electron-selective layer in inverted PSCs, which should be beneficial for the future development of high performance all-solution-processed or roll-to-roll processed PSCs.

Elimination of Burn-in Open-Circuit Voltage Degradation by ZnO Surface Modification in Organic Solar Cells

Photodegradation of inverted organic solar cells based on ZnO as an electron transport layer (ETL) was studied over short time scales of 5 min and 8 h. Devices with ZnO as ETL reproducibly exhibited a steep loss of open-circuit voltage, VOC, and shunt resistance, RSH, in a matter of minutes upon illumination. Removing the UV-content of illumination minimized VOC loss and impact on the devices shunting behavior, indicating its role in the loss. Application of an ultrathin layer of Al on ZnO led to almost negligible photoinduced VOC loss up to 8 h of exposure. By applying the fundamental Shockley diode equation, we approximated the VOC loss to be caused by dramatic increases in reverse saturation current I0. We attribute the increased rate of recombination to diminished carrier selectivity at the ZnO/organic interface. Devices with Al modified ZnO ETL demonstrated remarkable RSH (1.4 kΩ cm(2) at 1 sun), rectification ratio (10(6)) and reverse saturation current density (2.1 × 10(-7) mA/cm(2)).

Efficient Semitransparent Bulk-Heterojunction Organic Photovoltaic Cells With High-Performance Low Processing Temperature Indium–Tin Oxide Top Electrode

Abstract-An efficient semitransparent bulk-heterojunction zinc phthalocyanine (ZnPc): fullerene (C60)-based photovoltaic cell with a transparent cathode of Ag/LiF/indium-tin oxide (ITO) is demonstrated. The top ITO layer serves not only as an index matching layer to enhance the light in-coupling in semitransparent small molecule photovoltaic cells, but also improves current spreading due to its superior optical transparency and high electric conductivity. In order to avoid causing damages to the underlying functional photoactive organic layers, the ITO top electrode was formed at room temperature without intentional heating. Optimization of light distribution in the semitransparent ZnPc:C60 photovoltaic cells was performed using an optical admittance analysis. The performance of the semitransparent organic photovoltaic cells is optimized over the two competing parameter of power conversion efficiency (PCE) and optical transparency. Semitransparent bulk-heterojunction ZnPc:C60 photovoltaic cells with an average transmission of more than 40% in the visible light region and a PCE of ~3.0% measured under simulated AM1.5G illumination of 100 mW/cm2 were obtained.

Pure Blue-Light Emissive Poly(oligofluorenes) with Bifunctional POSS in the Main Chain

Emission of conjugated polymers is known to undergo bathochromic shift from solution to film formation due to π-π stacking in the solid state. In this report, a series of pearl-necklace-like hybrid polymers is designed via the hydrosilylation condensation between bifunctional polyhedral oligomeric silsesquioxanes (B-POSS) and oligofluorene segments. Optoelectronic analyses unequivocally show that the presence of these interconnecting B-POSS can effectively reduce red-shift in photoluminescence and electroluminescence during film formation. These hybrid poly(oligofluorenes) display stable blue emission with high color purity. Thermal analyses also indicate that they are vitrified polymers with high glass transition temperature (up to 125 °C). We believe that this strategy can be extended to other conjugated systems to control color purity in electroactive materials and holds promise as new emissive materials for various applications.

Ultra-high Seebeck coefficient and low thermal conductivity of a centimeter-sized perovskite single crystal acquired by a modified fast growth method

A centimeter-sized organic–inorganic hybrid lead-based perovskite CH3NH3PbI3 (MAPbI3) single crystal was obtained by using a modified fast and inverse-temperature growth method. The optical properties of this single crystal at room and low temperatures were studied in terms of optical absorption and photoluminescence measurements. The single crystal exhibited optical properties with a band-gap of 1.53 eV, which is comparable to a reported value. The temperature-dependent UV-vis spectra of this perovskite single crystal showed a unique structural phase transition as the temperatures varied. The thermoelectric properties of this MAPbI3 single crystal were studied, showing that the Seebeck coefficient of 920 ± 91 μV K−1 almost remained unchanged from room temperature to 330 K and it progressively increased with the increase in temperature and reached 1693 ± 146 μV K−1 at 351 K. In contrast, there was no very clear trend for thermal conductivities with changes in temperature. The thermal conductivities were maintained between 0.30 and 0.42 W m K−1 in the temperature range of 298–425 K. These thermoelectric characteristics would be useful for potential thermoelectric applications if the electrical conductivity of this crystal is improved by tuning its composition.

