Justin Searle

Investigating changes in corrosion mechanism induced by laser welding galvanised steel specimens using scanning vibrating electrode technique

Abstract A novel three-dimensional scanning vibrating electrode technique (3D SVET) apparatus is described, which uses a bifunctional probe to record topographical and current density data. This apparatus is used to investigate the localised corrosion occurring on 2 cm2 exposed areas of flat specimens of electroplated zinc and galvannealed (Zn-Fe alloy coated) 1·2 mm sheet steel and specimens of the same substrates laser welded together, freely corroding in near neutral, aerated, aqueous chloride electrolyte. On flat galvannealed (IZ) specimens anodic events are highly localised and occur at random over the exposed specimen surface during a 24 h immersion period. This reflects the progressive dezincification of zinc rich areas of the iron zinc intermetallic coating. By contrast on flat electroplated zinc (EZ) specimens anodic activity is localised but corrosion initiates at a single anodic centre, eventually spreading out to form a scar on the metallic surface. This concentration of anodic activity on the specimen leads to greater dezincification than for the IZ coating. The SVET data was used to provide an estimate of the total zinc loss from the 2 cm2 exposed area on the coupons of 544 μg for EZ and 236 μg for IZ respectively. The close physical proximity of anodic and cathodic events in the latter substrate is believed to lead to greater zinc (hydr) oxide formation and hence lower measured zinc loss. Laser welded specimens were prepared by joining IZ to IZ and IZ to EZ coated 1·2 mm steel panels. A 2 cm2 exposed area was investigated using SVET with ca. 1 cm2 exposed either side of the weld. The joining of IZ specimens together using a laser weld changes the localisation of anodic activity in neutral aerated sodium chloride solution dramatically. In this instance focal anodes initially concentrate next to the weld area in a zone enriched in zinc (and depleted in iron) as a result of the welding process. This localisation of anodic and cathodic activity next to the weld reduces the anodic damage on the IZ remote to the heat affected zone. When specimens of EZ and IZ are laser welded together all anodic activity becomes focussed on the EZ specimens with a total zinc loss over 24 h from the 1 cm2 exposed area measured as 489 μg, very close to that of the zinc loss from the EZ specimen (2 cm2) alone. By contrast there is no measurable zinc loss from the IZ portion specimen under these conditions. The increase in zinc loss per unit area from the EZ reflects the additional cathodic area provided by the connected IZ coupon and bimetallic coupling of the metallic coatings.

The role of fullerenes in the environmental stability of polymer:fullerene solar cells

Environmental stability is a common challenge for the commercialisation of low cost, encapsulation-free organic opto-electronic devices. Understanding the role of materials degradation is the key to address this challenge, but most such studies have been limited to conjugated polymers. Here we quantitatively study the role of the common fullerene derivative PCBM in limiting the stability of benchmark organic solar cells, showing that a minor fraction (<1%) of photo-oxidised PCBM, induced by short exposure to either solar or ambient laboratory lighting conditions in air, consistent with typical processing and operating conditions, is sufficient to compromise device performance severely. We identify the effects of photo-oxidation of PCBM on its chemical structure, and connect this to specific changes in its electronic structure, which significantly alter the electron transport and recombination kinetics. The effect of photo-oxidation on device current–voltage characteristics, electron mobility and density of states could all be explained with the same model of photoinduced defects acting as trap states. Our results demonstrate that the photochemical instability of PCBM and chemically similar fullerenes remains a barrier for the commercialisation of organic opto-electronic devices.

Digital imaging to simultaneously study device lifetimes of multiple dye-sensitized solar cells

In situ degradation of multiple dyes (D35, N719, SQ1 and SQ2) has been investigated simultaneously using digital imaging and colour analysis. The approach has been used to study the air stability of N719 and squaraine dyes adsorbed onto TiO2 films with the data suggesting this method could be used as a rapid screening technique for DSC dyes and other solar cell components. Full DSC devices have then been tested using either D35 or N719 dyes and these data have been correlated with UV-vis, IR and XPS spectroscopy, mass spectrometry, TLC and DSC device performance. Using this method, up to 21 samples have been tested simultaneously ensuring consistent sample exposure. Liquid electrolyte DSC devices have been tested under light soaking including the first report of D35 testing with I−/I3− electrolyte whilst operating at open circuit, short circuit, or under load, with the slowest degradation shown at open circuit. D35 lifetime data suggest that this dye degrades after ca. 370 h light soaking regardless of UV filtering. Control, N719 devices have also been light soaked for 2500 h to verify the imaging method and the N719 device data confirm that UV filtration is essential to protect the dye and I3−/I− electrolyte redox couple to maintain device lifetime. The data show a direct link between the colour intensity and/or hue of device sub-components and device degradation, enabling “real time” diagnosis of device failure mechanisms.

