Electrochemistry Communications 134 (2022) 107182
Contents lists available at ScienceDirect 
Electrochemistry Communications 
journal homepage: www.elsevier.com/locate/elecom 
Polypyridyl copper complexes as dye sensitizer and redox mediator for 
dye-sensitized solar cells 
Jeanet Conradie 1 
Department of Chemistry, University of the Free State, P.O. Box 339, Bloemfontein 9300, South Africa   
A R T I C L E  I N F O   A B S T R A C T   
Keywords: Developments on the application of polypyridyl-based copper complexes in dye-sensitized solar cells (DSSCs) are 
Copper briefly discussed in this mini review. Copper complexes in solar cells are special in that they can be used either as 
Dye dye sensitizer and/or as redox mediator (redox couple) in dye-sensitized solar cells (DSSCs). Both the abundance 
Mediator and low cost of copper motivates research on the use of copper complexes, as cheaper and non-toxic alternative 
Redox 
DFT to the mostly used iodide/triiodide (I
− /I −3 ) electrolyte redox couple, and the expensive and rare ruthenium- 
DSSC based dyes of good performance, which currently are best known.   
1. Introduction low light conditions [3]. In Section 2 the development of polypyridyl- 
based copper redox mediators is discussed. 
Dye-Sensitized Solar Cell (DSSC), invented in 1988 by O’Regan and The role of the dye sensitizer at the photoanode is for light harvesting 
Grätzel [1], is a low-cost solar cell working under a wide range of light and charge injection. The dye molecules are adsorbed onto a meso-
conditions, including low light conditions. DSSCs convert light energy porous TiO2 film on the anode. Both organic and inorganic-based dyes 
into electricity. The name originated from the fact that a dye is the are used as dye sensitizer. Ruthenium and osmium complex sensitizers 
component of the solar cell responsible for the maximum absorption of have been well tested in the past and are best known [4]. The rarity and 
the incident light. The three basic components of a DSSC are the dye- high cost of ruthenium has led to the development of many novel 
sensitized conducting photoanode, the redox electrolyte and counter cheaper dyes. Copper, as an abundant and nontoxic non-noble metal, is 
electrode or cathode. The functionality of a DSSC is based on the now considered as a good substitute for the expensive ruthenium in 
following steps: light absorption by the dye, electron injection from the DSSCs [5]. In Section 3 the development of polypyridyl-based copper 
electronically excited dye into the conduction band (CB) of the photo- dye sensitizers is discussed. 
anode (e.g. semi-conductor TiO2), the collection of charge, flow of In DSSCs the driving force of electron injection (ΔGinject) and dye 
electrons through the external circuit to the counter electrode (cathode) regeneration (ΔGregenerate) is evaluated by comparing the computed 
and into the redox electrolyte (e.g. I− /I −3 ). The redox system of the highest occupied molecular orbital energy (EHOMO) and lowest unoc-
electrolyte transports the electrons back to the dye molecules and re- cupied molecular orbital energy (ELUMO) of the dyes with the potential of 
generates the oxidized dye by reducing the dye-molecule, see Fig. 1. the CB of the TiO2 semi-conductor (ECB = -4.0 eV vs vacuum, or − 0.5 eV 
The role of the redox mediator (electrolyte) is to transport charges vs NHE [6]) and with the potential of the most widely used electrolyte 
between the electrodes and to regenerate the oxidized dye. Electrolytes redox couple I− /I −3 (-4.8 eV vs vacuum, or 0.3 eV vs NHE) [7]), 
used in DSSCs can be in the liquid, quasi-solid or solid states. The use of respectively. More negative HOMO energies than the I− /I −3 redox 
liquid electrolytes, such as the I− /I −3 redox couple, leads to leakage and couple imply a fast regeneration of the oxidized dyes. More positive 
corrosion of the solar cells and degradation of the dyes. Metal complexes LUMO energies than the ECB of TiO2 could ensure an effective injection 
as replacement of the liquid I− /I −3 redox couple, have been found to be a of excited electrons [8], see Fig. 2. The application of the ΔGinject and 
good alternative, due to their reversible one-step oxidation and reduc- ΔGregenerate driving force is important in both experimental and theo-
tion processes [2]. Studies on copper(II/I) complexes as redox mediators retical studies of redox mediators and dye sensitizers, in determining 
and hole conductors have shown surprisingly high performance under possible candidates for DSSCs. 
