Planar Black Silicon: disentangling optical and electrical properties of textured interfaces using transformation optics

• Applicants: Professor Dr. Carsten Rockstuhl; Professor Dr. Ralf B. Wehrspohn
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2019 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 413644979

Nanotexturing a solar cell’s absorber material leads to a strong and spectrally broadband absorption enhancement of incident sun light. However, nanotexturing the interfaces degrades simultaneously the electronic properties of the optoelectronic device. This denies the complete harvest of the absorption improvement and does not allow to translate it to a comparable improvement in device efficiency. In contrast, external light management structures that leave the absorber material undamaged are optically far from optimum and they provide substantially weaker absorption enhancement.To solve this problem, we propose to capitalize on the notion of transformation optics to design photonic structures that preserve the geometrical flatness of the absorber material, to leave it electrically intact, while acting optically like a textured interface with optimized antireflection and scattering properties. Following transformations optics procedures, the invariance of Maxwell’s equations can be exploited to deduce a material distribution that regulates the light flow in exactly the same way as a template surface texture, e.g. conventional black silicon, would do. By doing so, we are effectively deducing an inhomogeneous planar layer that, when placed on top of the absorber layer, can provide antireflection and light trapping, thus absorption enhancement, equivalent to the template surface texture without its electronic degradation. We will pursue the experimental realization of the dielectric graded refractive index light management structures and demonstrate its integration into a solar cell. As building blocks, we will utilize dielectric high refractive index nanostructures and thin film layers of different materials conformally deposited by an advanced atomic layer deposition technique.Our approach of deducing light management structures with transformation optics, to the best of our knowledge, is pioneering. Our design approach is a departure from currently common transformation optics applications and will open up applications of transformation optics concepts in designing structures for various real devices. Solar energy conversion is the obvious and most important application that would directly benefit from the findings of this project. However, the methods to be developed here may also contribute to the development of novel concepts in other fields of optics. For example, one can also deduce alternative light outcoupling structures in light emitting diodes (LEDs), which is the inverse of what we mainly aim to do here.

• DFG Programme: Research Grants