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25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 2569-2572, Copper light-induced plating onto the surface of existing screen-printed silver contacts of industrial Silicon solar cells was studied. Electrolyte and cell-in-electrolyte systems were analyzed to determine the optimal plating conditions. Two methods of protection of a cell rear side from dissolution (by insulating and by applying electrical potential) are compared. It was found that the parasite cells are created in the plating bath due to the difference of the redox potentials of silver, copper and aluminum. Previously reported equilibrium conditions for terminal and anode – rear side voltage must be modified to compensate the parasite cell current. The solar cell efficiency gain is found to be a nonmonotonic function of deposition rate and duration. Absolute solar cell efficiency was improved by 1,53%.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 2215-2217, This paper presents the results of a two-step front contact process consisting of a thin electroless nickelphosphorus (NiP) seed layer thickened by electrolytic deposition of silver. The electroless NiP layer can be used as a seed layer because it ensures satisfying adherence, good diffusion barrier properties and a low contact resistivity. Typical contact resistivities of 10-4 .cm2 have been obtained by Transmission Line Model measurements. The developed process is selective because the deposition occurs only onto the desired region of the silicon sample. Solar cells using this front metallization have been characterized by I-V measurements and compared with standard screen-printed solar cells. The major difficulty related to the use of nickel-phosphorous as seed layer is the potential shunt formation during annealing of the contact. Experimental conditions have been found to avoid pseudo-FF decrease after metallization. Finally, FF over 78 % were obtained with this metallization process.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 740-742, We report about a new low cost, fast and large area electroless technique for the formation of Ag nanoparticles with controlled size and density on the surface of SiNx:H and SiOx layers at room temperature and on the time scale of several minutes. The influence of the SiNx:H and SiOx layer stoechiometries on the size and density on Ag nanoparticles are discussed in details.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 3642-3645, Crystalline Silicon Thin-Films (CSiTF) solar cells are considered by some as a roadmap that is very likely to achieve the cost reduction which the photovoltaics industry is longing for. A bottleneck of this technology is the need for a suitable low-cost substrate. Sintered Silicon Powders is very promising since it reduces significantly the silicon consumption and allows the production of large area wafers. To obtain these substrates, the french company S’TILE has designed a process in two steps: the first one is to press at high temperature some micrometric grain size silicon powder in order to obtain a sintered wafer with dimensions of up to 200 x 200 mm². The second one is to recrystallize these wafers. These p+-doped multicrystalline silicon wafers have been used to make CSiTF solar cells. An epitaxial layer is grown on the substrate, with an in-situ emitter. A good cristallinity, a low resistivity and a low quantity of impurities were found in the recrystallized substrates. Complete characterization of solar cells is detailed.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 1302-1306, This study investigates the compensation effects at iso-carrier density on two intentionally compensated multicrystalline ingots grown from Solar-Grade Silicon feedstocks purified using metallurgical routes. The effects of the dopant compensation on both ingots were studied at the wafer and the solar cell levels at given equilibrium carrier densities, in order to allow a relevant investigation of the compensation effects on the overall electrical properties. At the wafer level the majority carrier mobility and the carrier lifetime were investigated. At the cell level, I-V characteristics under standard illumination were studied. In the light of all the characterization results, we propose physical mechanisms that explain the iso-carrier density differences in electrical properties observed between the two ingots.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 2043-2046, Selective emitter solar cells can provide a significant increase in conversion efficiency. However most approaches still need many technological steps and alignment procedures. In this paper we propose a single-step selective emitter process compatible with self-aligned electrochemical metal contact deposition. Silicon substrates with phosphosilicate glass and silicon nitride layers are used to demonstrate the laser-induced increase of dopant concentration in lightly doped phosphorous emitters. Sheet resistance variation from 100 Ω/sq to 40 Ω/sq is demonstrated with a nanosecond UV laser for optimised process parameters. Potential applications are discussed in view of doping profiles, passivation properties and electrical characterisation.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 548-552, Absorption enhancement of ultra-thin amorphous silicon (aSi) layers is demonstrated both theoretically and experimentally, through photonic crystal patterning of the absorbing layer. We show through simulation that an increase of the absorption by a factor of 1.5 is expected for a demonstrator of 100nm of aSi. The proposal is then validated by an experimental demonstration, showing a 50% relative increase of the absorption of 100nm of aSi deposited on glass, holographically lithographed and dry-etched in order to fabricate a one-dimensional photonic crystal. This shows new possibilities of increasing the efficiency of thin film photovoltaic cells. Photonic crystal based architecture of thin film cells are proposed and discussed.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 1868-1871, This work focuses on the passivation of highly doped silicon surfaces. We propose here to bring more knowledge on the emitter passivation by hydrogen during the thermal annealing of standard silicon nitride ARC layer. Silicon wafers were phosphorus-diffused, and the surfaces were subsequently covered by SiNx:H. Different rapid thermal annealing processes, varying in time and temperature, were then applied. Annealing time and temperature variations led to an optimal lifetime value. We also noticed a sheet resistance variation for different annealing parameters, although emitter had the same doping profile before annealing. This can be explained by the dopant deactivation due to the hydrogen release during annealing, leading to an effective doping decrease. Comparing the sheet resistance and the lifetime results, we found that the optimum passivation was correlated with the higher sheet resistance. Another important aspect to consider is therefore the contact resistance between emitter and metal. As hydrogen turns phosphorus inactive electrically, it can also lead to a contact resistance increase. TLM measurements allowed us to observe the evolution of the contact resistance, depending on firing temperature

