Fanny Meillaud | Ecole Polytechnique Federale de Lausanne (original) (raw)

Papers by Fanny Meillaud

IEEE Journal of Photovoltaics, 2012

Optimized transparent conductive oxide front electrodes are vital to further increase the efficie... more Optimized transparent conductive oxide front electrodes are vital to further increase the efficiency of thin-film silicon solar devices. We report details on the fabrication of multiscale textured zinc oxide substrates and their implementation in amorphous silicon/microcrystalline silicon tandem (micromorph) devices. Such substrates allow separate optimization of light trapping in the top and bottom cells, and efficient decoupling of transparency and conduction. We show in particular the need for sharp, nanoscale texturing for antireflection and light trapping in the top cell. We also show that smooth, micrometer-scale texturing can efficiently improve large-wavelength light management without degrading the quality of the silicon material grown on the substrate. By combining the appropriate morphologies, high currents can be reached in both the top and bottom subcells, while conserving the optimal electrical properties of the solar cells.

IEEE Journal of Photovoltaics, 2012

ZnO bilayer films were deposited by low-pressure chemical vapor deposition in a single process st... more ZnO bilayer films were deposited by low-pressure chemical vapor deposition in a single process step by controlling the differential doping of the nucleation and bulk parts of the layers. The resulting 2-μm-thick films are characterized by low free-carrier absorption and electron mobility over 40 cm2/Vs. They, therefore, combine high transparency in the infrared region and moderate sheet resistance that can be lowered below 20 Ω/sq. These properties make ZnO bilayers ideal candidates as electrodes for the development of micromorph thin-film solar cells with enhanced photogenerated current. The potential of such bilayer front electrodes for a further power improvement and cost reduction of industrial micromorph tandem modules is currently investigated at Oerlikon Solar. The first experiments already show a promising gain in the bottom μc-Si:H cell photogenerated current compared with the current generated with modules deposited on standard uniformly doped ZnO single-layer front contacts.

Solar Energy, 2005

Emergy (spelled with an m) is the energy of one kind, usually solar energy, which is required to ... more Emergy (spelled with an m) is the energy of one kind, usually solar energy, which is required to make a service or product. The yearly emergy consumption/production of a building is evaluated considering the Solar Energy Laboratory (LESO) building on the campus of the Swiss Federal Institute of Technology of Lausanne (Switzerland). This experimental building was constructed according to special environmental considerations, such as important the use of passive gains (heat emitted from solar radiations, electric appliances and building users). It is therefore characterized by a very low energy consumption, equal to 232 MJ/m 2 year. The LESO building is occupied by faculty and students. Undergraduate and graduate students as well as faculty represent information inputs to the system with their emergy accounting for 94.6% of the emergy inputs to the building, equal to 3.3E18 sej/year (solar emjoules per year).

There is general agreement that the future production of electric energy has to be renewable and ... more There is general agreement that the future production of electric energy has to be renewable and sustainable in the long term. Photovoltaic (PV) is booming with more than 7GW produced in 2008 and will therefore play an important role in the future electricity supply mix. Currently, crystalline silicon (c-Si) dominates the market with a share of about 90%. Reducing the cost per watt peak and energy pay back time of PV was the major concern of the last decade and remains the main challenge today. For that, thin film silicon solar cells has a strong potential because it allies the strength of c-Si (i.e. durability, abundancy, non toxicity) together with reduced material usage, lower temperature processes and monolithic interconnection. One of the technological key points is the transparent conductive oxide (TCO) used for front contact, barrier layer or intermediate reflector. In this paper, we report on the versatility of ZnO grown by low pressure chemical vapor deposition (ZnO LP-CVD) and its application in thin film silicon solar cells. In particular, we focus on the transparency, the morphology of the textured surface and its effects on the light in-coupling for micromorph tandem cells in both the substrate (n-i-p) and superstrate (p-i-n) configurations. The stabilized efficiencies achieved in Neuchâtel are 11.2% and 9.8% for p-i-n (without ARC) and n-i-p (plastic substrate), respectively.

