Leone Spiccia

Degradation observations of encapsulated planar CH3NH3PbI3 perovskite solar cells at high temperatures and humidity

The stability of encapsulated planar-structured CH3NH3PbI3 (MAPbI3) perovskite solar cells (PSCs) was investigated under various simulated environmental conditions. The tests were performed under approximately one sun (100 mW cm−2) illumination, varying temperature (up to 85 °C cell temperature) and humidity (up to 80%). The application of advanced sealing techniques improved the device stability, but all devices showed significant degradation after prolonged aging at high temperature and humidity. The degradation mechanism was studied by post-mortem analysis of the disassembled cells using SEM and XRD. This revealed that the degradation was mainly due to the decomposition of MAPbI3, as a result of reaction with H2O, and the subsequent reaction of hydroiodic acid, formed during MAPbI3 decomposition, with the silver back contact electrode layer.

A Fast Deposition‐Crystallization Procedure for Highly Efficient Lead Iodide Perovskite Thin‐Film Solar Cells

Thin-film photovoltaics based on alkylammonium lead iodide perovskite light absorbers have recently emerged as a promising low-cost solar energy harvesting technology. To date, the perovskite layer in these efficient solar cells has generally been fabricated by either vapor deposition or a two-step sequential deposition process. We report that flat, uniform thin films of this material can be deposited by a one-step, solvent-induced, fast crystallization method involving spin-coating of a DMF solution of CH3NH3PbI3 followed immediately by exposure to chlorobenzene to induce crystallization. Analysis of the devices and films revealed that the perovskite films consist of large crystalline grains with sizes up to microns. Planar heterojunction solar cells constructed with these solution-processed thin films yielded an average power conversion efficiency of 13.9±0.7% and a steady state efficiency of 13% under standard AM 1.5 conditions.

Nanomaterials: Applications in Cancer Imaging and Therapy

The application of nanomaterials (NMs) in biomedicine is increasing rapidly and offers excellent prospects for the development of new non-invasive strategies for the diagnosis and treatment of cancer. In this review, we provide a brief description of cancer pathology and the characteristics that are important for tumor-targeted NM design, followed by an overview of the different types of NMs explored to date, covering synthetic aspects and approaches explored for their application in unimodal and multimodal imaging, diagnosis and therapy. Significant synthetic advances now allow for the preparation of NMs with highly controlled geometry, surface charge, physicochemical properties, and the decoration of their surfaces with polymers and bioactive molecules in order to improve biocompatibility and to achieve active targeting. This is stimulating the development of a diverse range of nanometer-sized objects that can recognize cancer tissue, enabling visualization of tumors, delivery of anti-cancer drugs and/or the destruction of tumors by different therapeutic techniques.

High-efficiency dye-sensitized solar cells with ferrocene-based electrolytes

Dye-sensitized solar cells based on iodide/triiodide (I(-)/I(3)(-)) electrolytes are viable low-cost alternatives to conventional silicon solar cells. However, as well as providing record efficiencies of up to 12.0%, the use of I(-)/I(3)(-) in such solar cells also brings about certain limitations that stem from its corrosive nature and complex two-electron redox chemistry. Alternative redox mediators have been investigated, but these generally fall well short of matching the performance of conventional I(-)/I(3)(-) electrolytes. Here, we report energy conversion efficiencies of 7.5% (simulated sunlight, AM1.5, 1,000 W m(-2)) for dye-sensitized solar cells combining the archetypal ferrocene/ferrocenium (Fc/Fc(+)) single-electron redox couple with a novel metal-free organic donor-acceptor sensitizer (Carbz-PAHTDTT). These Fc/Fc(+)-based devices exceed the efficiency achieved for devices prepared using I(-)/I(3)(-) electrolytes under comparable conditions, revealing the great potential of ferrocene-based electrolytes in future dye-sensitized solar cells applications. This improvement results from a more favourable matching of the redox potential of the ferrocene couple with that of the new donor-acceptor sensitizer.

