Optimization of laser-firing processes for silicon-heterojunction solar-cell back contacts

One of the key steps to achieve high efficiencies in amorphous/crystalline silicon photovoltaic structures is to design low-ohmic-resistance backcontacts with good passivation in the rear part of the cell. A well known approach to achieve this goal is to use laser-fired contact (LFC) processes in which a metal layer is fired through the dielectric to define good contacts with the semiconductor. However, and despite the fact that this approach has demonstrated to be extremely successful, there is still enough room for process improvement with an appropriate optimization. In this paper, a study focused on the optimal adjustment of the irradiation parameters to produce laser-fired contacts in a-Si:H/c-Si heterojunctionsolarcells is presented. We used samples consisting of crystalline-silicon (c-Si) wafers together with a passivation layer of intrinsic hydrogenated amorphous silicon (a-Si:H(i)) deposited by plasma-enhanced chemical deposition (PECVD). Then, an aluminum layer was evaporated on both sides, the thickness of this layer varied from 0.2 to 1 μm in order to identify the optimal amount of Al required to create an appropriate contact. A q-switched Nd:YVO4laser source, λ = 532 nm, was used to locally fire the aluminum through the thin a-Si:H(i)-layers to form the LFC. The effects of laser fluences were analyzed using a comprehensive morphological and electrical characterization.

Influence of laser wavelength on Laser-Fired contacts for crystalline silicon solar cells

In the Laser-Fired Contact (LFC) process, a laser beam fires a metallic layer through a dielectric passivating layer into the silicon wafer to form an electrical contact with the silicon bulk [1]. This laser technique is an interesting alternative for the fabrication of both laboratory and industrial scale high efficiency passivated emitter and rear cell (PERC). One of the principal characteristics of this promising technique is the capability to reduce the recombination losses at the rear surface in crystalline silicon solar cells. Therefore, it is crucial to optimize LFC because this process is one of the most promising concepts to produce rear side point contacts at process speeds compatible with the final industrial application. In that sense, this work investigates the optimization of LFC processing to improve the back contact in silicon solar cells using fully commercial solid state lasers with pulse width in the ns range, thus studying the influence of the wavelength using the three first harmonics (corresponding to wavelengths of 1064 nm, 532 nm and 355 nm). Previous studies of our group focused their attention in other processing parameters as laser fluence, number of pulses, passivating material [2, 3] thickness of the rear metallic contact [4], etc. In addition, the present work completes the parametric optimization by assessing the influence of the laser wavelength on the contact property. In particular we report results on the morphology and electrical behaviour of samples specifically designed to assess the quality of the process. In order to study the influence of the laser wavelength on the contact feature we used as figure of merit the specific contact resistance. In all processes the best results have been obtained using green (532 nm) and UV (355 nm), with excellent values for this magnitude far below 1 mΩcm2.

High data rate modulation of high power 1060-nm DBR tapered lasers with separate contacts

Direct optical modulation at 2.5 Gb/s with amplitude of more than 0.5 W has been demonstrated in single longitudinal mode distributed Bragg reflector tapered lasers emitting at 1060 nm with separated injection of the ridge waveguide and tapered sections. The modulating signal of ~110 mA peak to peak was applied to the ridge waveguide section, yielding a high modulation efficiency of ~5 W/A. The large-signal frequency response of the experimental set-up was limited by the bandwidth of the electrical amplifier rather than by the internal dynamics of the laser, indicating that higher bit rates could be achieved with improved driving electronics.

Mathematical development of stress-strain law for rock block contacts

The fracture behavior of rock block contacts has been studied for many years. Unfortunately, up to now, there is not a rigorous formulation or a solid theoretical foundation to support it. A mathematical development to represent the failure mechanism which occurs in the contacts between rock blocks is presented to evaluate the performance of breaking mechanism of such blocks relating it to the morphology of the contact and mechanical parameters of the material. The examined framework includes the evaluation of the surface roughness of first order in the failure mechanism of the granular particles of large size and the development of a theoretical model describing the morphology of the contact between rock blocks.

Ti/Pd/Ag Contacts to n-Type GaAs for High Current Density Devices

The metallization stack Ti/Pd/Ag on n-type Si has been readily used in solar cells due to its low metal/semiconductor specific contact resistance, very high sheet conductance, bondability, long-term durability, and cost-effectiveness. In this study, the use of Ti/Pd/Ag metallization on n-type GaAs is examined, targeting electronic devices that need to handle high current densities and with grid-like contacts with limited surface coverage (i.e., solar cells, lasers, or light emitting diodes). Ti/Pd/Ag (50 nm/50 nm/1000 nm) metal layers were deposited on n-type GaAs by electron beam evaporation and the contact quality was assessed for different doping levels (from 1.3 × 1018 cm−3 to 1.6 × 1019 cm−3) and annealing temperatures (from 300°C to 750°C). The metal/semiconductor specific contact resistance, metal resistivity, and the morphology of the contacts were studied. The results show that samples doped in the range of 1018 cm−3 had Schottky-like I–V characteristics and only samples doped 1.6 × 1019 cm−3 exhibited ohmic behavior even before annealing. For the ohmic contacts, increasing annealing temperature causes a decrease in the specific contact resistance (ρ c,Ti/Pd/Ag ~ 5 × 10−4 Ω cm2). In regard to the metal resistivity, Ti/Pd/Ag metallization presents a very good metal conductivity for samples treated below 500°C (ρ M,Ti/Pd/Ag ~ 2.3 × 10−6 Ω cm); however, for samples treated at 750°C, metal resistivity is strongly degraded due to morphological degradation and contamination in the silver overlayer. As compared to the classic AuGe/Ni/Au metal system, the Ti/Pd/Ag system shows higher metal/semiconductor specific contact resistance and one order of magnitude lower metal resistivity.