There Is a Future for N-Heterocyclic Carbene Iron(II) Dyes in Dye-Sensitized Solar Cells: Improving Performance through Changes in the Electrolyte.

By systematic tuning of the components of the electrolyte, the performances of dye-sensitized solar cells (DSCs) with an N-heterocyclic carbene iron(II) dye have been significantly improved. The beneficial effects of an increased Li+ ion concentration in the electrolyte lead to photoconversion efficiencies (PCEs) up to 0.66% for fully masked cells (representing 11.8% relative to 100% set for N719) and an external quantum efficiency maximum (EQEmax) up to approximately 25% due to an increased short-circuit current density (JSC). A study of the effects of varying the length of the alkyl chain in 1-alkyl-3-methylimidazolium iodide ionic liquids (ILs) shows that a longer chain results in an increase in JSC with an overall efficiency up to 0.61% (10.9% relative to N719 set at 100%) on going from n-methyl to n-butyl chain, although an n-hexyl chain leads to no further gain in PCE. The results of electrochemical impedance spectroscopy (EIS) support the trends in JSC and open-circuit voltage (VOC) parameters. A change in the counterion from I- to [BF4]- for 1-propyl-3-methylimidazolium iodide ionic liquid leads to DSCs with a remarkably high JSC value for an N-heterocyclic carbene iron(II) dye of 4.90 mA cm-2, but a low VOC of 244 mV. Our investigations have shown that an increased concentration of Li+ in combination with an optimized alkyl chain length in the 1-alkyl-3-methylimidazolium iodide IL in the electrolyte leads to iron(II)-sensitized DSC performances comparable with those of containing some copper(I)-based dyes.

Sometimes the Same, Sometimes Different: Understanding Self-Assembly Algorithms in Coordination Networks.

The syntheses and characterizations of three new ligands containing two 4,2':6',4″-tpy or two 3,2':6',3″-tpy metal-binding domains are reported. The ligands possess different alkyloxy functionalities attached to the central phenylene spacer: n-pentyloxy in 3, 4-phenyl-n-butoxy in 4, benzyloxy in 5. Crystal growth under ambient conditions has led to the formation of {[Co(NCS)2(3)]·0.8C6H4Cl2}n and {[Co(NCS)2(4)]·1.6H2O·1.2C6H4Cl2}n, with structures confirmed by single crystal X-ray diffraction. Both the cobalt(II) center and ligand 3 or 4 act as 4-connecting nodes and both {[Co(NCS)2(3)]·0.8C6H4Cl2}n and {[Co(NCS)2(4)]·1.6H2O·1.2C6H4Cl2}n possess a 3D cds net despite the fact that 3 and 4 contain two 4,2':6',4″-tpy and two 3,2':6',3″-tpy units, respectively. Taken in conjunction with previously reported data, the results indicate that the role of the alkyloxy substituent is more significant than the choice of 4,2':6',4″- or 3,2':6',3″-tpy isomer in determining the assembly of a particular 3D net. The combination of Co(NCS)2 with 5 resulted in the formation of the discrete molecular complex [Co(NCS)2(MeOH)2(5)2]·2CHCl3·2MeOH in which 5 acts as a monodentate ligand. The pendant phenyls and both coordinated and non-coordinated 4,2':6',4″-tpy units are involved in efficient π-stacking interactions.

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

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 η 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.

Synthesis and Reactivity of Triazaphenanthrenes.

Pyridonaphthyridines (triazaphenanthrenes) were prepared in 4 steps and in 13-52% overall yield using Negishi cross-couplings between iodopicolines and 2-chloro-pyridylzinc derivatives. After chlorination, Gabriel amination and spontaneous ring-closure, the final aromatization leading to the triazaphenanthrenes was achieved with chloranil. This heterocyclic scaffold underwent a nucleophilic addition at position 6 leading to further functionalized pyridonaphthyridines. The influence of these chemical modifications on the optical properties was studied by steady-state and time-resolved optical spectroscopy. While the thiophene-substituted heterocycles exhibited the most extended absorption, the phenyl- and furan-substituted compounds showed a stronger photoluminescence, reaching above 20% quantum yield and lifetimes of several nanoseconds.