Assessment of photovoltaic junction position using combined focused ion beam and electron beam‐induced current analysis of close space sublimation deposited CdTe solar cells

Abstract

A 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.

The effects of junction interdiffusion and misfit dislocations on the efficiency of highly mismatched heterojunction photovoltaic devices

“…10 18 and 10 15 cm -3 respectively) the space charge region and band bending is largely confined to the CdTe layer. In this particular example W has a value of 1.1 µm, which is consistent with experiment [23].…”

“…In each case the lattice mismatch between bulk CdS and CdTe is offset by the net dislocation strain field. The CdTe absorber layer in real devices is polycrystalline with a typically columnar structure [8,23], so that the above situation is still applicable within the grain interiors, despite the presence of grain boundaries.…”

In-depth analysis of chloride treatments for thin-film CdTe solar cells

“…Following MgCl 2 treatment the junction is brought close to the interface, (as seen for CdCl 2 treatment2023), and the peak current is increased by approximately an order of magnitude relative to the as-grown case. Intermediate behaviour is seen for the ineffective chlorides—the junction is brought closer to the interface (NaCl ∼400 nm, KCl ∼590 nm), and the current peaks are increased—but the effects are much weaker for KCl than for NaCl, and neither is as effective as MgCl 2 .…”

“…For CdTe solar cells, this allows determination of the photovoltaic junction position within the cell structure to a high degree of accuracy71920. CdTe cells treated with MgCl 2 , KCl NaCl, and an as-grown sample were compared.…”

A comparative study of microstructural stability and sulphur diffusion in CdS/CdTe photovoltaic devices

“…One example of such a process is the chlorine-activation of the CdTe absorber layer. It has been empirically found [7][8][9] that the introduction of chlorine to the CdTe absorber layer, by various means, leads to a great increase in the conversion efficiency of CdTe-based thin film PV cells; however, a number of mechanisms have been put forward as to the cause of the improvement.…”