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Now showing items 1 - 16 of 35

  • Differences in photoinduced optical transients in perovskite absorbers for solar cells

    Pydzinska, Katarzyna   Karolczak, Jerzy   Szafranski, Marek   Ziolek, Marcin  

    Methylammonium lead iodide films and powdered crystals were studied by time-resolved absorption and emission spectroscopy on the time scales from femtoseconds to nanoseconds. Strikingly different transient absorption signals were observed, changing from strong long-wavelength band-edge bleach to weak signatures of band-shift, which depended on the absorber form (films or polycrystals) and preparation method (stoichiometric or non-stoichiometric). The observed differences were correlated with the variation in absorption and emission spectra, changes in photo-induced carrier lifetimes and solar cell efficiency. These differences also pointed out that similar perovskite absorbers can provide significantly different transient responses and emphasize that special care must be taken when interpolating the obtained findings to the processes occurring in the most efficient devices.
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  • Determination of Interfacial Charge-Transfer Rate Constants in Perovskite Solar Cells

    Pydzinska, Katarzyna   Karolczak, Jerzy   Kosta, Ivet   Tena-Zaera, Ramon   Todinova, Anna   Idigoras, Jesus   Anta, Juan A.   Ziolek, Marcin  

    A simple protocol to study the dynamics of charge transfer to selective contacts in perovskite solar cells, based on time-resolved laser spectroscopy studies, in which the effect of bimolecular electron-hole recombination has been eliminated, is proposed. Through the proposed procedure, the interfacial charge-transfer rate constants from methylammonium lead iodide perovskite to different contact materials can be determined. Hole transfer is faster for CuSCN (rate constant 0.20ns(-1)) than that for 2,2,7,7-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9-spirobifluorene (spiro-OMeTAD; 0.06ns(-1)), and electron transfer is faster for mesoporous (0.11ns(-1)) than that for compact (0.02ns(-1)) TiO2 layers. Despite more rapid charge separation, the photovoltaic performance of CuSCN cells is worse than that of spiro-OMeTAD cells; this is explained by faster charge recombination in CuSCN cells, as revealed by impedance spectroscopy. The proposed direction of studies should be one of the key strategies to explore efficient hole-selective contacts as an alternative to spiro-OMeTAD.
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  • Interrogating the ultrafast dynamics of an efficient dye for sunlight conversion

    Ziolek, Marcin   Yang, Xichuan   Sun, Licheng   Douhal, Abderrazzak  

    We report on studies of the recently synthesized compound (TPC1) with a promising potential use in dye-sensitized solar cells. We used steady-state as well as femtosecond (fs) to nanosecond (ns) time-resolved emission techniques to understand its behaviour under different conditions of solvation and light excitation. In polar solvents the equilibrium between TPC1 normal and anion structures was found to depend on solvent H-bond acceptor ability and concentration of the dye. We observed a correlation between the contribution of the normal form in the total absorption spectrum and solar energy conversion efficiency of the photovoltaic devices prepared in different baths, which are high in dichloromethane and low in tetrahydrofurane. Both forms exhibit a large charge transfer character in the excited state manifested by a large Stokes shift between absorption and emission maxima (up to 9000 cm(-1) in acetonitrile). The lifetime of the relaxed state of the normal structure varies significantly with the solvent polarity (from 80 ps in acetonitrile to 1.8 ns in n-hexane), and it is considerably shorter than that of the anion one (1.2-2.6 ns). The ultrafast relaxation processes are dominated by the solvation dynamics which is the fastest in acetonitrile (below 1 ps) and the slowest in ethanol (about 25 ps, the amplitude-averaged time). The results reported here should be relevant to a better understanding of the photobehaviour of metal-free dyes for solar cells and help in the design of new and more efficient dyes for conversion of light to electricity.
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  • Dynamics of local Stark effect observed for a complete D149 dye-sensitized solar cell

