James D. McGettrick

High throughput fabrication of mesoporous carbon perovskite solar cells

The screen printed mesoporous carbon perovskite solar cell has great potential for commercialisation due to its scalable deposition processes and use of inexpensive materials. However, each layer requires long high temperature heating steps to achieve the necessary sintering and porosity, which is very time and energy intensive for large scale production. Near infrared processing is demonstrated here to reduce the heating time of mesoporous layers within a fully printed lead halide perovskite solar cell from 2 hours to 30 seconds. A stabilised efficiency of 11% was achieved by processing in 30 seconds, identical to that of devices heated in 2 hours. For the first time the effect of residual binder in the carbon electrode on the electron lifetime and charge transfer within devices has been investigated. Furthermore cross section EDX mapping of perovskite infiltration provides a greater understanding into the processing requirements of these devices vital to enable commercialisation.

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.

Active removal of waste dye pollutants using Ta3N5/W18O49 nanocomposite fibres

A scalable solvothermal technique is reported for the synthesis of a photocatalytic composite material consisting of orthorhombic Ta3N5 nanoparticles and WOx≤3 nanowires. Through X-ray diffraction and X-ray photoelectron spectroscopy, the as-grown tungsten(VI) sub-oxide was identified as monoclinic W18O49. The composite material catalysed the degradation of Rhodamine B at over double the rate of the Ta3N5 nanoparticles alone under illumination by white light, and continued to exhibit superior catalytic properties following recycling of the catalysts. Moreover, strong molecular adsorption of the dye to the W18O49 component of the composite resulted in near-complete decolourisation of the solution prior to light exposure. The radical species involved within the photocatalytic mechanisms were also explored through use of scavenger reagents. Our research demonstrates the exciting potential of this novel photocatalyst for the degradation of organic contaminants, and to the authors’ knowledge the material has not been investigated previously. In addition, the simplicity of the synthesis process indicates that the material is a viable candidate for the scale-up and removal of dye pollutants on a wider scale.

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.

Impact of Aggregation on the Photochemistry of Fullerene Films: Correlating Stability to Triplet Exciton Kinetics

The photochemistry and stability of fullerene films is found to be strongly dependent upon film nanomorphology. In particular, PC61BM blend films, dispersed with polystyrene, are found to be more susceptible to photobleaching in air than the more aggregated neat films. This enhanced photobleaching correlated with increased oxygen quenching of PC61BM triplet states and the appearance of a carbonyl FTIR absorption band indicative of fullerene oxidation, suggesting PC61BM photo-oxidation is primarily due to triplet-mediated singlet oxygen generation. PC61BM films were observed to undergo photo-oxidation in air for even modest (≤40 min) irradiation times, degrading electron mobility substantially, indicative of electron trap formation. This conclusion is supported by observation of red shifts in photo- and electro-luminescence with photo-oxidation, shown to be in agreement with time-dependent density functional theory calculations of defect generation. These results provide important implications on the environmental stability of PC61BM-based films and devices.

Copper-complexed isonicotinic acid functionalized aluminum oxide nanoparticles

The functionalization of two different alumina morphologies (thin film and nanoparticle) with carboxylic acids is reported. Advancing contact angle measurements show that isonicotinic acid functionalization of a native aluminium oxide surface result in an increase in hydrophilicity. Isonicotinic acid and nicotinic acid functionalised alumina nanoparticles (iNA- NP and NA-NP, respectively) have been characterized by Fourier transform infrared-attenuated total reflectance spectroscopy (FTIR-ATR), thermo gravimetric analysis-infrared spectroscopy (TGA-IR), electron dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM). The surface grafting density for iNA-NP and NA-NP is determined to be 5.4 and 15.6 molecules/nm 2 , respectively. The reaction of iNA-NP with copper(II) acetate has been studied and the stability of the resulting complex species (CuA-iNA-NP) have been determined as a function of pH. UV-visible-near IR absorption spectroscopy indicates uptake of copper acetate from solution, and X-ray photoelectron spectroscopy (XPS) shows a small shift in the nanoparticle N 1 s binding energy after complex formation with copper(II) acetate.

