Timothy L. Kelly

Investigation of CH3NH3PbI3 degradation rates and mechanisms in controlled humidity environments using in situ techniques.

Perovskite solar cells have rapidly advanced to the forefront of solution-processable photovoltaic devices, but the CH3NH3PbI3 semiconductor decomposes rapidly in moist air, limiting their commercial utility. In this work, we report a quantitative and systematic investigation of perovskite degradation processes. By carefully controlling the relative humidity of an environmental chamber and using in situ absorption spectroscopy and in situ grazing incidence X-ray diffraction to monitor phase changes in perovskite degradation process, we demonstrate the formation of a hydrated intermediate containing isolated PbI6(4-) octahedra as the first step of the degradation mechanism. We also show that the identity of the hole transport layer can have a dramatic impact on the stability of the underlying perovskite film, suggesting a route toward perovskite solar cells with long device lifetimes and a resistance to humidity.

Effect of CH3NH3PbI3 thickness on device efficiency in planar heterojunction perovskite solar cells

Recent advances in the development of perovskite solar cells based on CH3NH3PbI3 have produced devices with power conversion efficiencies of >15%. While initial work in this area assumed that the perovskite-based cells required a mesoporous TiO2 support, many recent reports have instead focused on the development of planar heterojunction structures. A better understanding of how both cell architecture and various design parameters (e.g., perovskite thickness and morphology) affect cell performance is needed. Here, we report the fabrication of perovskite solar cells based on a ZnO nanoparticle electron transport layer, CH3NH3PbI3 light absorber, and poly(3-hexylthiophene) (P3HT) hole transport layer. We show that vapor-phase deposition of the PbI2 precursor film produces devices with performances equivalent to those prepared using entirely solution-based techniques, but with very precise control over the thickness and morphology of the CH3NH3PbI3 layer. Optimization of the layer thickness yielded devices with efficiencies of up to 11.3%. The results further demonstrate that a delicate balance between light absorption and carrier transport is required in these planar heterojunction devices, with the thickest perovskite films producing only very low power conversion efficiencies.

Plasmonic Enhancement of Dye Sensitized Solar Cells in the Red-to-near-Infrared Region using Triangular Core–Shell Ag@SiO2 Nanoparticles

Recently, plasmonic metal nanoparticles have been shown to be very effective in increasing the light harvesting efficiency (LHE) of dye-sensitized solar cells (DSSCs). Most commonly, spherical nanoparticles composed of silver or gold are used for this application; however, the localized surface plasmon resonances of these isotropic particles have maxima in the 400-550 nm range, limiting any plasmonic enhancements to wavelengths below 600 nm. Herein, we demonstrate that the incorporation of anisotropic, triangular silver nanoprisms in the photoanode of DSSCs can dramatically increase the LHE in the red and near-infrared regions. Core-shell Ag@SiO2 nanoprisms were synthesized and incorporated in various quantities into the titania pastes used to prepare the photoanodes. This optimization led to an overall 32 ± 17% increase in the power conversion efficiency (PCE) of cells made using 0.05% (w/w) of the Ag@SiO2 composite. Measurements of the incident photon-to-current efficiency provided further evidence that this increase is a result of improved light harvesting in the red and near-infrared regions. The effect of shell thickness on nanoparticle stability was also investigated, and it was found that thick (30 nm) silica shells provide the best protection against corrosion by the triiodide-containing electrolyte, while still enabling large improvements in PCE to be realized.

Supercapacitive properties of PEDOT and carbon colloidal microspheres.

The synthesis and characterization of a new PEDOT-carbon composite prepared using a microporous carbon template are described. The electrochemical behavior of this composite, as well as that of three other colloidal materials-PEDOT-silica, PEDOT, and microporous carbon particles-is investigated with respect to their suitability as electrode materials in supercapacitors. This was accomplished by a combination of cyclic voltammetry and galvanostatic charge/discharge cycles. It was found that the PEDOT-silica composite had the lowest specific capacitance of the four materials (ca. 60 F g(-1)) and also the worst retention of the capacitance at high scan rates. In the case of pure PEDOT, microporous carbon, or PEDOT-carbon microspheres, the specific capacitances of the materials were dramatically higher (C(M) = 115, 109, and 106 F g(-1), respectively). These values are higher than those of either unstructured electropolymerized PEDOT or commercially available high-surface-area carbon. The pure PEDOT materials retained this high capacitive behavior even at faster scan rates, although the capacitance of the carbon and PEDOT-carbon microspheres dropped substantially. These results are interpreted in the context of the local microstructure of the individual colloidal particles, as well as the overall film morphology. The morphologies of both the individual particles and the electrode films were investigated by field-emission scanning electron microscopy. Due to the monodisperse nature of the microspheres, films composed of these materials necessarily possess an interconnected network of interstitial pores that allow for facile ionic diffusion. This allows for more penetration of the conjugated polymer by the ionic electrolyte and therefore higher capacitances relative to the bulk materials. These results demonstrate the feasibility of utilizing monodisperse colloidal microparticles containing conjugated polymers as electrode materials for high-energy and high-power-density supercapacitors.

Template approaches to conjugated polymer micro- and nanoparticles

The control of nanostructure and morphology in conjugated polymers is of increasing importance to organic electronics and photonics. This tutorial review discusses template-based approaches to colloidal conjugated polymer particles having diameters in the 1-1000 nm range. Both soft and hard template approaches are covered, and particular emphasis is placed on those methodologies capable of producing spherical, monodisperse particles with good colloidal stability. Factors affecting the size and morphology of the conjugated polymer microspheres are discussed. Finally, emerging applications for particles of this type are highlighted, including those in the field of photonic crystals.