    Burdzinski, Gotard   Karolczak, Jerzy   Ziolek, Marcin  

    A complete, functioning dye-sensitized solar cell made of popular indoline D149 sensitizer is studied by means of transient absorption in visible light in the time scale of nanoseconds to seconds. Photocurrent and photovoltage decays are also measured under the same experimental conditions. A local electric field causing a Stark shift of the D149 absorption band is found to strongly influence the transient spectra and kinetics. The presence of electrons in titania has a major contribution to the Stark shift and the effect disappears over many time scales with an average rate of 5 x 10(3) s(-1). This is much slower than the decay of the oxidized dye (2 x 10(6) s(-1)) but, on the other hand, significantly faster than the decay of electrons in titania nanoparticles (3 x 10(2) s(-1) at standard AM1.5 irradiation and open circuit conditions). Possible explanations of this phenomenon are discussed. Electron recombination from the titania conduction band to the oxidized dyes proceeds at an average rate of 2-16 x 10(4) s(-1), depending on the excitation energy density, and does not influence the efficiency of dye regeneration.
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  • Femtosecond Dynamics of a Porphyrin Derivative Confined by the Human Serum Albumin Protein

    Synak, Anna   Ziolek, Marcin   Angel Organero, Juan   Douhal, Abderrazzak  

    The relaxation dynamics of 5,10,15,20-tetrakis(4-hydroxyphenyl)-porphyrin (p-THPP) in tetrahydrofuran (THF) and encapsulated within the human serum albumin (HSA) protein in water solution was investigated. The protein environment affects the B -> Q(y). and Q(x)-> Q(y) transition dynamics (from 80 and 140-200 fs in THF to 50 and 100 fs in HSA, respectively) as well as the lifetime of the relaxed Q. state (9.1 vs 9.9 ns). The most prominent differences are observed in the relaxation dynamics in the hot Q(x) state in HSA, which includes the energy transfer to the protein in similar to 1 ps and much slower solvent-assisted thermal equilibration component of about 20-30 ps.
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  • Virtues and Vices of an Organic Dye and Ti-Doped MCM-41 Based Dye-Sensitized Solar Cells

    Ziolek, Marcin   Martin, Cristina   Cohen, Boiko   Garcia, Hermenegildo   Douhal, Abderrazzak  

    We report on femtosecond to millisecond (fs-ms) studies of the interactions of an efficient triphenylamine organic dye (TPC1) for photovoltaics with titanium-doped high porous MCM-41 (TiMCM-41) silicate materials in half and complete dye-sensitized solar cells (DSSCs). Stationary UV-visible absorption results indicate a higher dye loading (3-5 times) per Ti atom in the TiMCM-41-based solar cells in comparison with those based on typical titania nanoparticles (NPs). However, the dye loading per Ti atom decreases with increasing the Ti doping, and the total dye content is still smaller in the TiMCM-41 solar cells than in the NP ones. Time-resolved emission studies showed that the average electron injection times (from TPC1 to the titania conduction band) are about 2.5 times longer for the TiMCM-41 solar cells (12 ps) than using the classical titania NP ones (4.5 ps). However, taking into account the slow internal deactivation of the dye in both materials, the yield of electron injection is > 90%. Nanosecond to millisecond (ns-ms) flash photolysis studies of films show that the (back) electron recombination from titania to the dye cation is slower using TiMCM-41 than that in the titania NPs (19 mu s vs 7 mu s). Similar experiments for complete solar cells indicate faster dye regeneration due to the electrolyte in the TiMCM-41 than in the titania NP devices (3 mu s vs 8 mu s). Therefore, from the point of view of interfacial charge separation, the results indicate comparable or even better performance of the TiMCM-41 than the typical titania nanoparticle. However, the solar cell performance and the total efficiency are much lower in the TiMCM-41 than in the NP devices. Additionally, the values of short-circuit current per Ti atom are higher for the NP devices, despite the higher dye content per Ti atom in TiMCM-41 samples. This suggests a strong limitation in the electron transport process along the TiMCM-41 channels. Hence, despite the promising interfacial properties observed in fast and ultrafast time scales, this kind of material needs further modification in terms of improving the total dye loading and the conductivity, in order to be suitable for DSSCs.
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  • Insights into the Femtosecond to Nanosecond Charge Carrier Kinetics in Perovskite Materials for Solar Cells

