Presentation Details
| Optimizing Monolithic (Ag, Cu)(In, Ga)Se2/Si Tandem Solar Cells via Bandgap Engineering and Bifacial Operation (yes) Julia Horstmann1, Tobias Gut1, Matthias Maiberg1, Mario Hanser2, Jana-Isabelle Polzin2, Martin Hermle2, Roland Scheer1. 1Martin-Luther-Universität Halle-Wittenberg, Halle (Saale), Germany.2Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Germany |
Abstract
Tandem solar cells provide a viable pathway to exceed the theoretical power conversion efficiency limit of single-junction devices. In this work, monolithic two-terminal (2T) tandem solar cells integrating wide-bandgap Ag-alloyed Cu(In,Ga)Se2 (ACIGSe) top cells with high efficiency TOPCon silicon bottom cells are investigated. In current-matched 2T tandem architectures, the photocurrent of the bottom cell is limited by the fraction of incident light transmitted through the top absorber. An optimal trade-off between top-cell absorber bandgap and thickness is therefore required to achieve the maximum current density in the tandem device. While (A)CIGSe is a mature absorber material for low-bandgap applications, increasing its bandgap above 1.6 eV by increasing the Ga-content, leads to reduced device efficiency. To analyze the impact of top cell absorber bandgap on overall tandem performance, ACIGSe bandgaps of 1.5 eV and 1.6 eV were compared. In the analyzed devices, the current density of the tandem solar cell was limited by the Si bottom cell. To enhance bottom cell current generation, bifacial illumination was therefore employed. To this end, a transparent back contact for the silicon sub cell is implemented thereby allowing additional photocurrent from albedo illumination. Monolithic bifacial ACIGSe/Si tandem devices with an ACIGSe bandgap of 1.6 eV demonstrate a power generation density of 15.3 mW/cm² under 30% albedo illumination for an active area of 0.6 cm², highlighting the potential of bifacial operation to relax current-matching constraints. Experimental results are supported by electro-optical simulations of top cell absorber and bandgap variation for different albedo illuminations.
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No part of this publication may be reproduced, distributed, or transmitted in any form or by any means, including photocopying, recording, or other electronic or mechanical methods, without the prior written permission of the author.