Abstract
ABSTRACTA major limitation of the cross‐section electron beam‐induced current method—the use of roughly fractured surfaces to provide the cross‐section—has been overcome with the use of focused ion beam microscope sample preparation. Using this method, it was possible to undertake a study of the relation between junction position and the corresponding external quantum efficiency (EQE) curves. For the case of cadmium telluride (CdTe) cells, it was demonstrated that the EQE curve shape that indicates a buried CdTe homojunction only arises if the junction is buried by more than 0.9 µm from the heterointerface. This highlights the limitations of interpreting EQE curve shape to determine junction position. It was also shown that extended postgrowth annealing degrades the cadmium sulfide by Kirkendall voiding, and this leads to efficiency loss. © 2014 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons, Ltd.
Highlights
Electron beam-induced current (EBIC) analysis is a scanning electron microscope-based technique wherein the current generated in a photovoltaic device under interrogation from an electron beam is recorded as a function of the beam position
It should be noted that because of the use of a Ga+ ion source for the focused ion beam (FIB) milling process, there is the potential that this may act as an n-type dopant within the cadmium telluride (CdTe) layer and cause a shift in the junction position observed by EBIC
Clean and smooth cross-sections were prepared that revealed surfaces displaying excellent crystallographic contrast under secondary electron imaging in the scanning electron microscope and were ideal for interrogation using EBIC: Interference from surface topography resulting from mechanical cleaving was absent
Summary
Electron beam-induced current (EBIC) analysis is a scanning electron microscope-based technique wherein the current generated in a photovoltaic device under interrogation from an electron beam is recorded as a function of the beam position. In the case of cadmium telluride (CdTe) solar cells, damage caused to the soft CdTe layer by cleaving may introduce image contrast artefacts that obscure genuine sample-related effects, a skilled operator may still produce useful results. An alternative to this is the use of focused ion beam (FIB) milling [6], to produce high quality cross-sectional cuts suitable for EBIC analysis. A series of four close space sublimation (CSS) deposited CdTe solar cell devices with various cadmium chloride (CdCl2) treatment times were compared using EBIC analysis of FIB-milled cross-sections to identify the junction position. It was found that while the EQE curve shapes may infer a shallow junction, EBIC revealed that the junctions were buried by up to 0.9 μm
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