学科进展系列报告

报告题目: Cu(In,Ga)Se2 at the micro- and nanometer scale

报告人：Dr. Sascha Sadewasser

International Iberian Nanotechnology Laboratory, 4715-330 Braga, Portugal

报告时间：2017年9月29日下午2:00 (星期五)

地    点：化学西楼一楼多功能厅

邀 请 人：乔亮   研究员

Dr. Sascha Sadewasser is the Principal Investigator of the Laboratory for Nanostructured Solar Cells at INL – International Iberian Nanotechnology Laboratory. The group of Sascha works on the development of advanced solar cell materials and devices implementing nano‐ and microstructures. Additionally, scanning probe microscopy methods, especially Kelvin probe force microscopy, are developed and applied for the characterization of the optoelectronic nanostructure of solar cell materials. Finally, the group also works on 2D chalcogenide materials. Sascha Sadewasser holds a Diploma (1995) in Physics from the RWTH Aachen, Germany and a PhD (1999) from the Washington University St. Louis, MO, USA. After 2 post‐docs in Berlin (Hahn‐Institute) and Barcelona (Centro Nacional de Microelectrónica), he became group leader and later deputy department head at the Helmholtz‐Zentrum Berlin, Germany. After his Habilitation in Experimental Physics from the Free University of Berlin, Germany (2011) he joined INL in 2011. Sascha has published more than 80 peer‐reviewed papers, with 2000 citations (h‐index 25). He has published 5 book chapters and 1 book and has been granted 3 patents. He is also a member of several scientific committees and evaluation boards.

Polycrystalline p-type Cu(In,Ga)Se₂ (CIGSe) semiconductors represent the absorber material in the thin film solar cells which currently reach the highest power conversion efficiency. The recent efficiency improvements have been achieved by the introduction of a potassium fluoride post-deposition treatment (KF-PDT). Direct and indirect effects of potassium at the interface and interface-near region in the CIGSe layer are thought to be responsible for this improvement. Using Kelvin probe force microscopy (KPFM), we show that also the electronic properties of grain boundaries (GBs) are beneficially modified by the KF-PDT. KPFM images spatially resolved surface potential of the CIGS surface. We observe that the KF-PDT increases the band bending at GBs by about 70% and results in a narrower distribution of work function values at the GBs [1]. This effect of the KF-PDT on the GB electronic properties is expected to contribute to the improved efficiency values observed for CIGSe thin-film solar cells with KF-PDT.

The second part of the presentation will be dedicated to solar cell concepts that rely on the incorporation of CIGSe micro- and nanostructures. The growth of CuInSe₂ (CISe) nanowires was achieved in a single step vacuum-based co-evaporation process. The resulting nanowires grow on top of a polycrystalline base layer, which makes them interesting for photovoltaic applications, especially for improved light absorption [2]. Micro-scale CISe islands were deposited by electro-deposition into holes in a SiO₂ matrix. Subsequently, the CISe islands were converted into solar cell devices, representing the first realization of micro solar cells deposited by a materials efficient growth process [3]. The devices can be used as micro-concentrator solar cells by adding a micro lens array. As a final example, a point-contact passivation layer for the realization of ultra-thin CIGSe solar cells will be presented. Thin (10-20 nm) Al₂O₃ layers were deposited directly on the Mo back contact and a regular hole pattern (diameters of a few 100 nm) was defined by electron beam lithography and opened by reactive ion etching. This point-contact passivation layer results in an increase from 8% to 11.8% for CIGSe devices with only 240nm absorber layer thickness [4].

References

[1] N. Nicoara, Th. Lepetit, L. Arzel, S. Harel, N. Barreau, and S. Sadewasser, Scientific Reports, in press (27.1.2017).

[2] H. Limborço, P.M.P. Salomé, J.P. Teixeira, D. Stroppa, R.-Ribeiro Andrade, N. Nicoara, K. Abderrafi, J. Leitão, J.C. González, and S. Sadewasser, CrystEngComm 18, 7147 (2016).

[3] S. Sadewasser, P. Salomé, and H. Rodriguez-Alvarez, Sol. Energy Mat. Sol. Cells 159, 496 (2017).

[4] B. Vermang, J.T. Wätjen, Ch. Frisk, V. Fjällström, F. Rostvall, M. Edoff, P. Salomé, J. Borme, N. Nicoara, S. Sadewasser, IEEE J. Photovoltaics 4, 1644 (2014).