    Pydzińska-Białek, Katarzyna   Szeremeta, Janusz   Wojciechowski, Konrad   Ziolek, Marcin  

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  • Confined Fast and Ultrafast Dynamics of a Photochromic Proton-Transfer Dye within a Zeolite Nanocage

    Gil, Michal   Ziolek, Marcin   Angel Organero, Juan   Douhal, Abderrazzak  

    We report on studies of salicylaldehyde azine (SAA) dissolved in dichloromethane solution and within the cages of the faujasite zeolite (NaX) using steady-state and femtosecond to nanosecond time-resolved spectroscopy. In solution, an excited-state intramolecular proton-transfer reaction takes place in less than 80 is, leading to a keto-type tautomer. In contrast within NaX zeolite, a zwitterionic (Z) form is present both at So and SI states, and a large hypsochromic shift of the stationary emission spectrum is observed, The increase in fluorescence lifetime upon encapsulation (from 54 ps to 0.2-2.8 ns) is mainly due to hindrance in twisting motion of the confined Z structure imposed by the nanocage. A significant dependence of the lifetimes on the guest concentration inside the zeolite indicates an interaction between neighboring guest molecules leading to a quenching of the fluorescence. The analysis of emission decays using stretched-exponential model suggests that the excited-state interactions between neighboring dyes play a key role in the deactivation of the trapped Z fluorophores. For the ultrafast relaxation dynamics of the SAA/NaX composite, intramolecular vibrational-energy redistribution and vibrational cooling process occur in longer times (up to 360 fs and 5 ps, respectively). Additionally, the presence of nonfluorescent twisted (n,pi*) state is suggested to form in 6-10 ps. We believe that our results are important for a better understanding of the photocycle of azine-based photochromic material when interacting with nanomaterials.
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  • Photochemistry and Photophysics in Silica-Based Materials: Ultrafast and Single Molecule Spectroscopy Observation.

    Alarcos, Noemi   Cohen, Boiko   Ziolek, Marcin   Douhal, Abderrazzak  

    Silica-based materials (SBMs) are widely used in catalysis, photonics, and drug delivery. Their pores and cavities act as hosts of diverse guests ranging from classical dyes to drugs and quantum dots, allowing changes in the photochemical behavior of the confined guests. The heterogeneity of the guest populations as well as the confinement provided by these hosts affect the behavior of the formed hybrid materials. As a consequence, the observed reaction dynamics becomes significantly different and complex. Studying their photobehavior requires advanced laser-based spectroscopy and microscopy techniques as well as computational methods. Thanks to the development of ultrafast (spectroscopy and imaging) tools, we are witnessing an increasing interest of the scientific community to explore the intimate photobehavior of these composites. Here, we review the recent theoretical and ultrafast experimental studies of their photodynamics and discuss the results in comparison to those in homogeneous media. The discussion of the confined dynamics includes solvation and intra- and intermolecular proton-, electron-, and energy transfer events of the guest within the SBMs. Several examples of applications in photocatalysis, (photo)sensors, photonics, photovoltaics, and drug delivery demonstrate the vast potential of the SBMs in modern science and technology.=20
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  • Improving the Spatial Resolution in Direct Laser Writing Lithography by Using a Reversible Cationic Photoinitiator

    Duocastella, Marti   Vicidomini, Giuseppe   Korobchevskaya, Kseniya   Pydzinska, Katarzyna   Ziolek, Marcin   Diaspro, Alberto   de Miguel, Gustavo  

