Aluminum-silicon Interdiffusion in Screen Printed Metal Contacts for Silicon based Solar Cells Applications (original) (raw)
Related papers
Screen-Printed Al Back Contacts on Si Solar Cells: Issues and Some Solutions
MRS Proceedings, 2009
This paper identifies some mechanisms that lead to problems in back Al contact formation. Major issues are related to a basic problem that the Al melt has a large surface tension and tries to ball up during the firing step. Other issues arise from dissolution of the Si-Al interface and entrapment of glass within the Si-Al alloy. Si diffusion into Al can be applied to control the melt, while cooling rate can help improve the structure of various regions of the back contact for a favorable series resistance. We also discuss a modified time-temperature profile that can lead to a deep and uniform back-surface field.
IEEE Transactions on Electron Devices, 2000
We study recombination properties and the formation of base contacts, which are realized by local laser ablation of a dielectric stack and the subsequent full-area screen printing of an Al paste. Based on charge-carrier lifetime measurements using the camera-based calibration-free dynamic infrared lifetime mapping technique, we determine contact reverse saturation current densities as low as J 0,cont = 9 × 10 2 fA/cm 2 on 1.5-Ωcm p-type float-zone silicon (FZ-Si) and J 0,cont = 2 × 10 4 fA/cm 2 on 200-Ωcm p-type FZ-Si. Scanning electron microscopy images reveal that the thickness of the highly Al-doped (Al-p + ) layer considerably depends on the contact size and the contact layout (e.g., point or line contacts). Based on this finding, we show that Al-p + layer thickness W Al−p+ significantly affects the contact recombination. As a result, we show which local contact geometry is most appropriate for the lowest contact recombination employing local Al-alloyed contacts.
Studies on Backside Al-Contact Formation in Si Solar Cells: Fundamental Mechanisms
MRS Proceedings, 2008
We have studied mechanisms of back-contact formation in screen-printed Si solar cells by a fire-through process. An optimum firing temperature profile leads to the formation of a P-Si/P+- Si/ Si-Al eutectic/agglomerated Al at the back contact of a Si solar cell. Variations in the interface properties were found to arise from Al-Si melt instabilities. Experiments were performed to study melt formation. We show that this process is strongly controlled by diffusion of Si into Al. During the ramp-up, a melt is initiated at the Si-Al interface, which subsequently expands into Al and Si. During the ramp-down, the melt freezes, which causes the doped region to grow epitaxially on Si, followed by solidification of the Si-Al eutectic. Any agglomerated (or sintered) Al particles are dispersed with Si. Implications on the performance of the cell are described.
2012 38th IEEE Photovoltaic Specialists Conference, 2012
The performance optimization of solar cells comprising a locally contacted rear surface is a trade-off between rear surface recombination losses and resistive losses. Considering local aluminum (Al)-alloyed contacts formed by screen-printing an additional challenge arises from the impact of the processing conditions on the contact properties. The aim of this work is the development of a guideline to find the optimum contact geometry based on analytical models and experimentally verified parameterizations. Our study reveals equal performance of point and line rear contact layouts and an optimum metallization fraction of around 20 % on 2 cm p-type Czochralski grown (Cz) Si.
Journal of Nanoscience and Nanotechnology, 2013
In this paper, we present a detailed study on the local back contact (LBC) formation of rearsurface-passivated silicon solar cells, where both the LBC opening and metallization are realized by one-step alloying of a dot of fine pattern screen-printed aluminum paste with the silicon substrate. Based on energy dispersive spectrometer (EDS) and scanning electron microscopy (SEM) characterizations, we suggest that the aluminum distribution and the silicon concentration determine the local-back-surface-field (Al-p + layer thickness, resistivity of the Al-p + and hence the quality of the Al-p + formation. The highest penetration of silicon concentration of 78.17% in aluminum resulted in the formation of a 5 m-deep Al-p + layer, and the minimum LBC resistivity of 0 92 × 10 −6 cm 2. The degradation of the rear-surface passivation due to high temperature of the LBC formation process can be fully recovered by forming gas annealing (FGA) at temperature and hydrogen content of 450 C and 15%, respectively. The application of the optimized LBC of rear-surface-passivated by a dot of fine pattern screen(-) printed aluminum paste resulted in efficiency of up to 19.98% for the p-type czochralski (CZ) silicon wafers with 10.24 cm 2 cell size at 649 mV open circuit voltage. By FGA for rear-surface passivation recovery, efficiencies up to 20.35% with a V oc of 662 mV, FF of 82%, and J sc of 37.5 mA/cm 2 were demonstrated.
