Miquel Planells

Energy levels, charge injection, charge recombination and dye regeneration dynamics for donor–acceptor π-conjugated organic dyes in mesoscopic TiO2 sensitized solar cells

Two new D–π–A type organic sensitizers, MP124 and MP-I-50, were synthesized and their electrochemical and spectroscopic properties studied. Efficiencies of DSSC devices utilizing these dyes were also investigated, where sensitization solvent, sensitization time and additive concentration were all varied. Under standard AM 1.5G simulated solar radiation, optimized MP124 devices show an efficiency of 7.45% (Voc = 0.73 V; Jsc = 14.44 mA cm−2; FF = 70%) while optimized MP-I-50 devices show an efficiency of 5.66% (Voc = 0.68 V; Jsc = 12.06 mA cm−2; FF = 69%). Transient absorption spectroscopy studies show that regeneration of dye cations by the red-ox electrolyte was more efficient in MP124cells which is attributed to its higher HOMO energy leading to greater driving force for the regeneration reaction. Transient photovoltage studies showed that electron lifetimes were longer lived in MP124 explaining the higher Voc for these cells compared to MP-I-50cells. DFT and MP2 calculations indicate that this is due to the greater tendency of MP-I-50 to form charge-transfer complexes with the I2 species in the electrolyte, due to the presence of an additional EDOT in its structure compared to MP124. This work highlights the effect that small changes to the sensitizer structure can have on the interfacial charge transfer reactions and ultimately on the device efficiency.

Diacetylene bridged triphenylamines as hole transport materials for solid state dye sensitized solar cells

We have synthesized and characterized a series of triphenylamine-based hole-transport materials (HTMs), and studied their function in solid-state dye sensitized solar cells (ss-DSSCs). By increasing the electron-donating strength of functional groups (–H < –Me < –SMe < –OMe) we have systematically shifted the oxidation potential and ensuing photocurrent generation and open-circuit voltage of the solar cells. Correlating the electronic properties of the HTM to the device operation highlights a significant energy offset required between the Dye – HTM highest occupied molecular orbital (HOMO) energy levels. From this study, it is apparent that precise control and tuning of the oxidation potential is a necessity, and usually not achieved with most HTMs developed to date for ss-DSSCs. To significantly increase the efficiency of solid-state DSSCs understanding these properties, and implementing dye-HTM combinations to minimize the required HOMO offset is of central importance.

The role of para-alkyl substituents on meso-phenyl porphyrin sensitised TiO2 solar cells: control of the eTiO2/electrolyte+ recombination reaction

We aim to investigate the effect of adding hydrophobic alkyl chains substituents to unsymmetrical free base tetra-phenyl porphyrins used for the preparation of dye sensitised solar cells (DSSC). We have used two different unsymmetrical meso-tetraphenyl substituted free base porphyrins attending to two objectives: (1) to observe how the substitution of three para positions of the meso-phenyl groups with hydrophobic alkyl chains influences the formation of molecular aggregates onto the semiconductor nanoparticles and (b) to deduce the influence that the substitution exerts over the eTiO2/electrolyte+ recombination reaction in operating devices. To achieve these goals we have focussed on the study of the electron transfer processes that take place at the different interfaces of the photovoltaic device using electrochemistry, steady-state and time resolved spectroscopic techniques.

Advances in high efficiency dye sensitized solar cells based on Ru(II) free sensitizers and a liquid redox electrolyte

Dye sensitized solar cells (DSCs) are one of the most promising cost effective emerging technologies for clean energy generation using solar radiation. Though many of the highest efficiencies have been associated with Ru(II) polypyridyl sensitizers, Ru(II) free sensitizers have made great progress recently and are now serious alternatives. Indeed the highest efficiency recorded in the literature of 12.05% under standard 100 mW cm−2 illumination was obtained using Ru(II) free sensitizers. In this highlight, recent progress in DSCs based on sensitizers such as porphyrins, phthalocyanines, squarines, indolenes, perylenes and donor–(π bridge)–acceptor (D–π–A) dyes is discussed.