    Direct laser writing (DLW) lithography has emerged as a competitive additive tool for the fabrication of detailed three-dimensional (3D) structures with a minimum feature size close to the nanometer scale. However, the minimal distance between adjacently written features with no overlapping, that is the writing resolution, is not in the same scale as the feature size. This is a consequence of the so-called "memory effect", namely, the accumulation of radicals between polymerized structures, which prevents the development of DLW for commercial applications. To overcome these limitations, we propose an original approach based on the reversible formation of the active species triggering the polymerization to decrease the impact of the "memory effect" on the writing resolution. We have selected the [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) molecule as the cationic photoinitiator in combination with an oxidizing agent, AgPF6, to trigger the polymerization of the photoresist. The two-photon absorption (2PA) ability of the PCBM material was explored by using the open aperture z-scan technique, obtaining a 2PA cross-section of similar to 400 GM. We have also utilized pump-probe spectroscopy to demonstrate the formation of the radical cation of the PCBM via a photoinduced electron transfer reaction with the Ag+ cation (Delta G < 0). Moreover, the regeneration of the primary photoinitiating system PCBM/AgPF6 was investigated with the flash photolysis technique, proving the absence of excited species in the mu s time scale. This is the key point of our approach: the reversible character of the electron transfer process allows the partial regeneration of the primary photoinitiator in the interstice between polymerized structures avoiding the "memory effect". The implementation of this approach with commonly used resists, SU-8 or Araldite, has resulted in a notable improvement of the spatial resolution, from 600 to 400 nm when using a conventional photoinitiator compared to our PCBM/AgPF6 system.
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  • What is the difference between the dynamics of anion- and keto-type of photochromic salicylaldehyde azine?

    Ziolek, Marcin   Gil, Michal   Angel Organero, Juan   Douhal, Abderrazzak  

    The normal and anion structures of salicylaldehyde azine (SAA) in solvents of different viscosities and polarities have been studied by means of femto- to nanosecond time-resolved emission techniques. In the normal form, an excited-state intramolecular proton-transfer (ESIPT) reaction takes place with a time constant shorter than 80 fs to produce an excited keto-type tautomer in which intramolecular-vibrational energy redistribution and vibrational cooling occur in 100 fs to 2 ps. The viscosity-dependent emission decay in the red part of the spectrum with 5-11 ps reflects a twisting motion leading to rotamers of these keto-type structures, most probably of (n, pi*) nature. For the anion type, the viscosity dependent rise-times (3 to 400 ps) at the red part of the emission, and the wavelength-dependent fluorescence lifetimes (20 to 1100 ps) indicate a stepwise formation of different conformers of the anions. The results reported here should be relevant to a better understanding of the photobehaviour of photochromic compounds and charged chromophores in biological systems.
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  • Interface Modification and Exceptionally Fast Regeneration in Copper Mediated Solar Cells Sensitized with Indoline Dyes

    Glinka, Adam   Gierszewski, Mateusz   Gierczyk, Błażej   Burdzinski, Gotard   Michaels, Hannes   Freitag, Marina   Ziolek, Marcin  

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  • Long-living structures of photochromic salicylaldehyde azine: polarity and viscosity effects from nanoseconds to hours

