Arthur Connell

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.

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.

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.

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.

Solvent issues during processing and device lifetime for perovskite solar cells

This paper considers the manufacturing issues associated with dimethyl formamide, γ-butyrolactone, dimethyl sulfoxide and chlorobenzene solvents, in particular the health and safety issues of using these solvents in scaled perovskite photovoltaic processing. Issues of device lifetime are also considered, for example the effects of atmospheric conditions (e.g. humidity).

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.

Surface interactions of half-squaraine dyes in dye-sensitized solar cells

This paper presents a summary of our recent work on half-squaraine dyes for use as sensitizers in dye-sensitized solar cells. Our data include a review of the highest efficiency half-squaraine dyes to date. The discussion also considers how Jsc varies with functionalization of the Hf-SQ dyes and how these modifications affect device lifetime. The paper also discusses recent papers which consider the influences of the position and number of carboxylate linker groups on the chromophore.

Convenient synthesis of EDOT-based dyes by CH-activation and their application as dyes in dye-sensitized solar cells

Precursors to three new 3,4-ethylenedioxythiophene (EDOT) incorportaing dyes have been synthesised via a one-pot C–H activation route using N,N-dimethylaniline as a donor group. We have extended this methodology to provide a convenient one-pot route to dye EDOT-Ph. The electrochemical and optical properties of the new dyes have been correlated with IV and EQE data for 1 cm2 dye-sensitized solar cell (DSSC) devices prepared using these dyes. The device data show that dye performance is strongly affected by the amount of chenodeoxycholic acid (CDCA) co-sorbent used. The best performance is for EDOT-Ph (η = 4.0%) at 10 mM CDCA compared to (η = 6.0% and η = 5.8%) for N719 and D205 control cells.