Revisited design optimization of metallic gratings for plasmonic light- trapping enhancement in thin organic solar cells Phuc Toan Dang a,b , Truong Khang Nguyen a,b , Khai Q. Le a,b,n a Division of Computational Physics, Institute for Computational Science, Ton Duc Thang University, Ho Chi Minh City, Vietnam b Faculty of Electrical and Electronics Engineering, Ton Duc Thang University, Ho Chi Minh City, Vietnam article info Article history: Received 31 May 2016 Received in revised form 11 July 2016 Accepted 29 July 2016 Keywords: Plasmonic gratings Solar cells Plasmonic absorption enhancement abstract We revisit previous studies of metallic gratings for optical absorption enhancement in an organic solar cell with a thin active layer. Our device geometry is designed for a real solar cell with full of functional layers. Various metallic gratings calibrated to generate periodic scatterers and low reflectors for broad- band light account for increases in short circuit current density of up to 47% when compared to its flat counterpart. We found that the tapered grating has greater performance than the regular rectangular grating for transverse magnetic (TM) polarization while the latter shows better performance for trans- verse electric (TE) polarization. The overall metallic grating induced absorption enhancement was found at all angles of incidence. The best configuration was realized for the tapered grating-based solar cell at 25° of inclination. & 2016 Elsevier B.V. All rights reserved. 1. Introduction Photovoltaic (PV) research is attempting to low-cost and high- manufacturability solar cells. Thin-film organic solar cell (OSC) is a promising candidate for such a route because of its cheaper pro- duction and higher manufacturing volume [1]. The primary ob- stacle to reach this route is the low efficiency of current OSCs. Significant improvement in the cell efficiency is required to make them competitive with grid power [2]. One of the most challenges in achieving higher efficiencies is absorbing enough light in the thin active layer and the short diffusion length of carriers. OSCs’ thicknesses are restricted by physical limitation due to the need to utilize electrical characteristics of the material. If the active layer becomes too thick, it will cause significant reduction in exciton collection from recombination within the active layer [3,4]. Ab- sorbed photons propagate through the active layer before enough may be converted to excitons, because of the thickness restraint. In addition, too thick active layer beyond carrier diffusion length will result in current loss due to recombination. Thus, the tradeoff between enough absorption and minimizing recombination is the key challenge in designing OSCs. Recently, the utility of metallic nanostructures within various forms such as nanoparticles, nanogratings, and nanocavities in solar cells has been demonstrated to provide optical field en- hancement, and improvement of the optical absorption [5–12]. This plasmonic light-trapping is able to absorb sunlight over a moderately broad bandwidth of operation owing to the excitation of tunable plasmonic modes in these metallic nanostructures. Properly placing metallic nanostructures inside the active layer can strongly enhance absorption via near-field enhancement ef- fects [5]. However, near-field absorbed sunlight may not con- tribute in the generation of photocurrent due to the metal losses and the induced quenching effect of the excited states at the metal/active layer interface [13]. Isolating the plasmonic nanos- tructures with an inert coating can reduce the electric losses, but decreases the near-field absorption. These detrimental effects are among the reasons why plasmonic light-trapping results in the limited improvement of the photocurrent generation in OSCs as reported in literatures [14,15]. The same scenarios happen with plasmonic gratings designed at the OSC's back reflector which is theoretically predicted to improve the light absorption [16], but experimentally reported on the less power conversion efficiency [17]. The absorbed light did not contribute to the generation of photocurrent and dissipated inside the metal heating up the electrode. Alternative ways to enable the light-trapping enhancement associated with the photocurrent generation improvement is to put the metallic nanoparticles/nanogratings on top of the cells [18–20]. These plasmonic nanostructures can act as periodic op- tical antennas for light and store energy in the localized surface plasmon resonance (LSPR). On the other hand, combined plas- monic gratings on the top and bottom of the polymer active layer Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/optcom Optics Communications http://dx.doi.org/10.1016/j.optcom.2016.07.080 0030-4018/& 2016 Elsevier B.V. All rights reserved. n Corresponding author at: Division of Computational Physics, Institute for Computational Science, Ton Duc Thang University, Ho Chi Minh City, Vietnam and Faculty of Electrical and Electronics Engineering, Ton Duc Thang University, Ho Chi Minh City, Vietnam. E-mail address: lequangkhai@tdt.edu.vn (K.Q. Le). Optics Communications 382 (2017) 241–245