Introduction to "Shaping the future using thin films and nanotechnology"

Aruna Ivaturi and Oomman Varghese introduce the Materials Advances themed issue on “Shaping the future using thin films and nanotechnology”

Improving carbon coated TiO2 films with a TiCl4 treatment for photocatalytic water purification

Using a simple thermal decomposition route, carbon-TiO2 hybrid films have been synthesized from a catechol-TiO2 surface complex. The coated films display enhanced visible region absorption, owing to the thin (~2 nm) layer of carbon encapsulating the TiO2. While photocatalytically active under visible light alone, it is demonstrated that the activity of the carbon coated films can be improved further by a hydrolytic treatment with TiCl4, leading to the introduction of small TiO2 particles (5-10nm) and doping of chlorine into the structure. The combination of the carbon layer and TiCl4 treatment gives increased photocatalytic performance for the photodegradation of dyes, phenolic pollutants and the reduction of toxic Cr(VI) to relatively benign Cr(III). In addition, the carbon coated films show improved bactericidal activity under UV irradiation, and hence have been successfully tested against the most common types of pollutant present in potential drinking waters

Indoor light harvesting perovskite solar cells on conducting oxide-free ultrathin deformable substrates

Perovskite solar cells (PSCs) are receiving renewed interest since they have reached high power conversion efficiency (PCE) and show potential for application not only on rigid and flexible substrates but also on mechanically deformable substrates for integration on nonplanar curvilinear surfaces. Here we demonstrate PSCs fabricated on transparent conducting oxide-free ultrathin polyethylene terephthalate substrates capable of efficiently harvesting indoor light even under compressive strain. Interface engineering with poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) improved the shunt resistance and band alignment at the perovskite-hole transport layer interface, which resulted in enhanced charge extraction, leading to 114% improvement in PCE from 5.57 to 11.91% under 500 lx indoor white LED (4000 K) illumination. The champion device exhibited a PCE of 18.37% under 250 lx cool white LED (4000 K) light. The maximum power output (Pmax) of the devices varied from 13.78 to 25.38 μW/cm2 by changing the indoor light illumination from 250 to 1000 lx, respectively. Moreover, the devices showed impressive performance even after mechanical deformation and retained 83 and 76% for 1 sun and indoor light, respectively, under 30% compressive strain. Our approach paves the way for fabrication of efficient indoor light harvesting PSCs on mechanically deformable substrates for integration on nonplanar surfaces prone to compressive strain

Mesocrystal TiO₂ films: in situ topotactic transformation and application in dye-sensitised solar cells

Thin film ceramics and semiconductors play an important role in energy- and environment- related areas such as photovoltaics, energy storage and water purification. The morphology and structure of materials significantly affect their properties and performance in applications. Mesocrystal materials with a hierarchical structure and designable overall shape possess not only the properties from nanosized building-blocks but also collective functions from the crystallographically ordered assembly, meeting the criteria of high performance candidates in various applications. In this study, a facile and versatile method was developed to prepare mesocrystal films by simply making a printable paste of the topotactic precursor, followed by in-situ topotactic transformation of printed films. Using TiO2 as the model material, mesocrystal TiO2 films made from NH4TiOF3 paste possess high surface area, crystallographic orientation of anatase nanoparticles and overall large particle size, performing well in dye-sensitised solar cells (DSSCs) as either single-layer photoanodes or additional scattering layers

Screen-printed stretchable supercapacitors based on tin sulfide-decorated face-mask-derived activated carbon electrodes with high areal energy density

