An investigation of the roles furan versus thiophene π-bridges play in donor–π-acceptor porphyrin based DSSCs

Dye-sensitized solar cells (DSSCs) continue to attract interest due to their lower cost production compared to silicon based solar cells and their improving power conversion efficiencies. Porphyrin-based sensitizers have become an important sub-class due to their strong absorption characteristics in the visible region, convenient modulation of properties through synthetic manipulation and class-leading power conversion efficiencies. In this article, we report the synthesis and characterization of two porphyrin-based dyes and their application as sensitizers in DSSCs. A thiophene and a furan moiety have been incorporated into the push–pull architecture as a π-bridge, allowing the systematic investigation of how these moieties influence the physical properties of the dyes and the performance of their resulting DSSCs. A significant difference in PCEs has been observed, with the furan containing dye (PorF, PCE = 4.5%) being more efficient than the thiophene-based analogue (PorT, PCE = 3.6%) in conjunction with the iodide/triiodide redox electrolyte

CdSe quantum dot (QD) and molecular dye hybrid sensitizers for TiO2 mesoporous solar cells: working together with a common hole carrier of cobalt complexes

Redox couples based on cobalt complexes were found to be effective in regenerating both inorganic CdSe quantum dot-and organic dye-sensitizers. The hybrid sensitizer composed of CdSe QD and ruthenium sensitizer (Z907Na) dye showed a maximum power conversion efficiency of 4.76% on using cobalt(o-phen)(3)(2+/3+) as a common redox mediator.close202

Ultrafast charge separation dynamics in opaque, operational dye-sensitized solar cells revealed by femtosecond diffuse reflectance spectroscopy

Efficient dye-sensitized solar cells are based on highly diffusive mesoscopic layers that render these devices opaque and unsuitable for ultrafast transient absorption spectroscopy measurements in transmission mode. We developed a novel sub-200 femtosecond time-resolved diffuse reflectance spectroscopy scheme combined with potentiostatic control to study various solar cells in fully operational condition. We studied performance optimized devices based on liquid redox electrolytes and opaque TiO2 films, as well as other morphologies, such as TiO2 fibers and nanotubes. Charge injection from the Z907 dye in all TiO2 morphologies was observed to take place in the sub-200 fs time scale. The kinetics of electron-hole back recombination has features in the picosecond to nanosecond time scale. This observation is significantly different from what was reported in the literature where the electron-hole back recombination for transparent films of small particles is generally accepted to occur on a longer time scale of microseconds. The kinetics of the ultrafast electron injection remained unchanged for voltages between +500 mV and –690 mV, where the injection yield eventually drops steeply. The primary charge separation in Y123 organic dye based devices was clearly slower occurring in two picoseconds and no kinetic component on the shorter femtosecond time scale was recorded

Enhanced Electron Collection Efficiency in Dye-Sensitized Solar Cells Based on Nanostructured TiO2 Hollow Fibers

Nanostructured TiO2 hollow fibers have been prepared using natural cellulose fibers as a template. This cheap and easily processed material was used to produce highly porous photoanodes incorporated in dye-sensitized solar cells and exhibited remarkably enhanced electron transport properties compared to mesoscopic films made of spherical nanoparticles. Photoinjected electron lifetime, in particular, was multiplied by 3-4 in the fiber morphology, while the electron transport rate within the fibrous photoanaode was doubled. A nearly quantitative absorbed photon-to-electrical current conversion yield exceeding 95% was achieved upon excitation at 550 nm and a photovoltaic power conversion efficiency of 7.2% reached under simulated AM 1.5 (100 mW cm(-2)) solar illumination

Electrochemical Properties of Cu(II/I)-Based Redox Mediators for Dye-Sensitized Solar Cells

