Air-annealing effects on polycrystalline Cu(In,Ga)Se2 heterojunctions

We compare the consequences of two different approaches for post-deposition treatments of ZnO/CdS/Cu(In,Ga)Se-2 heterojunction solar cells on their electronic properties: (i) air annealing of the bare, polycrystalline absorber and (ii) air annealing of the completed heterostructure. both in dry air at 200 degrees C. We use temperature dependent admittance spectroscopy to gain information on the position of the electron Fermi level at the CdSiCu(In,Ga)Se-2 interface and on the width of the space charge region within the Cu(In,Ga)Se-2 absorber layer of the differently oxidized samples. Changes which occur after annealing of the bare polycrystalline absorbers in dry air at 200 degrees C can be explained by assuming passivation of donor-like states at grain boundaries with oxygen. Annealing of the completed heterostructure has: a complementary impact on the electronic properties of the devices. Here, we find that annealing leads to a decrease of the space charge density. Field- and temperature-enhanced Cu-migration from the surface into the bulk of the absorber material has to be invoked to explain this result.