Hydrogenated microcrystalline silicon (μc-Si:H) has become a material of increasing interest thes... more Hydrogenated microcrystalline silicon (μc-Si:H) has become a material of increasing interest these last years mainly for its use in cost-effective production of tandem and triple junction thin film silicon based solar cells. Lately, the use of novel doped silicon oxide (SiOx) layers were shown to be very promising for increasing the solar cells efficiency [1,2]. We present in this study a detailed analysis on the possible reasons behind this significant increase of electrical performances. Complete solar cells were developed in an industrial type reactor with their intrinsic layer (i-layer) deposited at a high growth rate of 1 nm/s by VHF-PECVD. Different i-layer material quality and substrate roughness were systematically evaluated during this investigation. We demonstrate conversion efficiency increase of up to 29% when both these p-type and n-type doped SiOx layers are used instead of the regular microcrystalline ones, while keeping the bulk of intrinsic material unchanged and efficiencies over 8% are achieved for a wider range of plasma parameters and substrate roughness. Extensive material analysis is presented hereafter to understand the physical origins for the improvements observed. XRD, Raman and FTIR spectroscopy, intrinsic stress, FTPS and SIMS measurements were done along with SEM images of the solar cells. It is found that devices with very different efficiencies can lead to similar FTIR and FTPS spectrum. We show that the integration of doped SiOx layers reduces to some extent the influence of porous regions, i.e. microcracks, on the electrical properties of the solar cells, and the possible physical reasons for this improvement are discussed. The development of these extrinsic defects, not detected by FTPS and FTIR, is becoming especially detrimental on highly textured substrates, required for increased light trapping. This highlights the fundamental nature difference of intrinsic and extrinsic defects which c- n both drive the cells performances.

Progress in Photovoltaics, 2010

Microcrystalline silicon (µc-Si:H) cells can reach efficiencies up to typically 10% and are usual... more Microcrystalline silicon (µc-Si:H) cells can reach efficiencies up to typically 10% and are usually incorporated in tandem micromorph devices. When cells are grown on rough substrates, “cracks” can appear in the µc-Si:H layers. Previous works have demonstrated that these cracks have mainly detrimental effects on the fill factor and open-circuit voltage, and act as bad diodes with a high reverse saturation current. In this paper, we clarify the nature of the cracks, their role in post-oxidation processes, and indicate how their density can be reduced. Regular secondary ion mass spectrometry (SIMS) and local nano-SIMS measurements show that these cracks are prone to local post-oxidation and lead to apparent high oxygen content in the layer. Usually the number of cracks can be decreased with an appropriate modification of the substrate surface morphology, but then, the required light scattering effect is reduced due to a lower roughness. This study presents an alternative/complementary way to decrease the crack density by increasing the substrate temperature during deposition. These results, also obtained when performing numerical simulation of the growth process, are attributed to the enhanced surface diffusion of the adatoms at higher deposition temperature. We evaluate the cracks density by introducing a fast method to count cracks with good statistics over approximately 4000 µm of sample cross-section. This method is proven to be useful to quickly visualize the impact of substrate morphology on the density of cracks in microcrystalline and in micromorph devices, which is an important issue in the manufacturing process of modules. Copyright © 2010 John Wiley & Sons, Ltd.

