# Self-Assembled Multinuclear Complexes for Cobalt(II/III) Mediated Sensitized Solar Cells
## 3 Results and Discussion
### Photo-Electrochemical Characterization  
Figure 9a reports the photoaction spectra (IPCE vs. \(\lambda\)) of complexes **3** and **4** compared to **Z907**. The maximum photoconversion in the presence of the [Co(bpy)\({}_{3}\)]\({}^{2+/3+}\) couples is of the order of 65/70% and is achieved by complex **4**, which significantly outperformed the **Z907** (ca. 50% maximum conversion). Complex **4** was also slightly superior to **3**. The IPCE is the product of three terms according to IPCE = \(\Phi_{\text{inj}}\)\(\times\) n\({}_{\text{coll}}\)\(\times\) LHE. Since injection occurs from the **Z907** units based on our and on other evidence [33], the injection efficiency \(\Phi_{\text{inj}}\) can be considered close to unity. The LHE (calculating according to LHE = \(1-10^{-\text{A(\lambda max)}}\)) was 90% for **Z907** and 95% for the other two cases, hence the observed 20% conversion difference was mostly related to an improved n\({}_{\text{coll}}\) when the multinuclear species were used. This effect may arise from a slight acceleration in Ru(II) regeneration, following the ca. 100 meV increase in the driving force for Co(II) oxidation, but most importantly from a substantial reduction in the dark current (Figure 9b dashed line) due to a combination of steric and electrostatic shielding in the presence of **3** and **4**. This result is motivated by a combination of electrostatic repulsion and steric shielding of the TiO\({}_{2}\) surface, which is protected by a higher density of long alkyl chains and quaternary ammonium ions, which contribute to repel positively charged Co(III) ions present in the electrolyte. As a result, the efficiency of transparent cells increased from 1.9% (**Z907**) to 3.1% when **3** and **4** were used (Table 2 and Figure 9b). The Jsc of the cells agreed with the integrated photocurrent spectrum recorded during IPCE measurement (Figure S17). Actually, the integrated photocurrent was slightly higher than that observed under white light, where the cell probably suffers from partial diffusional limitations arising from the use of Co(II)/(III) electron mediators in association with a TiO\({}_{2}\) film made of small 18-20 nm particles. Indeed, the best conversion efficiency with redox mediators based on coordination compounds were reported with 30 nm particles, which created a sintered titania film with larger voids and channels for the unimpeded diffusion of bulky electron mediators [4]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **J/mA/cm\({}^{2}\)** & **Voc/V** & **FF\%** & \(\eta\) **\%** \\ \hline
**Z907** & 4.8 \(\pm\) 0.2 & 0.64 \(\pm\) 0.08 & 62 \(\pm\) 3 & 1.9 \(\pm\) 0.2 \\ \hline
**3** & & & & \\ (Z2907-Ag\({}^{+}\)) & 7.4 \(\pm\) 0.4 & 0.75 \(\pm\) 0.02 & 56 \(\pm\) 4 & 3.1 \(\pm\) 0.1 \\ \hline
**4** & & & & \\ (Z2907-Ag\({}^{+}\)-Ru(TMAM)) & 7.3 \(\pm\) 0.2 & 0.78 \(\pm\) 0.01 & 55 \(\pm\) 1 & 3.1 \(\pm\) 0.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Photovoltaic parameters of **Z907**, **3** (Z2907-Ag\({}^{+}\)), and **4** (Z2907-Ag\({}^{+}\)-Ru(TMAM)) sensitized cells recorded in the presence of Co(bpy)\({}^{2+/3+}\) redox mediator. The efficiency parameters were obtained by averaging the performance of a set of 5 cells. Figure 9: (**a**) IPCE spectra and (**b**) IV curves for **Z907** (black line), **3** (Z2907-Ag\({}^{+}\)) (red line) and **4** (Z2907-Ag\({}^{+}\)-Ru(TMAM)) (blue line) recorded under 0.1 W/cm\({}^{2}\) AM 1.5 G illumination. The electron lifetime \(\tau_{el}\) obtained from the open circuit photovoltage decay (Figure 10a) according to [PERSON] et al. [34] confirmed the superior screening achieved with the multinuclear complexes **3** and **4** (Figure 10a, red and blue symbols), with respect to **Z907** alone (black). In particular, **4** spanned the longest lifetime of the series (i.e., >0.1 s) within a photopotential range comprised between \(-\)0.2 V and \(-\)0.5 V. At the maximum photopotential (ca. \(-\)0.8 V), the electron lifetime of **3** and **4** achieved similar values (10-13 ms) and was longer than that of Z907 by a ca.