In this work, tin sulfide nanosheets decorated on face-mask-derived activated carbon have been explored as electrode material for electrochemical supercapacitors. A hydrothermal route was employed to grow tin sulfide on the surface and inside of high-surface-area face-mask-derived activated carbon, activated at 850 °C, to produce a hierarchical interconnected porous composite (ACFM-850/TS) structure. The presence of tin sulfide in the porous carbon framework exposed the surface active sites for rapid adsorption/desorption of electrolyte ions and ensured high utilization of the porous carbon surface. Furthermore, the porous ACFM-850 framework prevented the stacking/agglomeration of tin sulfide sheets, thereby enhancing the charge-transport kinetics in the composite electrodes. Benefiting from the synergistic effect of tin sulfide and ACFM-850, the resulting ACFM-850/TS composite exhibited an attractive specific capacitance of 423 F g–1 at a 0.5 A g–1 current density and superior rate capability (71.3% at a 30 A g–1 current density) in a 1.0 M Na2SO4 electrolyte. In addition, we fabricated a planar symmetric interdigitated supercapacitor on a stretchable Spandex fabric using an ACFM-850/TS composite electrode and carboxymethyl cellulose/NaClO4 as a solid-state gel electrolyte employing a scalable screen-printing process. The as-prepared stretchable supercapacitors displayed an ultrahigh energy density of 9.2 μWh cm–2 at a power density of 0.13 mW cm–2. In addition, they exhibited an excellent cyclic stability of 64% even after 10,000 charge–discharge cycles and 42% after 1000 continuous stretch (at 25% stretching)/release cycles. Such screen-printed interdigitated planar supercapacitors with activated carbon composite electrodes and a solid-state gel electrolyte act as promising low-cost energy-storage devices for wearable and flexible integrated electronic devices

Enhancement of Upconversion for Photovoltaics with β-NaYF4:Er3+ and Concentrating Integrated Optics

Renewable Energy and the Environment of the OSA Optics and Photonics Congress 2013, Tucson, USA, 3-6 November 2013The internal photoluminescence quantum yield of β-NaYF4:Er3+ is determined under broadband excitation and a photovoltaic-upconverter system with concentrating integrated optics is proposed to enhance the near-infrared response of silicon solar cells

Ultrasensitive electrochemical sensors based on Cu and Cu@Ag nanorods for simultaneous heavy metal detection

This work reports the development of ultrasensitive miniaturized electrochemical device for heavy metal sensing. A laser engraver based patterning of fluorine-doped tin oxide (FTO) sheet was done to draw an etched pattern forming a miniaturized 3-electrode configuration. A layer of Ag/AgCl ink served as pseudo-reference electrode. The sensing electrode was coated using low-cost Cu nanorods (CuNRs) grown radially along the {110} surface with aspect ratio of 8.0 and Cu@Ag core-shell nanorods (Cu@AgNRs) formed via galvanic displacement for simultaneous electrocatalytic detection of heavy metal ions (Pb(II), Cd(II), Hg(II), and Zn(II)) present in water. The electroactive surface area of the prepared devices is 0.026, 0.093 and 0.125 cm2 for bare FTO, CuNRs/FTO and Cu@AgNRs/FTO, respectively. Bimetal Cu@AgNRs/FTO sensor exhibited the lowest limit of detection of 1, 2, 5 and 6 nM, respectively, detecting Cd(II), Pb(II), Zn(II), and Hg(II) ions, and it was 2, 2, 3 and 4 nM, respectively, for simultaneous detection of Zn(II), Pb(II), Cd(II) and Hg(II). The Cu@AgNRs/FTO based device showed distinct peak-to-peak separation by 0.40, 0.25 and 0.51 V for Zn(II)-Cd(II), Cd(II)-Pb(II) and Pb(II)-Hg(II), respectively. This device was highly sensitive (583.6–1261.8 μA․μM−1․cm−2) for heavy metal detection over CuNRs/FTO (15.9–107.4 μA․μM−1․cm−2). The Cu@AgNRs/FTO based sensors demonstrated good reproducibility (relative standard deviation ≤ 5%) with recovery (>90%) in the case of all target heavy metals simultaneously present in environmental water samples. Hence, the Cu nanorods based miniaturized sensing platforms developed in the present study for simultaneous heavy metal detection are potential low-cost alternatives providing a repeatability of upto 4 cycles unlike the commercial screen-printed electrodes

