AVS 61st International Symposium & Exhibition
    Energy Frontiers Focus Topic Wednesday Sessions
       Session EN+AS+EM+SE-WeM

Paper EN+AS+EM+SE-WeM4
Effect of Chemical Wet Cleaning on Surface Composition and Work Function of Thin Film CZTS,Se

Wednesday, November 12, 2014, 9:00 am, Room 315

Session: Thin Film Photovoltaics
Presenter: Kasra Sardashti, University of California at San Diego
Authors: K. Sardashti, University of California at San Diego
E.A. Chagarov, University of California, San Diego
T. Kaufman-Osborn, University of California, San Diego
S.W. Park, University of California San Diego
R. Haight, IBM T.J. Watson Research Center
W. Wang, IBM T.J. Watson Research Center
D.B. Mitzi, IBM T.J. Watson Research Center
A.C. Kummel, University of California at San Diego
Correspondent: Click to Email
Polycrystalline Copper-tin-zinc-sulfide/selenide (CZTS,Se) compounds have received wide research interest due to their potential as inexpensive absorber materials composed earth-abundant elements. Photovoltaic devices fabricated on CZTS,Se has reached the highest (or record) conversion efficiency of the 12.6 %. One of the key parameters to further boost the conversion efficiency is to control the concentration of recombination sites at the surface, in the grain boundaries, and in the bulk. Surface states formed on the sample surface as a result of carbon and oxygen contamination can act as non-radiative recombination sites which limit the ultimate cell efficiency. Therefore, a surface-cleaning method which can effectively reduce the amount of surface oxygen and carbon is necessary for CZTS,Se processing. In this work, 2 μm thick CZTS,Se films were prepared by spin coating hydrazine-based precursor solutions onto Mo-coated soda lime glass substrates in a nitrogen-filled glove box. To clean the CZTS,Se surfaces, three different wet cleaning recipes were used: a) NH4OH only; b) HCl followed by NH4OH; 3) H2O2 followed by NH4OH. The effect of the wet cleaning on the surface composition including carbon and oxygen content has been studied via X-ray photoelectron spectroscopy (XPS) and femtosecond ultraviolet photoelectron spectroscopy (fs-UPS). Spatial variation of work function over the surface upon surface cleaning was measured via Kelvin Probe Force Microscopy (KPFM). The stability of the clean surface against reoxidation in ambient was modeled by density functional theory (DFT). The H2O2/NH4OH recipe showed the best result reducing the amount of surface O and C down to 5% and 20%, respectively. This is due to the oxidizing effect of H2O2 which converted the carbonaceous surfaces contaminants into oxides which were later removed by NH4OH. DFT calculations are consistent with a group VI surface being stable against oxidation by ambient moisture. KPFM measurements showed strongly non-homogeneous surfaces after both NH4OH-only and H2O2/NH4OH clean. Areas with work function different from CZTS could be the binary chalcogenides formed during the growth and were covered by the native oxide. NH4OH etch successfully removed the covering oxide and made those phases visible to KPFM.