Optimization of Light-Trapping in Thin-Film Solar Cells Enhanced with Plasmonic Nanoparticles Wael Itani
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Optimization of Light-Trapping in Thin-Film Solar Cells Enhanced with Plasmonic Nanoparticles Wael Itani To cite this version: Wael Itani. Optimization of Light-Trapping in Thin-Film Solar Cells Enhanced with Plasmonic Nanoparticles. 2021. hal-03129380v3 HAL Id: hal-03129380 https://hal.archives-ouvertes.fr/hal-03129380v3 Preprint submitted on 10 Mar 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Optimization of Light-Trapping in Thin-Film Solar Cells Enhanced with Plasmonic Nanoparticles Wael Itani Department of Electrical & Computer Engineering Technical University of Munich Munich, Germany [email protected] electronics-grade, pure, crystalline silicon [8], is Abstract—In this paper, we motivate the need for used to ensure long99 lifetime.999% of carriers for their collection. photovoltaic technology and its improvement before we overview the contributions of Akimov and Koh’s 2010 paper to Solar technology could be broadly classified by material plasmonic thin film cells. The paper results are complemented of active region [9]. Traditional, crystalline Silicon, solar cells, by the necessary theoretical description and formulation. We which current dominate the market could only be made so then conclude by recalling the limitations of the technology thin. Although the diminishing size improves the open circuit under review, and providing an outlook. The empirical nature voltage, it decreases the absorbance and corresponding of Akimov and Koh’s paper has led to focus on providing a photocurrent [9]. This puts a hard cap on their development, thorough theoretical background for readers, with a rich as nearly half of their production costs come from the choice appendix for comprehensiveness. of material [9]. Dye sensitized cells on the other hand, while they offer a cheap alternative, suffer from poor electrical Keywords—plasmonics, nanoparticles, thin film, solar cells, properties, and, thus, overall poor effectiveness. photovoltaics II. SUMMARY I. INTRODUCTION Akimov and Koh [1] study forward scattering spherical In the past decade, Akimov and Koh presented a seminal metallic nanoparticles embedded in the top transparent series of papers which have since taken plasmonic thin-film conductive layer ( ), in a square lattice arrangement, for solar cells mainstream [1]–[6], with thin films capturing 20푛푚 20% an amorphous silicon cell ( ) with the aim of achieving of the photovoltaic market share less than five years later [7]. maximal broadband light240-trapping푛푚 for optimized overall In this paper, we revisit their work which investigated the power absorption, and indirectly photoelectric carrier optimal material for the plasmonic nanoparticles, and put it in generation. They optimize the material, size (radius) and perspective with recent developments [1]. After motivating surface coverage for scattering in the dipolar and multipolar the need for plasmonic-enhanced thin-film solar cells, we regime in comparison with an ideal, non-dissipative non- review their underlying theory, its applications, and the dispersive (real negative frequency-independent permittivity), challenges that remain before concluding with an outlook. metal of and a control cell. The problem is modelled in A. The Landscape of the Energy Sector COMSOL with 3D Maxwell equations for a normal incident solar profile plane wave with periodic and perfectly As the global energy consumption is set to nearly double matched퐼퐴푀1.5 boundary conditions, and refractive indices in 2050 from its current figure of 17 TW in 2020 [8], and the interpolated from SOPRA. Results show that a perfect bells continues to toll warning us of climate change happening conductor, with infinitely negative permittivity is needed to as you read this paper, it is time to tap our alternative energy improve scattering, and to avoid parasitic absorption by sources. Solar energy an abundant clean source able to supply particles due to surface plasmon resonance. This corresponds ten thousand times our current needs with 176,000 TW to an infinitely high surface plasmon resonance frequency. radiation striking earth [8], [9]. That is covering the Venzuela Two maximas are obtained for the surface coverage and alone with 10% efficient panels would meet the globe’s radius, corresponding to dipolar and multipolar regimes. The demands. However, this goes against a fundamental former is preferred, whereas the latter reduces parasitic advantage of solar energy as a