E-mail address: conradj@ufs.ac.za.   
1 0000-0002-8120-6830. 
https://doi.org/10.1016/j.elecom.2021.107182    
Available online 14 December 2021
1388-2481/© 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
J. Conradie                                                                                                                                                                                        E  l e c  t r o  c h  e  m  i s t r  y  C   o m   m   u n  i c  a t i o  n  s 134 (2022) 107182
Fig. 1. Schematic energy level diagram of a DSSC, showing the basic operation of a Grätzel cell. The voltage output generated by different redox couples or 
electrolytes is listed. Reproduced from Ref. [2] with permission from the Royal Society of Chemistry. 
Fig. 2. Energy levels of a dye-sensitized solar cell, DSSC [9], where ECB is the conduction band potential and EVB the valence band potential of the TiO2 semi- 
conductor, ΔGinject is the driving force of electron injection, and ΔGregenerate of dye regeneration. 
Fig. 3. Structures of selected homoleptic Cu-polypyridyl complexes reported in literature [10,16,17,20–22], used as redox mediators (electrolytes) in DSSCs.  
2
J. Conradie                                                                                                                                                                                        E  l e c  t r o  c h  e  m  i s t r  y  C   o m   m   u n  i c  a t i o  n  s 134 (2022) 107182
Fig. 4. Structures of selected homoleptic Cu-polypyridyl complexes reported [23–27] as dyes in DSSCs.  
2. Polypyridyl copper complexes as redox mediator – a pseudo tetrahedral coordination, while oxidation to copper(II) leads to 
Experimental results a flattening of the geometry to compressed (flattened) tetrahedral, 
nearing a square planar geometry [11,12]. The change in oxidation state 
The first experimental report on using polypyridyl copper complexes during redox reactions leads to loss of efficiency of the whole DSSC 
as redox mediator to replace the conventional I− /I −3 redox couple in system, since a certain amount of energy is required for the reorgani-
DSSC, was a study by Fukuzumi et al. [10] in 2005, on redox couples [Cu zation of the coordination sphere of copper(I) [13]. It has been found 
(dmp) ]2+/+2 and [Cu(phen) 2+/+2] (see Fig. 3 for structures). Although a that bulky ortho substituents on the polypyridyl ligands, fix the geom-
lower power conversion efficiency (PCE) was obtained compared to the etry in an intermediate situation between tetrahedral and tetragonal, 
I− /I −3 electrolyte, a higher open-circuit voltage (Voc) of the cell has been leading to less dissipation and better efficiency of the DSSC [10,13,14]. 
attained than compared to I− /I −3 . Complexes with a smaller structural Since the first report, the application of different polypyridyl copper 
change between the copper(I) and copper(II) molecules, result in better complexes as redox mediator have been studied (Fig. 3 for selected 
electron-transport since less energy is dissipated by geometry rear- structures) by different groups: for excellent reviews, see [2,4,13]. 
rangement. The coordination geometry of the molecule is dependent on In 2015, researchers Freitag et al. were the first group to publish a 
the oxidation state of the metal. Copper(I) is generally characterized by study on solid-state dye-sensitized solar cells (ssDSSC), using electrolyte 
Fig. 5. Structures of well-known efficient Ru [45–47] and Zn [48,49] dyes used in DSSCs.  
3
J. Conradie                                                                                                                                                                                        E  l e c  t r o  c h  e  m  i s t r  y  C   o m   m   u n  i c  a t i o  n  s 134 (2022) 107182
Cu(dmp)2 as hole transport material (HTM) between the photoanode 
and cathode [15]. To make a ssDSSC initially was like liquid-injecting a 
DSSC with HTM, but instead of sealing the cell after electrolyte injection, 
the solvent was evaporated into the air and new injections were per-
formed, until the space between the photoanode and the counter elec-
trode was filled with solid HTM. Development of very high performing 
ssDSSCs followed, based on research using Cu(tmby)2 as HTM [16,17]. 
That study has found that by using Cu complexes, a driving force of 
about 0.1 eV is sufficient for efficient dye regeneration by the HTM [18]. 