Effect of gravity on the microstructure of Al-Si alloy for rear-passivated solar cells
Journal of Applied Physics, 2011
We study the influence of the gravity field orientation on the microstructure of Al-Si forming alloy. Due to the difference between the diffusivity of aluminum and silicon, Kirkendall void formations are normally found at the back of rear-passivated solar cells instead of an eutectic layer. We show that the voids may partially be avoided by sintering the samples with the solid/liquid interface oriented opposite to the direction of the gravity field. A local back-surface-field is found underneath the voids using this approach. This phenomenon strongly applies to rear-passivated solar cells, which exhibited strong fill factor losses.
Al-Si alloy formation in narrow p-type Si contact areas for rear passivated solar cells
Journal of Applied Physics, 2010
For high efficiency silicon solar cells, the rear surface passivation by a dielectric layer has significant advantages compared to the standard fully covered Al back-contact structure. In this work the rear contact formation of the passivated emitter and rear cell device structure is analyzed. Contrary to expected views, we found that the contact resistivity of fine screen printed Al fingers alloyed on narrow p-type Si areas depends on the geometry of the Al-Si alloy formation below the contacts, and decreases by reducing the contact area, while the contact resistance remains constant. At the solar cell level, the reduction in the contact resistivity leads to a minimization of the fill factor losses. At the same time, narrow Al-Si alloy formations increased the passivated area below the contacts, improving the optical properties of the rear side, reducing the short-circuit current and open-circuit voltage losses. Scanning electron microscopy analysis of the Al-Si alloy geometry is performed, in order to understand its influence on the contact resistivity. The analysis presented in this article has application in Al-Si alloying processes and advanced solar cells concepts, like back-contact and rear passivated solar cells.
IEEE Electron Device Letters, 2000
We present a detailed study on the rear contact formation of rear-surface-passivated silicon solar cells by full-area screen printing and alloying of aluminum pastes on the locally opened passivation layer. We demonstrate that applying conventional Al pastes exhibits two main problems: 1) high contact depths leading to an enlargement of the contact area and 2) low thicknesses of the Al-doped p + Si regions in the contact points resulting in poor electron shielding. We show that this inadequate contact formation can be directly linked to the deficiently low percentage of silicon that dissolves into the Al-Si melt during alloying. Thus, by intentionally adding silicon to the Al paste, we could significantly improve the contact geometry by reducing the contact depth and enlarging the Al-p + thickness in the contact points, enabling a simple industrially feasible way for the rear contact formation of silicon solar cells.
2011
Purpose: The aim of the paper was to apply a conventional method “screen printing” using micrometric pastes to improve the quality of forming front side metallization of monocrystalline solar cells. Design/methodology/approach: The topography of co-fired in the infrared belt furnace front contacts were investigated using confocal laser scanning microscope and scanning electron microscope with an energy dispersive X-ray (EDS) spectrometer for microchemical analysis. There were researched both surface topography and cross section of front contacts using SEM microscope. Phase composition analyses of chosen front contacts were done using the XRD method. Front contacts were formed on the surface with different morphology of the solar cells: textured with coated antireflection layer, textured without coated antireflection layer, non-textured with coated antireflection layer, non-textured without coated antireflection layer. The medium size of the pyramids was measured using the atomic for...
IEEE Journal of Photovoltaics, 2013
Silicon wafer solar cells with an aluminum local back surface field (Al-LBSF) are currently intensively investigated for industrial application. One of the main challenges for the Al-LBSF solar cell is the formation of the local Al rear contacts. In our previous work, we have introduced the relative photoluminescence (PL) intensity method to study the Al-Si local contact formation. In this study, we apply this method to experimentally investigate the impact of the geometry (lines or points) of the rear contacts and compare the experimental results with theoretical results that are obtained using Fischer's model. We find that the PL intensity strongly correlates with the p + layer thickness and inversely correlates with the void density at the rear surface. Al-LBSF solar cells with different rear contact geometries are fabricated. High R s was found, especially for those cells with narrower line widths and a large number of voids.