distributed resource, for which absorption in lower modes of resonance. Amongst the metals the geographic location, landuse, the weather, the season, and considered, aluminum comes closes to the perfect conductor the climate need to be considered [8] given the lifetime of with a surface plasmon frequency in the ultraviolet range, solar projects typically spanning several decades. In reality, away from the optimal frequency determined for solar energy “reserves”, are less than a third of the identified 2푒푉 resources, at 50,000 TW [8] which is still enough to meet a maximum absorption of 푊 , as opposed to the visible 430 2 hundred times our current demand. range for other metals. With푒푉 푚frequency being geometry dependent, aluminum, of permittivity, achieves B. The Solar Challenge optimal enhancement of −53 with푒푉 radius and Solar energy is already capturing an increasing share of the surface coverage, compared23% to 21푛푚 , 59% energy mix around the globe, with decreasing prices and enhancement, with radius−10~ and− 15 푒푣 5 − ,12% increasing competitiveness. However, to compete with fossil surface coverage for 23silver,− 30 gol푛푚d and copper in20 dipolar− 30% regime.3% fuels, and motivate the uprooting of existing power infrastructure for more renewable alternatives, the prices need III. THEORETICAL DESCRIPTION to be brought further down. While the material contributes to nearly of the cost of a crystalline silicon cell, a A. Principles of Photovoltaics significant50% portion of the bulk material does not contribute to Solar or photovoltaic cells convert light energy into efficient absorption [8]. On the contrary, it is required for electricity by the photoelectric effect, whereby a valence mechanical support as the silicon cells need to be cut with a electron is excited into the conduction band if the incoming minimum thickness of from wafers [8]. Moreover, photon provides it with enough energy to overcome the 200휇푚 bandgap. The process produces an electron-hole pair which Nanotechnology for Energy Systems ©2021 Wael Itani must be separated and collected at opposite electrodes. In a p- The hydrogenation of a-Si serves a dual purpose. For one, n junction, this separation occurs with diffusion, whereas a p- it improves electron mobility, and reduces recombination i-n junction allows an electric field to drive the drift of the losses by strengthening the bonds of the material [8]. Second, carriers. As is evident, there are many losses that could occur it resolves the Staebler-Wronski effects of degradation with through the process, from the photon not being absorbed due exposure to sunlight [8], by stabilizing the structure at shorter to very short or long wavelength, basically transmitted, or due exposure times, and, thus, higher performance. This is to reflection, to the charge carriers recombining before otherwise resolved by restorative thermal annealing, electric collection. The second figure in [10] shows the relative stressing [26] or introducing nanocrystalline Silicon in the significance of the losses. The losses are typically amorphous matrix. parametrized by the two quantum efficiencies, the external and internal ones, described in Appendix A. Thin film cells offer the flexibility of exploring new materials and architecture [27] to make the best use of B. Thin Film Solar Cells plasmonics, as the overall bulk material, and, thus, thickness Thin film cells, on the other hand, offer a middle ground is reduced. This advantage is typically characterized by the between cost of cell and performance. They benefit from mass-power ratio [9]: plasmonics to make the latter more competitive, especially improving light absorbance which deteriorates in the range 푚 푡휌 () near their bandgap, due the mismatch between optical and 푃 = 퐼퐴휂 carrier diffusion lengths. While the paper They are typically a where refers to the module, the power it provides, , or film, compared to typical of 푚 푃 푡 1 2 휇푚 200 − 300 휇푚 , , , its thickness, density, area, and efficiency crystalline silicon cells [9], deposited on a cheap substrate, respectively,휌 퐴 휂 and the incident solar flux. The decreased including but not limited to glass or plastic [16]. With thickness directly 퐼contributes to improving collection [9]. enhanced properties, they could be brought down all the way Ideally, the thickness should be comparable to minority carrier to the order of . The reduced thickness in turn 100 푛푚 [8] diffusion length [9]. With the enhancement provided by the reduced recombination losses due to abridged carrier diffusion inclusion of plasmonic nanoparticles for absorption, active