## 3 Results and Discussion  
### TCO Layers on Glass  
Figure 2a,b show sheet resistances (R\({}_{\rm sh}\)) of about 100-nm-thick ZnO:Al and ITO layers on glass, as well as the absorbed light expressed as an equivalent photo-current density (j\({}_{\rm loss}\)). The j\({}_{\rm loss}\) value is calculated by multiplying the spectrally resolved absorption of the samples with the AM1.5g spectrum, integrating over the wavelength range from 300 to 1200 nm, and subtracting the absorption of the planiclear glass. This method for estimating the absorption of the TCO layer is used due to the similarity of the samples prepared, since all of them are deposited on the same substrate with the same layer thickness and their refraction indexes and reflection are very similar. Both R\({}_{\rm sh}\) and j\({}_{\rm loss}\) are plotted against the oxygen partial flow during film deposition. Figure 2c shows Hall mobility (\(\mu\)) vs. charge carrier concentration (N\({}_{\rm e}\)) for the same layers. The R\({}_{\rm sh}\) value of ZnO:Al layers deposited with increasing oxygen ratio in the process gas increases from 120 \(\Omega\)/sq to 226 \(\Omega\)/sq (sputtered under r(O\({}_{2}\)) = 0% and r(O\({}_{2}\)) = 0.74%, respectively). N\({}_{\rm e}\) decreases from 2.2 \(\times\) 10\({}^{20}\) cm\({}^{-3}\) to 1.3 \(\times\) 10\({}^{20}\) cm\({}^{-3}\) for the same variation of O\({}_{2}\) partial flows, which is consistent with observations by other authors [10], and is often tentatively explained by a  
Figure 2: (**a**) Sheet resistance and (**b**) j\({}_{\rm loss}\) due to absorption in the 300–1200 nm range of the ZnO:Al (open triangles) and indium tin oxide (ITO) (filled squares) single layers on glass deposited under different O\({}_{2}\) concentrations. (**c**) Hall mobility as a function of charge carrier concentration. Note the axis breaks in panel (**b**). reduced density of oxygen vacancies in the TCO material since oxygen vacancies provide intrinsic doping on ZnO. The Hall mobility stays almost constant around 20 cm\({}^{2}\)/Vs for higher N\({}_{\rm e}\) and decreases to 18 cm\({}^{2}\)/Vs for the sample with the lowest N\({}_{\rm e}\). The R\({}_{\rm sh}\), \(\upmu\), and N\({}_{\rm e}\) values of the ITO layers follow a similar trend. R\({}_{\rm sh}\) increases from 55 \(\Omega\)/sq to 405 \(\Omega\)/sq for an O\({}_{2}\) concentration increasing from r(O\({}_{2}\)) = 0% to r(O\({}_{2}\)) = 5%. Hall mobility maximum values of 30-35 cm\({}^{2}\)/Vs are reached for layers with carrier concentrations of 1-2 \(\times\) 10\({}^{20}\) cm\({}^{-3}\), decreasing for both increasing and decreasing carrier concentrations. The Hall mobilities and charge carrier densities measured on our layers are in the same range as those reported in the literature for comparable sputtered ZnO:Al and ITO layers [10; 11]. The decreasing \(\upmu\) with decreasing N\({}_{\rm e}\) (for N\({}_{\rm e}\leq\) 1 \(\times\) 10\({}^{20}\) cm\({}^{-3}\)) can be explained using the grain boundary scattering model [12]. In this model, it is assumed that the boundaries between adjacent grains or crystallites have a relatively high amount of trap states that are able to reduce the effective amount of charge carriers by trapping them. The trap states become electrically charged by this process and represent an electrical potential barrier to the free charge carriers, reducing their Hall mobility. The increasing light absorption correlates with the increase of N\({}_{\rm e}\) extracted from the Hall mobility measurements [13]. From these measurements and calculations, we found that ZnO:Al and ITO layers deposited under r(O\({}_{2}\)) = 0.3% and r(O\({}_{2}\)) = 2.8%, respectively, exhibit similar R\({}_{\rm sh}\) (approx. 150 \(\Omega\)/sq) and optical absorption values (photocurrent-equivalent absorption loss 0.7 mA/cm\({}^{2}\) for the ZnO:Al and 0.5 mA/cm\({}^{2}\) for the ITO).