Residual layer etching techniques for NIL in
photovoltaics
Hubert Hauser, Ralph Müller, Oliver Höhn, Rita M. S. Freitas, Armin Richter,
Benedikt Bläsi
Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstraße 2, 79110 Freiburg, Germany
E-mail: hubert.hauser@ise.fraunhofer.deSilicon, as indirect bandgap semiconductor, suffers from weak absorption especially in thenear IR. In photovoltaics, this necessitates photon management schemes in order to prolong
internal light paths. Such photonic concepts gain even more in importance when realizing
silicon-based tandem solar cells. We already demonstrated the importance of light trapping ina multi-junction device (GaInP/GaAs//Si) by raising the cell efficiency from 31.4% to 33.3%
just by adding a nanoimprinted rear side diffraction grating [1]. This is to date the highest
conversion efficiency reached for a two-terminal silicon-based tandem solar cell [2].
The mentioned photonic structure was realized using interference lithography. The resulting
crossed grating with a period of 1 µm was replicated onto the rear of solar cells by applying athermally assisted UV-Roller-NIL process to pattern SU8 [3,4] with subsequent plasma
etching to remove the residual layer. This resist pattern was coated with silver (thermalevaporation), leading to a photonic mirror that additionally acts as the rear side contact of thesolar cell.
A detailed examination of the cells of the photonic rear side structure from Ref. [1] revealed
the following improvement potentials. We found that the plasma removal of the residual layer
causes two potentially negative aspects: (i) the remaining resist pattern shows a nano-roughsurface, which might give rise to enhanced parasitic absorption (see Fig. 1) [5]; (ii) thesurface passivation and thus the electrical quality of solar cells can degrade strongly.
Therefore, we established a wet chemical process using the so-called piranha solution(H2O2:H2SO4) to remove the residual layer without surface roughening and to avoid plasma
damages.
The influence of dry and wet chemical etching of residual layers was studied on single
junction silicon solar cell architectures. Optical properties were characterized via quantum
efficiency measurements. For both techniques, plasma and piranha etching, we found an
integrated gain of the short circuit current density (jsc) very similar to the 1.1 mA/cm²
reported in [1] (Fig. 2., left). Thus, we cannot confirm any harmful effects caused by thenano-roughness in the plasma etching process and degradation of the passivation for 100 nm
thick poly Si layers. However, we found that thinner poly-silicon passivation layers (15 nm),which have a lower free carrier absorption, strongly degraded due to plasma etching of theresidual layer. Using the wet chemical etching of residual layers no degradation was found(Fig. 2, right). Altogether, an improved wet chemical residual layer etch leads to acomparable optical and a superior electrical performance for the presented photonic rearconcept based on NIL.
References:[1]
R. Cariou et al., “III–V-on-silicon solar cells reaching 33% photoconversion
efficiency in two-terminal configuration,” Nature Energy 17, 183 (2018).
[2]
M. A. Green et al., “Solar cell efficiency tables (Version 53),” Progress in
Photovoltaics: Research and Applications 27, 3–12 (2019).
[3]
H. Hauser et al., “Honeycomb texturing of Silicon via nanoimprint lithography forsolar cell applications,” IEEE Journal of Photovoltaics 2, 114–122 (2012).
[4]
N. Tucher et al., “Characterizing the degradation of PDMS stamps in nanoimprint
lithography,” Microelectronic Engineering 180, 40–44 (2017).
[5]
S.-K. Kim et al., “Surface-plasmon-induced light absorption on a rough silver
surface,” Appl. Phys. Lett. 98, 11109 (2011).

Figure 1. Top left: Sketch of the solar cell structure applied for this study consisting of a p-type basewith both sides passivated poly-silicon contact layers. On the rear side the partially opened polymer
structure can be seen. Top right: Cross section SEM of the rear poly-silicon / SU8 / silver interfaces.
Bottom row: SEM micrographs of via NIL structured resist layers after plasma (left) and Piranha(right) treatment for the opening of the residual layer.
100

EQE [%]

80(Implied) Voc [mV]

Planar reference
Piranha etch
Plasma etch

60
40200950

1000

1050

1100

Wavenlength [nm]

1150

1200

PlanarGrating

700

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500

PlasmaPiranhaEtching techniqueFigure 2. Left: External quantum efficiency (EQE) measurement of single junction silicon solar cellswith plasma or piranha etched residual layer as well as a planar reference. Both etching techniqueslead to a gain in jsc of 1 mA/cm². Note: In this case, 100 nm thick passivation layers were applied.
Right: A graph visualizing influence of the residual layer etching technique on the passivation quality
for a 15 nm thick poly-silicon layer. While the passivation and thus the (implied) open circuit voltage
drastically degrades for the plasma treatment (based on minority carrier lifetime measurements), noeffect can be seen for the wet chemical approach (based on solar cell measurements).