Flexible monocrystalline Si films for thin film devices from transfer processes
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Flexible monocrystalline Si films for thin film devices from transfer processes Christopher Berge, Thomas A. Wagner, Willi Brendle, Cecilia Craff-Castillo, Markus B. Schubert, and Jürgen H. Werner Institute of Physical Electronics, University of Stuttgart, 70569 Stuttgart, Germany ABSTRACT Transfer of monocrystalline silicon films to arbitrary foreign substrates is a promising way for the fabrication of high quality silicon films on foreign substrates, demonstrated by solar cell efficiencies on glass as high as 16.6 % in the past. Transfer technologies also enable the use of flexible substrates. This paper investigates the mechanical stability of the separation layer for two different morphologies. First measurements on the minimum bending radius of unsupported silicon films are presented that allow us to estimate minimum curvatures for flexible monocrystalline devices. Finally, we report the first flexible monocrystalline thin film silicon solar cell of 4 cm² with an independently confirmed efficiency of 14.6 %. INTRODUCTION For a large range of electronic devices such as displays and solar cells, the use of thin and flexible materials is desirable, as it enables better integration in (mobile) consumer devices, but also new applications like wearable electronics. There are two main approaches to achieve this goal: i) the direct deposition of silicon on a foreign substrate, and ii) the transfer of monocrystalline films to a foreign substrate. While the first approach seems attractive due to the possibilities of cheap large-area fabrication, it suffers from the fact that for many types of devices the electronic quality is not sufficient. This in contrast is the main advantage of the transfer approach, that provides the high electronic quality of monocrystalline silicon, hence making it suitable even for flexible high frequency CMOS circuits. A number of transfer processes to foreign substrates have been under development at various companies and institutions, and some of them are commercially available. The most prominent approaches are the ELTRAN [1, 2] process and the Smart-Cut [3, 4], both serving primarily for the supply of silicon-on-insulator (SOI) wafers. A transfer process starts with the formation of a monocrystalline film that is − by some means − separated from the host wafer. When SOI wafers are fabricated, the layer is transferred before the beginning of a device processing sequence, and the transfer target is normally a silicon handle wafer, used as a substrate for further processing. A consequent development towards flexible electronics is the transfer of the monocrystalline film with the finished devices to a flexible substrate after the processing sequence, using the host wafer as a handle wafer during the process. In this way, one can make use of standard processes including high-temperature steps, and nearly arbitrary substrates can be used for the transfer once processing is finished. For thin film solar cells, several approaches are under investigation: The Epi-lift process developed at A
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