title: Investigation of multi-junction solar cells using electrostatic force microscopy methods.
abstract: Multi-junction III-V solar cells are designed to have a much broader absorption of the solar spectrum than Si-based or single junctions, thus yield the highest conversion. The conversion efficiency can be further scaled with sun concentration. The ability of high conversion efficiencies makes multi-junction prime candidates for fine-tuning explorations aimed at getting closer to the theoretical efficiencies. In this paper, we report on electrostatic force microscopy (EFM) measurements of the built-in potential of multi-junction III-V semiconductor-based solar cells. Kelvin probe force microscopy (KPFM) was employed to qualitatively study the width and electrical properties of individual junctions, i.e., built-in potential, activity, and thickness of the p-n junctions. In addition, the voltage drops across individual solar cell p-n junctions were measured using Kelvin probe microscopy under various operation conditions: dark; illuminated; short-circuit; and biased. We present a method which enables the measurement of a working structure, while focusing on the electrical characteristics of an individual junction by virtue of selecting the spectral range of the illumination used. We show that these pragmatic studies can provide a feedback to improve photovoltaic device design, particularly of operation under a current mismatched situation. This new analysis technique offers additional insights into behavior of the multi-junction solar cell and shows promise for further progress in this field.
authors.affiliation: Wrocław University of Technology, Faculty of Microsystem Electronics and Photonics, Division of Metrology of Micro- and Nanostructures, ul. Z. Janiszewskiego 11/17, 50-372 Wrocław, Poland. Electronic address: magdalena.moczala@pwr.wroc.pl.
authors.forename: M
authors.initials: M
authors.lastname: Moczała
authors.affiliation: IBM Thomas J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598, USA.
authors.forename: N
authors.initials: N
authors.lastname: Sosa
authors.affiliation: IBM Thomas J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598, USA.
authors.forename: A
authors.initials: A
authors.lastname: Topol
authors.affiliation: Wrocław University of Technology, Faculty of Microsystem Electronics and Photonics, Division of Metrology of Micro- and Nanostructures, ul. Z. Janiszewskiego 11/17, 50-372 Wrocław, Poland.
authors.forename: T
authors.initials: T
authors.lastname: Gotszalk
keywords: Epitaxial structures; High-efficiency solar cells; III–V Semiconductors; In-situ illumination; Kelvin probe force microscopy; Multi-junction solar cells