Enhanced efficiency has been obtained by co-sensitization of different 
sensitizers [19]. The introduction of bulky groups either at position C2 
and/or C9 of the phenanthroline ligand, such as in electrolytes Cu 
(dmp)2, Cu(dmpp)2 and Cu(dmmsp)2, is effective in reducing dark cur-
rent (residual electric current flowing when there is no incident light) by 
minimizing the recombination process, leading to higher efficiencies 
[20]. Complexes with groups exhibiting larger steric hindrance, such as 
electrolytes Cu(emp)2 and Cu(dep)2, showed more positive redox po-
Fig. 6. Structures of three selected anchoring ligands, as well as complex [Cu tentials, when compared to Cu(dmp)2 and Cu(bp)2 [21]. The redox po-
(L)(L )]+anchor anchored onto semi-conductor TiO2 in DSSCs [29,31]. tential of a redox couple should be higher than EHOMO of the sensitizer, 
70 63
60
60 56
50 4845
41 41 42
40
30
20
7.06 7.1 6.02 7.16 6.7 3.6 10.13 10.86
10 5.71 6.04 5.91 6.07 5.92 6.25 6.05
2.43 3.16 3.12
5.63
2.4 2.89 4.42 4.661.41
0
D1 D5 D7 D8 D2 D6 D4 D3
Voc (V) Jsc (mA cm-2) PCE (Cu-dye) (%) device efficiency (dye rel to N719) (%)
Fig. 7. Selected complexes reported as dye sensitizers in literature, with device efficiency ηrel > 40%: namely homoleptic dye D6 [37], and heteroleptic dyes D1, D2 
[31], D3, D4 [32], D5 [36], D7 [38] and D8 [39]. 
4
J. Conradie                                                                                                                                                                                        E  l e c  t r o  c h  e  m  i s t r  y  C   o m   m   u n  i c  a t i o  n  s 134 (2022) 107182
Fig. 8. Structures of various Cu(I)-polypyridyl complexes used as dye senstitizers [5,37,54–58], as studied theoretically by DFT computations.  
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J. Conradie                                                                                                                                                                                        E  l e c  t r o  c h  e  m  i s t r  y  C   o m   m   u n  i c  a t i o  n  s 134 (2022) 107182
in order to provide the required driving force for dye regeneration 4. Copper(I) dye coupled with a copper(I)/(II) electron shuttle – 
(Fig. 2). A more positive redox potential of a redox couple with respect Experimental results 
to the EHOMO level of the sensitizer, will lead to less energy loss during 
dye regeneration [2]. The combination of copper(I)-based dyes and [Cu(bpy) ]2+/3+3 type 
Further, in addition to the mentioned homoleptic polypyridyl copper electrolytes, called electron shuttles, has been an important step towards 
complexes, where all ligands are identical, it has been found that both the development of stable iodide-free copper(I) solar cells [34,50,51]. 
the absorption and redox properties of many heteroleptic complexes (e. High power conversion efficiencies have been obtained with full-copper 
g. Cu(bcp)(tmp)) are also very attractive for DSSC applications [22]. DSSCs, in which a heteroleptic copper(I) dye (bearing one 2,9-dimesityl- 
1,10-phenanthroline ligand, together with a 6,6′-dimethyl-2,2′-bipyr-
3. Polypyridyl copper complexes as dye sensitizer – idine-4,4′-dibenzoic acid as anchoring ligand, to anchor the dye onto the 
Experimental results TiO2 semi-conductor surface) was coupled with a copper(I)/(II) electron 
shuttle [52]. Electron shuttles are redox mediators that can reversibly be 
Probably the first mentioning of the sensitization effect of poly- oxidized and reduced, thereby repeatedly serving as electron carriers. By 
pyridyl copper(I) complexes in photochemical systems, was in 1983 on using heteroleptic bis(diimine)copper(I) dyes, coupled with homoleptic 
[Cu(dmp)2]+, [Cu(dpp) ]+2 and [Cu(tpp)2]+ complexes (see Fig. 4 for bis(diimine)copper(I)/(II) redox shuttles, photoconversion efficiencies 
structures) [23], covering a large fraction of the solar spectrum. Then in up to 2.06% (38.1% relative to the standard ruthenium N719 dye, set at 
1994 Sauvage and co-workers investigated [Cu(dcp-phen)2]+ as dye in a 100%) have been obtained. The best efficiency was obtained when using 
TiO2 solar cell, although the obtained photocurrents and photovoltages phosphonic acid as anchor ligand and 4,4′-dimethoxy-6,6′-dimethyl- 
were much smaller than those of the Grätzel cell itself [24]. The dye is 2,2′-bipyridine as the ancillary ligand in the dye, together with two 4,4′- 
the component of DSSCs responsible for the maximum absorption of the dimethoxy-6,6′-dimethyl-2,2′-bipyridine ligands in the electrolyte [53]. 
