Samuel Wiesendanger

Light trapping in periodically textured amorphous silicon thin film solar cells using realistic interface morphologies.

The influence of realistic interface morphologies on light trapping in amorphous silicon thin-film solar cells with periodic surface textures is studied. Realistic interface morphologies are obtained by a 3D surface coverage algorithm using the substrate morphology and layer thicknesses as input parameters. Finite difference time domain optical simulations are used to determine the absorption in the individual layers of the thin-film solar cell. The influence of realistic interface morphologies on light trapping is determined by using solar cells structures with the same front and back contact morphologies as a reference. Finally the optimal surface textures are derived.

Cloaked contact grids on solar cells by coordinate transformations: designs and prototypes

Nontransparent contact fingers on the sun-facing side of solar cells represent optically dead regions which reduce the energy conversion per area. We consider two approaches for guiding the incident light around the contacts onto the active area. The first approach uses graded-index metamaterials designed by two-dimensional Schwarz–Christoffel conformal maps, and the second uses freeform surfaces designed by one-dimensional coordinate transformations of a point to an interval. We provide proof-of-principle demonstrators using direct laser writing of polymer structures on silicon wafers with opaque contacts. Freeform surfaces are amenable to mass fabrication and allow for complete recovery of the shadowing effect for all relevant incidence angles.

Combining randomly textured surfaces and photonic crystals for the photon management in thin film microcrystalline silicon solar cells

Photon management aims at optimizing the solar cell efficiency by, e.g., incorporating supporting optical nanostructures for absorption enhancement. Their geometrical design, however, is usually a compromise since requirements in different spectral domains need to be accommodated. This issue can be mitigated if multiple optical nanostructures are integrated. Here, we present a photon management scheme that combines the benefits of a randomly textured surface and an opaline photonic crystal. Moreover, upon considering the device with an increasing complexity, we show that a structure that respects the mutual fabrication constraints has the best performance, i.e., a device where the photonic crystal is not perfect but to some extent amorphous as enforced by the presence of the texture.

A path to implement optimized randomly textured surfaces for solar cells

Randomly textured surfaces are nowadays routinely integrated into solar cells. Nonetheless, their performance is still not optimal. This became obvious while comparing their performance to optimized surfaces. Thus far, however, these optimized surfaces suffer from being either impossible to implement or only with expensive top-down nanofabrication technologies not suitable for large scale wafers. Here, we suggest a different approach to achieve optimized randomly textured surfaces. It exploits a self-assembled monolayer of spheres with a carefully balanced size distribution to define the random texture. Existing solar cells are outperformed with such realistic textures by up to 26%.

Effects of film growth modes on light trapping in silicon thin film solar cells

In this work, the impact of two different growth modes on the efficiency of an amorphous thin film solar cell comprising randomly textured interfaces is investigated. The two modes are the commonly used conformal growth which assumes identical textured interfaces and the isotropic growth, in which deposited material grows in the direction of the local surface normal. In the latter, the textures morphology can change significantly. The rivalling impact of these two growth modes on the solar cell absorption is not yet fully understood. Here, we show that the efficiency of a solar cell crucially depends on the growth mode. In different size regimes, they may outperform each other with regard to efficiency by almost 15%. The insights gained by this study will guide experimentalists in the future in selecting the optimised growth mode.

Single-pass and omniangle light extraction from light-emitting diodes using transformation optics

We present a light-extraction approach allowing for single-pass and omniangle outcoupling of light from light-emitting diodes (LED). By using transformation optics, we perceive a feasible graded-index structure that is a transition from the LED exit facet to a low refractive index region with expanded space that represents air. Apart from the material dispersion of the constituents, our approach is wavelength independent. The suggested extractor is geometrically compact with size parameters comparable to the width of an LED and therefore well adapted for pixelated LEDs. A beam-expanding functionality is possible while fully preserving the outcoupling efficiency by applying index and geometry truncation.

Cloaking of metal contacts on solar cells

We design by transformation optics, fabricate by three-dimensional direct laser writing, and characterize experimentally polymer-based cloaks for 20 μm wide gold-wire contacts on a silicon wafer. The contact shadowing effect is reduced by 90%.

Front and rear side photonic nanostructures for an optimal photon management

We investigate the light trapping of various photonic nanostructures integrated into the front and rear side of silicon solar cells. Combinations of textured surfaces and amorphous photonic crystals enhance the absorption by more than 15%.

Photon Management in Thin‐Film Solar Cells

We analyze the absorption enhancement in single and tandem solar-cells comprising nanostructures that increase the path of the photons inside the solar cell. For this purpose we exploit different physical phenomena in different material systems.