Fig. 1 schematically shows the combinatorial TiO2/Cu2O heterojunction
device library approach. FTO covered glass substrates
served as a joint front electrode onto which the TiO2 layer was
deposited with a linear thickness gradient parallel to the x-axis
(Fig. 1a) followed by the Cu2O absorber with a linear gradient
parallel to the y-axis (Fig. 1b) leading to a total heterojunction
thickness profile with a diagonal thickness gradient (Fig. 1c). The
left hand side in Fig. 1 shows a schematic representation, while
thickness data derived from optical measurements are shown on
the right hand side. The TiO2 thickness was ranging from 60 to
320 nm, while the Cu2O layer was between 180 to 580 nm thick.
Structural analysis was carried out before deposition of the Ag
back contacts by X-ray diffraction measurements (XRD) showing
clear peaks of Cu2O, TiO2 (anatase) and SnO2 which originated
from the FTO substrate (Supporting Information). SEM images
were recorded at different locations of the library corresponding to