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Development of Back Side Technology for Light Trapping and Photon Recycling in GaAs Solar Cells

: Micha, D.; Höhn, O.; Oliva, E.; Klinger, V.; Bett, A.; Dimroth, F.

Postprint urn:nbn:de:0011-n-5487276 (750 KByte PDF)
MD5 Fingerprint: 38a8e3f814a6614785707f2cf423b0de
Erstellt am: 30.09.2019

Progress in Photovoltaics 27 (2019), Nr.2, S.163-170
ISSN: 1062-7995
ISSN: 1099-159X
Zeitschriftenaufsatz, Elektronische Publikation
Fraunhofer ISE ()
Photovoltaik; III-V und Konzentrator-Photovoltaik; III-V Epitaxie und Solarzellen; reflector; solar cell; recycling; photovoltaic; film solar cell

In this work, we propose and realize three different design strategies to implement an optical cavity in GaAs thin film solar cells in order to confine its internal luminescence and hence to exploit photon recycling. The strategies are based on the definition of a highly reflective and very conductive back side, whereas front side light extraction is limited by total internal reflection. We show characterization results on the internal reflectivity of the back reflector and on the contact resistance at the rear side, important quantities for a good functioning of the final solar cell. First, a back side using only metal was optimized with a pure Ag layer leading to an internal reflectivity of 95.2% and a contact resistance of 1.0 × 10−4 Ω for a 1 cm2 device. With a metal‐dielectric stack at the back side and electrical contacts made by metals via point‐contacts, a second approach led to averaged internal reflectivity of 98.0% and contact resistance of 1.8 × 10−4 Ω for a 1 cm2 device. A third strategy in which a transparent conductive oxide in combination with a metal layer was used did not show the expected results in optical and electrical properties. We fabricated and characterized solar cells with the most promising back sides. When comparing with an ordinary reference GaAs solar cell, external radiative efficiency increased by factors of 150% and 90% for the thin film solar cells with pure Ag and with the metal‐dielectric stack at the back side, allowing enhancements of 19 and 13 mV in VOC, respectively.