Solar energy conversion using first row d-block metal coordination compound sensitizers and redox mediators

Abstract

The use of renewable energy is essential for the future of the Earth, and solar photons are the ultimate source of energy to satisfy the ever-increasing global energy demands. Photoconversion using dye-sensitized solar cells (DSCs) is becoming an established technology to contribute to the sustainable energy market, and among state-of-the art DSCs are those which rely on ruthenium(ii) sensitizers and the triiodide/iodide (I₃ ^-/I^-) redox mediator. Ruthenium is a critical raw material, and in this review, we focus on the use of coordination complexes of the more abundant first row d-block metals, in particular copper, iron and zinc, as dyes in DSCs. A major challenge in these DSCs is an enhancement of their photoconversion efficiencies (PCEs) which currently lag significantly behind those containing ruthenium-based dyes. The redox mediator in a DSC is responsible for regenerating the ground state of the dye. Although the I₃ ^-/I^- couple has become an established redox shuttle, it has disadvantages: its redox potential limits the values of the open-circuit voltage (V _OC) in the DSC and its use creates a corrosive chemical environment within the DSC which impacts upon the long-term stability of the cells. First row d-block metal coordination compounds, especially those containing cobalt, and copper, have come to the fore in the development of alternative redox mediators and we detail the progress in this field over the last decade, with particular attention to Cu²⁺/Cu⁺ redox mediators which, when coupled with appropriate dyes, have achieved V _OC values in excess of 1000 mV. We also draw attention to aspects of the recyclability of DSCs.

Fig. 1. The air mass (AM) 1.5 solar spectrum [https://commons.wikimedia.org/wiki/File:Solar_spectrum_en.svg].

Fig. 2. (a) A schematic representation of an n-type DSC. S = ground state of the dye; S* = excited state of the dye; E_F = Fermi level; E_cond = conduction band of the semiconductor; E_redox = redox potential of the redox shuttle (a component of the electrolyte); V_OC = open-circuit voltage. Working electrode = photoanode. The glass substrates may be replaced by polymer substrates. (b) Recombination processes: (i) decay of the excited state dye back to the ground state; (ii) recombination of the injected electron with the oxidized dye; (iii) recombination of the injected electron with the oxidized form of the redox shuttle. (c) A typical research DSC with glass/FTO/TiO₂/dye photoanode, glass/Pt counter electrode, and electrolyte. This particular DSC contains an N-heterocyclic iron(ii) dye and an I₃⁻/I⁻ redox mediator (photo: Dr Mariia Becker, University of Basel).

Scheme 1. The structure of chenodeoxycholic acid (cheno).

Scheme 2. Structures of the dyes N719 and SQ2.

Scheme 3. Structures of bpy, phen, bpm, dbbip, bpy-pz, 4,4′-Me₂bpy, 4,4′-^tBu₂bpy and 4,4′-di-tert-butyl-2,2′-bipyrimidine (4,4′-^tBu₂bpm).

Fig. 3. Schematic illustration of the relative E_redox levels (in red) for two representative Co³⁺/Co²⁺ redox mediators with respect to the I₃⁻/I⁻ couple and the effect on the value of V_OC (see Fig. 2a for complete DSC diagram).

Scheme 4. Structures of some of the high extinction coefficient metal-free and zinc(ii) porphyrin dyes used with cobalt(ii)/(iii) and/or copper(ii)/(i) redox mediators.

Scheme 5. Structures of the ruthenium(ii) dyes discussed in the text; see Scheme 2 for the structure of N719.

Scheme 6. Structures of some of N^N ligands used in copper(ii)/copper(i) redox mediators, and the structure of the [TFSI]⁻ anion.

Fig. 4. The structures of (a) trans-[Cu(TBP)₄(O₃SCF₃)₂] (CSD refcode IPEWAD), (b) the [Cu(Me₂phen)₂(NCMe)]²⁺ cation in the perchlorate salt (CSD refcode XIDWEP), and (c) the [Cu(Me₂phen)₂(TBP)]²⁺ cation from the structure of the TFSI⁻ salt; the cif was kindly provided by the authors of ref. . Hydrogen atoms are omitted for clarity. As a general note, 3D-structures in this review have been drawn using coordinates retrieved from the Cambridge Structural Database (CSD, version 2021.2.0) and using Mercury version 2021.2.0.

Scheme 7. Representation of the involvement of a strong Lewis base such as TBP in the redox cycle of the [Cu(Me₂phen)₂]²⁺/[Cu(Me₂phen)₂]⁺ mediator. Based on a scheme from the work of Bach and coworkers.

Scheme 8. Structures of the pentatdentate ligands tpe and tme and the related tetradentate ligand dbdpe, the bidentate ligands npbi and nbpbi, and a bpy-based ligand dbpy designed to form a double stranded copper(i) helicate.

Scheme 9. Structures of the metal-free dyes MS5 and XY1b.

Scheme 10. Structures of the conjugate acids of the tetradentate ligands [salen]²⁻ and [hybeb]⁴⁻.

Scheme 11. Structures of the commercially available dyes D131 and D205, and of the metal-free dyes MK2 and K4.

Fig. 5. The conjugate acid of the heteroscorpionate ligand [bdmpza]⁻ and the structures of (a) the manganese(iii) complex [Mn(bdmpza)₂]⁺ (CSD refcode ITEQOP) and (b) the iron(iii) complex [Fe(bdmpza)₂]⁺ (refcode ITEQEF) both in the [BF₄]⁻ salts.

