Matthew L. Davies

A Transparent Conductive Adhesive Laminate Electrode for High‐Efficiency Organic‐Inorganic Lead Halide Perovskite Solar Cells

A self-adhesive laminate solar-cell electrode is presented based on a metal grid embedded in a polymer film (x-y conduction) and set in contact with the active layer using a pressure-sensitive adhesive containing a very low quantity (1.8%) of organic conductor, which self-organizes to provide z conduction to the grid. This ITO-free material performs in an identical fashion to evaporated gold in high-efficiency perovskite solar cells.

Perovskite processing for photovoltaics: a spectro-thermal evaluation

Thermal analysis (TGA and DSC), coupled with evolved gas FTIR spectroscopy, has been used to study the changes occurring during, and differences between materials after, the annealing step of mixed-halide methylammonium lead halide perovskites. This is important because, to date, the material is the most efficient light harvester in highly efficient, 3rd generation perovskite photovoltaic devices, and processing plays a significant role in device performance. TGA-FTIR data show only solvent evolution during the annealing step, whilst post-annealing analysis shows that the resulting material still contains a significant amount of residual solvent; however, efficient DMF removal was possible using a silica gel desiccant for a period of 3 days. The data also show that methylammonium halide decomposition does not occur until temperatures well above those used for perovskite processing, suggesting that this is not a significant issue for device manufacture. The absence of a well-defined, reversible tetragonal – cubic phase change around 55 °C in the DSC data of the annealed material, and the presence of HCl in evolved gas analysed following thermal decomposition, demonstrates that CH3NH3I3−xClx does retain some Cl after annealing and does not simply form stoichiometric CH3NH3PbI3 as has been suggested by some workers.

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.

Development of selective, ultra-fast multiple co-sensitization to control dye loading in dye-sensitized solar cells

Enhancing the spectral response of dye-sensitized solar cells (DSC) is essential to increasing device efficiency and a key approach to achieve this is co-sensitization (i.e. the use of multiple dyes to absorb light from different parts of the solar spectrum). However, precise control of dye loading within DSC mesoporous metal oxide photo-anodes is non-trivial especially for very rapid processing (minutes). This is further complicated by dyes having very different partition (Kd) and molar extinction (e) coefficients which strongly influence dye uptake and spectral response, respectively. Here, we present a highly versatile, ultra-fast (ca. 5 min) desorption and re-dyeing method for dye-sensitized solar cells which can be used to precisely control dye loading in photo-electrode films. This method has been successfully applied to re-dye, partially desorb and re-dye and selectively desorb and re-dye photo-electrodes using examples of a Ru-bipy dye (N719) and also organic dyes (SQ1 and D149) giving η up to 8.1% for a device containing the organic dye D149 and re-dyed with the Ru dye N719. The paper also illustrates how this method can be used to rapidly screen large numbers of dyes (and/or dye combinations) and also illustrates how it can also be used to selectively study dye loading.

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.

A study of dye anchoring points in half-squarylium dyes for dye-sensitized solar cells

This paper reports the synthesis of a series of new half-squaraine dyes (Hf-SQ) based around a common chromophoric unit consisting of linked indoline and squaric acid moieties. Carboxylate groups have been incorporated onto this core structure at four different points to study the influence of the anchoring group position on dye-sensitized solar cell (DSC) device performance. Dyes have been linked to TiO2 directly through the squaric acid moiety, through a modified squaric acid unit where a vinyl dicyano group has replaced one carbonyl, via an alkyl carboxylate attached to the indole N or through a carboxylate attached to the 4 position of a benzyl indole. Contact angle measurements have been studied to investigate the hydrophobic/hydrophilic properties of the dyes and the results have been compared to N719 and Z907. Full characterization data of all the dyes and synthetic intermediates are reported including single-crystal X-ray structural analysis for dye precursors; the indole (2a) and the half-squarylium esters (3a) and (6b), as well as the dyes (4c), (8) and (12). Dye colours range from yellow to red/brown in solution (λmax range from 430 to 476 nm) with e ranging from 38000 to 133100 M−1 cm−1. The performance of the dyes in DSCs shows the highest efficiency yet reported for a Hf-SQ dye (η = 5.0%) for 1 cm2 devices with a spectral response ranging from 400 to 700 nm depending on the dye substituents. Co-sensitization of half-squarylium dye (7b) with squaraine dye (SQ2) resulted in a broader spectral response and an improved device efficiency (η = 6.1%). Density functional theory (DFT) calculations and cyclic voltammetry have been used to study the influence of linker position on dye HOMO–LUMO levels and the data has been correlated with I–V and EQE data.

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.

Effect of Aggregation on the Photophysical Properties of Three Fluorene—Phenylene—Based Cationic Conjugated Polyelectrolytes

The effect of aggregation on the photophysical properties of three cationic poly{9,9-bis[N,N-(trimethylammonium)hexyl] fluorene-co-l,4-phenylene} polymers with average chain lengths of ∼6, 12, and 100 repeat units (PFP-NR3(6(I),12(Br),100(Br))) has been studied by steady-state and time-resolved fluorescence techniques. Conjugated polyelectrolytes are known to aggregate in solution and for these PFP-NR3 polymers this causes a decrease in the fluorescence quantum yield. The use of acetonitrile as a cosolvent leads to the breakup of aggregates of PFP-NR3 in water; for PFP-NR3(6(I)), this results in an ∼10-fold increase in fluorescence quantum yield, a ca. 2-fold increase in the molar extinction coefficient at 380 nm, and an increase in the emission lifetime, as compared with polymer behavior in water. Fluorescence anisotropy also decreases with increasing aggregation, and this is attributed to increased fluorescence depolarization by interchain energy transfer in aggregate PFP-NR3 clusters. Förster resonance energy transfer along the polymer chain is expected to be very fast, with a calculated FRET rate constant of 7.3 × 10(12) s(-1) and a Förster distance of 2.83 nm (cf. the polymer repeat unit separation of 0.840 nm) for PFP-NR3(100(Br)). The complex polymer excited-state decay kinetics in aggregated PFP-NR3 systems have been successfully modeled in terms of intrachain energy transfer via migration and trapping at interchain aggregate trap sites, with model parameters in good agreement with data from picosecond time-resolved studies and the calculated theoretical Förster energy-transfer rates.

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.

Multiple linker half-squarylium dyes for dye-sensitized solar cells; are two linkers better than one?

The synthesis and full characterization of new half-squaraine dyes (Hf-SQ) containing two or three carboxylate-based linker units is reported and these dyes tested in dye-sensitized solar cell (DSC) devices. The data show improved device efficiency for a Hf-SQ dye with two linkers (η = 5.5%) compared to the highest efficiency Hf-SQ previously reported which had only a single linker (η = 5.0%); this is mainly due to improved Voc. To understand the effects of using multiple dye linker groups, device I–V data have been correlated with single crystal X-ray structural analysis and dye electrical properties (both in solution and adsorbed to TiO2) using UV-visible and ATR-IR spectroscopy along with cyclic voltammetry, and also theoretical studies using density functional theory (DFT) calculations. These data show that positioning the linkers near the dye LUMO and so that this enables complete linker chemisorption are key factors for device performance.