# Self-Assembled Multinuclear Complexes for Cobalt(II/III) Mediated Sensitized Solar Cells
## 1 Introduction  
Titania based dye sensitized solar cells (DSSCs), developed starting in the early 90s by [PERSON] and [PERSON] [1], represent a regenerative photoelectrochemical device type characterized by unique properties like transparency, low production cost, capability to harvest the diffuse light [2], and wide choice of color palette, which facilitate its building integration in both indoor and outdoor conditions [3, 4]. The functioning of DSSCs is based on photoinduced charge transfer from a molecular sensitizer adsorbed on a porous semiconductor followed by its regeneration from a hole transporting phase, which commonly consists of a redox couple dissolved in an organic electrolyte. As a result, photon absorption results in the separation of electrons and holes, which are confined in two different phases (semiconductor for electrons and redox electrolyte for holes) where selective transport of the carriers should ideally occur. The best efficiencies are now obtained with fast redox shuttles like polypropylene cobalt [5] and copper [6] coordination compounds. The fruitful employment of these redox mediators is subordinated to the possibility of achieving an appropriate TiO shielding against recombination involving the electron recombination with the oxidized mediator (namely Co(III) and Cu(II)). Donor-\(\pi\)-Acceptor (D-\(\pi\)-A) organic dyes like D35 and Y123 [7, 8] take advantage from the steric hindrance of substituted triphenylamine groups to sterically screen the titania surface from the oxidized redox couple. The typical D-\(\pi\)-A architecture thus requires a careful design and a multistep synthetic route, which leads to highly expensive molecular species. On the other hand, Ru(II) complexes, despite containing a rare metal, are normally less expensive and more abundant than the best panchromatic D-\(\pi\)-A dyes. Other approaches to suppress recombination caused by fast redox couples involve either the co-adsorption of siloxanes [9] to saturate the surface sites uncovered by the dye sensitizers and exposed to the electrolyte, or the formation of insulating thin metal oxide layers including alumina and magnesia [10]. In this latter case, a carefulcontrol of the thickness of the insulating layer is necessary in order to maintain an efficient charge injection by the dye. We thus investigated the possibility of realizing self-assembled molecular moieties that exploit Ag\({}^{+}\) as a bridging unit [11] between the NCS groups of sterically hindered **Z907** dyes (Figure 1a) and the cyano ligands of **1** ([Ru(TMAM)\({}_{2}\)(CN\({}_{2}\))]\({}^{4+}\) (TMAM = (bis-4,4'-bis(trimethylaminomethyl)2,2'bipyridine)) (Figure 1b). This coordination compound, named **4** (2Z907-Ag\({}^{+}\)-(Ru(TMAM))) (Figure 1c and Supplementary Materials (Figure S1f)) realizes a strong photon absorption and conversion in the 300-650 nm region thanks to the energy transfer (Figure 1d) from the [Ru(TMAM)\({}_{2}\)(CN\({}_{2}\))]\({}^{4+}\) antenna [12] to the **Z907** complexes, which undergo photoinduced electron transfer. At the same time, we exploit the quaternary ammonium ions present on the TMAM ligand [13; 14] and Ag\({}^{+}\) to add electrostatic repulsion to steric shielding of the surface against positively-charged Co(III) ions present in the electrolyte. This tetranuclear system comparatively outperforms **Z907** alone by a factor of ca. 2 when considering the energy conversion efficiency of dye sensitized solar cells based on the [Co(ppy)\({}_{3}\)]\({}^{3+/2+}\) redox mediator.