John N. Clifford

The origin of slow electron recombination processes in dye-sensitized solar cells with alumina barrier coatings

We investigate the effect of a thin alumina coating of nanocrystalline TiO2 films on recombination dynamics of dye-sensitized solar cells. Both coated and uncoated cells were measured by a combination of techniques: transient absorption spectroscopy, electrochemical impedance spectroscopy, and open-circuit voltage decay. It is found that the alumina barrier reduces the recombination of photoinjected electrons to both dye cations and the oxidized redox couple. It is proposed that this observed retardation can be attributed primarily to two effects: almost complete passivation of surface trap states in TiO2 that are able to inject electrons to acceptor species, and slowing down by a factor of 3–4 the rate of interfacial charge transfer from conduction-band states.

Stepwise Cosensitization of Nanocrystalline TiO2 Films Utilizing Al2O3 Layers in Dye‐Sensitized Solar Cells

Keywords: aluminum oxide ; electron transfer ; hole transfer ; sensitizers ; titanium dioxide ; Efficient Co-Sensitization ; Organic Sensitizers ; Recombination Dynamics ; Charge Recombination ; Highly Efficient ; Blocking Layers ; Electrodes ; Dendrimers ; Cyanine Reference EPFL-ARTICLE-160552doi:10.1002/anie.200802852View record in Web of Science Record created on 2010-11-30, modified on 2016-08-09

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.

A Robust Organic Dye for Dye Sensitized Solar Cells Based on Iodine/Iodide Electrolytes Combining High Efficiency and Outstanding Stability

Among the new photovoltaic technologies, the Dye-Sensitized Solar Cell (DSC) is becoming a realistic approach towards energy markets such as BIPV (Building Integrated PhotoVoltaics). In order to improve the performances of DSCs and to increase their commercial attractiveness, cheap, colourful, stable and highly efficient ruthenium-free dyes must be developed. Here we report the synthesis and complete characterization of a new purely organic sensitizer (RK1) that can be prepared and synthetically upscaled rapidly. Solar cells containing this orange dye show a power conversion efficiency of 10.2% under standard conditions (AM 1.5G, 1000 Wm−2) using iodine/iodide as the electrolyte redox shuttle in the electrolyte, which is among the few examples of DSC using an organic dyes and iodine/iodide red/ox pair to overcome the 10% efficiency barrier. We demonstrate that the combination of this dye with an ionic liquid electrolyte allows the fabrication of solar cells that show power conversion efficiencies of up to 7.36% that are highly stable with no measurable degradation of initial performances after 2200 h of light soaking at 65°C under standard irradiation conditions. RK1 achieves one of the best output power conversion efficiencies for a solar cell based on the iodine/iodide electrolyte, combining high efficiency and outstanding stability.

Synthesis, Spectroscopy, Crystal Structure, Electrochemistry, and Quantum Chemical and Molecular Dynamics Calculations of a 3-Anilino Difluoroboron Dipyrromethene Dye

An asymmetrically substituted fluorescent difluoroboron dipyrromethene (BODIPY) dye, with a phenylamino group at the 3-position of the BODIPY chromophore, has been synthesized by nucleophilic substitution of 3,5-dichloro-8-(4-tolyl)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene. The solvent-dependent spectroscopic and photophysical properties have been investigated by means of UV-vis spectrophotometry and steady-state and time-resolved fluorometry and reflect the large effect of the anilino substituent on the fluorescence characteristics. The compound has a low fluorescence quantum yield in all but the apolar solvents cyclohexane, toluene, and chloroform. Its emission maxima in a series of solvents from cyclohexane to methanol are red-shifted by approximately 50 nm in comparison to classic BODIPY derivatives. Its oxidation potential in dichloromethane is at ca. 1.14 V versus Ag/AgCl. The absorption bandwidths and Stokes shifts are much larger than those of typical, symmetric difluoroboron dipyrromethene dyes. The values of the fluorescence rate constant are in the (1.4-1.7) x 10(8) s(-1) range and do not vary much between the solvents studied. X-ray diffraction analysis shows that the BODIPY core is planar. Molecular dynamics simulations show that there is no clear indication for aggregates in solution.

