Cecile Charbonneau

Ultra-fast sintered TiO2 films in dye-sensitized solar cells: phase variation, electron transport and recombination

With the application of near-infrared radiation (NIR), TiO2 films for dye-sensitized solar cells (DSCs) on metallic substrates can be sintered in just 12.5 seconds. The photovoltaic performance of devices made with NIR sintered films match those devices made with conventionally sintered films prepared by heating for 1800 seconds. Here we characterise the electron transport, electron lifetime and phase-morphological properties of ultrafast NIR sintered films, using impedance spectroscopy, transient photovoltage decay and X-ray diffraction measurements. An important factor in NIR processing of TiO2 films is the peak metal temperature (PMT) and we show that during the 12.5 second heat treatment, a PMT of around 635 °C gives near identical electron transport, electron lifetime and morphological properties, as well comparable photovoltaic performance to a conventionally sintered (500 °C, 30 min) film. We demonstrate that the rapid heating of the TiO2 (to temperatures of up to 785 °C) does not lead to a large scale rutile phase transition. As such photovoltaic performance of resultant DSC devices is maintained since the heating period is insufficient to induce a significant transition from anatase to rutile or morphology changes which result in a loss of photocurrent.

Rapid processing of perovskite solar cells in under 2.5 seconds

A rapid annealing technique for CH3NH3PbI3 perovskite solar cells is presented. We report a co-deposited Al2O3–perovskite device annealed in under 2.5 seconds with a PCE of 10.0% compared to 10.9% for a 45 minute oven-annealed device.

Performance enhancement of solution processed perovskite solar cells incorporating functionalized silica nanoparticles

High efficiency, solution processed organic–inorganic trihalide perovskite solar cells are now a reality, meaning that perovskite photovoltaics have the potential to challenge more established photovoltaic technologies. To date, some of the most efficient solution processed perovskite solar cells feature a pre-deposited Al2O3 scaffold and we have shown in a previous communication, that it is possible to make efficient devices by co-depositing the Al2O3 nanoparticles with the perovskite precursor solution. In this work, we have substituted the alumina nanoparticles with 3-aminopropyl (3-oxobutanoic acid) functionalized silica nanoparticles (f-SiO2). We observe performance enhancements in planar heterojunction (PHJ) devices made with up to 0.75 wt% f-SiO2 nanoparticles present in the precursor solution, yielding power conversion efficiencies (PCE) of up to 12.4%, compared to the maximum PCE of 10.5% in the equivalent PHJ devices made without f-SiO2 nanoparticles. The performance enhancement arises in part from an average increase to VOC by up to 50 mV when the nanoparticles are present in the precursor solution and is attributed to substrate passivation within pinholes formed in the perovskite film during processing.

Aqueous Solution Synthesis of Crystalline Anatase Nanocolloids for the Fabrication of DSC Photoanodes

The production of anatase nanocolloids via a novel aqueous solution synthesis method is described. The process involves forced hydrolysis of 0.2 M TiCl 4 aqueous solution at low temperature (80°C) leading to fast nucleation of Ti0 2 anatase nanoparticles (4―5 nm). The isolated nanopowders were found to consist predominantly of anatase as the major phase ( ∼ 85 wt %), possess a very high surface area (up to 250-350 m 2 /g), and a relatively elevated surface hydroxyl content in comparison with the P25 reference material. Single-layer (∼ 12 μm thick) photoanodes prepared with aqueous-synthesized powders were found to have a specific surface area of ∼ 130 m 2 /g and the anatase nanocrystallites to have grown to ∼12 nm and assume a higher degree of crystallinity but still carry ∼ 1.5% surface OH groups. The pore size of the film was determined to be ∼8 nm. By comparison, the film prepared with P25 nanoparticles had the following properties: 50 m 2 /g, 30 nm crystallite size, 0.2% surface OH, and 25 nm pore size. Dye-sensitized solar cell (DSC) devices assembled with the newly fabricated photoanodes showed equivalent photoelectrochemical performance [power conversion efficiency η, short-circuit current voltage I sc , open-circuit voltage V oc , and Fill Factor (FF)] to cells prepared using the benchmark P25 titania powder.

Preparation and DSSC Performance of Mesoporous Film Photoanodes Based on Aqueous-Synthesized Anatase Nanocrystallites

In this work, anatase nanocrystallites are synthesized from an aqueous titanium(IV) chloride solution by hydrolysis and are used to fabricate mesoporous TiO 2 film photoanodes. The aqueous-synthesized titania nanocrystallites possess high specific surface area (250-350 m 2 /g), 4-5 nm crystallite size, enlarged bandgap, and enhanced surface hydroxylation. Current-voltage measurements of dye-sensitized solar cells (DSSCs) based on the aqueous-synthesized titania showed higher photocurrent (13.0 mA/cm 2 ) and conversion efficiency (5.0%) when compared to cells built from commercial Dyesol (10.7 mA/cm 2 , 4.8%) or P25 (8.5 mA/cm 2 , 4.1%) benchmark nanotitania materials. The enhanced performance is discussed in terms of improved dye loading and associated electron injection and transport facilitated by the nanocrystallite surface characteristics.

