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Dongheon Ha1 2 Nikolai Zhitenev1

1, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
2, University of Maryland, College Park, Maryland, United States

Due to earth’s finite natural resources and greenhouse effect, developing high efficient photovoltaics is one of the primary interests in energy research society. Recently, dielectric nano-resonators-based coating atop solar cells has been considered as a solution to make high absorptive, efficient photovoltaics as they are relatively cheap and can be deposited atop solar cells without surface damage [1]. However, the evaluation and optimization of the nano-resonator coatings have been made only with macro-/mesoscale experiments and calculations. Therefore, characterization in nanoscale where photo-excited carrier generations and collections actually take place is necessary to fully establish the physical origin of the efficiency enhancements. In this presentation, for the first time, we show how excited optical resonances within a monolayer of nano-resonators improve photocurrent of solar cells in nanoscale via a novel near-field scanning photocurrent microscopy technique [2]. We employ a tuning fork-based non-contact mode atomic force microscopy probe attached with a metal-coated multimode optical fiber to inject light in nanoscale. At a wavelength of whispering gallery mode excitation, we image strong resonance-induced local photocurrent enhancements over each nano-resonator. Based on nanoscale photocurrent imaging as well as macroscale measurements and calculations, we conclude that collective optical resonances between whispering gallery modes and thin-film interference improve the photocurrent of solar cells by more than 20 %. We also examine the properties of multiple layers of nano-resonators by the suggested near-field scanning photocurrent microscopy to evaluate the role of possible manufacturing imperfections [3]. We show that the optical resonances and their resultant photocurrent enhancements are dependent on the number of nano-resonator layers and the size of nano-resonators. The in-depth understanding of physical origins of enhancements from macro-/nanoscale experiments and calculations as well as easy, inexpensive, and scalable fabrication process would propel the application of this novel coating on high efficient commercial photovoltaics.

References
[1] Dongheon Ha, Chen Gong, Marina S. Leite, and Jeremy N. Munday, “Demonstration of resonance coupling in scalable dielectric microresonator coatings for photovoltaics, ACS Applied Materials and Interfaces, 8, 24536-24542, 2016 (Cover)
[2] Dongheon Ha, Yohan Yoon, and Nikolai B. Zhitenev, “Nanoscale imaging of photocurrent enhancement by resonator array photovoltaic coatings,” Advanced Optical Materials (under review)
[3] Dongheon Ha and Nikolai B. Zhitenev, “Nano- and macroscale characterization of antireflection coatings made of nanosphere arrays,” Journal of Applied Physics (under review)

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