A high work function anode interfacial layer via mild temperature thermal decomposition of a C60F36 thin film on ITO

A high work function anode interfacial layer has been developed via a mild temperature thermal decomposition of fluorinated fullerene (C60F36) on ITO at 120 °C. As revealed by in situ ultraviolet photoelectron spectroscopy (UPS) measurements, after the interfacial modification, the ITO electrode work function can be as high as ∼5.62 eV. It also possesses very good air stability even after the exposure to air for more than one day. The thermal annealing induced carbon–fluorine bond breaking was confirmed by in situ X-ray photoelectron spectroscopy (XPS) measurements. The residual F atoms are chemically bonded onto the ITO surface. Taking advantage of such a high work function anode interfacial layer on ITO, enhanced performance of a chloroaluminium phthalocyanine (ClAlPc)/fullerene (C60) planar heterojunction based organic solar cell was observed. The performance enhancement is attributed to the higher anode WF together with the optimal nanoscale morphology, and hence better hole collection efficiency.

Fully printable organic and perovskite solar cells with transfer-printed flexible electrodes

The perovskite solar cells (PSCs) and organic solar cells (OSCs) with high performance were fabricated with transfer-printed top metal electrodes. We have demonstrated that PSCs and OSCs with the top Au electrodes fabricated by using the transfer printing method have comparable or better performance than the devices with the top Au electrodes fabricated by using the conventional thermal evaporation method. The highest PCE of the PSCs and OSCs with the top electrodes fabricated using the transfer printing method achieved 13.72% and 2.35%, respectively. It has been investigated that fewer defects between the organic thin films and Au electrodes exist by using the transfer printing method which improved the device stability. After storing the PSCs and OSCs with the transfer-printed electrodes in a nitrogen environment for 97 and 103 days without encapsulation, the PSCs and OSCs still retained 71% and 91% of their original PCEs, respectively.

Low band-gap weak donor–strong acceptor conjugated polymer for organic solar cell

By insertion of an additional weak acceptor into a donor–acceptor conjugated polymer backbone, a new weak donor–strong acceptor alternating copolymer PTTBOBT-DFBT was synthesized and it showed a low band-gap of 1.64 eV with a deep HOMO energy level (−5.44 eV). The bulk heterojunction (BHJ) solar cell fabricated from polymer PTTBOBT-DFBT displayed a remarkable power conversion efficiency (PCE) of 5.36% (Jsc = 11.04 mA cm−2, FF = 63.65%, Voc = 0.76 V).

High-performance thermoelectric materials based on ternary TiO2/CNT/PANI composites

In the present work, we report the fabrication of high-performance thermoelectric materials using TiO2/CNT/PANI ternary composites. We showed that a conductivity of ∼2730 S cm-1 can be achieved for the binary CNT (70%)/PANI (30%) composite, which is the highest recorded value for the reported CNT/PANI composites. We further demonstrated that the Seebeck coefficient of CNT/PANI composites could be enhanced by incorporating TiO2 nanoparticles into the binary CNT/PANI composites, which could be attributed to lower carrier density and the energy scattering of low-energy carriers at the interfaces of TiO2/a-CNT and TiO2/PANI. The resulting TiO2/a-CNT/PANI ternary system exhibits a higher Seebeck coefficient and enhanced thermoelectric power. Further optimization of the thermoelectric power was achieved by water treatment and by tuning the processing temperature. A high thermoelectric power factor of 114.5 μW mK-2 was obtained for the ternary composite of 30% TiO2/70% (a-CNT (70%)/PANI (30%)), which is the highest reported value among the reported PANI based ternary composites. The improvement of thermoelectric performance by incorporation of TiO2 suggests a promising approach to enhance power factor of organic thermoelectric materials by judicial tuning of the carrier concentration and electrical conductivity.