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The improvement of optical confinement on the back crystalline silicon solar cell is one of the factors leading to its better performance. Porous silicon (PS) layer can be used as a back reflector (BR) in solar cells. In this work, single layers of porous silicon were grown by electrodeposition on a single crystalline silicon substrate. The measurement of the total reflectivity (RT) on Si/PS surface showed a significant improvement in optical confinement compared to that measured on Si/standard Al back surface field (BSF). The internal reflectivity (RB) extracted from total reflectivity measurements achieved 86 % for the optimized single PS layer (92 nm thick layer with 60 % porosity) in the wavelength range between 950 and 1200 nm. This improvement was estimated as more than 17 % compared to that measured on the surface of Si/BSF Al contact. To improve the stability and passivation properties of PS layer BR, silicon nitride layer (SiNx) was deposited by PECVD on a PS layer. The maximum measured total reflectivity for PS/SiNx achieved approximately 56 % corresponding to an improved RBof up to 83 %. The PS formation process in combination with the PECVD SiNx, can be applied in the photovoltaic cell technology and offer a promising technique to produce high-efficiency and low-cost c-Si solar cells.

GaAsPN semiconductors are promising material for the development of high-efficiency tandem solar cells on silicon substrates. GaAsPN diluted-nitride alloy is studied as the top-junction material due to its perfect lattice matching with the Si substrate and its ideal bandgap energy allowing a perfect current matching with the Si bottom cell. The GaP/Si interface is also studied in order to obtain defect-free GaP/Si pseudo-substrates suitable for the subsequent GaAsPN top junctions growth. Result shows that a double-step growth procedure suppresses most of the microtwins and a bi-stepped Si buffer can be grown, suitable to reduce the anti-phase domains density. We also review our recent progress in materials development of the GaAsPN alloy and our recent studies of all the different building blocks toward the development of a PIN solar cell. GaAsPN alloy with energy bandgap around 1.8 eV, lattice matched with the Si substrate, has been achieved. This alloy displays efficient photoluminescence at room temperature and good light absorption. An early-stage GaAsPN PIN solar cell prototype has been grown on a GaP(001) substrate. The external quantum efficiency and the I–Vcurve show that carriers have been extracted from the GaAsPN alloy absorber, with an open-circuit voltage above 1 eV, however a low short-circuit current density obtained suggests that GaAsPN structural properties need further optimization. Considering all the pathways for improvement, the 2.25% efficiency and IQE around 35% obtained under AM1.5G is however promising, therefore validating our approach for obtaining a lattice-matched dual-junction solar cell on silicon substrate.

In this paper, we show through both calculations and Hall effect measurements that incomplete ionization of dopants has a greater influence on the majority-carrier density in p -type and n-type compensated Si than in uncompensated Si with the same net doping. The factors influencing incomplete ionization at room temperature are shown to be the majority-dopant concentration, its ionization energy and type, and the compensation level. We show that both the majority- and the minority-carrier mobilities are lower in compensated Si than expected by Klaassen's model and that the discrepancy increases with the compensation level at room temperature. The study of the temperature dependence of the majority-carrier mobility shows that there is no compensation-specific mechanism and that the reduction of the screening in compensated Si cannot explain alone the observed gap between experimental and theoretical mobility.

In this paper we present the results of solar cell’s screen printed contacts thickening by light induced plating (LIP) of Cu and Ag. Light-induced and light-assisted plating techniques are compared. Plating bath current versus illumination intensity, the potentials of metallic anode, front and back surfaces of the solar cell were measured and analyzed for all possible plating modes. The current-voltage behaviour of cells under illumination in a plating bath for the cases of electrically insulated and non-insulated back-surface of a cell is analyzed and discussed. Finally, the exact values of current/voltage/illumination intensity for optimal depositions and a general recipe for their determination for different plating materials are given.

In this paper we present the results of solar cell’s screen printed contacts thickening by light induced plating (LIP) of Cu and Ag. Light-induced and light-assisted plating techniques are compared. Plating bath current versus illumination intensity, the potentials of metallic anode, front and back surfaces of the solar cell were measured and analyzed for all possible plating modes. The current-voltage behaviour of cells under illumination in a plating bath for the cases of electrically insulated and non-insulated back-surface of a cell is analyzed and discussed. Finally, the exact values of current/voltage/illumination intensity for optimal depositions and a general recipe for their determination for different plating materials are given. The quality of deposited layers, state of contact-line and anti-reflective coating surface were controlled after plating by cross-section and planar-view SEM observations. The electrical properties of plated contacts and their effect on overall efficiency of solar cell were investigated by measuring contact-line resistance and dark- and light I–V solar cell measurements. The absence of short-circuits was verified by imaging with an infra-red camera.

Abstract: The formation of the grain structure in multicrystalline photovoltaic Silicon relies on basic phenomena which are not well understood and mastered in industrial processes. Nucleation, growth modes (facetted or not), grain competition and kinetics, coarsening are at the origin of the mean grain size and of the morphology of the grain structure which impacts drastically on the photovoltaic properties. During solidification, solute rejection (especially carbon and nitrogen) at the growth interface leads to an increase of the impurity concentration in the liquid phase and then to the precipitation of silicon nitride (Si3N4) and silicon carbide (SiC). As a consequence, the grain structure of the ingot changes from columnar to small grains, also known as grits. A new electromagnetic levitation set up has been developed in order to measure the undercooling versus impurity concentration. The impurity concentration in the levitated Si droplet is controlled by the partial pressure of nitrogen or hydrocarbon gas. The concentration of nitrogen and carbon dissolved will be compared with theoretical predictions (phase diagram). And a new analytical model is proposed to understand the precipitation of SiC during the solidification process of an ingot.

