Search

We fabricated amorphous silicon n-i-p solar cells with two types of nanopatterned back reflectors using stencil lithography. One reflector type has a plasmonic grating that is embedded in the ZnO layer; the other one has a metallic grating patterned on top of the Ag layer. From comparing the short-circuit current densities of the two device types, we conclude that light trapping through grating coupling is more efficient than coupling of light through the excitation of localized surface plasmons. The back reflectors were patterned with dot arrays by evaporation of Ag through millimeter-size stencil membranes. The stencils themselves were patterned by wafer-scale nanosphere lithography. The dot arrays have a periodicity of 428 nm and efficiently scatter light in the near-infrared wavelength range. Both back reflectors types lead to the same morphology for the silicon films. This allows us a fair comparison of the two light coupling mechanisms. We found a 14% and 19% short-circuit current density enhancement for the plasmonic and for the metallic grating, respectively. The external quantum efficiency gains between 550 and 650 nm show similar guided modes resonances for both device types, but the excitation is stronger for the device with the metallic grating.

High reflectivity is essential when a metal is used as back contact and reflector in thin film silicon solar cells. We show that thermal annealing at 150 °C improves the reflectivity of silver films deposited by sputtering at room temperature on nanotextured substrates. The annealing provokes two interlinked effects: rearrangement of the silver layer with a modification of its morphology and an increase of up to 42 in the grain size of the polycrystalline film for the preferential orientation as measured by X ray diffraction. The main consequence of these two mechanisms is a large increase in the reflectivity of silver when measured in air. This reflectivity increase is also noticeable in devices: amorphous silicon thin film solar cells grown on annealed silver films yield higher internal and external quantum efficiencies compared to cells grown on as deposited silver. The morphology modification smoothes down the substrate which is revealed by a clear increase of the open circuit voltage and fill factor of the cells grown on top. An amorphous silicon cell with a 200 nm nominally thick i layer fabricated on a flexible plastic substrate yielded an initial efficiency close to 10 with 15.9 mA/cm 2 of short circuit current using highly reflective annealed textured silver. We also propose for industrial purpose the sputtering of thin silver layer (120 nm) under moderate substrate temperature (~150 °C) to increase the layer reflectivity which avoids lengthening of the back reflector fabrication. © 2011 Elsevier B.V. All rights reserved.

Silver can be used as the back contact and reflector in thin film silicon solar cells. When deposited on textured substrates, silver films often exhibit reduced reflectance due to absorption losses by the excitation of surface plasmon resonances. We show that thermal annealing of the silver back reflector increases its reflectance drastically. The process is performed at low temperature (150°C) to allow the use of plastic sheets such as polyethylene naphthalate and increases the efficiency of single junction amorphous solar cells dramatically. We present the best result obtained on a flexible substrate: a cell with 9.9% initial efficiency and 15.82 mA/cm2 in short circuit current is realized in n-i-p configuration.

UV imprinting is an interesting, low cost technique to produce large area thin film solar cells incorporating nanometric textures. Here, we review and present new results confirming that replicas of the most common textures used in photovoltaics can be obtained by UV imprinting with an excellent fidelity. The use of these replicas as substrates for amorphous and micromorph thin film silicon solar cells is also shown, together with a comparison with devices obtained on the original textures.

Several thin-film solar cell technologies require light-trapping schemes that are predominantly based on depositing the solar cells on rough surfaces. While this approach efficiently increases the density of photo-generated carriers, open-circuit voltage and fill factor generally decrease. Substrates that decouple the growth interface from the light-scattering interface were previously proposed as a solution to this dilemma, and proof-of-concepts were demonstrated in thin film-silicon solar cells. In this contribution, we review as an introduction the problematic of rough versus smooth interface for n-i-p single-junction lc-Si:H cells. Then, the benefits of the newly developed substrate that decouples the growth and scattering interfaces are investigated in n-i-p triple-junction a-Si:H/lc-Si:H/lc-Si:H solar cells for the first time. Conversion efficiencies of 13.7% (initial) and 12.5% (stabilized) are obtained, which are among the highest ever reported for such devices.

We investigate the influence of the oxygen content in boron-doped nanocrystalline silicon oxide films (p-nc-SiOx) and introduce this material as window layer in n-i-p solar cells. The dependence of both, optical and electrical properties on the oxygen content is consistent with a bi-phase model which describes the p-nc-SiOx material as a mixture of an oxygen-rich (O-rich) phase and a silicon-rich (Si-rich) phase. We observe that increasing the oxygen content enhances the optical gap E-04 while deteriorating the activation energy and the planar conductivity. These trends are ascribed to a higher volume fraction of the O-rich phase. Incorporated into n-i-p a-Si: H cells, p-nc-SiOx layers with moderate oxygen content yield open circuit voltage (V-oc) up to 945mV, which corresponds to a relative gain of 11% compared to an oxygen-free p-layer. As a similar gain is obtained on planar and on textured substrates, we attribute the increase in V-oc to the higher work function of the p-nc-SiOx layer made possible by its wider band gap. These results are attained without changing the dilution ratio of the 250 nm thick intrinsic layer. We also observe an enhancement of 0.6mA cm(-2) in short circuit current density in the short wavelengths due to the higher transparency of the p-nc-SiOx layer. Finally, an initial efficiency of 9.9% for a single junction 250 nm a-Si:H n-i-p solar cell on plastic foil is achieved with the optimization of the p layer thickness, the doping ratio of the front transparent conductive oxide, and the optical properties of the back reflector. (C) 2011 American Institute of Physics. [doi:10.1063/1.3669389]

We study the plasmonic and the non-plasmonic regimes of operation of a thin-film amorphous-silicon solar cell. By rigorous calculation, we discuss the impact of the cell geometry on the nature of its optical resonances.

