Towards increased performance of iron(II)-based dye sensitized solar cells

Today, because of the current climate situation, renewable energy sources are of high importance. Over the last few decades, solar cells have become a vital technology due to their ability to convert sunlight into electrical energy. Most of the commercially available solar cells are made of crystalline silicon. Silicon-based solar cells are already well known in the market, but their costly and tedious fabrication motivates the examination of alternative systems. Since the pioneering work of O’Regan and Grätzel in 1991, dye-sensitized solar cells (DSCs) have become a promising substitute. The sandwich-type DSC structure is easy to manufacture, and a broad variety of sensitizers offers lower costs of materials. However, the use of metals such as ruthenium with low natural abundances still significantly increases the price and reduces the sustainability of DSCs. In this work, the focus is set on n-type DSCs, which combine the use of Earth abundant iron(II) coordination complexes as dyes and the advantageous effects of using different additives in electrolytes. Tuning of electrolyte composition can also remarkably enhance the photoconversion efficiency (PCE) and, as shown for other dyes, has the potential to make iron-sensitizers a promising alternative to ruthenium-based compounds. It has been demonstrated that both the redox couple and the components of the electrolyte have a critical influence on the PCE, and this effect originates from its role as a charge transfer medium. The effects of lithium salts, ionic liquids with different counter-ions and solvents while retaining an I-/I3- redox shuttle are presented. Sometimes small changes might lead to significant progress. The design of an alternative iron(II) based dye is proposed with a corresponding synthetic route. The synthesis towards the target complex is presented. Moreover, a statistical study of electrochemical impedance (EIS) measurements was performed. EIS offers a possibility to study complex electronic systems and is commonly used for solar cells, but there is a general tendency in the literature to present impedance data only for one device. At the same time, the current density–voltage plots can illustrate that measurements may vary within one set of DSCs with identical components. The multiple DSCs impedance measurements are presented on the example of two dyes and provide the statistical analysis for their reproducibility between the cells

The influence of alkyl chains on the performance of DSCs employing iron(II) N-heterocyclic carbene sensitizers

The photovoltaic performances of DSCs employing two new iron(II) N-heterocyclic carbene (NHC) sensitizers are presented. The presence of n-butyl side chains had a significant impact on DSC performace. The improvement in DSC performance up to 0.93-0.95% was observed for a new heteroleptic sensitizer bearing one carboxylic acid anchoring group. The photovoltaic performance was remarkably affected by sensitization time and by a presence/absence of coadsorbent on the semiconductor surface. The highest photoconversion efficiencies (PCE) were achieved for DSCs sensitized over 17.5 hours without addition of coadsorbents. However, for a shorter dipping time of 4 hours, the presence of chenodeoxycholic acid improved the PCE from 0.46% (no coadsorbents) to 0.74%, respectively. The performance of DSCs based on a new homoleptic complex bearing two n-butyl side chains and a carboxylic acid anchor on each NHC-ligand was improved from 0.05 to 0.29% via changes in dye-bath concentration and sensitization time. The changes in the dye load on the semiconductor surface depending on the sensitization conditions were confirmed using solid-state UV-Vis spectroscopy and thermogravimetric analysis. Electrochemical impedance spectroscopy was used to gain information about the processes occurring at the different interfaces in the DSCs. The impedance response was strongly affected by the immersion time of the photoanodes in the dye-bath solutions. In the case of the homoleptic iron(II) complex, a Gerischer impedance was observed after 17.5 hours immersion. Shorter dipping times resulted in a decrease in the resistance in the system. For the heteroleptic complex, values of the chemical capacitance and electron lifetime were affected by the immersion time. However, the diffusion length was independent of sensitization conditions

Are Alkynyl Spacers in Ancillary Ligands in Heteroleptic Bis(diimine)copper(I) Dyes Beneficial for Dye Performance in Dye-Sensitized Solar Cells?