Scheme 12. Structures of the metal-free dyes Carbz-PAHTDTT and RR9, and the ruthenium(ii) dyes [Ru(tpyCO₂H)(ttpy)][PF₆]₂ (tpy = 2,2′:6′,2′′-terpyridine) and the commercially available Ruthenizer-505.

Fig. 6. (a) The nickel(iv)/(iii) bis(dicarbollide), [Ni(C₂B₉H₁₁)₂]/[Ni(C₂B₉H₁₁)₂]⁻, redox couple, and (b) the structure of the nickel(iv) complex [Ni(C₂B₉H₁₀Ph)₂] (CSD refcode HABQEI).

Scheme 13. The sensitizer designed by Sauvage and coworkers, and the preorganized nature of the phen metal-binding domain compared to the conformational change required by bpy.

Fig. 7. Structures of ligands 1–3, and the structure of [Cu(3)₂]⁺ in the [PF₆]⁻ salt (CSD refcode JOHXIO). The space-filling representation is used to emphasize the protection imparted by the 6- and 6′-methyl groups.

Scheme 14. Structures of the bis(arylimino)acenaphthene compounds 4 and Na₂[5], and ligand 6.

Fig. 8. A schematic representation of structural design for a heteroleptic bis(diimine)copper(i) sensitizer facilitating electron transfer through the dye molecule to the n-type semiconductor.

Scheme 15. Structures of functionalized bpy ligands 7–11 which contain carboxylic or phosphonic acid anchoring groups.

Fig. 9. The SALSAC approach to in situ assembly of a heteroleptic copper(i) dye on an electrode surface using ligand exchange.

Scheme 16. The structures of (a) [Cu(12)(tmpDMP)]⁺ reported by Ashbrook and Elliott, and (b) a copper(i) dye reported by our group tested with a Co³⁺/Co²⁺ redox mediator. Both heteroleptic dyes were assembled in situ using the SALSAC approach.

Scheme 17. Structures of some anchoring ligands used with heteroleptic copper(i) dyes. See also Scheme 15.

Scheme 18. Structures of ancillary ligands which are derivatives of bpy used in complexes in Table 5. Note the use of electron-releasing methoxy groups in some of the ligands (see text).

Scheme 19. Structures of ancillary ligands which are derivatives of phen used in complexes in Table 5.

Scheme 20. Structures of a series of related ancillary ligands used in heteroleptic copper(i) dyes; complexes with ligands in the family 49–51 were the subject of a theoretical study (see text).

Fig. 10. Solid-state absorption spectra of FTO/TiO₂ electrodes with adsorbed dyes [Cu(11)(45)]⁺ (red) and SQ2 (blue). The high-energy tail arises from TiO₂ absorption (spectra recorded by Frederik Malzner, University of Basel).

Scheme 21. Anchoring and ancillary ligands used in the HETPHEN approach to copper(i) dyes.

Fig. 11. Two views of the structure of a [Cu(51)(phenazine)]⁺ derivative (CSD refcode RNAFAP) showing the π-stacking interaction between one mesityl group and the phen unit of the second ligand.

Scheme 22. Structures of the porphyrin H₂CPI, and three Hdipyrrin ligands.

Scheme 23. Structures of the ligands POP, 61 and 62, and the structures of the hetermetallic complex 63 and of the dinuclear copper(i) complexes from Jayapal et al. In 61 and 62, the N atoms shown in blue are the metal-binding sites.

Scheme 24. Structures of ancillary ligand 68, and of ligand 69 used in a Cu²⁺/Cu⁺ redox shuttle.

Fig. 12. Examples of ferrocenyl dithiocarbamate metal complexes used as sensitizers, (a) a cobalt(iii) complex, and (b) a zinc(ii) complex (CSD refcode EJAYUL). Both have hydroxyl anchoring groups. (c) Example of a ferrocenyl dithiophosphonate complex of nickel(ii) with P–OH anchors (refcode XORKAT).

Scheme 25. Structures of the homoleptic NHC iron(ii) complexes 70–73 and of [Fe(4′-HO₂Ctpy)₂]²⁺ (74).

Fig. 13. The 1,2,3-triazol-5-ylidene-containing compound [H₂btz][PF₆]₂ and the structure of [Fe(bpy)(btz)₂]²⁺ (CSD refcode NOVVAX).

Scheme 26. Structures of some ionic liquids (ILs) used in DSC electrolytes.

Scheme 27. Structures of the heteroleptic NHC iron(ii) complexes 75–83.

Fig. 14. Part of the 1D-coordination polymer [Ni(en)₂(azobc)]_n (CSD refcode SIQNEN01).

Fig. 15. The SALSAC approach to in situ assembly of a heteroleptic bis(terpyridine)zinc(ii) dye on an electrode surface using a stepwise strategy. ZnX₂ is typically Zn(OAc)₂ or ZnCl₂.

Scheme 28. Structures of derivatives of tpy used as anchoring (84–86) and ancillary (87–93) ligands in zinc(ii) dyes.

Scheme 29. Schiff base zinc(ii) D–π–A dyes.

Scheme 30. Structures of ligands 100–104, H105 and H₂106 used in zinc(ii) co-sensitizers with N719.