Solar Energy Materials and Solar Cells, 2011

High conversion efficiency for (amorphous/microcrystalline) ''micromorph'' tandem solar cells req... more High conversion efficiency for (amorphous/microcrystalline) ''micromorph'' tandem solar cells requires both a dedicated light management, to keep the absorber layers as thin as possible, and optimized growth conditions of the microcrystalline silicon (mc-Si:H) material. Efficient light trapping is achieved here by use of textured front and back contacts as well as by implementing an intermediate reflecting layer (IRL) between the individual cells of the tandem. This paper discusses the latest developments of IRLs at IMT Neuchâtel: SiO x based for micromorphs on glass and ZnO based IRLs for micromorphs on flexible substrates were successfully incorporated in micromorph tandem cells leading to high, matched, current above 13.8 mA/cm 2 for p-i-n tandems. In n-i-p configuration, asymmetric intermediate reflectors were employed to achieve currents of up to 12.5 mA/cm 2 . On glass substrates, initial and stabilized efficiencies exceeding 13% and 11%, respectively, were thus obtained on 1 cm 2 cells, while on plastic foils with imprinted gratings, 11.2% initial and 9.8% stable efficiency could be reached. Recent progress on the development of effective front and back contacts will be described as well.

In the present work, we apply a 2D self consistent model of SiH4/H2 discharges in order to invest... more In the present work, we apply a 2D self consistent model of SiH4/H2 discharges in order to investigate the transition from microcrystalline to amorphous silicon growth. The model is used to examine the relative importance of ions and atomic hydrogen in the film growth process. This examination also involves monitoring the changes that take place in the electrical properties of the plasma, the gas phase as well as the surface chemistry near the transition between the two growth regimes. Based on these results, the discussion is extended to the mechanism of a-Si:H to μc-Si:H growth transition and the use of plasma diagnostics that can be used to monitor this transition.

We report on results of tandem amorphous/microcrystalline (a-Si:H/µc-Si:H) silicon solar cells de... more We report on results of tandem amorphous/microcrystalline (a-Si:H/µc-Si:H) silicon solar cells developed in commercial Oerlikon Solar KAI PECVD reactors, at an excitation frequency of 40.68 MHz. The cell structure consists of a stack of glass/front contact/pin a-Si:H/intermediate reflector/pin µc-Si:H/back contact. LPCVD (low-pressure chemical vapor deposition) ZnO (zinc oxide) is applied as front and back transparent conductive contacts. The silicon oxide based intermediate reflector (SOIR) is deposited in-situ.

We report on the latest results of tandem micromorph (a-Si:H/µc-Si:H) silicon solar cells fabrica... more We report on the latest results of tandem micromorph (a-Si:H/µc-Si:H) silicon solar cells fabricated in commercial Oerlikon Solar KAI-S and KAI-M PECVD reactors. First developments of in-situ silicon oxide based intermediate reflector (SOIR) in KAI reactors are as well presented.

Applied Physics Letters, 2010

The deposition of thin-film silicon solar cells on highly textured substrates results in improved... more The deposition of thin-film silicon solar cells on highly textured substrates results in improved light trapping in the cell. However, the growth of silicon layers on rough substrates can often lead to undesired current drains, degrading performance and reliability of the cells. We show that the use of a silicon oxide interlayer between the active area and the back contact

Solar Energy Materials and Solar Cells, 2009

The effect of substrate morphology on the growth and electrical properties of single-junction mic... more The effect of substrate morphology on the growth and electrical properties of single-junction microcrystalline silicon cells is investigated. A large variety of V-shaped and U-shaped substrates are characterized by scanning electron microscopy (SEM) and the growth of thin-film microcrystalline silicon (mc-Si:H) devices is observed by cross-sectional transmission electron microscopy (TEM). It is shown that enhanced electrical properties of solar cells are obtained when U-shaped substrates are used and the effect is universal, i.e. independent of the substrate or feature size. U-shaped substrates prevent the formation of two dimensional ''cracks'', which are identified as zones of porous material, from propagating throughout the active part of the solar cell. A numerical growth simulation program reproduces satisfactorily these experimental observations. According to these simulations, shadowing effect due to surface morphology and low adatom surface diffusion length are responsible for the formation of cracks in mc-Si:H material.

