## 5 Optical Properties, Band Bending and Device Improvement  
Next to improved stability and better device performance, a general observation upon doping with Zinc(II) is an enhanced emission intensity and a longer emission lifetime. Quantitative data of emission yield and lifetimes are easily influenced by various factors so that uniformity around data of different groups is not observed, but the general trend is that enhanced emission and longer lifetimes are reported for a certain doping level (relative to undoped materials), see Table 1. A representative example [11] is given in Figure 9. The optimum in optical properties observed here is clearly at \(\sim\)3%, which is also reported by other groups. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Material** & **x (Zn)** & **\(\Upsilon\) (ns) Undoped (x = 0)** & **\(\Upsilon\) (ns) Doped** & **Reference** \\ \hline CsPb\({}_{1-x}\)Zn\({}_{x}\)Br\({}_{3}\) & \(<\)0.1 & 17 & 15 & [10]1 [FOOTNOTE:1]Footnote 1: Value of x is not exactly indicated, but is “low”, x \(<\) 0.1 for these nanoparticles in solution. 2 Lifetimes obtained with TPC (transient photo-current). 3 Average values on glass surface. 4 Average values on ITO surface. 5 Long components of emission of the center and grain boundaries of crystals on ITO surface. In References [14; 15; 26] we find no indication of chloride components used. PEAI is phenethylammonium iodide. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Photoluminescence lifetimes of different undoped and Zn(II) doped perovskite materials on glass or quartz (unless indicated). Additives are indicated in square brackets. Figure 8: EDS Elemental mapping of Zn atoms in the perovskite lattice. The Zn atoms are evenly dispersed over the structure and not aggregated at grain boundaries. MA(Pb:Zn)\({}_{3}\)-[Cl\({}^{-}\)]. Reproduced from _Sustain. Energy Fuels_**2018**, 1093–1100, Reference [13], with permission from The Royal Society of Chemistry. It is important to note that for all thin films, Zn(II) incorporation enhances the photoluminescence lifetime for all doping levels. Furthermore, the lifetime of the composition reported by [PERSON] et al. changes from 12 ns on ITO [16] to 1548 ns on quartz [24], indicating a quenching factor of 129 by ITO interaction (charge injection). This 1548 ns value is similar to the long emission lifetimes (2835 and 1105 ns) reported for recent record-breaking material compositions [25; 26]. The addition of transition metals like Zn(II) can also be used to tailor the band bending [9]. These bands are important with regards to the barrier for charge injection into the charge transport layers of the solar cell devices. If we assume that the tailoring of the band bending for Zn(II) follows the same trend as for Co(II), an optimum between 1.6 and 6% of Zn(II) content [9] is anticipated. These results clearly set the stage for further optimization, where a clear distinction between MAPbI\({}_{3}\) and FAPbI\({}_{3}\)-based materials has to be made. It also has to be noted that also the chemical source of Zn(II) can play a role, where zinc acetate, zinc chloride and zinc iodide not necessarily result in the exact same behavior. Several groups have reported improved solar device performance upon Zn(II) incorporation. Power conversion efficiencies up to 20.06% [23] (with 1% of Zn(II) content) and enhancement factors of 1.07 to 1.33 (when comparing undoped and doped devices) are the trend. A PCE of 18.2% was reported by using 3% of ZnCl\({}_{2}\) in 2017 [11] and 18.35% in 2018 [14] with 3% ZnI\({}_{2}\). A similar value (PCE = 18.25%) was found for 5% Zn(II) [15]. The largest enhancement factor of 1.33 was obtained for 0.1% Zn(II), attaining a final PCE of 16.3% [13].