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Electrolyte tuning in dye-sensitized solar cells with N-heterocyclic carbene (NHC) iron(II) sensitizers.

ABSTRACT: We demonstrate that the performances of dye-sensitized solar cells (DSCs) sensitized with a previously reported N-heterocyclic carbene iron(II) dye in the presence of chenodeoxycholic acid co-adsorbant, can be considerably improved by altering the composition of the electrolyte while retaining an I^-/I₃ ^- redox shuttle. Critical factors are the solvent, presence of ionic liquid, and the use of the additives 1-methylbenzimidazole (MBI) and 4-tert-butylpyridine (TBP). For the electrolyte solvent, 3-methoxypropionitrile (MPN) is preferable to acetonitrile, leading to a higher short-circuit current density (J _SC) with little change in the open-circuit voltage (V _OC). For electrolytes containing MPN, an ionic liquid and MBI (0.5 M), DSC performance depended on the ionic liquid with 1-ethyl-3-methylimidazolium hexafluoridophosphate (EMIMPF) > 1,2-dimethyl-3-propylimidazolium iodide (DMPII) > 1-butyl-3-methylimidazolium iodide (BMII) ? 1-butyl-3-methylimidazolium hexafluoridophosphate (BMIMPF). Omitting the MBI leads to a significant improvement in J _SC when the ionic liquid is DMPII, BMII or BMIMPF, but with EMIMPF the removal of the MBI additive results in a dramatic decrease in V _OC (542 to 42 mV). For electrolytes containing MPN and DMPII, the effects of altering the MBI concentration have also been investigated. Although the addition of TBP improves V _OC, it causes significant decreases in J _SC. The best performing DSCs with the NHC-iron(II) dye employ an I^-/I₃ ^--based electrolyte with MPN as solvent, DMPII ionic liquid (0.6 M) with no or 0.01 M MBI; values of J _SC = 2.31 to 2.78 mA cm^-2, V _OC = 292 to 374 mV have been achieved giving ? in the range of 0.47 to 0.57% which represents 7.8 to 9.3% relative to an N719 reference DSC set at 100%. Electrochemical impedance spectroscopy has been used to understand the role of the MBI additive in the electrolytes.

SUBMITTER: Karpacheva M

PROVIDER: S-EPMC6317411 | biostudies-literature | 2018

REPOSITORIES: biostudies-literature

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Electrolyte tuning in dye-sensitized solar cells with N-heterocyclic carbene (NHC) iron(II) sensitizers.

Karpacheva Mariia M Housecroft Catherine E CE Constable Edwin C EC

Beilstein journal of nanotechnology 20181221

We demonstrate that the performances of dye-sensitized solar cells (DSCs) sensitized with a previously reported N-heterocyclic carbene iron(II) dye in the presence of chenodeoxycholic acid co-adsorbant, can be considerably improved by altering the composition of the electrolyte while retaining an I-/I3 - redox shuttle. Critical factors are the solvent, presence of ionic liquid, and the use of the additives 1-methylbenzimidazole (MBI) and 4-tert-buty ...[more]

PMID: 30643705

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Project description:A new generation of octahedral iron(ii)-N-heterocyclic carbene (NHC) complexes, employing different tridentate C^N^C ligands, has been designed and synthesized as earth-abundant photosensitizers for dye sensitized solar cells (DSSCs) and related solar energy conversion applications. This work introduces a linearly aligned push-pull design principle that reaches from the ligand having nitrogen-based electron donors, over the Fe(ii) centre, to the ligand having an electron withdrawing carboxylic acid anchor group. A combination of spectroscopy, electrochemistry, and quantum chemical calculations demonstrate the improved molecular excited state properties in terms of a broader absorption spectrum compared to the reference complex, as well as directional charge-transfer displacement of the lowest excited state towards the semiconductor substrate in accordance with the push-pull design. Prototype DSSCs based on one of the new Fe NHC photosensitizers demonstrate a power conversion efficiency exceeding 1% already for a basic DSSC set-up using only the I-/I3 - redox mediator and standard operating conditions, outcompeting the corresponding DSSC based on the homoleptic reference complex. Transient photovoltage measurements confirmed that adding the co-sensitizer chenodeoxycholic acid helped in improving the efficiency by increasing the electron lifetime in TiO2. Time-resolved spectroscopy revealed spectral signatures for successful ultrafast (<100 fs) interfacial electron injection from the heteroleptic dyes to TiO2. However, an ultrafast recombination process results in undesirable fast charge recombination from TiO2 back to the oxidized dye, leaving only 5-10% of the initially excited dyes available to contribute to a current in the DSSC. On slower timescales, time-resolved spectroscopy also found that the recombination dynamics (longer than 40 μs) were significantly slower than the regeneration of the oxidized dye by the redox mediator (6-8 μs). Therefore it is the ultrafast recombination down to fs-timescales, between the oxidized dye and the injected electron, that remains as one of the main bottlenecks to be targeted for achieving further improved solar energy conversion efficiencies in future work.

Project description:Six new bio-inspired flavylium salts were synthesized and investigated by a combined computational and experimental study for dye-sensitized solar cell applications. The compounds were characterized by FT-IR, UV-Vis, NMR spectroscopy, and LC-MS spectrometry techniques. The pH-dependent photochromic properties of the flavylium dyes were investigated through a UV-Vis spectroscopy study and revealed that they follow the same network of chemical reactions as anthocyanins upon pH changes. The structural and electronic properties of the dyes were investigated using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). Geometry optimization calculation revealed that all dyes, regardless of the specie, flavylium cations or quinoidal bases, present a planar geometry. The photovoltaic performances of the dyes, in both flavylium and quinoidal base forms, were evaluated by the HOMO and LUMO energies and by calculating the light-harvesting efficiencies, the free energy change of electron injection, and the free energy change regeneration. The MO analysis showed that all dyes can inject electrons into the conduction band of the TiO2 upon excitation and that the redox couple can regenerate the oxidized dyes. The results obtained for the free energy change of electron injection suggest that the quinoidal bases should inject electrons into the semiconductor more efficiently than the flavylium cations. The values for the free energy change regeneration showed that the redox electrolyte can easily regenerate all dyes. Dipole moment analysis was also performed. DSSCs based on the dyes, in both flavylium and quinoidal base forms, were assembled, and their photovoltaic performances were evaluated by measuring the open-circuit voltage, the short circuit current density, the fill factor, and the energy conversion efficiency. Results obtained by both experimental and computational studies showed that the overall performances of the DSSCs with the quinoidal forms were better than those obtained with the flavylium cations dyes.

Dataset Information

Electrolyte tuning in dye-sensitized solar cells with N-heterocyclic carbene (NHC) iron(II) sensitizers.

Publications

Electrolyte tuning in dye-sensitized solar cells with <i>N</i>-heterocyclic carbene (NHC) iron(II) sensitizers.

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