Plasmonic nanostructures for light trapping in thin-film solar cells

The optical properties of localized surface plasmon resonances (LSPR) sustained by self-assembled silver nanoparticles are of great interest for enhancing light trapping in thin film photovoltaics. First, we report on a systematic investigation of the structural and the optical properties of silver nanostructures fabricated by a solid-state dewetting process on various substrates. Our study allows to identify fabrication conditions in which circular, uniformly spaced nanoparticles are obtainable. The optimized NPs are then integrated into plasmonic back reflector (PBR) structures. Second, we demonstrate a novel procedure, involving a combination of opto-electronic spectroscopic techniques, allowing for the quantification of useful and parasitic absorption in thin photovoltaic absorber deposited on top of the PBR. We achieve a significant broadband useful absorption enhancement of 90% for 0.9 um thick uc-Si:H film and demonstrate that optical losses due to plasmonic scattering are insignificant below 730 nm. Finally, we present a successful implementation of a plasmonic light trapping scheme in a thin film a-Si:H solar cell. The quantum efficiency spectra of the devices show a pronounced broadband enhancement resulting in remarkably high short circuit current densities (Jsc)

Colloidal plasmonic back reflectors for light trapping in solar cells

Novel plasmonic scattering structures are presented, composed of self-assembled arrays of monosized colloidal gold nanospheres, for light trapping in photovoltaics

Broadband photocurrent enhancement in a-Si:H solar cells with plasmonic back reflectors

The authors acknowledge Francesco Ruffino for the AFM measurements. This work was funded by the EU FP7 Marie Curie Action FP7-PEOPLE-2010-ITN through the PROPHET project (Grant No. 264687), the bilateral CNR/AVCR project "Photoresponse of nanostructures for advanced photovoltaic applications", the MIUR project Energetic (Grant no. PON02_00355_3391233) and by the Portuguese Science Foundation (FCT-MEC) through the Strategic Project PEst-C/CTM/LA0025/2013-14 and the research project PTDC/CTM-ENE/2514/2012.Plasmonic light trapping in thin film silicon solar cells is a promising route to achieve high efficiency with reduced volumes of semiconductor material. In this paper, we study the enhancement in the opto-electronic performance of thin a-Si:H solar cells due to the light scattering effects of plasmonic back reflectors (PBRs), composed of self-assembled silver nanoparticles (NPs), incorporated on the cells' rear contact. The optical properties of the PBRs are investigated according to the morphology of the NPs, which can be tuned by the fabrication parameters. By analyzing sets of solar cells built on distinct PBRs we show that the photocurrent enhancement achieved in the a-Si:H light trapping window (600 - 800 nm) stays in linear relation with the PBRs diffuse reflection. The best-performing PBRs allow a pronounced broadband photocurrent enhancement in the cells which is attributed not only to the plasmon-assisted light scattering from the NPs but also to the front surface texture originated from the conformal growth of the cell material over the particles. As a result, remarkably high values of J(sc) and V-oc are achieved in comparison to those previously reported in the literature for the same type of devices. (C)2014 Optical Society of Americapublishersversionpublishe

Self-assembled Plasmonic Nanostructures for Thin Film Photovoltaics

The aim of this thesis is to explore the optical properties of localized surface plasmon resonance sustained by self-assembled metallic nanoparticles (NPs) for the light trapping application in thin lm photovoltaics (PV). Photovoltaics is able provide safe and clean electricity of the future, inparticular, thin lms solar cells have a potential to increase the competitiveness of PV through a substantial reduction of the manufacturing cost. However, an essential step it to develop an e cient, reliable and inexpensive light trapping scheme in order to maximize absorption of the near-infrared radiation in the cell and balance the reduced volume of semiconductor material. Recently there is a growing interest in the application of subwavelength metallic NPs for light trapping as they can scatter light e ciently over a broad wavelength range of the solar spectrum, due to the to the phenomena known as localized surface plasmon (LSP) resonance. A systematic study of the correlation between the structural and the optical properties of self-assembled silver nanostructures fabricated on soda-lime glass by a solid-state dewetting (SSD) process, which consist in thermallyinduced morphology transformation from a thin lm to an array of islands or nanoparticles is reported. It is shown that four distinct types of morphology tend to form in speci c ranges of fabrication parameters, which is quantitatively summarized by a proposed structural-phase diagram and allows to identify the fabrication conditions in which preferable, uniformly spaced and circular NPs are obtainable. The optical properties of the NPs stay in qualitative agreement with the trends predicted by Mie theory, and correlate with the surface coverage (SC) distributions and the mean SC size. As a step forward towards the implementation in thin lm photovoltaics, the NPs are incorporated on the rear side of thin silicon fillm in two distinct arrangements, namely superstrate and substrate. In superstrate configration,The coupling e ciency increases with NPs' average size, decreases with increasing distance between silicon, and is signi cantly smaller for spherical than for hemispherical NPs, which stay in qualitative agreement with theoretical predictions. A novel procedure, involving a combination of phothermal de ection spectroscopy and fourier transform photocurrent spectroscopy, employed for substrate con guration lms allowed for the quanti cation of useful and parasitic absorption. It is demonstrated that the optical losses in the NPs are insigni cant in the 500-730nm wavelength range, beyond which they increase rapidly with increasing illumination wavelength. Furthermore, a broadband enhancement of 89.9% of useful absorption has been achieved. Susequantly, a successful implementation of a plasmonic light trapping scheme implemented in a thin lm a-Si:H solar cell in plasmonic back re ector (PBR) con guration. The optical properties of the PBRs are systematically investigated according to the morphology of the self-assembled silver nanoparticles (NPs), which can be tuned by the fabrication parameters. By analyzing sets of solar cells built on distinct PBRs, it is shown that the photocurrent enhancement achieved in the a-Si:H light trapping window (600-800 nm) stays in linear relation with the PBRs di use re ection. The best-performing PBRs allow a pronounced broadband photocurrent enhancement in the cells which is attributed not only to the plasmon-assisted light scattering from the NPs but also to the front surface texture originated from the conformal growth of the cell material over the particles. As a result, remarkably high values of Jsc and Voc are achieved in comparison to those previously reported in the literature for the same type of devices. Furthermore an attempt on implementation of the plasmonic light trapping in the industrial a-Si/ c-Si double junction solar cells is reported

