The chemical and electronic properties of a-
Si:H(B)/ZnO:Al and μc-Si:H(B)/ZnO:Al thin-film solar cell structures
are studied by hard X-ray photoelectron spectroscopy (HAXPES).
Using a combination of different X-ray excitation energies
and deliberate sample design, we were able to select the probed
volume, i.e., the silicon capping layer only or the silicon and zinc
oxide layer (including the buried interface). For the a-Si:H(B) material,
we find a higher deposition rate and a smaller value for the
modified Auger parameter than for μc-Si:H(B). In addition, we
find indications of a pronounced band bending limited to the very
surface of the a-Si:H(B) and the μc-Si:H(B) layers, which is more
distinct in the latter case.
The electronic structure of the interface between the boron-doped oxygenated amorphous silicon “window layer” (a-SiOx:H(B)) and aluminum-doped zinc oxide (ZnO:Al) was investigated using hard x-ray photoelectron spectroscopy and compared to that of the boron-doped microcrystalline silicon (μc-Si:H(B))/ZnO:Al interface. The corresponding valence band offsets have been determined to be (−2.87 ± 0.27) eV and (−3.37 ± 0.27) eV, respectively. A lower tunnel junction barrier height at the μc-Si:H(B)/ZnO:Al interface compared to that at the a-SiOx:H(B)/ZnO:Al interface is found and linked to the higher device performances in cells where a μc-Si:H(B) buffer between the a-Si:H p-i-n absorber stack and the ZnO:Al contact is employed.