A simple theoretical model for pin/nip-type mc-Si:H solar cells is presented. It is based on a su... more A simple theoretical model for pin/nip-type mc-Si:H solar cells is presented. It is based on a superposition of a drift-dominated collection model and of the classical driftdiffusion transport model of the pn-diode. The model is the basis for a solar cell equivalent circuit, identical to the one introduced by Merten et al. [1], for amorphous cells. The equivalent circuit serves as framework for the diagnosis of faulty solar cells, by selected experimental tools, such as: J(V) curves, Quantum efficiency (QE) curves, Raman spectroscopy, Fourier-Transform Photo Spectroscopy (FTPS), Variable Intensity Measurements (VIM) [1]. The main parameter that characterizes solar cell "quality" is the fill factor (FF). For best cells FF is over 75%. FF can be reduced by (1) collection problems (characterized by a drop in the collection voltage Vcoll); (2) shunts (characterized by low shunt resistance Rshunt); (3) excessive series resistance. Thanks to VIM analysis, it is possible to discriminate experimentally between these 3 types of deficiencies. It is also possible to measure Vcoll very easily and link it to fill factor reduction DFF. Selected examples of solar cell series and case studies of defective and degraded cells are given. V appl J total R shunt R series J diode J rec J gen J shunt 1572 1-4244-0016-3/06/$20.00 ©2006 IEEE R slope 1 = J sc J m J ) , ( * * * m m V J MPP oc V V MPP A * MPP A ) , ( m m V J MPP 0 A A D initial raded deg diode J * rec J

Solar Energy Materials and Solar Cells, 2006

Basic limitations of single-junction and tandem p-n and p-i-n diodes are established from thermod... more Basic limitations of single-junction and tandem p-n and p-i-n diodes are established from thermodynamical considerations on radiative recombination and semi-empirical considerations on the classical diode equations. These limits are compared to actual values of short-circuit current, open-circuit voltage, fill factor and efficiency for amorphous (a-Si:H) and microcrystalline (mc-Si:H) silicon solar cells. For single-junction cells, major efficiency gains should be achievable by increasing the short-circuit current density by better light trapping. The limitations of p-i-n junctions are estimated from recombination effects in the intrinsic layer. The efficiency of double-junction cells is presented as a function of the energy gap of top and bottom cells, confirming the 'micromorph' tandem (a-Si:H/mc-Si:H) as an optimum combination of tandem solar cells.

Thin Solid Films, 2006

Production volume of PV modules increases at > 35% per year, but one is yet far from making a glo... more Production volume of PV modules increases at > 35% per year, but one is yet far from making a global impact on energy supply. One of the obstacles is given by the present high production costs of PV modules. A possibility to reduce costs are thin-film PV modules on glass. The specific option of thin-film silicon is considered. The combination of amorphous and microcrystalline silicon thin films within a tandem solar cell corresponds to a theoretical optimum. In practice, stabilized efficiencies of 10% to 12% have so far been obtained in the laboratory with such tandem solar cells. Silicon being a material with an indirect band gap, its absorption coefficient is relatively low, and therefore light management in the solar cell has to be further optimized. Thin-film silicon can be deposited by plasma-enhanced CVD, as used for AM-LCD displays. The use of modified fabrication equipment from the AM-LCD Display Industry is therefore a promising way to implement low-cost mass production. D

We report here on the latest research developments of tandem micromorph (amorphous/ microcrystall... more We report here on the latest research developments of tandem micromorph (amorphous/ microcrystalline) silicon solar cells in our laboratory. High conversion efficiency for micromorph cells requires both a dedicated light management to keep the absorber layers as thin as possible, and optimized growth conditions of the microcrystalline silicon (μc-Si:H) material. We will show that an improved cell design based on the use of silicon oxide doped layers permits to achieve high efficiencies on substrates that are usually considered as inappropriate for μc-Si:H because of their roughness. Furthermore, recently, new front contacts based e.g. on bi-layers or Ultraviolet nanoimprint lithography were developed, leading to very promising results in micromorph solar cells. While efficiencies of 12.7% initial and 11.3% stable could be achieved with only 1.1 μm of bottom cell on a front rough Low Pressure Chemical Vapor Deposited (LPCVD) ZnO, a remarkable 12% initial was also reached on textured replica (as on the master). Emphasis is also laid in our lab on increasing the deposition rate of mc-Si:H while maintaining high quality material. This is done by reducing the interelectrode gap while working at high deposition pressure, in "powder free" processes at 40 MHz. We could observe that high pressure-low hydrogen dilution process conditions lead to dense high quality material. So far, conversion efficiencies up to 8.5% have been achieved at 1 nm/s for single junction μc-Si:H solar cells with 1.8 μm thick absorber layer. We also report a promising micromorph tandem initial efficiency of 11.9% with the μc-Si:H i-layer at 0.9 nm/s. High efficiency micromorph solar cells could thus be fabricated under conditions that are favorable to industrial, low-cost, fabrication of micromorph modules. Recent results of tandems combining original substrates and improved deposition processes suggest that stabilised efficiencies close to 13% can be - xpected in a near future.

