Recent Investigations on the Use of Copper Complexes as Molecular Materials for Dye-Sensitized Solar Cells

Abstract

Three decades ago, dye-sensitized solar cells (DSSCs) emerged as a route for harnessing the sun's energy and converting it into electricity. Since then, an impressive amount of work has been devoted to improving the global photovoltaic efficiency of DSSCs, trying to optimize all components of the device. Up to now, the best efficiencies have usually been reached with ruthenium(II) photosensitizers, even if in the last few years many classes of organic compounds have shown record efficiencies. However, the future of DSSCs is stringently connected to the research and development of cheaper materials; in particular, the replacement of rare metals with abundant ones is an important topic in view of the long-term sustainability of DSSCs intended to replace the consolidated fossil-based technology. In this context, copper is a valid candidate, being both an alternative to ruthenium in the fabrication of photosensitizers and a material able to replace the common triiodide/iodide redox couple. Thus, recently, some research papers have confirmed the great potential of copper(I) coordination complexes as a cheap and convenient alternative to ruthenium dyes. Similarly, the use of copper compounds as electron transfer mediators for DSSCs can be an excellent way to solve the problems related to the more common I₃^-/I^- redox couple. The goal of this mini-review is to report on the latest research devoted to the use of versatile copper complexes as photosensitizers and electron shuttles in DSSCs. The coverage, from 2022 up to now, illustrates the most recent studies on dye-sensitized solar cells based on copper complexes as molecular materials.

Keywords: copper complexes; dye-sensitized solar cells; electron shuttles; photosensitizers.

Figure 3

Structure of dyes 3a–3g.

Figure 5

Structure of dyes 5a, PPV-SF-Cd, 5b, and PBDTT-SF-Cd.

Figure 6

Structure of dyes 6, 7, Co-Sal, and 8.

Figure 7

Structure of dyes 9a–9d.

Figure 10

Structure of dyes ZS4 and ZS5 and of the redox couple [(tmby)₂Cu]^+/2+.

Figure 1

Structure of reference dyes N719 and N3.

Scheme 1

General structure of Cu(I) complexes bearing bidentate N^N ligands: compound A represents a generical homoleptic complex bearing electron-accepting moieties, compound B an heteroleptic complex based on a push-pull structure, while C the novel class of complexes presenting asymmetrical ligands, as proposed in 2023 by Housecroft et al. The electron-withdrawing substituents are marked with A, while the electron-donating ones are marked with D.

Scheme 2

Structure of copper-based dye Cu1–4.

Figure 2

Structure of dyes 1a, 1b, 2, Ni-Fc and Co-Fc.

Figure 4

Structure of dyes 4a–4e.

Figure 8

Structure of dye TY6 and of the redox couple [(dmp)₂Cu]^+/2+.

Figure 9

Structure of dye MK-2 and of redox couples [(mbpbi)₂Cu]^+/2+, [(tbbpbi)₂Cu]^+/2+, [(mbpbi)₃Co]^2+/3+ and [(tbbpbi)₃Co]^2+/3+.