incident light. Therefore, in 2002 a new copper(I) dye [Cu(tmdcbpy) ]+2
with higher photoresponse was successfully used in a DSSC, with 30% 5. Polypyridyl copper complexes – Theoretical studies 
IPCE (incident monochromatic photon-to-current conversion efficiency) 
[25]. Further, introduction of anchoring ligands, by adding carboxylate The suitability and performance of dyes for effective DSSC applica-
groups in the [Cu(4,4′-COOH-dmby) ]+2 and [Cu(4,4′-CHCH-COOH- tion can also be computed by theoretical density functional theory (DFT) 
dmby)2]+ dyes, has led to effective bonding or anchoring of these calculations. During such theoretical studies, computed properties of the 
homoleptic dyes to the TiO2 nanoparticles of the semi-conductor. dyes, such as frontier orbitals, UV–vis absorbance spectrum, charge 
However, the efficiency of these homoleptic Cu(I) dyes (IPCE values transfer characteristics, the driving force of electron injection (ΔGinject) 
38.6 and 50.1%) were over 4 times lower than that of the N719 ruthe- and dye regeneration (ΔGregenerate), light harvesting efficiency (LHE) 
nium sensitizer (Fig. 5), although the cost of copper(I) sensitizers is an and the excited-state lifetime (τ), are reported [5,54–58]. EHOMO and 
order of magnitude lower than Ru [26,27]. Due to the rarity of ruthe- ELUUMO are of importance, see Fig. 2. DFT studies have found that 
nium, research on the application of copper(I)-polypyridyl as dyes in different oxidation states of the redox mediator result in different ge-
DSSCs has continued [28]. ometries: for example the geometry of reduced [Cu(bpy) ]1+2 is four- 
Subsequently, ligand exchange reactions between various [CuL +2] coordinate pseudo tetrahedral, while the geometry of oxidized [Cu 
complexes (L = substituted bipyridine) and TiO 2+2-anchored bipyridine- (bpy)2] is four-coordinate compressed (flattened) tetrahedral [12]. 
based ligands, containing CO2H or PO(OH)2 anchoring groups, were In 2010, a comprehensive DFT study on the polypyridyl copper(I) 
applied to the dyes. This produced surface-anchored heteroleptic copper complexes [CuL2]+ and [CuL2][PF ′6] (with L = 6,6 -dimethyl-2,2′- 
(I) complexes, [Cu(L)(L +anchor)] (Fig. 6), with increased efficiencies bipyridine-4,4′-dimethylformate) was published (see structures 1A and 
[29–32] compared to homoleptic Cu(I) dyes. In heteroleptic dyes, one 1A[PF6] in Fig. 8). The electronic structures and spectral properties of 
ligand (Lanchor) carries a group effective for anchoring the dye to the these homoleptic Cu(I) complexes, both in the gas phase and in methyl 
TiO2 semiconductor, while the second ligand (L) can be electronically cyanide (MeCN) solution, were investigated on the B3LYP/6-31G(d) and 
and structurally tuned to enhance electron transfer from electrolyte to B3LYP/DZVP levels of theory [54]. Results indicate that the five HOMOs 
dye, by adding appropriate substituents [33]. Incorporation of extended are Cu-3d based orbitals, whereas the four LUMOs are the bipyridine 
π-conjugation in substituted bipyridine as ligand (L), enhances dye ligand π* orbitals. The maxima of the UV/vis spectra in the range of 
performance of [Cu(L)(L +anchor)] [29,30,34]. Introduction of an aro- 400–600 nm have been found to originate from metal-to-ligand charge- 
matic spacer into the phosphonic acid anchoring ligand, has led to transfer (MLCT) transitions. These results indicate that the copper-based 
increased photon-to-current conversion of the heteroleptic dyes, with complexes might be effective sensitizers for a next-generation dye- 
IPCE up to 48.5%, compared to the standard high performance ruthe- sensitized solar cell. In 2011 the same authors followed up on this study 
nium dye N719 (100%) [31]. An advantage of heteroleptic complexes is by a DFT computational study (at B3LYP/6-31G* level of theory), on a 
the possibility to finely adjust their opto-electronic properties more series of eight polypyridyl Cu(I)-based complexes with general formula 
accurately than for homoleptic complexes [35]. Heteroleptic dyes (such [CuLL′]+ (where L and L′ represent heteroleptic bipyridyl ligands) (see 
as D1 – D5, D7 – D8 in Fig. 7, as obtained from literature) generally lead structures 1B – 8B in Fig. 8), as well as on two experimentally well- 
to enhanced conversion efficiencies compared to homoleptic dyes (D6). known Ru(II) sensitizers, N3 and CYC-B11 (see structures in Fig. 5). 