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 1623-1627, In this article, we used 2D simulation to evaluate the influence of the Metal-Insulator-Semiconductor (MIS) and fixed charge density contained in the dielectric layers which may be used for the passivation of the rear side of silicon solar cells. Negative fixed charge densities were found in PECVD SiOx:H and SiNx:H dielectrics by dark-capacitance-voltage measurements, using a new method more adapted to solar cell applications. Next, multicrystalline silicon solar cells based on the i-PERC process were made with PECVD SiOx:H / SiNx:H stacks of different thicknesses. I-V characterization showed open-circuit voltage and fill factor degradations for the processed cells compared to the reference. These degradations were analyzed by spectral response measurements, scanning electron microscopy and infrared thermography. It was found that the serigraphy paste and the laser impacts on grain boundaries were responsible for the degradations.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 1430-1433, Electroless nickel plating is a very promising process for metallization of silicon solar cells. It can be used as a seed layer for an electrolytic deposition of copper because it ensures the required adherence, a low contact resistance and prevent copper from diffusing into the silicon. Two nickel plating methods are studied. The first one consists on a selective electroless nickel-phosphorous deposition from an alkaline bath containing sodium hypophosphite as a reducing agent and ammonia for pH control. The second process called “Dynamic Chemical Plating” developed by Stremsdoerfer & al. is a new direct plating industrially carried out by Jet Metal Technologies company. This paper presents results of experiments on the properties of these nickel deposits. Observations with a Scanning Electron Microscope (SEM) permit to check the quality of the deposits. Then, thin nickel layers of about 0.1 μm on silicon are annealed under different conditions by Rapid Thermal Annealing (RTA) and Rutherford Backscattering Spectrometry (RBS) is used to obtain information on silicide formation versus temperature. These results are correlated with contact resistivity measurements by Transmission Line Model (TLM). Typical contact resistivity of 10-4 Ω.cm2 is obtained.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 2601-2603, The main issue of solar cell research field is the reduction of the fabrication cost. Due to the combination of thin film absorbing silicon (

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 2107-2109, Minority carrier lifetime t is one of the most important electrical parameters to qualify silicon for photovoltaic applications. Amongst the available techniques for measuring this parameter, photoluminescence has regained interest as it can be done on raw material as well as on completed solar cells in a non destructive way. Although very efficient mapping systems can provide fast information on multicrystalline samples, quantitative t values require a calibration that is normally achieved by correlation with other techniques like photoconductance decay. In this paper we show that t may be evaluated in a simple way from the dependence of the room temperature photoluminescence signal on the excitation beam intensity using an analytical model based on 1D carrier diffusion. Using this 1D model, the diffusion length was calculated in photovoltaic grade silicon materials, thin epitaxial layer and metallurgic grade silicon.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 415-417, Photoluminescence investigations were performed on amorphous silicon nitride (-SiNx) films containing silicon quantum dots (Si QDs) prepared by plasma enhanced chemical vapour deposition with different deposition parameters. It was found that the optimization of the plasma conditions such as plasma power, Ton and Toff times are essential to the preparation of high density Si QDs. Metal-insulator-semiconductor devices were formed using gold (Au) contact on the SiNx surface. Electronic properties have been investigated under light exposition and photocurrent (PC) spectra have been obtained. Two main peaks located at 1.22 eV and 1.67 eV were observed and were attributed to the confined carriers within two different size QDs. Moreover, the PC spectra show an increase of the photogenerated current in the blue-UV range. The current density deduced from PC measurements was extrapolated under equivalent solar flux (AM1.5) and was found to be ~ 5 μA.cm-2 for Si QDs spacing of 7 nm.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 2258-2260, Epitaxial Lateral Overgrowth (ELO) is a method which enables to grow epitaxial layers using liquid phase epitaxy (LPE) on a growth substrate partly covered by a dielectric film. This method is used to obtain layers of better quality than the original growth substrate, due to the dielectric film which prevents defects propagation into the ELO layer. This feature makes it a promising technology to obtain good silicon thin film for photovoltaic applications, using low quality Si substrate. Solar cells were fabricated using ELO structures prepared with different growth conditions allowing coalescence between the different seeding windows. I-V, external quantum efficiency and Light Beam Induced Current characterizations were performed. Analysis of these results shows that there is a strong recombination effect at the interface between ELO and the dielectric film. We demonstrate that it is due to a small gap existing between them caused by the growth mechanism. We discuss on the possibility to overcome this problem.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 372-374, Transport properties of minority carriers are evaluated by Light Beam Induced Current and Internal Quantum Efficiency measurements in multicrystalline, monocrystalline Cz and epitaxial silicon samples. Diffusion length values are calculated from experimental data, using analytical models and numerical simulation. Surface passivation appears to be a necessary step to determine the diffusion length in thin samples.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 1793-1796, We propose to study the emitter formation using Lydop technology, which is low pressure phosphorus diffusion. Various emitters are obtained by varying diffusion parameters, i.e., temperature, drive in time, POCl3/O2 ratio and preoxidation time. This allows to observe the evolution of the emitter dead layer as a function of diffusion parameters and to conclude on the emitter quality. c-Si and mc-Si are then elaborated for different kinds of emitter. POCl3/O2 ratio and preoxidation time are of particularly interest for enhancing emitter performances. While POCl3/O2 ratio decrease enhances the emitter performance, it leads to a series resistance increase. Preoxidation, consisting in the formation of a thermal oxide prior to diffusion, leads to a Joe enhancement, and a 1% absolute increasing of efficiency.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 491-494, The absorption properties of silicon nanocrystals (Si-nc) with different sizes ranging from 4.5 to 3.2 nm embedded in silicon nitride matrix have been studied in the energy range 1.5-5 eV with spectroscopic ellipsometry. The influence of the Si-nc size on the proper absorption coefficient of Si-nc (aSi-nc) has been investigated. It was shown that the aSi-nc decreases when the Si-nc becomes smaller. The evolution of the direct energy gaps of Si-nc with their size is also discussed.