Three Cu(II/I)-phenanthroline and Cu(II/I)-bipyridine redox mediators are studied on various electrodes and in variety of electrolyte solutions using cyclic voltammetry and impedance spectroscopy on symmetrical dummy cells. Graphene-based catalysts provide comparably high activity to PEDOT, and both catalysts outperform the activity of platinum. The charge-transfer kinetics and the diffusion rate significantly slowdown in the presence 4-tert-butylpyridine. This effect is specific only for Cu-mediators (is missing for Co-mediators), and is ascribed to a sensitivity of the coordination sphere of the Cu(II)-species to structural and substitutional changes. The 'Zombie Cells' made from symmetrical PEDOT/PEDOT devices exhibit enhanced charge-transfer rate and enhanced diffusion resistance. Electrochemically clean Cu(II)-bipyridine species are prepared, for the first time, by electrochemical oxidation of the parent Cu(I) complexes. Our preparative electrolysis brings numerous advantages over the standard chemical syntheses of the Cu(II)-bipyridine complexes. The superior performance of electrochemically grown clean Cu(II)-bipyridine complex is demonstrated on practical dye-sensitized solar cells. (C) 2016 Elsevier Ltd. All rights reserved

Temperature-Dependent Ordering Phenomena of a Polyiodide System in a Redox-Active Ionic Liquid

Iodine added to iodide-based ionic liquids leads to dramatic changes of their physical properties which may have implications for technological applications. Here we study the phase diagram of 1-methyl 3-propylimidazolium iodide, the temperature versus iodine (I2) concentration. Above a threshold I2 concentration of 3.9 M, polyiodides are found to be the major determinant of the thermodynamic properties, where nucleation occurs at reduced temperatures leading to a crystalline phase followed by a nematic phase. At the highest concentrations and for increasing temperatures a phonon mode develops which gives indication of mesophases with both improved orientational order of the polyiodide chains and a degree of positional order close to melting. These novel results are important for the fundamental understanding of the physical properties in molten salts and for applications where ionic liquids are used as charge-transporting media such as in batteries and dye-sensitized solar cells

Reduction in the Interfacial Trap Density of Mechanochemically Synthesized MAPbI(3)

Organo-lead halide perovskites have emerged as promising light harvesting materials for solar cells. The ability to prepare high quality films with a low concentration of defects is essential for obtaining high device performance. Here, we advance the procedure for the fabrication of efficient perovskite solar cells (PSCs) based on mechanochemically synthesized MAPbI3. The use of mechano-perovskite for the thin film formation provides a high degree of control of the stoichiometry and allows for the growth of relatively large crystalline grains. The best device achieved a maximum PCE of 17.5% from a current-voltage scan (J-V), which stabilized at 16.8% after 60 s of maximum power point tracking. Strikingly, PSCs based on MAPbI(3) mechanoperovskite exhibit lower "hysteretic" behavior in comparison to that comprising MAPbI(3) obtained from the conventional solvothermal reaction between PbI2 and MAI. To gain a better understanding of the difference in J V hysteresis, we analyze the charge/ion accumulation mechanism and identify the defect energy distribution in the resulting MAPbI(3) based, devices. These results indicate that the use of mechanochemically synthesized perovskites provides a promising strategy for the formation of crystalline films demonstrating slow charge recombination and low trap density

Hill climbing hysteresis of perovskite-based solar cells: a maximum power point tracking investigation

The surge of the power conversion efficiency of metal halide lead perovskite solar cells comes with concerns, such as the long-term ecotoxicity of lead compounds, their sensitivity toward moisture and oxygen, or the scarcity of some of their components. Most perovskite solar cells still suffer from serious stability problems when measured under real working conditions (maximum power point tracking at 60 degrees C). In the long run, stability will certainly decide on the fate of CH3NH3PbI3 and related lead perovskites for their use in photovoltaic modules. Herein, we show an effective and inexpensive strategy to perform ageing of perovskite solar cells under maximum power point tracking. For the first time, we analyze the issue of power extraction from solar cells exhibiting hysteresis. We show that a standard tracking algorithm such as perturb and observe fails to converge to the maximum power point of the solar cell if it exhibits j(V) hysteresis, and we present an effective strategy to stabilize the algorithm. We show that enforcing oscillations in forward bias can boost the mean power output of some perovskite solar cells by more than 10%, in contrast to a reference crystalline silicon solar cell. Copyright (c) 2017 John Wiley & Sons, Ltd