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Department Optics for Solar Energy


Nanoimprint lithography

enlarged view

Fig. 1 Illustrating the replication process using a PDMS stamp.

Nanoimprint Lithography (NIL) is a surface patterning technique first introduced in 1996 by Prof. Stephen Chou and his colleagues [1,2]. This new technology had such an important impact in so many research fields that it has been listed in 2003 by the Massachusetts Institute of Technology as one of 10 emerging technologies that will strongly impact the world [3]. Indeed beside its ultrahigh resolution reproduction capability (up to 5 nm) this technology is also very cost effective, easy to process and therefore makes it a serious candidate for the implementation of up-scaling applications and large scale industrial fabrication.

The principle of NIL (sometimes also called “hot embossing”) is very simple. A template or mold (e.g. a silicon wafer textured with nanoscale features) is pressed with a controlled pressure and temperature onto a substrate coated with a defined layer of polymeric material. After the mold removal an inverse reproduction of the features will then be directly imprinted onto the substrate.  Recently new UV-curable polymer materials (also called resists) have appeared on the market and have been used for nanoimprinting technology. This UV-NIL process makes it even simpler and faster than the traditional hot embossing technique because it can be done at room temperature. The only requirement is then to have a transparent mold which allows the UV light to pass through for the curing of the resist. The most common process is to make first a so-called transparent stamp of the original template and then use this stamp for the replication process of the resist dispersed onto the substrate. Poly(dimethyl siloxane) block polymer (PDMS) is commonly used for the fabrication of the transparent stamp. This material offers a very high resolution patterning and is also easy to separate from the mold due to its low surface energy.

enlarged view

Fig 2 PDMS stamp replication of three fields (6x8 mm2) with a hexagonal lattice structure with periods ranging from 600 to 800 nanometers.

In the Nano-SIPPE group we are working with UV-NIL technology in order to imprint silica nanostructures directly onto glass substrates. A UV-curable hybrid Sol-Gel resist is patterned using PDMS stamps. This process allows us to have a high degree of control over the different features of our structure like the periodicity, the diameter (in the case of nanopillar or nanohole structures) or even the height by varying the thickness of the resist on the substrate. These silica textured substrates are then used as templates for the investigation and development of silicon based new light trapping nanostructures and photonic crystals.

Optical simulations

Spectrum - enlarged view

Numerical and experimental 1 - R spectra for sinusoidally textured layer stacks. The textures have 750 nm pitch. Numerical results were calculated with two corrections, which differ because not all diffraction orders that are present in glass can propagate into air. Simulation results are shown for two angles of incidence: θin = 0° (thin lines) and θin = 8° (thick lines). Experimental results were obtained with theta θin = 8° [5].

Numerical techniques are a powerful tool to develop effective light management architectures for improved solar cell performance. We mainly use the state-of-the-art time-harmonic finite element solver JCMsuite, which is developed by JCMwave GmbH, a spin-off of the Zuse Institute Berlin (ZIB). The strengths of the finite-element-method (FEM) and the JCMsuite-software are its outstanding accuracy and convergence, which can outperform comparable methods such as FDTD in respective benchmarks [1].

The Department Optics for Solar Energy is a founding partner of the Berlin Joint Lab for Optical Simulations for Energy Research (BerOSE) which was founded by HZB, ZIB and the Free University Berlin in 2014. BerOSE forms an ideal environment for the interaction between experts in 3D optical computation and scientists in the synthesis of nanostructured materials for solar electricity generation, energy storage and photonics.

We use 3D optical simulations primarily to study nanophotonic light trapping for highly efficient silicon thin-film solar cells and to investigate large area photonic crystals. Further, we contribute with our numerical expertise to the Helmholtz Innovation Lab HySPRINT.

Besides rigorous Maxwell solvers, which are mainly suited for periodic architectures, we are also very experienced in other simulation techniques that can be used for solar devices. Among those are the coherent-incoherent net radiation method for planar layer stacks [2,3], which can be expanded with the scalar scattering theory for non-periodic nanotextures [4] and ray tracing for large textures.