## 2 Materials and Methods  
_Preparation of TiO\({}_{2}\):ZnO/CuO_  
TiO\({}_{2}\):ZnO/CuO solar cells were prepared using reactive, direct-current magnetron sputtering (DC-MS). Details of the sputtering conditions and the approximate value of the thickness of the TiO\({}_{2}\):ZnO and CuO layers are shown in Tables 1 and 2. More details on the method and the apparatus used can be found in the Supplementary Materials. The substrate temperature during layer growth was kept at 300 \({}^{\circ}\)C. The deposition process was as follows. First, thin films of n-type TiO\({}_{2}\):ZnO were deposited on glass substrates with ITO (indium tin oxide) with dimensions of 10 \(\times\) 10 \(\times\) 1.1 mm and commercial silicon plates(N-type Si, 100), using ZnOTiO\({}_{2}\) 4-3 at% (Z1-9057-D3 commercial target from Testbourne) for Samples #14, #15, and #18 and the self-produced Ti:ZnO target for Samples #26 and #27 (see the preparation of the Ti-ZnO target in [21]). Next, a Cu buffer layer, a p-type copper oxide absorber layer, and a thin Cu film were grown successively using a Cu target (99.995% from Kurt J. Lesker Company). For the Cu buffer layer production in particular, the following process parameters and procedures were used:  
* For Sample #14: deposition time 10 s, argon flow rate 1 cm\({}^{3}\)/s; the magnetron shutter was closed, flows were set for deposition of the CuO layer, the plasma beam was stabilized for 20 s with the shutter closed, and the deposition of the CuO layer began;
* For Samples #15 and #18: no Cu buffer between TiO\({}_{2}\):ZnO and CuO was used;
* For Sample #26: deposition time 5 s, argon flow 4 cm\({}^{3}\)/s; the oxygen flow was switched on, and the CuO layer deposition was started smoothly,
* For Sample #27: deposition time 5 s, argon flow 1 cm\({}^{3}\)/s; the oxygen flow was switched on, the argon flow was set to 1 cm\({}^{3}\)/s, and the CuO layer deposition was started smoothly. The remaining process parameters were the same for the CuO layers; see Table 2. A scheme of the TiO\({}_{2}\):ZnO/CuO thin-film solar cells and a real view of TiO\({}_{2}\):ZnO/CuO/Cu\({}_{2}\)O. Sample #27 are shown in Figure 1a,b. \begin{table}
\begin{tabular}{c c c c c c} \hline  & **\#14** & **\#15** & **\#18** & **\#26** & **\#27** \\ \hline Time [min] & 30 & 40 & 30 & 20 & 25 \\ Power [W] & 100 & 100 & 100 & 100 & 100 \\ Pressure [mbar] & \(9.89\times 10^{-3}\) & \(9.89\times 10^{-3}\) & \(9.89\times 10^{-3}\) & \(8.99\times 10^{-3}\) & \(8.99\times 10^{-3}\) \\ Distance between the source and substrate [mm] & & & & & \\ Oxygen flow rate & 3.5 & 3.5 & 3.5 & 3 & 2.5 \\ \([\)cm\({}^{3}\)/s\(]\) & 0.5 & 0.5 & 0.5 & 1 & 1 \\ Substrate & 300 & 300 & 350 & 300 & 300 \\ temperature [\({}^{\circ}\)C] & 86 & 43 & 57 & 245 & 354 \\ \hline \end{tabular}
\end{table} 
Table 1: Growth parameters of the TiO\({}_{2}\) + ZnO window layer. \begin{table}
\begin{tabular}{c c c c c} \hline  & **\#14** & **\#15** & **\#18** & **\#26** & **\#27** \\ \hline Time [min] & 30 & 30 & 30 & 30 & 30 \\ Power [W] & 70 & 70 & 70 & 70 & 70 \\ Pressure [mbar] & \(9.23\times 10^{-3}\) & \(8.75\times 10^{-3}\) & \(9.41\times 10^{-3}\) & \(1.05\times 10^{-2}\) & \(1.11\times 10^{-2}\) \\ Distance between the source and substrate [mm] & & & & \\ Oxygen flow rate & 3.5 & 3.5 & 3.5 & 3.5 & 3.5 \\ \([\)cm\({}^{3}\)/s\(]\) & 0.5 & 0.5 & 0.5 & 1 & 1 \\ Substrate & & & & & \\ temperature [\({}^{\circ}\)C] & 300 & 300 & 300 & 300 & 300 \\ Thickness [nm] & 747 & 723 & 650 & 1654 & 1487 \\ \hline \end{tabular}
\end{table} 
Table 2: Growth parameters of the CuO absorber layer.