Bio-inspired Design of Quasi-random Structure for Effective Photon Management

Through millions of years of evolution, structural colors originated from the so-called quasi-random patterns have been widely adopted by natural creatures to fulfill their complex biological functions, such as camouflage, intimidation, display, and social communication. Possessing local correlation but little long-range order, these quasi-random structures feature rich Fourier spectra and extraordinary optical functionalities. Their distinctive capability in controlling the flow of light has thus inspired the design of a broad range of photonic devices. I’ll present our recent progress in exploring the new design strategies for the quasi-random nanophotonics structures and their applications for effective photon management to enhance the efficiency of thin film solar cells. I will first talk about topology optimization to design quasi-random light trapping structure for thin-film solar cells, in contrast to the commonly adopted methods relying the deterministic strategy. Mimicking the natural evolution process to gradually improve light trapping structure from random through iterative procedures, this method produces the most favorable structure with substantial enhancement in light absorption. Nearly 100% absorption has been accomplished for single incident wavelength and 30% average absorption for solar spectrum without back reflector, with spectral enhancement factor twice the Yablonovitch limit. While searching for the cost-effective solution to fabricate the quasi-random structure, we noticed that they also exist in Blu-ray movie discs, an already mass-produced consumer product. We uncover that Blu-ray disc patterns are surprisingly well suited for light-trapping applications. As a proof-of-concept, imprinting polymer solar cells with the Blu-ray patterns indeed increases their efficiencies by 18.2%. Finally, recognizing structural orders underlying the low-cost pattern formation process and the unique Fourier spectra of the quasi-random structures found in Nature, we recently developed a new design strategy that employs spectral density function to represent quasi-random structures in Fourier space. We have demonstrated that the optimized design in Fourier space corresponds to infinite sets of quasi-random structures in real space with equally optimized performance but completely different morphological or even topological characteristics. The observed one-to-multiple mapping between the Fourier-space design and the resultant quasi-random real-space structures was further validated experimentally using light-trapping structures in a thin film solar cell as a model system.