Browsing by Author "Hassan, Mohamed"

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Open Access
Laser doping for silicon solar cells : modeling and application
(2024) Hassan, Mohamed; Werner, Jürgen H. (Prof. Dr. rer. nat. habil.)
In meiner Dissertation geht es um die Simulation des Laserdotierungsprozess der Oberfläche des Siliziumwafers um hoch effizienten Solarzellen herzustellen. Die Simulation ermöglicht die genaue Vorhersage der Dimensionen eines dotierten Bereiches. Das hat ermöglicht, nicht nur die Abhängigkeit des ergebenden Schichtleitwerts von der benutzten Rastergeschwindigkeit des Laserstrahls auf die Siliziumoberfläche zu verstehen, sondern auch der Schichtleitwert einer laserdotierten Schicht basierend auf ein einfaches geometrisches Modell vorherzusagen.
Open Access
Sheet conductance of laser-doped layers using a Gaussian laser beam : an effective depth approximation
(2024) Hassan, Mohamed; Werner, Juergen H.
Laser doping of silicon with pulsed and scanned laser beams is now well-established to obtain defect-free, doping profile tailored, and locally selectively doped regions with a high spatial resolution. Picking the correct laser parameters (pulse power, pulse shape, and scanning speed) impacts the depth and uniformity of the melted region geometry. This work performs laser doping on the surface of single crystalline silicon, using a pulsed and scanned laser profile with a Gaussian intensity distribution. A deposited boron oxide precursor layer serves as a doping source. Increasing the local inter-pulse distance xirrbetween subsequent pulses causes a quadratic decrease of the sheet conductance Gshof the doped surface layer. Here, we present a simple geometric model that explains all experimental findings. The quadratic dependence stems from the approximately parabolic shape of the individual melted regions directly after the laser beam has hit the Si surface. The sheet resistance depends critically on the intersection depth dchand the distance xirrof overlap between two subsequent, neighboring pulses. The intersection depth dchquadratically depends on the pulse distance xirrand therefore also on the scanning speed vscanof the laser. Finally, we present a simple model that reduces the complicated three dimensional, laterally inhomogeneous doping profile to an effective two-dimensional, homogeneously doped layer which varies its thickness with the scanning speed.
Open Access
Solar cells with laser doped boron layers from atmospheric pressure chemical vapor deposition
(2022) Zapf-Gottwick, Renate; Seren, Sven; Fernandez-Robledo, Susana; Wete, Evariste-Pasky; Schiliro, Matteo; Hassan, Mohamed; Mihailetchi, Valentin; Buck, Thomas; Kopecek, Radovan; Köhler, Jürgen; Werner, Jürgen Heinz
We present laser-doped interdigitated back contact (IBC) solar cells with efficiencies of 23% on an area of 244 cm2 metallized by a screen-printed silver paste. Local laser doping is especially suited for processing IBC cells where a multitude of pn-junctions and base contacts lay side by side. The one-sided deposition of boron-doped precursor layers by atmospheric pressure chemical vapor deposition (APCVD) is a cost-effective method for the production of IBC cells without masking processes. The properties of the laser-doped silicon strongly depend on the precursor’s purity, thickness, and the total amount of boron dopants. Variations of the precursor in terms of thickness and boron content, and of the laser pulse energy density, can help to tailor the doping and sheet resistance. With saturation-current densities of 70 fA/cm2 at sheet resistances of 60 Ohm/sq, we reached maximum efficiencies of 23% with a relatively simple, industrial process for bifacial IBC-cells, with 70% bifaciality measured on the module level. The APCVD-layers were deposited with an inline lab-type system and a metal transport belt and, therefore, may have been slightly contaminated, limiting the efficiencies when compared to thermal-diffused boron doping. The use of an industrial APCVD system with a quartz glass transport system would achieve even higher efficiencies.
Open Access
Unified model for laser doping of silicon from precursors
(2021) Hassan, Mohamed; Dahlinger, Morris; Köhler, Jürgen R.; Zapf-Gottwick, Renate; Werner, Jürgen H.
Laser doping of silicon with the help of precursors is well established in photovoltaics. Upon illumination with the constant or pulsed laser beam, the silicon melts and doping atoms from the doping precursor diffuse into the melted silicon. With the proper laser parameters, after resolidification, the silicon is doped without any lattice defects. Depending on laser energy and on the kind of precursor, the precursor either melts or evaporates during the laser process. For high enough laser energies, even parts of the silicon’s surface evaporate. Here, we present a unified model and simulation program, which considers all these cases. We exemplify our model with experiments and simulations of laser doping from a boron oxide precursor layer. In contrast to previous models, we are able to predict not only the width and depth of the patterns on the deformed silicon surface but also the doping profiles over a wide range of laser energies. In addition, we also show that the diffusion of the boron atoms in the molten Si is boosted by a thermally induced convection in the silicon melt: the Gaussian intensity distribution of the laser beam increases the temperature-gradient-induced surface tension gradient, causing the molten Si to circulate by Marangoni convection. Laser pulse energy densities above H > 2.8 J/cm2 lead not only to evaporation of the precursor, but also to a partial evaporation of the molten silicon. Without considering the evaporation of Si, it is not possible to correctly predict the doping profiles for high laser energies. About 50% of the evaporated materials recondense and resolidify on the wafer surface. The recondensed material from each laser pulse forms a dopant source for the subsequent laser pulses.