Thin Film Tin Selenide (SnSe) Thermoelectric Generators Exhibiting Ultralow Thermal Conductivity

Tin selenide (SnSe) has attracted much attention in the field of thermoelectrics since the discovery of the record figure of merit (ZT) of 2.6 ± 0.3 along the b-axis of the material. The record ZT is attributed to an ultralow thermal conductivity that arises from anharmonicity in bonding. While it is known that nanostructuring offers the prospect of enhanced thermoelectric performance, there have been minimal studies in the literature to date of the thermoelectric performance of thin films of SnSe. In this work, preferentially orientated porous networks of thin film SnSe nanosheets are fabricated using a simple thermal evaporation method, which exhibits an unprecedentedly low thermal conductivity of 0.08 W m-1 K-1 between 375 and 450 K. In addition, the first known example of a working SnSe thermoelectric generator is presented and characterized.

A perspective on using experiment and theory to identify design principles in dye-sensitized solar cells

ABSTRACT Dye-sensitized solar cells (DSCs) have been the subject of wide-ranging studies for many years because of their potential for large-scale manufacturing using roll-to-roll processing allied to their use of earth abundant raw materials. Two main challenges exist for DSC devices to achieve this goal; uplifting device efficiency from the 12 to 14% currently achieved for laboratory-scale ‘hero’ cells and replacement of the widely-used liquid electrolytes which can limit device lifetimes. To increase device efficiency requires optimized dye injection and regeneration, most likely from multiple dyes while replacement of liquid electrolytes requires solid charge transporters (most likely hole transport materials – HTMs). While theoretical and experimental work have both been widely applied to different aspects of DSC research, these approaches are most effective when working in tandem. In this context, this perspective paper considers the key parameters which influence electron transfer processes in DSC devices using one or more dye molecules and how modelling and experimental approaches can work together to optimize electron injection and dye regeneration. Graphical Abstract This paper provides a perspective that theory and experiment are best used in tandem to study DSC devices

Investigation into the effects of surface stripping ZnO nanosheets

ZnO nanosheets are polycrystalline nanostructures that are used in devices including solar cells and gas sensors. However, for efficient and reproducible device operation and contact behaviour the conductivity characteristics must be controlled and surface contaminants removed. Here we use low doses of argon bombardment to remove surface contamination and make reproducible lower resistance contacts. Higher doses strip the surface of the nanosheets altering the contact type from near-ohmic to rectifying by removing the donor-type defects, which photoluminescence shows to be concentrated in the near-surface. Controlled doses of argon treatments allow nanosheets to be customised for device formation.

Screen printed carbon CsPbBr3 solar cells with high open-circuit photovoltage

Screen printed mesoporous carbon solar cells (mC-PSC) are a promising fully printable technology that does not require organic hole conductors, expensive metal contacts or vacuum processing. However, when infiltrated with the archetypal CH3NH3PbI3 perovskite, mC-PSCs show low voltage which limits their use in innovative applications such as indoor light harvesting. Here we investigate both planar (C-PSC) and mesoporous (mC-PSC) carbon cells, based on all-inorganic CsPbBr3. Pure CsPbBr3 is a yellow material with an orthorhombic crystal structure at room temperature and a 2.3 eV band gap, which is not ideal for solar cell applications. However, CsPbBr3 is thermally stable up to over 400 °C and high-voltage planar carbon solar cells, with open circuit voltages of up to 1.29 V and efficiencies up to 6.7% have been reported in the literature. We focus on the effect of the post-annealing temperature on the material properties and photovoltaic activity. XPS and XRD results show a non-linear trend with temperature, with significant improvements in composition between 200 and 300 °C. Both the mesoporous and planar champion devices were obtained after heat processing at 400 °C, reaching PCEs of 8.2% and 5.7% respectively. The average Voc for the planar and mesoporous devices were 1.33 V and 1.27 V respectively with a record 1.44 V for the best mC-PSC.