Enhanced light trapping in realistic thin film solar cells using one-dimensional gratings

Christophe Ballif, Franz-Josef Haug, Hans Peter Herzig, Ali Naqavi, Vincent Paeder, Toralf Scharf
Spie-Int Soc Optical Engineering, Po Box 10, Bellingham, Wa 98227-0010 Usa, 2011
Article de conférence

Résumé

Finding the optimal structure to enhance light trapping in thin film silicon solar cells has attracted much attention in the previous decades. However, because of problems in integrating theory and experiment, there are only few comprehensive contributions that provide guidelines for the optimal design of such structures. In this work, a realistic thin film solar cell with almost conformal layers based on a one-dimensional metallic grating back-reflector is investigated through experiment and theory. The external quantum efficiency of the cell is obtained with the aid of both theory and experiment for different angles of incidence and in both polarizations to validate the computational method and to show the impact of guided mode excitation. Different substrate shapes that are compatible with solar cell fabrication are then considered and the effect of geometrical parameters on the short circuit current density of the device is investigated. Calculations show that among the investigated shapes, trinagular gratings with a very sharp slope in one side, so called sawtooth gratings, are the most promising one-dimensional grating for light trapping. Furthermore, the role of material property is discussed specifically in the back-reflector by simulating aluminum and silver back-reflectors. It is shown that the blue response of the solar cells is similar almost regardless of the back-reflector material but their red response is viable to change due to variation in resonant properties of the structure.

Official source

https://infoscience.epfl.ch/record/171320?ln=fr

À propos de ce résultat

Cette page est générée automatiquement et peut contenir des informations qui ne sont pas correctes, complètes, à jour ou pertinentes par rapport à votre recherche. Il en va de même pour toutes les autres pages de ce site. Veillez à vérifier les informations auprès des sources officielles de l'EPFL.

Concepts associés

Chargement

Publications associées

Chargement

Résumé

Official source

https://infoscience.epfl.ch/record/171320?ln=fr

À propos de ce résultat

Concepts associés (20)

Concepts associés

Chargement

Publications associées (17)

Publications associées

Chargement

Absorption enhancement in thin-film silicon solar cells

Ali Naqavi

Thin-film silicon technology is a major candidate to comply with the ever-increasing global energy demand. The small thickness of silicon allows high industrial throughput and low material usage and subsequently opens new avenues to mass-production of low-cost solar cells. This small thickness, together with the indirect bandgap of silicon and thus its relatively weak absorption of long-wavelength light, requires methods to improve absorption. This is often called light trapping. Light absorption can be enhanced in a thin film solar cell by introducing light scattering functionalities, for example textured interfaces. Of common interest is to know: 1) how far the photocurrent and efficiency of a cell can be improved by applying these geometrical changes, 2) which geometries can satisfy the criterion of high photocurrent generation, and 3) how much the incident angle affects the two previous points. This thesis addresses the mentioned questions. Specifically, amorphous silicon (a-Si) cells are the main subject of focus in the thesis, however, the results and conclusions are applicable to other types of solar cells, and similar structures such as light-emitting diodes. The first part of the thesis is devoted to the electromagnetic theory for thin film multilayers and the numerical methods which were used for optical simulations during the PhD work. These methods are described and compared, and some common sources of numerical error in them are identified. To address the first and the third questions, the limits of light absorption enhancement, photocurrent generation and efficiency in thin-film solar cells are studied. As a result, we obtain the limits of absorption enhancement in thin films with periodic texture, over a wide angular and wavelength range. More specifically, first we extend the statistical temporal coupled-mode theory to the case of thin films with wavelength-scale grating couplers. Then, we use this theory to study the effect of the incident angle and the grating period on the absorption enhancement in an idealized thin film with a thickness of 200 nm and refractive index n=4. We show that absorption in a thin-film solar cell depends strongly on the grating period and angle of incidence; therefore, consideration of oblique incident of light in these cells is a necessity. We provide guidelines for the design of thin-film solar cells with periodic texture. Afterwards, we obtain the limit of absorption enhancement for different structures including a full thin-film a-Si solar cell stack for different grating geometries. We show that for thin-films, hexagonal gratings enhance absorption more significantly compared to square gratings. We identify parasitic absorption as a major bottleneck for photocurrent generation. To deal with realistic cases, we investigate the guided modes of thin-film a-Si solar cells by rigorous simulations (the second and the third question). First, we extract the guided modes of the cells and study them in an equivalent planar model. We show that a plasmonic mode exists for very thin buffer layers. Then, we focus on the effect of texture geometry over a broad angular range by comparing the short-circuit current density (Jsc) of the cells for different grating patterns. We find that based on the cell configuration, the optimal texture may be symmetric or asymmetric. We show that TM polarized light produces higher photocurrent at large incident angles regardless of the texture geometry. In the final part of the thesis, we study two novel configurations for thin-film solar cells. First a plasmonic a-Si cell is considered which does not include a buffer layer. We demonstrate that the Jsc of the plasmonic cell is sensitive to the thickness of the n-doped silicon layer and we find that for an n-Si thickness of less than 10 nm, the plasmonic cell outperforms a conventional a-Si cell. Then we simulate an a-Si cell with a periodic array of ZnO nanowires inside the active layer. Our simulations indicate that assuming a periodicity of around 500 nm, the Jsc is highest for a nanowire diameter of about 300 nm.

