The syntheses of 4,4′-bis(4-dimethylaminophenyl)-6,6′-dimethyl-2,2′-bipyridine (1), 4,4′-bis(4-dimethylaminophenylethynyl)-6,6′-dimethyl-2,2′-bipyridine (2), 4,4′-bis(4-diphenylaminophenyl)-6,6′-dimethyl-2,2′-bipyridine (3), and 4,4′-bis(4-diphenylaminophenylethynyl)-6,6′-dimethyl-2,2′-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)(L_ancillary)]⁺ dyes in which 5 is the anchoring ligand ((6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid) and L_ancillary = 1–4 have been assembled on fluorine-doped tin oxide (FTO)-TiO₂ electrodes in dye-sensitized solar cells (DSCs). Performance parameters and external quantum efficiency (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 (J_SC), confirmed by lower values of EQE_max. 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 J_SC values for [Cu(5)(1)]⁺.

The effects of different I₂ 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^–/I₃^– redox shuttle have been investigated. Either no I₂ was added to the electrolyte, or the initial I₂ 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 I₂ 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 I₂ 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 I₂ was added to the initial electrolyte, or when [I₂] was less than 0.05 M. Higher [I₂] led to decreases in the diffusion resistance in the electrolyte and the counter electrode resistance. The electron lifetime and diffusion length depended upon the [I₂]. Overall, DMII was the most beneficial IL. A combination of DMII and 0.1 M I₂ in the electrolyte produced the best performing DSCs with an average maximum photoconversion efficiency of 0.65% for a series of fully-masked cells.

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 Ln³⁺‐based complexes with 4′‐phenyl‐2,2′ : 6′,2“‐terpyridine γ‐[Ln₄(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 Ln³⁺ 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 state.

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 (EQE_max) up to approximately 25% due to an increased short-circuit current density (J_SC). 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 J_SC 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 J_SC and open-circuit voltage (V_OC) 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 J_SC value for an N-heterocyclic carbene iron(II) dye of 4.90 mA cm⁻², but a low V_OC 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.

DSC photoconversion efficiencies of up to 0.93–0.95% have been achieved for a new heteroleptic iron(ii) N-heterocyclic carbene sensitizer employing n-butyl chains.

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) offers 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 different dyes and present a statistical analysis regarding the reproducibility.