Self-assembled polymetallic square grids ([2 × 2] M4, [3 × 3] M9) and trigonal bipyramidal clusters (M5)—structural and magnetic properties

New self-assembled grids and clusters are reported, with square [2 × 2] M4 (M = Mn(II)4, Cu(II)4), trigonal-bipyramidal Mn(II)5, and square [3 × 3] M9 (M = Mn(II), Cu(II)) examples. These are based on a series of ditopic and tritopic hydrazone ligands involving pyridine, pyrimidine and imidazole end groups. In all cases the metal centres are bridged by hydrazone oxygen atoms with large (>125°) bridge angles, leading to antiferromagnetic exchange for all the Mn systems (J = −2 to −5 cm−1), which results in S = 0 (Mn4), and S = 5/2 (Mn5, Mn9) ground states. The copper systems have a 90° alternation of the Jahn–Teller axes within the Cu4 and Cu8 grid rings (Cu9), which leads to magnetic orbital orthogonality, and dominant ferromagnetic coupling. For the Cu9 grid antiferromagnetic exchange between the ring and the central copper leads to a S = 7/2 ground state, while for the Cu4 grids S = 4/2 ground states are observed. The magnetic data have been treated using isotropic exchange models in the cases of the Cu4 and Cu9 grids, and the Mn5 clusters. However due to the enormity of a fully isotropic calculation a simplified model is used for the Mn9 grid, in which the outer Mn8 ring is treated as the equivalent of an isolated magnetic chain, with no coupling to the central metal ion.

Highly Stable Porous Silicon–Carbon Composites as Label-Free Optical Biosensors

A stable, label-free optical biosensor based on a porous silicon-carbon (pSi-C) composite is demonstrated. The material is prepared by electrochemical anodization of crystalline Si in an HF-containing electrolyte to generate a porous Si template, followed by infiltration of poly(furfuryl) alcohol (PFA) and subsequent carbonization to generate the pSi-C composite as an optically smooth thin film. The pSi-C sensor is significantly more stable toward aqueous buffer solutions (pH 7.4 or 12) compared to thermally oxidized (in air, 800 °C), hydrosilylated (with undecylenic acid), or hydrocarbonized (with acetylene, 700 °C) porous Si samples prepared and tested under similar conditions. Aqueous stability of the pSi-C sensor is comparable to related optical biosensors based on porous TiO(2) or porous Al(2)O(3). Label-free optical interferometric biosensing with the pSi-C composite is demonstrated by detection of rabbit IgG on a protein-A-modified chip and confirmed with control experiments using chicken IgG (which shows no affinity for protein A). The pSi-C sensor binds significantly more of the protein A capture probe than porous TiO(2) or porous Al(2)O(3), and the sensitivity of the protein-A-modified pSi-C sensor to rabbit IgG is found to be ~2× greater than label-free optical biosensors constructed from these other two materials.

Fatigue resistance of a flexible, efficient, and metal oxide-free perovskite solar cell†

Although the high efficiencies of perovskite solar cells have attracted the most attention from the photovoltaic community, one of their most attractive attributes is that flexible devices can be prepared by depositing the perovskite on plastic substrates using solution-based processing techniques. Highly flexible devices have the potential to be fabricated through roll-to-roll manufacturing, which is a fast and easy method for light weight thin-film solar cell production. In order to determine the flexibility of these perovskite-based devices, we have carried out fatigue resistance measurements on flexible perovskite solar cells by bending the devices over a cylinder with a 4 mm radius of curvature for up to 2000 cycles. We show that the main reason for the drop in performance of these devices is the formation of cracks in the indium oxide-based transparent conductive electrode. To improve device flexibility, we substituted the metal oxide electrode with a layer of highly conductive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The resulting devices were entirely metal oxide free, and displayed power conversion efficiencies as high as 7.6% with very little hysteresis. By comparing the fatigue resistances of these metal oxide-free devices with those of polymer-based solar cells, we were able to evaluate the inherent flexibility of CH3NH3PbI3 films for the first time.

Carbon and Carbon/Silicon Composites Templated in Rugate Filters for the Adsorption and Detection of Organic Vapors

inorganic salts, [ 9 ] proteins [ 10 ] and metal chalcogen semiconductors. [ 11 ] Despite the breadth of work in this fi eld and the commercial availability of the porous membranes, the sizes and shapes of the pores (and therefore of the templated nanostructures) are limited. In typical AAO membranes the pores are restricted to straight, cylindrical channels with diameters in the range of 5–500 nm. [ 1 , 2 ] In contrast, the morphology of porous silicon (pSi) layers prepared via electrochemical anodization can be controlled in a systematic way through variation of the current density, etch time, wafer resistivity, and electrolyte concentration. [ 12 ] More complex structures, including double layers, [ 13 , 14 ] Bragg stacks, [ 15 , 16 ]

Decomposition and Cell Failure Mechanisms in Lead Halide Perovskite Solar Cells.

Perovskite solar cells have experienced a remarkably rapid rise in power conversion efficiencies, with state-of-the-art devices now competing with multicrystalline silicon and thin-film cadmium telluride in terms of efficiency. Unfortunately, the lead halide perovskite absorbers suffer from a lack of chemical stability and decompose in response to a variety of environmental stimuli. In this Forum Article, we provide a brief overview of the decomposition mechanisms in lead halide perovskite thin films, as well as the processes contributing to cell failure in finished devices. We finish by briefly surveying recent efforts to extend the device lifetime. Ultimately, if perovskite solar cells can be made stable, they will be an exciting, highly complementary addition to existing photovoltaic technologies.