Andreas Gombert, Benedikt Blasi, Wolfgang HoRfeld, Volker Kubler, Michael Niggemann, Peter Nitz, Gunther Walze, Fraunhofer-Institut fur Solare Energiesysteme ISE, Heiden — hofstr. 2, 79110 Freiburg, Germany

Jorg Mick, Institut fur Mikrosystemtechnik, Albert-Ludwigs-Universitat, Georges-Kohler — Allee, 79110 Freiburg, Germany

An overview of known methods to modify the optical properties of solar energy ma­terials by using micro-structured surfaces is given. Applications for micro struc­tures in solar energy components are wavelength-selective absorbers, heat mirrors, light traps for PV cells, wavelength-selective concentrators for solar radiation, day­lighting components, antireflective zero-order gratings, and radiation emitters with selective optical properties based on grating resonances. This paper addresses the design and the whole experimental process chain from the microstructure origina­tion on large areas to the replication. The need for cost-effective production tech­nologies and durable structured materials is emphasised.

Introduction

A wide variety of solar energy systems from PV cells to buildings exists. All the very differ­ent systems have in common that they demand sophisticated optical solutions for an effi­cient transport, collection and conversion of the solar radiation. Modifying the optical prop­erties of surfaces or planar devices by coatings and microstructures is often used in order to optimise the radiation power management of solar energy systems. Publications in which diffractive structures were proposed to fine-tune the optical properties date back to the 1970’s [1 — 4].

In fact, advances in diffractive optics like the first approaches to solve the problem of dou — bly-periodic gratings were driven by scientists having solar applications in mind, e. g. Mc Phedran and Maystre [2]. Since the fundamental work of the mentioned authors, diffractive structures in solar energy systems were published for a variety of components in solar en­ergy systems, e. g. [5 — 15]. In Table 1, the quoted publications are classified according to the components for which the diffractive structures were proposed. Additionally, the re­quired optical properties and the proposed structures are listed.

The advantage of considering periodic microstructures is the possibility to model their opti­cal properties rigorously, i. e. by solving the Maxwell equations [16]. Thus, the optical prop­erties of periodically micro-structured surfaces can be simulated with rather high accuracy. This is not the case for aperiodic structures.

The challenge of using microstructures is the requirement on very precise manufacturing technologies. Such technologies exist in the field of microelectronics or microsystems technology but are in general not suitable to structure large areas homogeneously. Thus, many of the published approaches are difficult to realise especially due to the mismatch of dimensions between the microstructures and the areas which have to be micro-structured in solar applications.

From the technical point of view, homogeneous origination of precise microstructures on large areas is the most difficult step in the process chain. Because of its ability to share the high origination cost with a large number of products, microreplication is very promising from the commercial point of view. Microreplication is a suitable process for some of the applications listed in Table 1 but not for all of them.

At Fraunhofer ISE, we have picked up the idea of using periodic microstructures in solar energy applications at the beginning of the 1990s. Since then we have been working on the design and the manufacturing techniques for the following optical components: antireflective surfaces, light traps for PV cells, sun protection systems, and wavelength — selective radiation emitters. By using interference lithography we were able to originate microstructures on areas of up to 4800 cm2.