Nano-scale compositional analysis of surfaces and interfaces in earth-abundant kesterite solar cells
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nnis Paul Physical Electronic, Inc., Chanhassen, MN 55317, USA
Chuck Hitzman Stanford Nano Shared Facilities, Stanford, CA 94305, USA
John Hammond Physical Electronic, Inc., Chanhassen, MN 55317, USA
Richard Haight IBM T J Watson Research Center, Yorktown Heights, NY 10598, USA
Andrew C. Kummela) Department of Chemistry and Biochemistry, University of California–San Diego, La Jolla, CA 92093, USA (Received 31 July 2016; accepted 29 September 2016)
Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) absorbers are considered promising alternatives to commercial thin film technologies including CdTe and Cu(In,Ga)Se2 (CIGSe) owing to the earth abundance and non-toxicity of their constituents. However, to be competitive with the existing technologies, the photovoltaic performance of CZTSSe solar cells needs to be improved beyond the current record conversion efficiency of 12.6%. In this study, nanoscale elemental mapping using Auger nanoprobe microscopy (NanoAuger) and nano secondary ion mass spectrometry (NanoSIMS) are used to provide a clear picture of the compositional variations between the grains and grain boundaries in Cu2ZnSn(S,Se)4 kesterite thin films. NanoAuger measurements revealed that the top surfaces of the grains are coated with a Zn-rich (Zn,Sn)Ox layer. While thick oxide layers were observed at the grain boundaries, their chemical compositions were found to be closer to SnOx. NanoSIMS elemental maps confirmed the presence of excess oxygen deeper within the grain boundary grooves, as a result of air annealing of the CZTSSe films.
I. INTRODUCTION
The market for thin film photovoltaics (PV) has been expanding over the past decade due to the technological advances in developing low-cost and high-efficiency chalcogenide-based devices with CdTe and Cu(In,Ga)Se2 absorbers. However, the total power generation capacity per year for these chalcogenide-based technologies is projected to saturate at about 100 GWp.1,2 This is mainly due to the limited accessible reserves of In and Te in the earth’s crust, as well as the toxicity of Cd.3 Alternatively, In and Ga in the CIGSe films can be replaced with more earth-abundant, non-toxic elements such as Zn and Sn forming Cu2ZnSn(S,Se)4 (CZTSSe) which has kesterite crystal structure. The current record
Contributing Editor: Sam Zhang a) Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2016.389
conversion efficiency of polycrystalline thin film CZTSSe solar cell is 12.6%; 2.4% short of the efficiency threshold needed to make this material competitive with the commercial thin film technologies.2,4 This deficit in performance has been mostly ascribed to the limited open-circuit voltage (Voc) due in large to the extent of carrier recombination at the defects sites within the bulk of the absorbers or at the interfaces that exist in the PV devices.5–8 One of the critical interfaces in polycrystalline thin films is the one between adjacent grains, called grain boundaries. If not properly passivated, grain boundar
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