Direct evidence of enhanced chlorine segregation at grain boundaries in polycrystalline CdTe thin films via three‐dimensional TOF‐SIMS imaging

Abstract

For more than 25 years, the CdTe photovoltaic research and manufacturing communities have been subjecting CdTe materials to a CdCl 2 treatment or activation step to improve performance. However, little work has been carried out using imaging to elucidate the spatial distribution of chlorine in the CdTe devices after this treatment. This work addresses fundamental questions about the spatial distribution of chlorine in the CdTe absorber material after a CdCl 2 treatment comparable to industrial practices. We used a state-of-the-art, time-of-flight secondary ion mass spectrometer (ION-TOF GmbH) (Muenster, Germany) with a lateral resolution of about 80 nm to complete three-dimensional depth-profiling and imaging of two CdTe devices. The results clearly demonstrate enhanced chlorine concentration along grain boundaries, supporting the hypothesis that chlorine plays an important role in passivating grain boundaries in CdTe solar cells. The results are discussed in terms of possible passivation mechanisms, and the effect of chlorine on grain interiors and grain boundaries. The data are also used to estimate the free energy of segregation of chlorine to grain boundaries in CdTe.

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

“…There is also an increase in the chlorine counts within the CdS compared with the CdTe, as is often observed for CdCl 2 -treated cells10. This is at least in part due to the change in matrix upon sputtering from the CdTe into the CdS: the sputter yield of chlorine from the two materials is different and this acts to increase the count rate from CdS11 rather than indicating a genuine increase in concentration. In contrast, both the NaCl and KCl treatments give only a minor increase in the chlorine concentration, and even then this is confined to the CdTe nearest to the diffusion surface—deeper in the CdTe, the chloride level does not rise above its background value.…”

Stoichiometric effects in polycrystalline CdTe

“…The positive effect of CdCb on minority-carrier lifetime is well known and anticipated to be due to the elimination of deep-level traps, particularly in the grain boundaries (GB) [II]. Recent studies have demonstrated a large concentration of chlorine near the GB, indicating its role in their passivation [12]. EELS profiling by Chen Li et al demonstrated Te reduction and CI enrichment while having same relative Cd concentration in the grain boundary [13].…”

Current Enhancement of CdTe-Based Solar Cells

“…The poor performance is mainly attributed to a low shunt resistance (282 Ω·cm 2 ), which is an indication of insufficient GB passivation. After the CdCl 2 treatment, which is known to incorporate Cl into CdTe and facilitate GB passivation [14], [15], the cell efficiency is improved to ∼14.2%. J SC is further increased to 26.4 mA/cm 2 , which is rather high for a CdTe cell with no ARC.…”