EPFL2013

Chargement

This thesis investigates amorphous (a-Si:H) and microcrystalline (μc-Si:H) solar cells deposited by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) in the n-i-p or substrate configuration. It focuses on processes that allow the use of non transparent and flexible substrates such as plastic foil with Tg < 180°C like poly-ethylene-naphtalate (PEN). In the first part of the work, we concentrate on the light trapping properties of a variety of device configurations. One original test structure consists of n-i-p solar cells deposited directly on glass covered with low pressure chemical vapour deposition (LP-CVD) ZnO. For this device, silver is deposited below the LP-CVD ZnO or white paint is applied at the back of the substrate as back reflector. This avoids the parasitic plasmonic absorptions in the back reflectors, which are observed for conventional rough metallic back contacts. Furthermore, the size and morphology of the LP-CVD ZnO is varied and the relation between the substrate morphology and the short circuit current density (Jsc) is experimentally explored. As a result, the Jsc can be increased by 23% for a-Si:H and 28% for μc-Si:H solar cells compared to the case of flat substrate and the role of the size and shape can be clearly separated. We also explore the optical behavior of single and multi-junction devices prepared with different back and front contacts. The back contact consists either of a 2D periodic grid with moderate slope, or of LP-CVD ZnO with random pyramids of various sizes. The front contacts are either a 70 nm thick, nominally flat ITO or a rough 2 microns thick LP-CVD ZnO. We observe that, for a-Si:H, the cell performance is critically dependent on the combination of thin flat or thick rough front TCOs and the back contact. Indeed, for a-Si:H, a thick LP-CVD ZnO front contact provides more light trapping on the 2D periodic substrate. The Jsc relatively increases by 7 % with LP-CVD ZnO compared to ITO. Then, we study the influence of the thick and thin TCOs in conjunction with thick absorbers like triple junction or mc-Si:H solar cells. Because of the different nature of the optical systems, thick (> 1 micron) against thin (

University of Neuchâtel2009

Chargement

Angular behavior of the absorption limit in thin film silicon solar cells

Christophe Ballif, Corsin Battaglia, Franz-Josef Haug, Hans Peter Herzig, Ali Naqavi, Vincent Paeder, Toralf Scharf, Karin Söderström

We investigate the angular behavior of the upper bound of absorption provided by the guided modes in thin film solar cells. We show that the 4n2 limit can be potentially exceeded in a wide angular and wavelength range using two-dimensional periodic thin film structures. Two models are used to estimate the absorption enhancement; in the first one, we apply the periodicity condition along the thickness of the thin film structure but in the second one, we consider imperfect confinement of the wave to the device. To extract the guided modes, we use an automatized procedure which is established in this work. Through examples, we show that from the optical point of view, thin film structures have a high potential to be improved by changing their shape. Also, we discuss the nature of different optical resonances which can be potentially used to enhance light trapping in the solar cell. We investigate the two different polarization directions for one-dimensional gratings and we show that the transverse magnetic polarization can provide higher values of absorption enhancement. We also propose a way to reduce the angular dependence of the solar cell efficiency by the appropriate choice of periodic pattern. Finally, to get more practical values for the absorption enhancement, we consider the effect of parasitic loss which can significantly reduce the enhancement factor.

Wiley-Blackwell2014

Chargement

Absorption enhancement in thin-film silicon solar cells

Ali Naqavi

EPFL2013

University of Neuchâtel2009