Some important measurements used to characterize the effectiveness Compared with the efficient well-known Ru(II) sensitizers, these heter-
of dyes used as sensitizers in DSSCs, include the PCE, short-circuit oleptic polypyridyl Cu(I)-based complexes under study exhibited similar 
photocurrent density (Jsc), open-circuit voltage (Voc), device efficiency optical properties, but an improving trend of photoresponse and DSSCs 
(ηext) and percentage device efficiency (ηrel = PCE/PCEref × 100%). performance [55]. 
Fig. 7 shows selected polypyridyl-based copper complexes from litera- The same group of authors continued to publish further theoretical 
ture, with ηN791 > 40% [31,32,36–39], where device efficiency is studies on a variety of polypyridyl Cu(I)-based dye-sensitizer complexes, 
measured relative to the standard ruthenium dye molecule. More data namely: Cu(I)-based sensitizers, containing 6,6′-dimethyl-4,4′-dicar-
on these heteroleptic dyes is found in references [26,35,40–43]. The boxylate-2,2′-bipyridine with functionalized 2,9-dimethyl-1,10-phenan-
synthesis and characterization of these different dye complexes are throline ligands (at B3LYP/DZVP level, in CH2Cl2 solution) (see 
described in the references provided. The DSSC fabrication (for testing structures 1C – 8C in Fig. 8) [56]; a series of 14 heteroleptic Cu(I) 
of solar cell application) is generally based on the method of Grätzel and complexes integrating dicarboxylic acid dimethyl bipyridine/phenan-
co-workers [44]. throline with functionalized chromophores (B3LYP/6-31G(d) + DZVP 
basis set, in CH2Cl2 solution) (see structures B1– B7 and P1 – P7 in Fig. 8) 
6
J. Conradie                                                                                                                                                                                        E  l e c  t r o  c h  e  m  i s t r  y  C   o m   m   u n  i c  a t i o  n  s 134 (2022) 107182
that incorporation of thiophene motifs into the ligands of these copper(I) 
complexes (structure 1G in Fig. 8) for use as dyes in DSSCs, has led to a 
bathochromic shift of the MLCT absorbance. Thiophene further also has 
led to an increase in the computed HOMO energy level and an increase 
in efficiency compared to the related complex lacking the thiophenes 
(structure 2F in Fig. 8) [37]. EHOMO and ELUUMO of many of these pol-
ypyridyl copper complexes compared favourably with the potential of 
the TiO2 semi-conductor CB and the potential of the electrolyte I− /I −3
redox couple [5,55–58] (Fig. 2). 
Light harvesting efficiency (LHE), defined as the fraction of light 
intensity absorbed by the dye at a certain wavelength, was calculated in 
some of the above studies [55–57]. For a good PCE, the LHE of the dye 
should be as high as possible. The relative LHE (RLHE) of different 
copper(I) dyes, relative to the well-known N3 Ru(II) sensitizer, has 
varied as follows: Variation between 1.00 and 1.44 for the heteroleptic 
[CuLL′]+ sensitizers (with L and L′ = bipyridyl ligands, see structures 1B 
– 8B in Fig. 8) [55], and variation of 1.00 – 1.79 for the Cu(I)-based 
sensitizers containing 6,6′-dimethyl-4,4′-dicarboxylate-2,2′-bipyridine, 
with functionalized 2,9-dimethyl-1,10-phenanthroline ligands (see 
structures 1C – 8C in Fig. 8) [56]. 
