Photon management structures for solar cells

Abstract

Since micro- and nanostructures for photon management are of increasing importance in novel high-efficiency solar cell concepts, structuring techniques with up-scaling potential play a key role in their realization. Interference lithography and nanoimprint processes are presented as technologies for origination and replication of fine-tailored photonic structures on large areas. At first, these structure origination and replication technologies are presented in detail: With the interference pattern of two or more coherent waves, a wide variety of structures with feature sizes ranging from 100 nm to 100 µm can be generated in photoresist by interference lithography. Examples are linear gratings, crossed gratings, hexagonal structures, three dimensional photonic crystals or surface-relief diffusers. The strength of this technology is that homogeneous structures can be originated on areas of up to 1.2 x 1.2 m2. The structures in photoresist, the so-called master structures, can serve as an etching mask for a pattern transfer, as a template for infiltration with different materials or they can be replicated via electroplating and subsequent replication processes. Especially in combination with replication steps, the industrially feasible production of elaborate structures is possible. As a particularly interesting process, nanoimprint lithography (NIL) is described in detail. As a way towards industrial production, a roller NIL tool is presented. After the description of the basic technologies, three application examples for solar cells are presented with details about the design of the structures, the structuring processes, sample characterization and evaluation: (1) honeycomb structures for the front side texturization of multicrystalline silicon wafer solar cells, (2) diffractive rear side gratings for absorption enhancement in the spectral region near the band gap of silicon, and (3) plasmonic metal nanoparticle arrays manufactured by combined imprint and lift off processes

Nanoimprinted diffraction gratings for crystalline silicon solar cells: implementation, characterization and simulation

“…This technique allows fine-tailored nanostructures to be realized on large areas with high throughput: a necessity for industrialization. A detailed description of the process chain can be found in [8,15]. A brief description is given here.…”

Comparison between periodic and stochastic parabolic light trapping structures for thin-film microcrystalline Silicon solar cells

“…Figure 3 shows the absorption calculated for a crystalline Si solar cell with a thickness of 1.1 μm deposited onto the texture shown in Fig. 1(a), using the SST method described in the previous section and Ref [18] and also using the simulation tool ASA [22]. Also shown is the measured EQE of the actual solar cell.…”

“…Provided a similar etching process is used as for the stochastic structure, similar shapes should be obtained for the periodic structure. More details about the investigated structures and a direct comparison of their light trapping properties can be found in Ref [18]. Fig.…”

“…1(a) with a scaling factor S l and the height with a scaling factors Sh. This procedure is an extension of a method described in Refs [18] and [28]. The simulated impact of this variation on the absorbed current j ph is shown in Fig.…”

Development of nanoimprint processes for photovoltaic applications

“…10 A large diversity of patterns can be realized including periodic (1-D, 2-D, and 3-D photonic crystals) and stochastic features (e.g., well-defined isotropic or anisotropic diffuser structures) as well as combinations thereof. 11 Minimum feature sizes are determined by the laser wavelength. An argon-ion laser emitting at 363.8 nm was used; therefore, minimum periods are in the range of 200 nm.…”