Nanostructured Hybrid Metal Mesh as Transparent Conducting Electrodes: Selection Criteria Verification in Perovskite Solar Cells.

Nanostructured metal mesh structures demonstrating excellent conductivity and high transparency are one of the promising transparent conducting electrode (TCE) alternatives for indium tin oxide (ITO). Often, these metal nanostructures are to be employed as hybrids along with a conducting filler layer to collect charge carriers from the network voids and to minimize current and voltage losses. The influence of filler layers on dictating the extent of such ohmic loss is complex. Here, we used a general numerical model to correlate the sheet resistance of the filler, lateral charge transport distance in network voids, metal mesh line width and ohmic losses in optoelectronic devices. To verify this correlation, we prepared gold or copper network electrodes with different line widths and different filler layers, and applied them as TCEs in perovskite solar cells. We show that the photovoltaic parameters scale with the hybrid metal network TCE properties and an Au-network or Cu-network with aluminum-doped zinc oxide (AZO) filler can replace ITO very well, validating our theoretical predictions. Thus, the proposed model could be employed to select an appropriate filler layer for a specific metal mesh electrode geometry and dimensions to overcome the possible ohmic losses in optoelectronic devices.

The role of PbI2 in CH3NH3PbI3 perovskite stability, solar cell parameters and device degradation.

We report a systematic investigation on the role of excess PbI2 content in CH3NH3PbI3 perovskite film properties, solar cell parameters and device storage stability. We used the CH3NH3I vapor assisted method for the preparation of PbI2-free CH3NH3PbI3 films under a N2 atmosphere. These pristine CH3NH3PbI3 films were annealed at 165 °C for different time intervals in a N2 atmosphere to generate additional PbI2 in these films. From XRD measurements, the excess of PbI2 was quantified. Detailed characterization using scanning electron microscopy, X-ray diffraction, UV-Visible and photoluminescence for continuous aging of CH3NH3PbI3 films under ambient condition (50% humidity) is carried out for understanding the influence of different PbI2 contents on degradation of the CH3NH3PbI3 films. We find that the rate of degradation of CH3NH3PbI3 is accelerated due to the amount of PbI2 present in the film. A comparison of solar cell parameters of devices prepared using CH3NH3PbI3 samples having different PbI2 contents reveals a strong influence on the current density-voltage hysteresis as well as storage stability. We demonstrate that CH3NH3PbI3 devices do not require any residual PbI2 for a high performance. Moreover, a small amount of excess PbI2, which improves the initial performance of the devices slightly, has undesirable effects on the CH3NH3PbI3 film stability as well as on device hysteresis and stability.

TiO2 Nanorods Decorated with Pd Nanoparticles for Enhanced Liquefied Petroleum Gas Sensing Performance.

Development of highly sensitive and selective semiconductor-based metal oxide sensor devices to detect toxic, explosive, flammable, and pollutant gases is still a challenging research topic. In the present work, we systematically enhanced the liquefied petroleum gas (LPG) sensing performance of chemical bath deposited TiO2 nanorods by decorating Pd nanoparticle catalyst. Surface morphology with elemental mapping, crystal structure, composition and oxidation states, and surface area measurements of pristine TiO2 and Pd:TiO2 nanorods was examined by high resolution transmission electron microscopy with energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and nitrogen adsorption-desorption characterization techniques. LPG sensing performance of pristine TiO2 and Pd:TiO2 nanorods was investigated in different LPG concentration and operating temperature ranges. The LPG response of 21% for pristine TiO2 nanorods is enhanced to 49% after Pd catalyst decoration with reasonably fast response and recovery times. Further, the sensor exhibited long-term stability, which could be due to the strong metal support (Pd:TiO2) interaction and catalytic properties offered by the Pd nanoparticle catalyst. The work described herein demonstrates a general and scalable approach that provides a promising route for rational design of variety of sensor devices for LPG detection.

Effect of Thermal and Structural Disorder on the Electronic Structure of Hybrid Perovskite Semiconductor CH3NH3PbI3.

In this Letter, we investigate the temperature dependence of the optical properties of methylammonium lead iodide (MAPbI3 = CH3NH3PbI3) from room temperature to 6 K. In both the tetragonal (T > 163 K) and the orthorhombic (T < 163 K) phases of MAPbI3, the band gap (from both absorption and photoluminescence (PL) measurements) decreases with decrease in temperature, in contrast to what is normally seen for many inorganic semiconductors, such as Si, GaAs, GaN, etc. We show that in the perovskites reported here, the temperature coefficient of thermal expansion is large and accounts for the positive temperature coefficient of the band gap. A detailed analysis of the exciton line width allows us to distinguish between static and dynamic disorder. The low-energy tail of the exciton absorption is reminiscent of Urbach absorption. The Urbach energy is a measure of the disorder, which is modeled using thermal and static disorder for both the phases separately. The static disorder component, manifested in the exciton line width at low temperature, is small. Above 60 K, thermal disorder increases the line width. Both these features are a measure of the high crystal quality and low disorder of the perovskite films even though they are produced from solution.

Compact Layers of Hybrid Halide Perovskites Fabricated via the Aerosol Deposition Process-Uncoupling Material Synthesis and Layer Formation.

We present the successful fabrication of CH3NH3PbI3 perovskite layers by the aerosol deposition method (ADM). The layers show high structural purity and compactness, thus making them suitable for application in perovskite-based optoelectronic devices. By using the aerosol deposition method we are able to decouple material synthesis from layer processing. Our results therefore allow for enhanced and easy control over the fabrication of perovskite-based devices, further paving the way for their commercialization.

Low temperature synthesis and visible light driven photocatalytic activity of highly crystalline mesoporous TiO2 particles.

Mesoporous TiO2 powder materials with a high crystallinity have been prepared by evaporation induced self assembly (EISA) process using titanium tetraisopropoxide (TTIP) and pluronic P123 surfactant (EO20PO70EO20) as titanium source and structure-directing reagent, respectively. The prepared materials were characterized by low and wide-angle X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), optical absorption, and N2 adsorption-desorption experiments. The crystallinity of the materials was controlled by varying the calcination temperature. The resulting TiO2 materials showed highly crystalline structure with uniform particle size which increases from 11.8 to 23.8 nm with increasing the calcination temperature from 400 to 600 degrees C, respectively, whereas the specific surface area decreases from 125 to 40 m2/g. TEM and XRD results revealed that the calcination temperature of 600 degrees C is the best condition to obtain highly crystalline mesoporous TiO2. The photocatalytic activity of the TiO2 mesoporous materials with different crystallinity and textural parameters has been studied in the decomposition of methylene blue (MB) dye molecules under visible light irradiation. Among the mesoporous TiO2 materials studied, the material with the highest crystallinity, prepared at 600 degrees C, showed the best photocatalytic performance in the decomposition of MB under visible light in a short time.

Growth of shape- and size-selective zinc oxide nanorods by a microwave-assisted chemical bath deposition method: effect on photocatalysis properties.

Herein, we demonstrate the shape- and size-selective growth of ZnO nanostructures on indium tin oxide-coated glass substrates by using a microwave-assisted chemical bath deposition method. By systematically controlling the deposition parameters, it is possible to produce shape- and size-selective nanostructures with high alignment and uniformity. Specifically, the pH of the bath can be used to control the shape of rods from bundled structures to tapered and flat tips. Furthermore, the deposition temperature can be used to control the size of the ZnO array from 770 to 125 nm. The prepared rods were active catalysts in the degradation of methylene blue under UV radiation, and exhibited size-dependent activity.