Near Infrared Radiation as a Rapid Heating Technique for TiO2Films on Glass Mounted Dye-Sensitized Solar Cells

Near infrared radiation (NIR) has been used to enable the sintering of TiO2 films on fluorine-doped tin oxide (FTO) glass in 12.5 s. The 9 µm thick TiO2 films were constructed into working electrodes for dye-sensitized solar cells (DSCs) achieving similar photovoltaic performance to TiO2 films prepared by heating for 30 min in a convection oven. The ability of the FTO glass to heat upon 12.5 s exposure of NIR radiation was measured using an IR camera and demonstrated a peak temperature of 680°C; glass without the 600 nm FTO layer reached 350°C under identical conditions. In a typical DSC heating step, a TiO2 based paste is heated until the polymeric binder is removed leaving a mesoporous film. The weight loss associated with this step, as measured using thermogravimetric analysis, has been used to assess the efficacy of the FTO glass to heat sufficiently. Heat induced interparticle connectivity in the TiO2 film has also been assessed using optoelectronic transient measurements that can identify electron lifetime through the TiO2 film. An NIR treated device produced in 12.5 seconds shows comparable binder removal, electron lifetime, and efficiency to a device manufactured over 30 minutes in a conventional oven.

Effect of TiO2 Photoanode Porosity on Dye Diffusion Kinetics and Performance of Standard Dye-Sensitized Solar Cells

Low-cost water-based P25-TiO2 pastes were formulated and used to produce porous TiO2 films in application to the fabrication of dye-sensitized solar cells. The structural properties of the films were characterized using a variety of techniques such as stylus profilometry, FEG-SEM imaging, BET surface area, and BJH pore size analyses. These were compared to films produced from a commercial paste, DSL 18 NR-AO Dyesol. The major difference was in the fraction of macroporosity: 23% of the total pore volume for films produced with the commercial material and 67–73% for the P25-TiO2 films owing to the vast difference in dispersion and size distribution of the particles in the two types of pastes. The macroporosity was found to have a dramatic effect on the dye diffusion kinetics measured using in situ UV-Vis reflectance spectroscopy. The sensitization of P25-based films was much faster for heavily macroporous P25-TiO2 films >90% saturation at 15–35 mins than for their commercial analogue >90% saturation at 110 mins. DSC devices built with optimized P25-TiO2 photoanodes showed better performance at short dye immersion time 30 mins and 1 hr due to faster percolation of the dye molecules through the film.

Facile self-assembly and stabilization of metal oxide nanoparticles

This paper describes a facile method of self-assembling different metal oxide nanoparticles into nanostructured materials via di-carboxylate linkers (oxalic acid) using TiO2 as an example. In this method, the di-carboxylate linkers react with surface hydroxyls on metal oxide nanoparticles forming covalent, ester-like bonds, which enable the binding of two metal oxide particles, one at either end of the linker and facilitates efficient self-assembly of one group of metal oxide nanoparticles homogeneously distributed onto the surface of another group. The oxalate linkers can then be removed by thermal decomposition. This approach is shown to be effective using differently-sized TiO2 nanoparticles, namely in-house synthesized 3-5nm anatase nanocrystals and Degussa P25 titania particles (mean 21nm particle size). Our data show that the application of a high temperature heat treatment (450°C for 30min), conventionally applied to achieve a stable porous structure by thermal decomposition of the linker molecules and by inducing inter-particle necking, damages the surface area of the nanostructured material. However, here we show that sintering at 300°C for 30min or by flash near infrared radiation sintering for 12s efficiently decomposes the oxalate linkers and stabilizes the nanostructure of the material whilst maintaining its high surface area.

Low temperature sintering of binder-containing TiO2/metal peroxide pastes for dye-sensitized solar cells

Nano-structured metal oxide films are key components of dye-sensitized (DSC) solar cells. Scaling such devices requires lower temperature processing to enable cheaper substrates to be used. In this context, we report a new and scalable method to sinter binder-containing metal oxide pastes to make DSC photo-electrodes at lower temperatures. Metal peroxide powders (CaO2, MgO2, or ZnO2) were added to terpineol-based P25 pastes containing ethyl cellulose binder or to commercial TiO2 paste (DSL18NR-T). Thermal analysis shows that binder decomposition occurs at 300 °C instead of the standard 450 °C for a TiO2-only paste and suggests that the metal peroxides act as combustion promoters releasing heat and oxygen within the film while heating. The data show that this heat and oxygen release coincide best with binder combustion for ZnO2 and DSC device tests show that adding ZnO2 to TiO2 pastes produces the best performances affording η = 7.5% for small devices (0.26 cm2) and η = 5.7% at 300 °C or 450 °C for DSL18NR-T/ZnO2 for larger (1 cm2) devices. To the best of our knowledge, the performance of the (0.26 cm2) cells is comparable to the highest efficiency devices reported for DSCs fabricated using low temperature methods. The device efficiency is most strongly linked with Jsc; BET and dye sorption measurements suggest that Jsc is linked with the metal oxide surface area and dye loading. The latter is linked to the availability of surface sorption sites for dye molecules which is strongly negatively affected by any residual organic binder which resulted from incomplete combustion.

Compositions, colours and efficiencies of organic–inorganic lead iodide/bromide perovskites for solar cells

Abstract Developments in the field of organic–inorganic lead halide perovskite based solar cells have metaphorically opened the door to an exciting new solid state photovoltaic technology that retains the low cost and processability benefits of dye sensitised solar cells (DSCs). Encouraging device efficiencies of greater than 10% on both titania and alumina, and as a planar device, have been reported in the literature. Here we report the characteristics of a series of lead iodide/bromide perovskites. As one might expect, changing the halide(s) alters the crystal structure and band gap of the perovskite which results in the possibility of vivid and colourful solar cells; a characteristic which is seen as one of the main benefits of DSCs. X-ray diffraction and scanning electron microscopy with elemental mapping via energy dispersive X-ray analysis has been used to characterise perovskites on sensitised thin films. UV-vis and diffuse reflectance has been used to study the optical characteristics of crystallised perovskite thin films. Finally device performance is evaluated and suitability for use in photovoltaic devices is discussed.