Angus Rockett

Abstract:

Chalcogenide semiconductors, in particular CdTe and Cu(In,Ga)Se2 (CIGS), have achieved great progress in performance of photovoltaic materials recently. However, they present a challenge to understand, as they perform better as polycrystalline materials than as single crystals. Therefore, it is necessary to understand how the films grow and in particular how grain boundaries affect the devices. This talk presents an overview of two scanning probe methods, scanning tunneling microscopy (STM) and atomic force microscopy (AFM) applied to this question. In particular, two modifications to AFM are applied, conductive AFM and scanning microwave impedance microscopy (SMIM) to understand local variations in carrier density and conduction. The results show how band edge fluctuations at the atomic scale can prevent atomic resolution imaging and how grain boundary properties differ from the bulk behaviors. CIGS shows large variations in both band edges on the atomic scale. Thus, choosing one tip bias results in large changes in current driven by the fluctuations and hence to changes in tip height not resulting from atomic features but rather from electronic behaviors. AgInSe2 shows smaller variations in the valence band edge and produces excellent atomic-resolution images when tunneling out of the surface but not when tunneling into the conduction band. It is shown that CdTe grain boundaries exhibit depleted majority hole concentrations relative to the grain bulk. This will induce collection of electrons to the boundaries. The resulting separation of the electrons from the holes reduces recombination. Conductive AFM shows that the boundaries are pathways for collection of current by the device. Remarkably, current injected directly into the boundary travels in the boundary rather than crossing into the grains while current injected in the grains stays in the grains. By contrast CIGS does not show this behavior and grain boundaries are very similar in overall behavior to grains. Implications of these results for the devices are described.