Unravelling the chloride dopant induced film improvement in all-inorganic perovskite absorber

CsPbI2Br perovskite material has been the focus of much recent research, thanks to its improved stability over CsPbI3, useful bandgap of 1.9 eV and enhanced thermal stability over hybrid perovskite materials with volatile organic components. It has great potential for both single junction solar cells for indoor applications, and implementation in tandem cells. However, moisture stability has remained an issue. In order to overcome this roadblock towards commercialisation, metal chloride dopants have been widely investigated to improve film quality and reduce damage from humidity. The majority of the research to date on this topic has focussed on device performance and bulk film characteristics, with limited attention paid to grain-level crystallinity and whether the dopant is proportionally incorporated into the film. In the present work, cathodoluminescence (CL) and electron backscatter diffraction (EBSD) are utilised to investigate the effects of a lead chloride dopant, both on emission and crystal structure at a grain level, with the findings supported by X-ray diffraction (XRD). Confirmation of proportional incorporation of the dopant into the final prepared films is provided by wavelength dispersive X-ray (WDX) spectroscopy. This work provides a valuable insight into the impact dopants have on all-inorganic perovskite absorbers, helping to influence future dopant design

High Absorption Coefficient Cyclopentadithiophene Donor-Free Dyes for Liquid and Solid-State Dye-Sensitized Solar Cells

We report a series of "donor-free" dyes featuring moieties of oligo(4,4-dihexyl-4H-cyclopenta[1,2-b:5,4-b′]dithiophene) (CPDT) functionalized with cyanoacrylic end groups for mesoscopic titania solar cells based on I-/I3 - or Co(II)/Co(III) redox couple and spiro-OMeTAD hole transporter. These were compared with similar cells using an oligo(3-hexylthiophene) dye (5T), which we reported before. Extending the CPDT moiety of the dye molecules from one to three (denoted as CPDT-1, CPDT-2, and CPDT-3) widens the photoresponse overlap with the solar spectrum, increases the molar absorption coefficient up to 75-000 M-1 cm-1, and improves the short-circuit current (JSC), open-circuit voltage (VOC), and power conversion efficiency (PCE) for all types of DSSCs. Among these sensitizers, CPDT-3 shows the highest PCE of 6.7%, 7.3%, and 3.9% with I-/I3 -, Co(II)/Co(III) redox couple, and spiro-OMeTAD hole transporter, respectively, compared with 7.6%, 9.0%, and 4.0% for 5T. Benefiting from the high absorption of CPDT-3, we demonstrate 900 nm thick mesoporous TiO2 film with remarkable JSC of 10.9 mA cm-2 in solid-state DSCs

Ultra-broadband near-infrared upconversion for solar energy harvesting

Upconversion – the absorption of two or more photons resulting in radiative emission at a higher energy than the excitation – has the potential to enhance the efficiency of solar energy harvesting technologies, most notably photovoltaics. However, the required ultra-high light intensities and the narrow absorption bands of lanthanide ions limit efficient solar utilisation. In this paper, we report results from exciting upconverters with concentrated sunlight at flux densities up to 2300 suns, where the radiation is restricted to photon energies below the bandgap of silicon (corresponding to a wavelength λ = 1200 nm). Upconversion to λ = 980 nm is achieved by using hexagonal erbium-doped sodium yttrium fluoride (β-NaYF4: Er3+) in a fluoropolymer matrix. Upconversion has a nonlinear relation with irradiance, therefore at a high irradiance a threshold occurs where the process becomes linear. For β-NaYF4:25%Er3+, we find a two-photon threshold under concentrated sunlight at 320 suns. Notably, this threshold is lower than under corresponding laser excitation and can be related to all resonantly excited Er3+ ion levels and excited stated absorption. These results highlight a pathway that utilises a far broader portion of the solar spectrum for photovoltaics