Cross-sectional electrostatic force microscopy of thin-film solar cells

In a recent work, we showed that atomic force microscopy (AFM) is a powerfu l technique to image cross sections of polycrystalline thin films. In this work, we apply a modification of AFM, namely, electrostatic force microscop y (EFM), to investigate the electronic properties of cleaved II-VI and mult ijunction thin-film solar cells. We cleave the devices in such a way that t hey are still working with their nominal photovoltaic efficiencies and can be polarized for the measurements. This allows us to differentiate between surface effects (work function and surface band bending) and bulk device pr operties. In the case of polycrystalline CdTe/CdS/SnO2/glass solar cells, w e find a drop of the EFM signal in the area of the CdTe/CdS interface (+/- 50 nm). This drop varies in amplitude and sign according to the applied ext ernal bias and is compatible with an n-CdS/p-CdTe heterojunction model, the reby invalidating the possibility of a deeply buried n-p CdTe homojunction. In the case of a triple-junction GaInP/GaAs/Ge device, we observe a variat ion of the EFM signal linked to both the material work-function differences and to the voltage bias applied to the cell. We attempt a qualitative expl anation of the results and discuss the implications and difficulties of the EFM technique for the study of such thin-film devices. (C) 2001 American I nstitute of Physics.