The syntheses of 4,40 -bis(4-dimethylaminophenyl)-6,60 -dimethyl-2,20 -bipyridine (1 ), 4,40 -bis(4-dimethylaminophenylethynyl)-6,60 -dimethyl-2,20 -bipyridine (2 ), 4,40 -bis(4- diphenylaminophenyl)-6,60 -dimethyl-2,20 -bipyridine (3 ), and 4,40 -bis(4-diphenylaminophenylethynyl)- 6,60 -dimethyl-2,20 -bipyridine (4 ) are reported along with the preparations and characterisations of their homoleptic copper(I) complexes [CuL2 ][PF6 ] (L = 1 -4 ). The solution absorption spectra of the complexes exhibit ligand-centred absorptions in addition to absorptions in the visible region assigned to a combination of intra-ligand and metal-to-ligand charge-transfer. Heteroleptic [Cu(5 )(Lancillary )]+ dyes in which 5 is the anchoring ligand ((6,60 -dimethyl-[2,20 -bipyridine]- 4,40 -diyl)bis(4,1-phenylene))bis(phosphonic acid) and Lancillary = 1 -4 have been assembled on fluorine-doped tin oxide (FTO)-TiO2 electrodes in dye-sensitized solar cells (DSCs). Performance parameters and external quantum e ciency (EQE) spectra of the DSCs (four fully-masked cells for each dye) reveal that the best performing dyes are [Cu(5 )(1 )]+ and [Cu(5 )(3 )]+ . The alkynyl spacers are not beneficial, leading to a decrease in the short-circuit current density (JSC ), confirmed by lower values of EQEmax . Addition of a co-absorbent (n -decylphosphonic acid) to [Cu(5 )(1 )]+ lead to no significant enhancement of performance for DSCs sensitized with [Cu(5 )(1 )]+ . Electrochemical impedance spectroscopy (EIS) has been used to investigate the interfaces in DSCs; the analysis shows that more favourable electron injection into TiO2 is observed for sensitizers without the alkynyl spacer and confirms higher JSC values for [Cu(5 )(1 )]

How Reproducible are Electrochemical Impedance Spectroscopic Data for Dye-Sensitized Solar Cells?

Dye-sensitized solar cell (DSC) technology has been broadly investigated over the past few decades. The sandwich-type structure of the DSC makes the manufacturing undemanding under laboratory conditions but results in the need for reproducible measurements for acceptable DSC characterization. Electrochemical impedance spectroscopy (EIS) o ers the possibility to study complex electronic systems and is commonly used for solar cells. There is a tendency in the literature to present impedance data only for one representative device. At the same time, as current density-voltage plots illustrate, measurements can vary within one set of DSCs with identical components. We present multiple DSC impedance measurements on "identical" devices prepared using two di erent dyes and present a statistical analysis regarding the reproducibilit

Air-stable Solid-state Photoluminescence Standards for Quantitative Measurements Based on 4'-phenyl-2,2':6',2''-Terpyridine Complexes with Trivalent Lanthanides

Correct photoluminescence quantum yield (PLQY) determination in the solid state is vital for numerous application fields, such as photovoltaics, solid lighting or the development of phosphors. In order to increase the limited number of suitable standards for such determinations, two new Ln 3+ -based complexes with 4′-phenyl-2,2′ : 6′,2"-terpyridine γ-[Ln 4 (OAc) 12 (ptpy) 2 ] ( 1-Eu with europium and 1-Tb with terbium) are presented. The corresponding complexes show solid-state QYs of 58(4) % and 46(3) %, respectively, exhibiting broadband absorption in the UV range from 380-200 nm. As Ln 3+ ions in general exhibit narrow f - f transitions, spectral regions with a broadness of 20-35 nm can be checked. Both complexes have suitable thermal stability, up to 270 °C, and are stable with respect to air and humidity, for 1-Eu up to 75 % and for 1-Tb up to 53 % relative humidity. These complexes are altogether suitable as standards to increase the reliability of PLQY determination and proposed to be used for a relative PLQY determination in the solid stat

Electrolyte Tuning in Iron(II)-Based Dye-Sensitized Solar Cells: Different Ionic Liquids and I2 Concentrations

The effects of different I 2 concentrations and different ionic liquids (ILs) in the electrolyte on the performances of dye-sensitized solar cells (DSCs) containing an iron(II) N -heterocyclic carbene dye and containing the I - /I 3 - redox shuttle have been investigated. Either no I 2 was added to the electrolyte, or the initial I 2 concentrations were 0.02, 0.05, 0.10, and 0.20 M. The short-circuit current density ( J SC ), open-circuit voltage ( V OC ), and the fill factor ( ff ) were influenced by changes in the I 2 concentration for all the ILs. For 1-hexyl-3-methylimidazole iodide (HMII), low V OC and low ff values led to poor DSC performances. Electrochemical impedance spectroscopy (EIS) showed the causes to be increased electrolyte diffusion resistance and charge transfer resistance at the counter electrode. DSCs containing 1,3-dimethylimidazole iodide (DMII) and 1-ethyl-3-methylimidazole iodide (EMII) showed the highest J SC values when 0.10 M I 2 was present initially. Short alkyl substituents (Me and Et) were more beneficial than longer chains. The lowest values of the transport resistance in the photoanode semiconductor were found for DMII, EMII, and 1-propyl-2,3-dimethylimidazole iodide (PDMII) when no I 2 was added to the initial electrolyte, or when [I 2 ] was less than 0.05 M. Higher [I 2 ] led to decreases in the diffusion resistance in the electrolyte and the counter electrode resistance. The electron lifetime and diffusion length depended upon the [I 2 ]. Overall, DMII was the most beneficial IL. A combination of DMII and 0.1 M I 2 in the electrolyte produced the best performing DSCs with an average maximum photoconversion efficiency of 0.65% for a series of fully-masked cells

