Fraunhofer-Gesellschaft

Publica

Hier finden Sie wissenschaftliche Publikationen aus den Fraunhofer-Instituten.

Nanoimprint lithography for solar cell texturisation

Nanoimprint Lithographie für die Solarzellentexturierung
 
: Hauser, H.

:
Fulltext ()

Freiburg, 2013, III, 202 pp. : Ill.
Freiburg/Brsg., Univ., Diss., 2013
URN: urn:nbn:de:bsz:25-opus-90774
English
Dissertation, Electronic Publication
Fraunhofer ISE ()

Abstract
The technology of nanoimprint lithography (NIL) offers completely new possibilities in terms of achievable pattern resolutions for photovoltaic applications. However, existing tools and processes were mostly aiming for microelectronic or biological sensing applications. Therefore, the aim of this thesis was the development of tools and processes adapted to the requirements in photovoltaics. The concrete application of NIL was the defined texturisation of wafer based silicon solar cells via subsequent plasma etching processes.
Within this thesis, a Roller-NIL tool was designed and constructed to combine the high-resolution capability of NIL technology with in-line feasible processing. Using soft stamps and UV-curing resist materials, this tool was proven to be able to pattern of etching masks on large areas of up to 156 x 156 mm². What is especially remarkable is that this is possible on very thin (tested down to 100 um), opaque, brittle and also rough as-cut multicrystalline silicon substrates.
Two exemplary applications were investigated to verify the high potential of this novel texturing sequence based on NIL and plasma etching. These were the honeycomb texturing for multicrystalline silicon solar cells and the origination of diffractive rear side gratings as photon management structures, which are of particular interest for forthcoming thinner wafers.
The honeycomb texturing of the front side of multicrystalline silicon via NIL leads to excellent optical properties of around 14 % weighted reflectance (without ARC). This very high optical quality was confirmed on small area (2 x 2 cm²) FZ silicon solar cells by very high short-circuit current densities of 40.1 mA/cm² and 40.7 mA/cm² for a single and a double layer ARC respectively. The up-scaling capability of the developed texturing process chain were demonstrated on 125 x 125 mm² wafers.
As a second application, diffractive rear side gratings were and their considerable absorption enhancement was demonstrated. Both linear and crossed gratings with a period of 1 um were realised. For the crossed gratings, absolute absorption enhancements of up to 52 % absolute were measured at a wavelength of around 1100 nm for 200 um thick wafers. In this context a concept for minimising parasitic absorption by applying a dielectric buffer layer in the liquid phase was introduced. Furthermore, a concept for decoupling electrical and optical properties of the solar cell rear incorporating a photonic rear side structure was successfully demonstrated.

 

Die Technologie der Nanoimprint Lithographie (NIL) eröffnet in punkto erreichbarer Strukturauflösungen bisher nicht denkbare Möglichkeiten für photovoltaische Anwendungen. Bisherige Entwicklungen zielten vorwiegend auf Anwendungen beispielsweise im Bereich der Mikroelektronik oder der Biowissenschaften ab. Aus diesem Grund lag die Zielstellung der vorliegenden Arbeit insbesondere auf der für die PV maßgeschneiderten Anlagen- und Prozessentwicklung dieser Technologie mit der Anwendung der Texturierung wafer-basierter Siliciumsolarzellen. Die NIL ist dabei lediglich ein Schritt in einer komplexen Prozesskette bestehend aus vorgeschalteten (Herstellung der Urform sowie Replikation polymerer Stempel) sowie sich wiederholender Schritte (NIL, Ätzen und Reinigung). Exemplarisch wurden zwei Anwendungsbeispiele für Oberflächentexturen untersucht, um das hohe Potential dieser innovativen Prozesskette zur Solarzellentexturierung aufzuzeigen. Dabei handelt es sich zum Einen um die Honeycomb-Texturierung, die insbesondere für multikristallines Silicium sehr interessant ist, sowie zum Anderen um die Realisierung diffraktiver Rückseitengitter als Photonen-management Strukturen, die speziell für zukünftige Zellgenerationen mit sehr dünnen Substraten wichtig werden können.

: http://publica.fraunhofer.de/documents/N-389300.html