Key Developments in CdTe Thin Film Solar Cell Back-Contact

In CdTe-based devices the "back-contact" is a crucial subject of scientific and industrial importance, as it affects the conversion efficiency and the long-term stability of solar cells. It is common use to Te-enrich the CdTe surface by chemical etching. It is proposed a new way for obtaining a CdTe film with a Te-rich surface taking advantage of the original treatment in Chlorine atmosphere, alternative to the use of CdCl2, without doing any CdTe etching. The back-contact formation on this Te-rich surface occurs by depositing a buffer layer, which indifferently could be Sb2Te3, As2Te3 or Bi2Te3, deposited by sputtering or by evaporation at low temperature (≈ 200°C), followed by the deposition of a metal contact (Mo). In this condition, the device I-V characteristic shows the typical roll-over behavior, distinctive of a rectifying back-contact. Performing an annealing of all the device, at a temperature of 200°C, in air for a few minutes, the roll-over disappears and the fill factor increases considerably. If the annealing is not done in air or in presence of Oxygen, the rollover doesn't disappear. Preliminary accelerated aging tests have shown a good contact stability of these devices. In this article, we will explain why the CdTe surface is Te-rich only after an annealing in air at 200°C and how this can contribute to form an ohmic and stable back-contact in presence of Sb-, As- and Bibased chalcogenide.