Water-oxidation catalysis by manganese in a geochemical-like cycle

Water oxidation in all oxygenic photosynthetic organisms is catalysed by the Mn₄CaO₄ cluster of Photosystem II. This cluster has inspired the development of synthetic manganese catalysts for solar energy production. A photoelectrochemical device, made by impregnating a synthetic tetranuclear-manganese cluster into a Nafion matrix, has been shown to achieve efficient water oxidation catalysis. We report here in situ X-ray absorption spectroscopy and transmission electron microscopy studies that demonstrate that this cluster dissociates into Mn(II) compounds in the Nafion, which are then reoxidized to form dispersed nanoparticles of a disordered Mn(III/IV)-oxide phase. Cycling between the photoreduced product and this mineral-like solid is responsible for the observed photochemical water-oxidation catalysis. The original manganese cluster serves only as a precursor to the catalytically active material. The behaviour of Mn in Nafion therefore parallels its broader biogeochemistry, which is also dominated by cycles of oxidation into solid Mn(III/IV) oxides followed by photoreduction to Mn²⁺.

Solar driven water oxidation by a bioinspired manganese molecular catalyst

A photoelectrochemical cell was designed that catalyzes the photooxidation of water using visible light as the sole energy source and a molecular catalyst, [Mn(4)O(4)L(6)](+) (1(+), L = bis(methoxyphenyl)phosphinate), synthesized from earth-abundant elements. The essential features include a photochemical charge separation system, [Ru(II)(bipy)(2)(bipy(COO)(2))], adhered to titania-coated FTO conductive glass, and 1(+) embedded within a proton-conducting membrane (Nafion). The complete photoanode represents a functional analogue of the water-oxidizing center of natural photosynthesis.

Sustained Water Oxidation Photocatalysis by a Bioinspired Manganese Cluster

The creation of efficient catalysts for splitting water into H2 and O2 is one of the greatest challenges for chemists working on the production of renewable fuel. The water oxidizing center (WOC) within photosynthetic organisms is the only natural system able to efficiently photooxidize water using visible light, and is thus a blueprint for catalyst design. One of the atomic structural models of the WOC derived from X-ray diffraction involves a “cubelike” core comprised of a {CaMn3O4} unit tethered to a fourth manganese atom through one or two bridging oxo units. A few nonbiological tetramanganese complex mimics of this site have been prepared that contain an incomplete or distorted cubic {Mn4Ox} core [4–7] or are part of a larger Mnx–oxo lattice. [4] However, none of these have shown activity towards water oxidation. We have previously synthesized a prototypical molecular manganese–oxo cube [Mn4O4] n+ in a family of “cubane” complexes [Mn4O4L6], where L is a diarylphosphinate ligand (p-R-C6H4)2PO2 (R=H, alkyl, OMe). The diphenylphosphinate complex (1, R=H, Figure 1) assembles spontaneously from manganese(II) and permanganate salts in high yield in non-aqueous solvents. The release of O2 by the {Mn4O4} 6+ core in 1 was shown to be possible on thermodynamic grounds, but cannot take place because of the rigidity of the core arising from the six diarylphosphinate ligands, which bridge pairs of manganese atoms on the six cube faces. The assembly of 1 is also driven by intramolecular van der Waals forces that attract three aryl rings from adjacent phosphinate ligands. The cubic core in 1 is a much stronger oxidant than any known dimanganese complex with {Mn2O2} 3+ cores. Cubane 1 abstracts hydrogen atoms from various organic substrates by breaking O H and N H bonds with dissociation energies greater than 390 kJmol . Titrations of 1 against compounds containing either amine or phenol groups reach an end point after the abstraction of four successive hydrogen atoms, yielding two water molecules (from corner oxo groups) plus [L6Mn4O2], the so-called “pinned butterfly” complex 2 (Scheme 1). {Mn4O4} cubane complexes are unique in releasing an O2 molecule upon photoexcitation of the Mn !O charge transfer band, which reaches a maximum at 350 nm. This process, which occurs with high quantum efficiency only in the gas phase, involves the core oxygen atoms and is triggered by ejection of one phosphinate ligand, thereby generating the [L5Mn4O2] + “butterfly” complex 3 (Scheme 1). In contrast, noncuboidal manganese molecular complexes possessing {Mn2O}, {Mn2O2}, and {Mn3O6} cores in the Mn or Mn oxidation states fail to release O2, but instead photodecompose into multiple fragments. Thus, O2 release is favored by complexes with a {Mn4O4} cubane core. The composition of the butterfly complexes 2 and 3 differs only by one phosphinate ligand (Scheme 1). This finding suggests the possibility of creating a catalytic cycle that could oxidize two water molecules bound to 2 along the reverse pathway in Scheme 1 (1-3H!1-2H!1-H!1), eventually forming 3 by photochemical release of O2 and a phosphinate ligand. Thus far it has proved impossible to realize a catalytic cycle, as in Scheme 1, because O2 is not photodissociated from 1 or 1 (the one-electron oxidized cubane) in condensed phases. This was attributed to a large activation barrier for O2 release when all the phosphinate ligands remain ligated or re-ligate by fast geminate recombination. Figure 1. X-ray crystal structure of 1.