Plasmonic nanostructures for light trapping in photovoltaic

Metallic nanoparticles (NPs), sustaining localized surface plasmon resonances, are currently of great interest for enhancing light trapping in thin film solar cells. To be directly applicable in the photovoltaic industry, the NPs fabrication needs to be simple, reliable, low-cost and scalable. As such, self-assembly processes are most commonly used, and Ag is the preferred material, due to its high radiative efficiency and low imaginary permittivity⁠. After exploring the correlation between structural and optical properties of Ag NPs fabricated by solid- state dewetting process on various substrates, we identified the fabrication conditions in which desirable NPs are obtained, but we also evidenced unexpectedly high parasitic absorption, main obstacle for photovoltaics. Thus, we introduced a novel spectroscopic method which enables the quantification of absorption enhancement and parasitic losses and demonstrated that the optical losses in the NPs are insignificant in the wavelength range of interest, while the NPs provides up to 90% useful absorption enhancement, which can be attributed to both the random front surface texture, originated from the conformal growth of the material over the NPs and to the scattering of light by the plasmonic NPs. Our optimized plasmon-enhanced thin film solar cell shows a pronounced broadband enhancement of external quantum efficiency and remarkably high short circuit current density in comparison to those reported in the literatur

Colloidal Self-assembled Nanosphere Arrays for Plasmon-enhanced Light Trapping in Thin Film Silicon Solar Cells☆

AbstractTo realize high-efficiency thin-film silicon solar cells it is crucial to develop light-trapping methods that can increase absorption of the near- bandgap light in the silicon material. That can be achieved using the far-field scattering properties of metal nanoparticles (MNP) sustaining surface plasmons. The MNPs should be inserted in the back of the cell, embedded in the transparent conductive oxide (TCO) layer which separates the rear mirror from the silicon layers. In this way, a plasmonic back reflector (PBR) is constructed that can redirect light at angles away from the incidence direction and thereby increase its path length in the cell material.In this work, a novel technique is presented to fabricate PBRs (composed of Ag mirror/TCO/MNPs/TCO) containing colloidal gold MNPs patterned with a self-assembly wet-coating method. The method allows the construction of long-range ordered arrays of MNPs with monodisperse size and shape using fast, scalable, low-cost and low-temperature (<120°C) procedures.Colloidal MNPs are synthesized with spherical shapes, so their scattering properties are analytically modeled with Mie theory. Such formalism allowed the computation of the preferential MNP sizes that provide the best scattering performance for light-trapping in amorphous and microcrystalline thin-film silicon solar cells

Plasmonic and diffractive nanostructures for light trapping—an experimental comparison

Metal nanoparticles and diffractive nanostructures are widely studied for enhancing light trapping efficiency in thin-film solar cells. Both have achieved high performance enhancements, but there are very few direct comparisons between the two. Also, it is difficult to accurately determine the parasitic absorption of metal nanoparticles. Here, we assess the light trapping efficiencies of both approaches in an identical absorber configuration. We use a 240 nm thick amorphous silicon slab as the absorber layer and either a quasi-random supercell diffractive nanostructure or a layer of self-assembled metal nanoparticles for light trapping. Both the plasmonic and diffractive structures strongly enhance the absorption in the red/nearinfrared regime. At longer wavelengths, however, parasitic absorption becomes evident in the metal nanoparticles, which reduces the overall performance of the plasmonic approach. We have formulated a simple analytical model to assess the parasitic absorption and effective reflectivity of a plasmonic reflector and to demonstrate good agreement with the experimental data

Direct measurement of optical losses in plasmon-enhanced thin silicon films (Conference Presentation)

Plasmon-enhanced absorption, often considered as a promising solution for efficient light trapping in thin film silicon solar cells, suffers from pronounced optical losses i.e. parasitic absorption, which do not contribute to the obtainable photocurrent. Direct measurements of such losses are therefore essential to optimize the design of plasmonic nanostructures and supporting layers. Importantly, contributions of useful and parasitic absorption cannot be measured separately with commonly used optical spectrophotometry. In this study we apply a novel strategy consisting in a combination of photocurrent and photothermal spectroscopic techniques to experimentally quantify the trade-off between useful and parasitic absorption of light in thin hydrogenated microcrystalline silicon (μc-Si:H) films incorporating self-assembled silver nanoparticle arrays located at their rear side. The highly sensitive photothermal technique accounts for all absorption processes that result in a generation of heat i.e. total absorption while the photocurrent spectroscopy accounts only for the photons absorbed in the μc-Si:H layer which generate photocarriers i.e. useful absorption [1]. We demonstrate that for 0.9 μm thick μc-Si:H film the optical losses resulting from the plasmonic light trapping are insignificant below 730 nm, above which they increase rapidly with increasing illumination wavelength. For the films deposited on nanoparticle arrays coupled with a flat silver mirror (plasmonic back reflector), we achieved a significant broadband enhancement of the useful absorption resulting from both surface texturing and plasmonic scattering, and achieving 91% of the theoretical Lambertian limit of absorption