A dye with longer lifetime in the excited state is expected to be more 
facile for electron injection into the CB of the semiconductor. The re-
ported calculated excited state lifetime (τ) was computed for two groups 
of heteroleptic Cu(I) based dyes with different anchoring groups, based 
on substituted bipyridine and 2,9-dimethyl-1,10-phenanthroline ligands 
(structures 1D– 5D and 1E – 5E in Fig. 8). The excited lifetime (τ values) 
of these Cu(I) dyes were at the nanosecond timescale, between 2.5 and 
6.5 ns [5]. This τ value is similar to τ = 11.7 ns [8], calculated for the 
well-known Zn-based sensitizer YD2-o-C8 (Fig. 5), which is an experi-
mentally efficient Zn-porphyrin based sensitizer [49]. 
6. Conclusions 
A vast amount of experimental research has been done over the past 
30 years in optimizing the different parts of DSSCs. The main importance 
has been to find cost-effective functional materials that can be produced 
economically under environmentally friendly conditions. Theoretical 
computational studies assisted in the interpretation of experimental 
results, as well as in the screening and prediction of new dyes and redox 
couples with improved performance, for implementation in the next- 
generation DSSCs. Experimental studies have shown that copper com-
Fig. 9. Structure of the dye(B2)/(TiO2)38 system, anchored onto the anatase 
TiO semi-conductor [5]. plexes are excellent alternative redox mediators to the currently used 2 expensive iodide/triiodide redox couple. Copper complexes have shown 
good performance even at very low lighting conditions and can be 
[57]; and two groups of heteroleptic Cu(I) based dyes with different combined with copper dyes to produce eco-sustainable, iodide-free, low- 
anchoring groups, based on substituted bipyridine and 2,9-dimethyl- cost DSSCs soon. 
1,10-phenanthroline ligands (B3LYP/6-31G(d) + DZVP basis set, in 
CH2Cl2 solution) (see structures 1D– 5D and 1E – 5E in Fig. 8) [5]. The CRediT authorship contribution statement 
latter study included DFT calculations on the dyes, adsorbed onto the 
anatase mineral form of TiO2 ( 101) surface, namely computations on Jeanet Conradie: Conceptualization, Resources, Validation, Meth-
dye/(TiO2)38 systems [5], see example in Fig. 9. A further study on five odology, Funding acquisition, Writing – review & editing. 
homoleptic molecular systems of the type [CuL2]+, with differently 
substituted bipyridine ligands (computed at the M06/LANL2DZ + DZVP Declaration of Competing Interest 
level of theory) (see structures 1F – 5F in Fig. 8) [58], which are to be 
used as sensitizers in dye-sensitized solar cells, suggested potential ap- The authors declare that they have no known competing financial 
plications for these copper complexes in photovoltaic devices. interests or personal relationships that could have appeared to influence 
These theoretical studies also produced simulated UV/vis spectra the work reported in this paper. 
[58,59], with an analysis of the orbitals involved in the important 
electronic transitions relating to the absorbance maxima. These Acknowledgement 
confirmed MLCT bands [43,58], ILCT (intra-ligand charge transfer) 
transitions [59], as well as whether ligand-based oxidation [59] or Cu(I/ This work has received support from the South African National 
II) oxidation [37] occurs first. In the case of surface-anchored hetero- Research Foundation (grant numbers 129270 and 132504). 
leptic copper(I) complexes, for the sake of efficient electron injection, 
the LUMOs should be localized on the anchoring domain of the dye Ethics statement 
[32,36]. 
A combined experimental and theoretical DFT study demonstrated, This work does not require any ethical statement. 
7
J. Conradie                                                                                                                                                                                        E  l e c  t r o  c h  e  m  i s t r  y  C   o m   m   u n  i c  a t i o  n  s 134 (2022) 107182
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