Silicon nitride (Si-N) and oxynitride (Si-O-N) thin films are obtained by low pressure rapid thermal chemical vapor deposition (RT-LPCVD) by using the reaction of diluted silane (SiH4/Ar = 10%) with ammonia (NH3) or a mixture of ammonia and nitrous-oxide (N2O) at various gas ratios (R = NH3/SiH4 and R′ = [N2O + N2O]). Deposition kinetics of silicon nitride films have been studied in the medium to high temperature range (700–850°C) at fixed total pressure of 9.5 mbar and gas flow ratio (R = 10). Oxynitride deposition rates have been also studied as a function of R′ at fixed temperature and total pressure parameters of 750 °C and 9.5 mbar respectively. Rutherford backscattering spectrometry results showed that the RT-LPCVD Si-N films are silicon-rich and that the composition stoichiometry is mainly controlled by the process parameters, particularly the gas flow ratios. Si3N4 silicon nitride stoichiometry can only be approached for R ≥ 10. Fourier transform spectrometry measurements revealed that the Si-O-N films have a particular structure formed by a mixture of Si-O and Si-N bonds with no apparent separated phases. In addition, in contrast to the few 10 at % usually reported for classical CVD processes, no significant traces of hydrogen bonds (Si-H or N-H) were detected for R ≥ 10. Furthermore, secondary ion mass spectrometry analysis always indicated that Si-N and Si-O-N RT-LPCVD deposited layers are homogeneous along the whole film thickness.

An original process has been developed enabling the fabrication of CVD insulating (Si3N4) thin films by means of an in situactivation of the reactions at T < 400 °Cand under P = 1to 2 torr. Mono-Si substrates were nitrided using a mixture of argon containing SiH4and NH3. O2has also been added to the reaction gases. The activation was performed by a DC electrical discharge. The substrate was not used as an electrode, and was placed parallel to the discharge current. This configuration minimized the contamination of the films during their formation. The layers have been analyzed using FT-IR and SIMS spectroscopy. MIS structures have been realized using these nitrided layers. The flat-band shift ΔVFB(about −1 V) and the interface state density Nithas been measured. Good electrical characteristics are obtained with T ⩽ 225 °C.

Silicon On Porous Layer (SOPL) substrate was fabricated by applying the Smart Cut technology to silicon substrate having a superficial porous Si layer. It consists in a non-porous/porous single crystalline Si bilayer transferred onto a handling wafer to form Silicon-On-Insulator (SOI) like structure. Subsequent bonding to a final wafer and separation induced in the fragile embedded porous Si layer led to the double transfer of the single crystalline non porous Si layer. Changes in structural and mechanical properties of the embedded porous silicon layer as a function of the annealing temperature were also determined. Thickness homogeneity of the transferred thin Si layer was estimated after SOPL fabrication and after double layer-transfer achievement. The perfect crystallinity of the active Si layer was conserved during the whole process. SOI-like structure fabrication and subsequent separation of thin high quality Si film are reported in order to demonstrate capability of SOPL structure with double-sided layer processing for device integration.

Electroless nickel plating is a very promising process for metallization of silicon solar cells. It can be used to form a seed layer for an electrolytic deposition of copper or silver because it ensures the required adherence, a low contact resistance, and prevent copper from diffusing into the silicon. The nickel plating technique developed in this work consists of a selective electroless nickel–phosphorus (NiP) deposition without the activation step from an alkaline bath containing sodium hypophosphite as a reducing agent and ammonia for pH control. Experimental results of the deposit properties are presented. Thin NiP layers of about 0.1μmon silicon were annealed under different conditions. Rutherford backscattering spectrometry, glancing incidence X-ray diffraction, and scanning electron microscopy were used to obtain information on silicide formation vs temperature. These results were correlated with contact resistivity measurements by the transmission line model. A typical contact resistivity of 10−4Ωcm2has been obtained.

The absorption of thin hydrogenated amorphous silicon layers can be efficiently enhanced through a controlled periodic patterning. Light is trapped through coupling with photonic Bloch modes of the periodic structures, which act as an absorbing planar photonic crystal. We theoretically demonstrate this absorption enhancement through one or two dimensional patterning, and show the experimental feasibility through large area holographic patterning. Numerical simulations show over 50% absorption enhancement over the part of the solar spectrum comprised between 380 and 750nm. It is experimentally confirmed by optical measurements performed on planar photonic crystals fabricated by laser holography and reactive ion etching.

Among high-efficiency cells, interdigitated back-contact (IBC) solar cells present many advantages over conventional structures. In most cases, fabrication processes involving many masks and alignments are needed. In this paper, we present an alternative approach to simplify the IBC solar cell fabrication process. Here, only one mask without alignment was used to define the total solar cell. Separation between emitter and base comb is reached by cantilevers which prevent interconnections during Al deposition. Cantilevers are formed by a low-cost chemical etching solution. With this process, we have obtained cells with a fill factor of 77%.