Constraints on the KS0→μ+μ−K^0_S \rightarrow \mu^+ \mu^- Branching Fraction

International audienceA search for the decay KS0→μ+μ- is performed using proton-proton collision data, corresponding to an integrated luminosity of 5.6  fb-1 and collected with the LHCb experiment during 2016, 2017, and 2018 at a center-of-mass energy of 13 TeV. The observed signal yield is consistent with zero, yielding an upper limit of B(KS0→μ+μ-)<2.2×10-10 at 90% C.L.. The limit reduces to B(KS0→μ+μ-)<2.1×10-10 at 90% C.L. once combined with the result from data taken in 2011 and 2012

Measurement of the Λb0→J/ψΛ\Lambda^0_b\rightarrow J/\psi\Lambda angular distribution and the Λb0\Lambda^0_b polarisation in pppp collisions

International audienceThis paper presents an analysis of the ${\Lambda}_b^0$→ J/ψΛ angular distribution and the transverse production polarisation of ${\Lambda}_b^0$ baryons in proton-proton collisions at centre-of-mass energies of 7, 8 and 13 TeV. The measurements are performed using data corresponding to an integrated luminosity of 4.9 fb$^{−1}$, collected with the LHCb experiment. The polarisation is determined in a fiducial region of ${\Lambda}_b^0$ transverse momentum and pseudorapidity of 1 < p$_{T}$< 20 GeV/c and 2 < η < 5, respectively. The data are consistent with ${\Lambda}_b^0$ baryons being produced unpolarised in this region. The parity-violating asymmetry parameter of the Λ → pπ$^{−}$ decay is also determined from the data and its value is found to be consistent with a recent measurement by the BES III collaboration.[graphic not available: see fulltext

Measurement of the electron reconstruction efficiency at LHCb

International audienceThe single electron track-reconstruction efficiency is determined using a sample corresponding to $1.3~\mathrm{fb}^{-1}$of $pp$ collision data recorded with the LHCb detector in 2017. This measurement exploits $B^+\to J/\psi (e^+e^-)K^+$ decays, where one of the electrons is fully reconstructed and paired with the kaon, while the other electron is reconstructed using only the information of the vertex detector. Despite this partial reconstruction, kinematic and geometric constraints allow the $B^+$-meson mass to be reconstructed and the signal to be well separated from backgrounds. This in turn allows the electron reconstruction efficiency to be measured by matching the partial track segment found in the vertex detector to tracks found by LHCb's regular reconstruction algorithms. The agreement between data and simulation is evaluated, and corrections are derived for simulated electrons in bins of kinematics. The presented method allows LHCb to measure branching fractions involving single electrons with a an electron reconstruction systematic uncertainty below $1\%$

Measurement of ∣Vcb∣|V_{cb}| with Bs0→Ds(∗)−μ+νμB_s^0 \to D_s^{(*)-} \mu^+ \nu_{\mu} decays

International audienceThe element |Vcb| of the Cabibbo-Kobayashi-Maskawa matrix is measured using semileptonic Bs0 decays produced in proton-proton collision data collected with the LHCb detector at center-of-mass energies of 7 and 8 TeV, corresponding to an integrated luminosity of 3  fb-1. Rates of Bs0→Ds-μ+νμ and Bs0→Ds*-μ+νμ decays are analyzed using hadronic form-factor parametrizations derived either by Caprini, Lellouch and Neubert (CLN) or by Boyd, Grinstein and Lebed (BGL). The measured values of |Vcb| are (41.4±0.6±0.9±1.2)×10-3 and (42.3±0.8±0.9±1.2)×10-3 in the CLN and BGL parametrization, respectively. The first uncertainty is statistical, the second systematic, and the third is due to the external inputs used in the measurement. These results are in agreement with those obtained from decays of B+ and B0 mesons. They are the first determinations of |Vcb| at a hadron-collider experiment and the first using Bs0 meson decays