Interfacial photo-induced charge transfer reactions in perylene imide dye sensitised solar cells

The interfacial electron transfer reactions of a perylene imide based dye sensitised solar cell (DSSC) with an efficiency of 3.15% under standard conditions have been characterised. The observed interfacial charge transfer dynamics under different sensitisation conditions have been correlated with the device efficiency.

Supramolecular interactions in dye-sensitised solar cells

A new Li+-coordinating perylene monoimide dye has been synthesised. The dye was used as a sensitiser in dye-sensitised solar cells (DSCs), and the effect of Li+ coordination on the device performance was analyzed using various spectroscopic techniques.

A colorimetric molecular probe for Cu(II) ions based on the redox properties of Ru(II) phthalocyanines

The one-electron oxidation of a Ru(II) phthalocyanine induces a dramatic colour change and provides a useful tool for the selective and highly sensitive colorimetric detection of copper(II) ions in neat aqueous solutions.

The effect of molecular aggregates over the interfacial charge transfer processes on dye sensitized solar cells

The electron transfer reaction between the photoinjected electrons in the nanocrystalline TiO2 mesoporous sensitized films and the oxidized electrolyte in dye sensitized solar cells (DSSC) plays a major role on the device efficiency. In this communication we show that, although the presence of molecular aggregates on the free base porphyrin DSSC limits the device photocurrent response under illumination, they form an effective hydrophobic barrier against the oxidized electrolyte impeding fast back-electron transfer kinetics. Therefore, their drawback can be overcome by designing dyes with peripheral moieties that prevent the formation of the aggregates and are able to achieve efficiencies as high as 3.2% under full sun.

'Donor-free' oligo(3-hexylthiophene) dyes for efficient dye-sensitized solar cells

The common trend in designing dyes for use in DSSCs with iodide-based electrolyte is based on a donor–π spacer–acceptor (D–π–A) architecture. Here, we report two ‘donor-free’ cyanoacrylic end-functionalized oligo(3-hexylthiophene) dyes (5T and 6T). Despite having no donor group, both dyes show reversible first oxidation process. Both 5T and 6T have n-hexyl alkyl chains to retard aggregation at different positions as well as different numbers of thiophene moieties. However, the dyes showed similar absorption properties and redox potentials. The DSSCs based on these dyes give power conversion efficiencies of more than 7%, although a significant difference in the VOC and FF has been observed. Using electrochemical impedance spectroscopy, this is attributed to the presence of more trap states when 6T attaches to TiO2 and modifies the surface, mainly affecting the fill factor. Overall, these dyes introduce a new and effective design concept for liquid-electrolyte DSSC sensitisers.

Oligothiophene Interlayer Effect on Photocurrent Generation for Hybrid TiO2/P3HT Solar Cells

A series of conjugated 3-hexylthiophene derivatives with a cyanoacrylic acid group has been prepared with conjugation length from one up to five thiophene units (1T-5T). The UV-vis spectra, photoluminescence spectra, electrochemical data and DFT calculations show lowering of LUMO energies and red-shift of absorption into the visible as the thiophene chain length increases. TiO2/P3HT solar cells were prepared with prior functionalization of the TiO2 surface by 1T-5T and studies include cells using undoped P3HT and using P3HT doped with H-TFSI. Without H-TFSI doping, photocurrent generation occurs from both the oligothiophene and P3HT. Doping the P3HT with H-TFSI quenches photocurrent generation from excitation of P3HT, but enables very effective charge extraction upon excitation of the oligothiophene. In this case, photocurrent generation increases with the light harvesting ability of 1T-5T leading to a highest efficiency of 2.32% using 5T. Overall, we have shown that P3HT can act in either charge generation or in charge collection, but does not effectively perform both functions simultaneously, and this illustrates a central challenge in the further development of TiO2/P3HT solar cells.