Activation of Thin Film CdTe Solar Cells Using a Cadmium Bromide Treatment

Abstract

The activation of CdTe with a cadmium chloride annealing treatment is a vital step in the fabrication of high efficiency solar cells. Thin film MZO/CdTe cells have been activated using CdBr2 instead of CdCl2 with a lower activation process temperature. Using this method, CdBr2 does activate the cell as revealed by J-V and EQE measurements. TEM and EDX elemental maps from device cross-sections confirm that bromine is present in the grain boundaries. TEM shows that the treatment removes stacking faults at 425 °C. CdBr2 treatment resulted in a relatively modest conversion efficiency of 5.49% when treated at 375 °C. Nevertheless, the experiments shed further light on the mechanisms involved in the activation.

Analysis of MZO/CdTe photovoltaic device treated with cadmium bromide

“…The SIMS data (Fig. 5) shows the bromine behaving very similar to chlorine [15], [16], while Br has been shown by TEM/EDX to segregate to the grain boundaries [12]. Areas with high bromine concentration are also present in the grain bulk and are likely to be present in twin boundaries, with Br migrating down the twin defects, this has been observed with Cl previously [16].…”

“…The ~3.2 µm CdTe layer was deposited by close space sublimation at a substrate temperature of 500° C [14]. The CdBr2 treatment was performed in a graphite box set up in a tube furnace as described previously [12]. CdBr2 material (99.9985 % metals basis, ultra-dry Alfa Aesar) was used in a graphite box as shown schematically in Fig.…”

“…Previous studies have investigated the effects of bromine, iodine and fluorine as a bulk dopant [6]- [10]. CdBr2 has previously been demonstrated to produce a 5.4 % CdTe solar device [12]. CdF2 added to the CdCl2 chemical bath treatment step has improved all JV parameters in a CdTe device [11].…”