Homogeneous and highly controlled deposition of low viscosity inks and application on fully printable perovskite solar cells

Abstract The fully printed, hole-transporter-free carbon perovskite solar cell structure incorporating a triple mesoscopic layer has emerged as a possible frontrunner for early industrialisation. It is an attractive structure because it can be fabricated by the simple sequential screen printing and sintering of titania, zirconia, and carbon. The device is finalised by manual dropping of a perovskite precursor solution onto the carbon which subsequently infiltrates. This stage in device fabrication is inhomogeneous, ineffective for large areas, and prone to human error. Here we introduce an automated deposition and infiltration system using a robotic dispenser and mesh which delivers the perovskite precursor uniformly to the carbon surface over a large area. It has been successfully used to prepare perovskite solar cells with over 9% efficiency. Cells, prepared by this robotic mesh deposition, showed comparable performance to reference cells, made by standard drop deposition, confirming this approach to be effective and reliable. X-ray diffraction and Raman spectroscopy were used to confirm the uniformity of the deposition over a large area.

Perovskite solar cells in N-I-P structure with four slot-die-coated layers

The fabrication of perovskite solar cells in an N-I-P structure with compact titanium dioxide blocking, mesoporous titanium dioxide scaffold, single-step perovskite and hole-transport layers deposited using the slot-die coating technique is reported. Devices on fluorine-doped tin oxide-coated glass substrates with evaporated gold top contacts and four slot-die-coated layers are demonstrated, and best cells reach stabilized power conversion efficiencies of 7%. This work demonstrates the suitability of slot-die coating for the production of layers within this perovskite solar cell stack and the potential to transfer to large area and roll-to-roll manufacturing processes.

Sequential Slot-Die Deposition of Perovskite Solar Cells Using Dimethylsulfoxide Lead Iodide Ink

This work demonstrates a sequential deposition of lead iodide followed by methylammonium iodide using the industrially compatible slot-die coating method that produces homogeneous pin-hole free films without the use of the highly toxic dimethylformamide. This is achieved through the careful selection and formulation of the solvent system and coating conditions for both the lead iodide layer and the methylammonium iodide coating. The solvent system choice is found to be critical to achieving good coating quality, conversion to the final perovskite and for the film morphology formed. A range of alcohols are assessed as solvent for methylammonium iodide formulations for use in slot-die coating. A dimethylsulfoxide solvent system for the lead iodide layer is shown which is significantly less toxic than the dimethylformamide solvent system commonly used for lead iodide deposition, which could find utility in high throughput manufacture of perovskite solar cells.

Characterising the discharge cycle of CaCl 2 and LiNO 3 hydrated salts within a vermiculite composite scaffold for thermochemical storage

Transpired solar collectors (TSC) are an efficient means of building heating but due to the demand / use mismatch their capabilities are maximised when paired with a suitable storage technology. The Hydration and / dehydration of inorganic salts provides an appropriate energy storage medium which is compatible with the air temperature provided by a conventional TSC (<70 C). The study reports on technical appraisal of materials which are compatible with building scale energy storage installations. Two salts (CaCl2, and LiNO3) were impregnated into porous vermiculite to form a salt in matrix (SIM). Their performance during the discharge portion of the cycle at high packing density was examined using a laboratory scale reactor. Reactor and exit temperature increases were considerably lower than those predicted from first principles. Peak reactor temperature rises of only 14 C were observed with a reduction in temperature output from this initial peak over 60 hours. Poor salt utilization resulting from deliquescence near the reactor inlet was identified as being the source of the reduced performance. Changes in reactor size, orientation and cycling between input periods of moist and dry air did not improve reactor performance. The investigation has identified that moist air transit through the packed SIM reactor column is limited to approximately 100 mm AC CE PT ED M AN US CR IP T from the air inlet. This has implications for reactor design and the operation of any practical building scale installation. Predictions of building scale energy storage capabilities based on simple scaling of laboratory test considerably under estimate the volume and complexity of equipment required.