Microelectrical characterizations of junctions in solar cell devices by scanning Kelvin probe force microscopy.

Scanning Kelvin probe force microscopy was applied to the microelectrical characterizations of junctions in solar cell devices. Surface Fermi-level pinning effects on the surface potential measurement were avoided by applying a bias voltage (V(b)) to the device and taking the V(b)-induced potential and electric field changes. Two characterizations are presented: the first is a direct measurement of Bi-induced junction shift in GaInNAs(Bi) cells; the second is a junction-uniformity measurement in a-Si:H devices. In the first characterization, using Bi as a surfactant during the molecular beam epitaxy growth of GaInNAs(Bi) makes the epitaxial layer smoother. However, the electrical potential measurement exhibits a clear Bi-induced junction shift to the back side of the absorber layer, which results in significant device degradation. In the second characterization, the potential measurement reveals highly non-uniform electric field distributions across the n-i-p junction of a-Si:H devices; the electric field concentrates much more at both n/i and i/p interfaces than in the middle of the i-layer. This non-uniform electric field is due possibly to high defect concentrations at the interfaces. The potential measurements further showed a significant improvement in the electric field uniformity by depositing buffer layers at the interfaces, and this indeed improved the device performance.

Electrically benign behavior of grain boundaries in polycrystalline CuInSe2 films.

The classic grain-boundary (GB) model concludes that GBs in polycrystalline semiconductors create deep levels that are extremely harmful to optoelectronic applications. However, our first-principles density-functional theory calculations reveal that, surprisingly, GBs in CuInSe2 (CIS) do not follow the classic GB model: GBs in CIS do not create deep levels due to the large atomic relaxation in GB regions. Thus, unlike the classic GB model, GBs in CIS are electrically benign, which explains the long-standing puzzling fact that polycrystalline CIS solar cells with remarkable efficiency can be achieved without deliberate GB passivation. This benign electrical character of GBs in CIS is confirmed by our scanning Kelvin probe microscopy measurements on Cu(In,Ga)Se2 chalcopyrite films.