Highly active nickel oxide water oxidation catalysts deposited from molecular complexes

Nickel oxide (NiOx) water oxidation catalysts with high catalytic activity have been electrodeposited from [Ni(en)3]Cl2 (en = 1,2-diaminoethane, NiOx-en) in a 0.10 M borate buffer (NaBi) solution (pH = 9.2). Electrolysis experiments at a fixed applied potential of 1.1 V (vs. Ag/AgCl) established that the NiOx-en films sustain a stable current of 1.8 mA cm−2 for extended periods, compared with 1.2 mA cm−2 for films derived from [Ni(OH2)6](NO3)2 and [Ni(NH3)6]Cl2 when tested in a 0.60 M NaBi buffer. XAS studies indicate that the γ-NiOOH phase is formed in each case whereas SEM studies revealed significant differences in film morphology. The NiOx-en films were found to be more homogenous and to have a higher electroactive surface area, as determined from capacitance measurements. The results highlight the influence that the choice of molecular precursor can have on the activity and robustness of electrodeposited NiOx water oxidation catalysts.

Highly Efficient p-Type Dye-sensitized Solar Cells based on Tris(1,2-diaminoethane)Cobalt(II)/(III) Electrolytes

Co-produced: using [Co(en)(3)](2+/3+) based-electrolytes in p-type dye-sensitized solar cells (p-DSCs) gives record energy conversion efficiencies of 1.3 % and open-circuit voltages up to 709 mV under simulated sun light. The increase in photovoltage is due to the more negative redox potential of [Co(en)(3)](2+/3+) compared to established mediators.

Dye Regeneration Kinetics in Dye-Sensitized Solar Cells

The ideal driving force for dye regeneration is an important parameter for the design of efficient dye-sensitized solar cells. Here, nanosecond laser transient absorption spectroscopy was used to measure the rates of regeneration of six organic carbazole-based dyes by nine ferrocene derivatives whose redox potentials vary by 0.85 V, resulting in 54 different driving-force conditions. It was found that the reaction follows the behavior expected for the Marcus normal region for driving forces below 29 kJ mol(-1) (ΔE = 0.30 V). Driving forces of 29-101 kJ mol(-1) (ΔE = 0.30-1.05 V) resulted in similar reaction rates, indicating that dye regeneration is diffusion controlled. Quantitative dye regeneration (theoretical regeneration yield 99.9%) can be achieved with a driving force of 20-25 kJ mol(-1) (ΔE ≈ 0.20-0.25 V).