Large area interference lithography
as mastering technique to patternsilicon-based tandem solar cells
Benedikt Bläsi, Volker Kübler, Hubert Hauser
Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstraße 2, 79110 Freiburg, Germany
E-mail: benedikt.blaesi@ise.fraunhofer.de

In this presentation, we will introduce our capabilities in the field of mastering using
interference lithography and will discuss the business case of silicon-based tandem solar cells
as a potential field of application.
An efficiency increase of approx. 0.6 % per year can be observed in silicon PV industry in thepast [1] (Fig.1, left). Yet, there will be an end to these improvements as physics limits theefficiency of single junction silicon solar cells to 29.4 % [2]. One way to circumvent this deadend is based on the well-known method of stacking semiconductors of higher bandgaps
preferably on silicon. This trend can be seen in research and is already listed in thetechnology roadmap of the PV industry, where projections already predict a production ofsuch cells from 2023 on [3]. Until 2030 the PV market will grow from currently~ 100 GWpeak to over 1000 GWpeak annual production [4]. With a price of approx. 10 cent per
Wpeak this will be a total market volume in the range of ~ 100 billion US $.
The current record for a two terminal silicon-based tandem device incorporates a photonic
rear side structure, raising the cell efficiency from 31.4 % to 33.3 % [5]. This photonic rear isbased on a master structure realized via interference lithography [6], which is subsequently
replicated via Roller-NIL [7] (Fig. 2).
In interference lithography a laser beam is split into two or more beams, which then arefiltered, expanded and superimposed on a photoresist coated sample. The resulting
interference pattern is applied to expose photoresists, leading to a surface relief after the wet
chemical development. We use an argon-ion laser emitting at 363.8 nm for this process. By
varying the setup, this technology can be applied to realize patterns with periods ranging from
200 nm up to 100 µm arranged in different symmetries (e.g. linear, crossed or hexagonalpatterns) and also to realize (tailored) diffuser structures. Also modifications of the structure
profile are possible (e.g. prismatic or continuous spherical/aspherical shapes). The big
strength of this technology is the possibility to realize seamless micro and nanostructures onvery large areas. The largest sample size patterned in our labs up two now is 1.2 x 1.2 m²(Fig.2, right). Besides PV, potential fields of application for interference lithography arephotonic structures for LED´s (e.g. light out-coupling, directed emission), displays (e.g.
polarization optical elements) or sensing (e.g. plasmonics or resonant-waveguide gratings).
Reference:[1] M. Hermle, ETIP PV Manufacturing Conference 2017[2] A. Richter et al. “Reassessment of the limiting efficiency for crystalline silicon solarcells,” IEEE Journal of Photovoltaics 3, 1184–1191 (2013).
[3] ITRPV, International Technology Roadmap for Photovoltaic. 2018 Results 10th Edition,
2019.
[4] F. Creutzig et al. “The underestimated potential of solar energy to mitigate climatechange,” Nat. Energy 2, 17140 (2017).
[5] R. Cariou, et al. “III–V-on-silicon solar cells reaching 33% photoconversion efficiency intwo-terminal configuration,” Nature Energy 17, 183 (2018).
[6] A. J. Wolf et al. “Origination of nano- and microstructures on large areas by interference
lithography,” Microelectron Eng 98, 293–296 (2012).
[7] H. Hauser et al. “Honeycomb texturing of Silicon via nanoimprint lithography for solar
cell applications,” IEEE Journal of Photovoltaics 2, 114–122 (2012).

Figure 1. Left: Efficiency increase of ~ 0.6 %/year based on data from industrial fabrication fordifferent silicon solar cell architectures (adapted from Ref. [1]). The intrinsic efficiency limit around
29 % is also sketched. Right: Technology roadmap of the PV industry showing the emergence ofsilicon based tandem solar cells in 2023 (red circle) [3].

Figure 2. Left: Photograph of the largest seamlessly patterned sample at Fraunhofer ISE up to now.
A hexagonal grating with a period of 1 µm was realized on 1.2 x 1.2 m². Right: Photograph of theRoller-NIL developed at Fraunhofer ISE. This tool has the functionality of a roll-to-plate tool with aUV-light source realized within a quartz roller. Thus it can be applied to pattern UV-curable
materials on opaque and stiff substrates.