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C8.04 - Light Coupling into the Whispering Gallery Modes of a Fiber Array Organic Solar Cell to Achieve Mechanically Fixed Sun Tracking 
April 23, 2014   11:30am - 11:45am

In organic solar cells (OSCs) where the thickness of the active material is, typically, 100 nm, several approaches have been implemented to increase light absorption by increasing the effective light path. Using the light scattered by plasmonic nanoparticles an enhancement in the light absorption was recently demonstrated and single junction OSCs with a power conversion efficiency (PCE) above 8% were obtained [1]. More recently, a disordered multilayer architecture with properties similar to those from one-dimensional photonic crystals has been used to enhance near infrared and near UV light absorption in semi-transparent OSCs which exhibited a PCE above 5% [2]. In other thin film cells, whispering gallery (WG) resonant modes from a low quality-factor spherical nanoshell structure were used to facilitate the coupling of light and substantially enhance the light path in the active material [3]In the current work, we propose the use of WG coupling but in a drastically different configuration based on implementing an organic cell on the back side of an array of parallel fibers. Such fibers serve as the substrate for an OSC device. The different layers of the cell architecture are on the back side of the array relative to the incoming sunlight incident on the front side of the array. Light is coupled into the low quality factor WG resonant modes of the fibers. Such coupling becomes more effective as the angle for the incident light relative to the normal of the fiber array increases up to 55 deg. In any case, light absorption at the active layer is at all angles larger for the fiber array organic cell (FAOC) when compared to the standard planar configuration. We show that the EQE for the FAOC is largely improved at almost the entire absorption wavelength range for the active material while, at the same time, the short circuit current can be increased up to 30% relative to the planar cell.In conclusion, we demonstrated that an array of fibers can be used as substrate to significantly improve light harvesting capacity for OSC when compared to the standard planar device. The incident light is coupled to some low-Q WG modes of the fibers. Such coupling is effective for a broad wavelength range as required when converting light into electricity from wideband sources. In the event that the longitudinal axis of the FAOC would be oriented in the meridian direction, the angular increase in PCE would provide, over the course of one year, an energy harvesting capacity 17% times larger than the one from a mechanically fixed cell. Such an increase in energy harvesting capacity is very close to the one obtained from one-axis sun tracking systems.[1] L. Lu, Z. Luo, T. Xu, L. Yu, Nano Lett. 2013, 13, 59.[2] R. Betancur, P. Romero-Gomez, A. Martinez-Otero, X. Elias, M. Maymó, J. Martorell, Nature Photonics, 2013 [online DOI: 10.1038/nphoton.2013.276] [3] Y. Yao, J. Yao, V. K. Narasimhan, Z. Ruan, C. Xie, S. Fan, Y. Cui, Nature Communications, 2012, 3, 664

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