    Ziolek, Marcin   Burdzinski, Gotard   Douhal, Abderrazzak  

    In this study, we report on the effects of solvent viscosity and polarity on the photochromic salicylaldehyde azine (SAA) molecule by examining the steady-state and UV-visible absorption results in the time scale from nanoseconds to hours, in solution and in a polymer film. For the neutral structure, the viscosity strongly affects the lifetime of the photochromic (trans-keto) tautomer by suppressing the second order quenching process, and thus increasing the photochrome lifetimes in highly viscous solvents to 500 mu s in polar triacetine, and to 65 mu s in non-polar squalane. Trapping SAA in a non-polar polymer film (polyethylene) results in further elongation of the photochromic lifetime (700 mu s) by one order of magnitude (with respect to that in squalane), due to the retardation of the intramolecular back-isomerization. Another species, living significantly longer and absorbing more in the UV comparing to the photochrome, was identified as the syn-enol tautomer. The lifetime of this tautomer, created in a competitive mechanism to the photochrome creation, is much longer in non-polar solvents (hundreds of minutes) than in polar ones (tens of minutes), opposite to the trend observed for the photochrome. For the SAA anion, the transient living on the ns-mu s time scale can be exclusively assigned to the triplet state, which is not observed for the neutral form at room temperature.
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  • Comparison of charge transfer dynamics in polypyridyl ruthenium sensitizers for solar cells and water splitting systems

    Gradzka, Iwona   Gierszewski, Mateusz   Karolczak, Jerzy   Ziolek, Marcin  

    Standard ruthenium components of dye-sensitized solar cells (sensitizer N719) and dye-sensitized photoelectrochemical cells (sensitizer RuP and water oxidation catalyst RuOEC) are investigated in the same solar cell configuration to compare their photodynamics and charge separation efficiency. The samples are studied on time scales from femtoseconds to seconds by means of transient absorption, time-resolved emission and electrochemical impedance measurements. RuP shows significantly slower electron injection into a mesoporous titania electrode and enhanced fast (sub-ns) electron recombination with respect to those of N719. Moreover, RuOEC is found to be responsible for partial light absorption and electron injection with low efficiency. The obtained results reveal new insights into the reasons for the lower charge separation efficiency in water splitting systems with respect to that in solar cells. The important role of the initial processes occurring at the dye-titania interface within the first nanoseconds in this efficiency is emphasized.
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  • Photochromic cycle of 2 '-hydroxyacetophenone azine studied by absorption and emission spectroscopy in different solvents

    Filipczak, Katarzyna   Karolczak, Jerzy   Lipkowski, Pawel   Filarowski, Aleksander   Ziolek, Marcin  

    This paper reports on the investigations of the synthesized di-(o-hydroxyaryl ketoimine) compound by the steady state absorption and emission techniques as well as picosecond time resolved emission and femtosecond transient absorption methods in different solvents. The results of the experimental observation have been supported by the theoretical DFT and TD-DFT calculations. The theoretical data have revealed the completed influence of the environmental polarity on particular conformers of studied compound. Dependencies between the activation rate constant and polarizability function as well as Kamlet-Abbond-Taft hydrogen-bonding parameter have been obtained in different solvent. The mechanism of photodynamic changes of di-(o-hydroxyaryl ketoimine) is presented. (C) 2013 AIP Publishing LLC.
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  • Insights into the limitations of solar cells sensitized with ruthenium dyes revealed in time-resolved spectroscopy studies

    Gierszewski, Mateusz   Gradzka, Iwona   Glinka, Adam   Ziolek, Marcin  

    The substitution of iodide electrolytes with cobalt ones has led to the current champion laboratory efficiencies for dye-sensitized solar cells (DSSCs). However, unlike with organic dyes, this strategy does not work with classical ruthenium dyes. Therefore, we compare DSSCs sensitized with a popular Ru dye (N719) using both types of electrolytes by exploring the electron dynamics occurring from sub-ps to seconds. An important limitation in the photocurrent of cobalt-based cells is revealed to be due to electron recombination between titania and oxidized Ru dyes, which is much higher than that in iodide-based cells and occurs on the time scale of tens and hundreds of ps. Electron recombination between titania and the electrolyte, taking place on the millisecond time scale, is responsible for further lowering of the photovoltage and fill factor of cobalt-based cells. Ruthenium dyes also exhibit lower absorption coefficients with respect to their organic counterparts. For this reason, we also investigate the effect of the changes in the titania layer thickness, addition of scattering nanoparticles and modifications in the TiCl4 treatment on DSSC performance.
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