1:30pm - 2:00pmKeynote
Search for High-Performance Earth-Abundant Multinary Chalcogenide Absorbers
1Duke University, United States; 2University of Oklahoma, United States
Thin-film photovoltaic (PV) devices offer the prospect of low-cost manufacturing coupled with high power conversion efficiency, thereby providing a pathway to cost-competitive solar energy. Currently, the fastest growing commercial thin-film PV technologies rely on direct bandgap chalcogenide semiconductors CdTe and Cu(In,Ga)(S,Se)2 (CIGS), with cumulatively >15GW of capacity manufactured. In order to overcome prospective scalability issues related to elemental scarcity (Te, In) and/or heavy-metal toxicity (Cd) with these systems, this talk will present recent progress on alternative predominantly chalcogenide-based systems, including zinc-blende-related Cu2ZnSn(S,Se)4 (CZTSSe), with abundant Zn/Sn replacing In/Ga, and more recent candidates, Cu2(Ba,Sr)Sn(S,Se)4 (CBTSSe), offering the potential of reduced atomic site disorder [1,2]. If desirable electronic structure tunability associated with the multi-element stoichiometry (e.g., bandgap control using S:Se ratio) can be combined with control over disorder and defect formation in these complex systems, multinary chalcogenide semiconductors are expected to offer a bright path forward in the quest for high-performance, low-cost and scalable PV technologies.
 D. Shin, B. Saparov, D. B. Mitzi, Adv. Energy Mater. DOI: 10.1002/aenm.201602366 (2017).
 D. Shin, B. Saparov, T. Zhu, W. P. Huhn, V. Blum, D. B. Mitzi, Chem. Mater. 28, 4771 (2016).
2:00pm - 2:30pmInvited
Development of Cu2ZnSn(SXSe1-X)4 Thin Film Solar Cells with 11.8% Cell Efficiency by Sputtering Method
Chonnam National University, South Korea
Cu2ZnSn(Sx,Se1-x)4 thin film solar cells have been fabricated using sputtered Cu/Sn/Zn metallic precursors on Mo coated sodalime glass substrates without using a toxic H2Se and H2S atmosphere. The as-deposited metallic precursors were sulfo-selenized in a graphite box containing S and Se powder using a rapid thermal annealing (RTA) furnace. Thin film solar cells were fabricated after sulfo-selenization process using a 40~60 nm CdS buffer layer, a 100 nm intrinsic ZnO, a 600 nm Al doped ZnO, and Al/Ni top metal contact. The Effect of annealing process pressure and CdS buffer layer thickness on the morphological, structural and electrical properties have been studied using field emission scanning electron microscopy, X-ray diffraction, Raman spectroscopy, I-V and quantum efficiency measurement system, and time resolved photoluminescence spectroscopy, respectively. The fabricated Cu2ZnSn(S,Se)4 thin film solar cell shows the best conversion efficiency of 11.80% (Voc : 484.6 mV, Jsc : 37.50 mA/cm2, FF: 64.91%, and active area : 0.3 cm2). Details about other experimental results will be discussed during the presentation.
2:30pm - 3:00pmInvited
Interface Engineering: Towards 10% Energy Conversion Efficiency Pure Sulfide Cu2ZnSnS4 Thin Film Solar Cells
1School of Photovoltaic and Renewable Energy Engineering, Faculty of Engineering, The University of New South Wales, Sydney, Australia; 2NREL (National Renewable Energy Lab), United States
Intensive studies have been carried out on kesterote solar cells in the past ten years and confirmed 12.6% and 9.5% reocrd efficiency have been achieved for kesterite Cu2ZnSnSxSe4-x (CZTSSe) and Cu2ZnSnS4 (CZTS) solar cells by IBM and UNSW respectively. Se-free kesterite, CZTS is one of the most promising next-generation photovoltac cell material due to its abundance, non-toxicity constituents and suitable bandgap of 1.4-1.5 eV. The major performance loss of CZTS solar cell lies in its low open circuit voltage (i.e. large Voc deficit compared with its bandgap). Studies on the typical CZTS devices with CdS buffer have shown that the recombination is dominated in the interface region. The properties of hetero-interface, i.e. the surface of CZTS absorber prior to and after the deposition of the following overlying buffer, are therefore necessary to be looked into to improve the CZTS solar cell performance.