Synthesis, structure and properties of [Pt(2,2 '-bipyridyl-5,5 '-dicarboxylic acid)(3,4-toluenedithiolate)]: tuning molecular properties for application in dye-sensitised solar cells

The platinum diimine dithiolate complex, [Pt(2,2′-bipyridyl-5,5′-dicarboxylicacid)(3,4-toluenedithiolate)] ([Pt(5,5′-dcbpy)(tdt)]) and its tetrabutylammonium salt [TBA]2[Pt(5,5′-dcbpy)(tdt)] have been prepared, spectroscopically and electrochemically characterised and attached on to TiO2 substrate to be used as solar cell sensitisers. A single-crystal X-ray structure was obtained for [TBA]2[Pt(5,5′-dcbpy)(tdt)]·EtOH·EtOAc. The effect of the position of the two carboxylic acid substituents on the electrochemistry of the 5,5′-disubstituted complexes is discussed in comparison with the previously reported [Pt(4,4′-dcbpy)(tdt)]. Electrochemical studies show no major change in the HOMO after movement of the carboxylic acid groups, consistent with assignment of the HOMO as largely dithiolate based. Movement of the carboxylic acid groups makes the diimine electronic character and hence the LUMO of the complexes different. Electrochemical studies show a change to lower energy of the LUMO represented by changes in reduction potential of the compound on moving the carboxylic acid substituents from the 4,4′ to the 5,5′ positions. Both [Pt(5,5′-dcbpy)(tdt)] and [TBA]2[Pt(5,5′-dcbpy)(tdt)] have been used as solar cell sensitisers, with the di-TBA salt giving lower dye loading but superior photovoltaic performance. The consequences of tuning the complex through the position of the carboxylic acid groups are discussed.

Metal-free organic sensitizers with narrow absorption in the visible for solar cells exceeding 10% efficiency

A novel family of six donor–acceptor type organic sensitizers for dye-sensitized solar cells (DSSCs) is reported. The dyes have been designed to have outstanding light absorption properties in the visible range and being able to achieve high photon-to-electrical current conversion for BIPV (building-integrated photovoltaic). Moreover, stability tests under illumination at 1 Sun and 65 °C showed a great stability for some of the devices, with less than 6% decrease of power conversion efficiency after 3000 hours. The differences in the performance of the six sensitizers under standard illumination conditions can be correlated with the observed differences in the photo-induced transient photovoltage and in charge extraction measurements. We report the use of one of the dyes for the fabrication of semi-transparent solar modules showing an active area of 1400 cm2 and a power output of 10.5 W m−2.

Photo-induced charge transfer dynamics in efficient TiO2/CdS/CdSe sensitized solar cells

Nanocrystalline TiO2 films were co-sensitized with CdS and CdSe, deposited using the successive ionic layer adsorption and reaction (SILAR) process. These films were used to make optimized solar cell devices using polysulfide electrolyte and gold counter electrodes with the highest efficiency of 3.60% recorded under AM 1.5G 1 sun illumination using 6 cycles of CdS and 8 cycles of CdSe. CdS(6)/CdSe(8) co-sensitized devices were compared to devices containing only CdS and CdSe with the co-sensitized cells being superior in terms of photocurrent, cell voltage and overall efficiency. Photo-induced processes were investigated using both Transient Absorption Spectroscopy and Transient Photovoltage measurements. Transient absorption kinetics show that following electron injection from CdS/CdSe into the TiO2 film, the polysulfide electrolyte regenerates holes in the CdS/CdSe layer resulting in a long-lived species which lives up to several hundreds of milliseconds. Comparison with transient photovoltage decays indicates that this long-lived species is due to electrons in the TiO2 that eventually recombine with the polysulfide electrolyte. Electron lifetimes are also shown to be longest in the co-sensitized devices in agreement with the larger Voc observed for these cells.

Co-sensitized DSCs: dye selection criteria for optimized device Voc and efficiency

Co-sensitization of nanocrystalline TiO2 with the organic dye D2 and the zinc phthalocyanine dye TT1 improves light harvesting, Jsc and efficiency of DSC devices. However, the Voc of the co-sensitized cell is markedly inferior (∼130 mV) when compared to the reference device made with D2 only. We discuss the implications of our results with regard to selection criteria for dyes for co-sensitized DSCs.

Slow charge recombination in dye-sensitised solar cells (DSSC) using Al2O3 coated nanoporous TiO2 films

The conformal growth of an overlayer of Al2O3 on a nanocrystalline TiO2 film is shown to result in a 4-fold retardation of interfacial charge recombination, and a 30% improvement in photovoltaic device efficiency.