The Stanford Photonics Research Center

March 14, 2009

Development of photovoltaic materials comprised of non-toxic, abundant elements is an important step toward increasing the economic viability of solar energy to meet growing global energy needs. The quaternary kesterite, Cu2ZnSnS4 (CZTS), is a potential photovoltaic absorber material that has recently gained attention. It has a direct energy band gap of ~1.45 eV, which is nearly ideal for the terrestrial solar spectrum, and is comprised of elements that are inexpensive, abundant, and non-toxic. Vacuum deposition routes have been used to make cells with reported efficiencies of 6.7%.1 In this presentation, we will report fabrication and testing of CZTS solar cells by two approaches: solution chemical bath deposition (CBD) and reactive sputter deposition. With CBD, multi-layer chemical bath deposition of sulfide films, followed by metal cation exchange, was used to fabricate layered precursor films. Films deposited by CBD exhibited nanosized grains, facilitating efficient replacement of metal ions. Final concentrations were regulated by controlling the starting film thicknesses using time of ion exchange and solution concentration. Conversion of the multilayer film to CZTS was accomplished by a sulfidizing heat treatment under vacuum in a tube furnace.  Using reactive sputter deposition, we have grown CZTS films with controllable stoichiometry. Deposition was performed by co-deposition from elemental targets using DC biasing for Cu and Sn and RF biasing for Zn. This reactive deposition approach forms the desired phase in-situ during growth and thus avoids the defects associated with post-deposition sulfidization. Results of characterization using X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, scanning Auger spectroscopy, optical absorption, and photocurrent measurements will be presented.
 
1. H. Katagiri, et al., Applied Physics Express, Vol 1 (2008) 041201.