This work reports the interface engineering strategies developed at UNSW towards 10% efficiency CZTS solar cell. Interface defect and unfavorable conduction band alignment are the two major issues contributing to the CZTS/CdS heterojunction recombination. The quality of CZTS absorber surface poses critical impact on the performace of heterojuncton as it directly determines quality of overlying grown buffer layer and the hetero-interface. A combined S and SnS atmosphere is found to make a big difference in the microstructure and chemistry of the surface of CZTS and defects at its adjacent heterointerface. Based on such processed CZTS absorber, surface passivation as well as alternative ZnCdS buffer replacing traditional CdS buffer and shifting the CBO from "cliff-like" towards more favorable "spike-like" are develoepd to further reduce the heterojunction recombination. These interface engineering strategies together result in the confirmed 9.5% efficiency CZTS cell. Further analysis showing directions towards beyond 10% efficiency CZTS solar cell is also discussed in this work.
3:00pm - 3:15pmOral
Depth Resolving Electronic and Structural Properties of a Pentanary Cu(In,Ga)(S,Se)2 Thin-Film Solar Cell by Nano-Foucsed X-ray Techniques
National Synchrotron Radiation Research Center, Taiwan
Utilizing a nanofocusing X-ray Laue diffraction and X-ray absorption spectroscopy (XAS), we studied the crystalline properties and electronic structure in the pentanary Cu(In,Ga)(S,Se)2 (CIGSSe) thin-film solar cell as a function of sample depth on measuring the thickness-gradient sample. A novel approach is proposed for studying the depth-dependent information of thin films, which can provide a gradient thickness and a wide cross section of the sample by polishing process. The depth-dependent X-ray Laue diffraction pattern presents the crystalline behavior and the stress distribution over the thin-film device. We find that the randomly oriented grains in the bottom region of the absorber layer and the different residual stress between the underlying Mo and the absorber interface, which can deteriorate the electrical performance due to peeling-off effect. A rich containment of Ga and S atoms are segregated at the bottom of absorber layer. This distinctive compositional distribution in the absorber layer formed by a two-stage process is described according to the thermodynamic reaction and the manufacturing process. In addition, the near-edge X-ray absorption fine structure and extended X-ray absorption fine structure shown the sulfate compounds such as SiSO4 and ZnSO4 were formed in the glass and ZnO window layer respectively. The deficiencies of Cu and Se are found at the top of the absorber layer and the interstitial defects of S and Se are found at the top and bottom of the absorber layer, respectively. These acceptor state defect will act as hole traps or recombination centers to decrease the short circuit current (Jsc). Moreover, the corner-shared Cu(S,Se)4 tetrahedra structure separates the GaS4 and InSe4 tetrahedra structure in the practical CIGSSe local environments.
3:15pm - 3:30pmOral
Design of Overlayers for Enhanced Photocatalytic Performance of CZTS based Photocathode
1Nanyang Technological University, Singapore; 2University of Tokyo, Japan; 3Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), Singapore
Cu2ZnSnS4 (CZTS) has shown promising characteristics for the application of a photocathode since its first application as a solar cell due to its p-type nature, suitable band gap and favourable conduction band position with respect to water reduction potential. However, despite many studies since then, the photocurrent has only improved slightly from 9mAcm-2 to 9.3mAcm-2 by incorporating a dual buffer layer of CdS and In2S3 with CZTS and finally to 12mAcm-2 by changing the electrolyte to a potassium phosphate buffer. This gap in performance between CZTS and other photocathodes such as Cu(In,Ga)Se2 (~32mAcm-2) remains large, limiting its commercial usability. Moreover, this photocurrent is lower than that observed in photovoltaic cells (~20mAcm-2), showing its potential for improvement and thus highlighting its limitation of charge transfer from the absorber to the electrolyte. Deposition of appropriate overlayers have been demonstrated to improve charge transfer kinetics in other photocathode materials such as Cu(In,Ga)Se2 and CuGaS2, improving both its photocurrent and stability. In our study, we have fabricated a solution processed CZTS photocathode and have conducted a systematic study of the different overlayers and their impacts on the charge transfer kinetics, band alignment, stability and onset potential of the photocathode through the use of X-ray Photoelectron and Ultraviolet Photoelectron Spectroscopy, Time-resolved Photoluminescence and Open Circuit voltage decay measurement.