The influences of the deposition pressure and silane depletion on the efficiency of single-juncti... more The influences of the deposition pressure and silane depletion on the efficiency of single-junction microcrystalline silicon solar cells has been investigated. The efficiency is found to correlate with the ion energy which affects the density of states in the absorber material. Cell with efficiency of 7.3% at a deposition rate of 1 nm/s, and, respectively, 7.8% at 0.35 nm/s were deposited in R&D KAI M industrial reactor. Silicon oxide based intermediate reflector layers were developed in KAI reactor for incorporation in micromorph devices. Material with an index of refraction of 1.7 at 600 nm and low lateral conductivity were deposited. Micromorph devices incorporating these intermediate reflector layers were fabricated with initial efficiency of 12.3% at a deposition rate of 0.35 nm/s and 10.8% at 1 nm/s.

The increasing demand for photovoltaic devices and the associated crystalline silicon feedstock d... more The increasing demand for photovoltaic devices and the associated crystalline silicon feedstock demand scenario have led in the past years to the fast growth of the thin film silicon industry. The high potential for cost reduction and the suitability for building integration have initiated both industrial and research laboratories dynamisms for amorphous silicon and micro-crystalline silicon based photovoltaic technologies. The recent progress towards higher efficiencies thin film silicon solar cells obtained at the IMT-EPFL in Neuchatel in small-area laboratory and semi-large-area industrial Plasma Enhanced Chemical Vapor Deposition (PE-CVD) systems are reviewed. Advanced light trapping schemes are fundamental to reach high conversion efficiency and the potential of advanced Transparent Conductive Oxides (TCO) is presented, together with issues associated to the impact of the substrate morphology onto the growth of the silicon films. The recent improvements realized in amorphous-microcrystalline tandem solar cells on glass substrate are then presented, and the latest results on 1 cm 2 cells are reported with up to 13.3 % initial efficiency for small-area reactors and up to 12.3 % initial for large-area industrial reactors. Finally, the different strategies to reach an improved light confinement in a thin film solar cell deposited on a flexible substrate are discussed, with the incorporation of asymmetric intermediate reflectors. Results of micromorph solar cells in the n-i-p configuration with up to 9.8 % stabilized efficiency are reported.

Solar Energy Materials and Solar Cells

... Nicolay, Corsin Battaglia, Gregory Bugnon, Laura Ding, Fanny Meillaud, Franz-Josef Haug, Chri... more ... Nicolay, Corsin Battaglia, Gregory Bugnon, Laura Ding, Fanny Meillaud, Franz-Josef Haug, Christophe Ballif. Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering (IMT), Photovoltaics and Thin Film Electronics Laboratory, Rue A.-L. Breguet ...

IEEE Journal of Photovoltaics, 2012

Solar Energy, 2005

Progress in Photovoltaics, 2010

Solar Energy Materials and Solar Cells, 2011

Applied Physics Letters, 2010

Solar Energy Materials and Solar Cells, 2009

Solar Energy Materials and Solar Cells, 2006

Thin Solid Films, 2006

Solar Energy Materials and Solar Cells