Investigating the Properties of ZnO:Al Buffer Layers, Deposited By ALD, for Thin-Film CIGS Solar Cells

CIGS is a tetrahedrally-bonded semiconductor material that can be used in thin film photovoltaic cells. Particularly, it is a very promising and growing absorber technology for the p-type absorber layer of CIGS solar cells. On the other hand, the standard material used for the n-type buffer layer in CIGS solar cells is Cadmium sulfide (CdS). However, CdS is toxic and absorbs unwanted blue light and UV light.

We would like to find a good alternative buffer layer as a possible replacement for CdS. Such buffer layers can be effectively deposited by thin film deposition technique called ALD. Atomic layer deposition (ALD) is capable of depositing a variety of materials by gas-surface reaction methods. The motivation to use ALD to deposit n-type buffer layers is due the conformality of ALD-deposited films on rough films, as well as the uniformity in the resulting alloy composition both laterally and with regards to thickness.

One buffer layer material that we have deposited via ALD on CIGS is Zinc Oxide (ZnO). Zno is used in channels called point contacts, which have been found to increase CIGS solar cell performance. To reduce the influence of interference recombination, one can passivate the p-n junction surface, except in a few places where one can create conductive channels with a transparent conductor (i.e. ZnO). Typically the conductivity of a buffer layer made of ZnO can be increased by 10-100 times when doped with Al. In our lab, we have been varying this Al:ZnO ratio, as well as the temperature of the ALD chamber to achieve lower resistivity values.