Progress in Pulse Plating Atomic Layer Deposition (PP-ALD)

A family of absorber materials of interest for high volume production of photovoltaics is referred to as CZTS, or Cu₂ZnSn(S,Se)₄.  The primary advantages of these materials are that they are made from nontoxic earth abundant elements.   This group has been working on electrochemical atomic layer deposition (E-ALD) as a method for the formation of PV absorber materials for many years.  E-ALD involves the use of surface limited reactions to form deposits an atomic layer at a time, in order to achieve atomic layer control during the growth process. E-ALD has been based on the alternating of precursor solutions of the desired elements and the use of UPD to limit growth to an atomic layer at a time.   The E-ALD method has important advantages for the formation of nanofilms of materials, including: conformal growth, atomic layer control over deposition, room temperature deposition, and scalable principles.  There are a range of new PV designs involving nanostructured electrodes and plasmonic layers where the absorber layer thickness can be considerably thinner than the 4 µm used for say the formation of CdTe based thin layer photovoltaics.  However, even a µm of CdTe formed using present E-ALD technology would take at least 100 time longer then the presently used industrial methods.  Present E-ALD technology is can be used to grow films from a nm up, but is most applicable to where conformal, high quality nanofilms are needed.   The rate of growth using E-ALD is presently limited by the time it takes to change solutions, and the solution should be changed at least twice per atomic layer.   The present report involves efforts by this group to develop of a variant of E-ALD based on changing the potential, pulsing, rather than solution exchanges, greatly decreases the cycle time.   Presently, the Authors referred to it as potential pulse ALD (PP-ALD).   Preliminary results on the formation of Cu₂Se, ZnS, and other possible binary compounds which could serve as components of a CZTS absorber layer will be discussed.