Article history:Received 31 July 2011Received in revised form 23 January 2012Available online 8 March 2012Keywords:Thin film silicon solar cell;Off-stoichiometric silicon oxide;Open-circuit voltage;Built-in voltage The introduction of a nc-SiOx:H material as window layer in single junction a-Si:H n-i-p solar cell leads to aV oc enhancement of 80 mV compared to a μc-Si:H p-layer. According to numerical modeling of the V oc , boththe higher work function p-layer and the conduction band offset (CBO) at the i/p interface match well withthe experimental V oc increase with the oxygen content. Using the differential temperature method, the built-in voltage (V bi ) of the cells with the two different p-layers is measured to be similar, agreeing well with theCBO model. Thus we attribute the improvement of the V oc to the reduction of recombination at the i/p inter-face, as a consequence of the CBO in this region.© 2012 Elsevier B.V. All rights reserved. 1. IntroductionIn amorphous silicon solar cells, the open-circuit voltage (V

In this paper, we investigate tandem amorphous/microcrystalline silicon solar cells with asymmetric intermediate reflectors grown in the n–i–p substrate configuration. We compare different types of substrates with respect to their light-trapping properties as well as their influence on the growth of single-junction microcrystalline cells. Our most promising back reflector combines a textured zinc oxide film grown by low-pressure chemical vapor deposition, a silver film for reflection, and a zinc oxide buffer layer. Grown on this substrate, microcrystalline cells exhibit excellent response in the infrared while keeping high open-circuit voltage and fill factor, leading to efficiencies of up to 10.0%. After optimizing the morphology of the asymmetric intermediate reflector, we achieve an n–i–p micromorph solar cell stabilized efficiency of 11.6%, using 270 nm and 1.7 mm of silicon for the absorber layer of the amorphous top cell and the microcrystalline bottom cell, respectively. Using this original device architecture, we reach efficiencies close to those of state-of-the-art n–i–p and p–i–n micromorph devices, demonstrating a promising route to deposit high-efficiency thin-film silicon solar cells on opaque substrates.

An amorphous silicon solar cell on a periodic nanocone back reflector with a high 9.7% initial conversion efficiency is presented. The optimized back-reflector morphology provides powerful light trapping and enables excellent electrical cell performance. Up-scaling to industrial production of large-area modules should be possible using nanoimprint lithography.

We present an experimental study of light scattering back reflectors in thin film silicon solar cells. The results suggest that novel strategies based on localized plasmon resonances in silver nano-particles cannot rival conventional reflector geometries.

This paper focuses on our latest progress in n-i-p thin-micromorph solar-cell fabrication using textured back reflectors and asymmetric intermediate reflectors, both deposited by low-pressure chemical vapor deposition of zinc oxide. We then present microcrystalline bottom cells with high crystallinity, which yield excellent long wavelength response for relatively thin absorber thickness. In a 1.5-µm-thick µc-Si:H single-junction n-i-p solar cell, we thus obtain a short-circuit current density of 25.9 mA·cm−2, resulting in an initial cell efficiency of 9.1%. Subsequently, the roughness of the intermediate reflector layer is adapted for the growth of high-performance amorphous silicon (a-Si:H) top cells. Combining bottom cells with high current, an optimal intermediate reflector morphology and a 0.22-µm-thick a-Si:H top cell, we reach high initial open-circuit voltages of 1.45 V, and we obtain a stabilized cell with an efficiency of 11.1%, which is our best stable efficiency for n-i-p solar cells.

We study n i p amorphous silicon solar cells with light scattering nanoparticles in the back reflector. In one configuration the particles are fully embedded in the zinc oxide buffer layer; In a second configuration the particles are placed between the buffer layer and the flat back electrode. We use stencil lithography to produce the same periodic arrangement of the particles and we use the same solar cell structure on top thus establishing a fair comparison between a novel plasmonic concept and its more traditional counterpart. Both approaches show strong resonances around 700 nm in the external quantum efficiency the position and intensity of which vary strongly with the nanoparticle shape. Moreover disagreement between simulations and our experimental results suggests that the dielectric data of bulk silver do not correctly represent the reality. A better fit is obtained by introducing a porous interfacial layer between the silver and zinc oxide. Without the interfacial layer e.g. by improved processing of the nanoparticles our simulations show that the nanoparticles concept could outperform traditional back reflectors. © 2013 Optical Society of America.