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Laser Enhanced Contact Optimization – A Novel Technology for Metal-Semiconductor-Contact Optimization for Crystalline Silicon Solar Cells
Abstract
pp. 1–25, ISBN: 3-936338-73-6
... M ETAL semiconductor contacts are commonly made by screen printing of metal pastes on the Si wafer and the formation of low-ohmic contacts has been a major topic in research over the years [1]- [3]. A process for contact improvements is laser-enhanced contact-optimization (LECO), which has been reported previously [4], [5]. Contrary to other used laser-based treatments within the cell manufacturing process, like laser cutting [6], laser edge isolation [7], laser fired contacts [8], and laser doped selective emitters [9], in the LECO treatment, the laser is used for nondestructive carrier injection, whereas the driving force of the treatment is the current induced by the LECO process. ...
... The process takes place after the fast firing process of screen-printed solar cells. After the LECO treatment solar cells show a strongly reduced contact resistance Manuscript allowing the contact formation even on low-doped emitters [5]. New pastes adapted for the LECO process have been reported [4], [10] exhibiting also improved open-circuit voltages as well as a small increase in short-circuit current. ...
... The LECO treatment was performed on the commercially available LECO Labtool by CE Cell Engineering with a processing time of 0.8 s (half-cell). As described in [5] during the LECO process a focused laser beam laterally scans the cell, inducing charge carriers locally. Free carriers are separated by applying a reverse bias, resulting in a high reverse current. ...
A technique to improve contacts of solar cells laser enhanced contact optimization (LECO) has been studied in terms of micro scale contact formation at industrial PERC solar cells. High-resolution diagnostics by focused ion beam techniques and scanning electron and transmission electron microscopy indicate the LECO-induced formation of microscopic contacts at the buried interface between the screen-printed silver finger and the silicon wafer. A large quantity of these micro contacts where found exclusively for LECO processed cells. Target preparation and three-dimension cross-section investigation reveal an interdiffusion of the Ag and Si material as the underlying root cause for improved local contact resistivity. We propose a descriptive model for local ohmic contact formation maintaining surface passivation.
... It is a downstream inline-solution, that can easily be added to a production line at manageable effort and cost. It has proven to decrease the off-spec share [2] and increase the peak efficiency of PERC cells [3][4][5] likewise. While latest publications show that the LECO treatment has no significant impact one long-term stability of solar cells [6] and modules [7], a first physical model describing the micro-scale LECO-contact formation was published in the beginning of 2021 by Großer et al. [8]. ...
... Indeed, the presented cell batch was declared by the manufacturer as the lowest efficiency group. Considering the discussed even larger impact of LECO on low efficiency cells additionally to the here found FF dependence as in references [2], [5], the calculated revenue is nevertheless in a realistic order of magnitude. ...
The Laser Enhanced Contact Optimization treatment has been shown to be beneficial for yield and efficiency improvement of PERC cells in the past three years. In this contribution, we analyze the impact of the LECO treatment on three batches of industrial PERC cells by different manufacturers applying statistical data analysis. Especially a correlation analysis method is used to identify the most relevant cell parameter for a beneficial LECO process. In the end, the data produced in this work are fed into a techno-economic analysis, to evaluate the economic benefit of integrating LECO into a PERC cell production line.
... This laserbased process results in improved metal-semiconductor contacts for crystalline silicon solar cells. It was reported that its application can even increase the overall cell efficiency of PERC solar cells [1]. LECO is a very sensitive treatment which does not cause any laser or heat induced damage [2]. ...
... Since we want to analyse the PID-s sensitivity for cells with different series resistance values (due to the LECO driven R s improvement), the parallel conductance (G p ) was used as comparison criterium. It can be calculated by taking the reciprocal value of the shunt resistance: 1) and is independent of the series resistance. In Figure 4 (b) the PID-s sensitivity on the G p is plotted for the untreated cells (batch 3) and LECO treated cells (batch 4). ...
In this work the influence of the LECO treatment on the typical PERC degradation mechanisms was investigated. It was found that the treatment has no significant impact on the Boron-Oxygen related Light Induced Degradation and Light and elevated Temperature Induced Degradation behavior. Furthermore, it is shown that stabilized cells remain stable after the LECO treatment. Also, we did not find any indication of LECO influencing the Potential Induced Degradation susceptibility. Finally, the variation of the process order of the LECO treatment and the B-O Stabilization Process was analysed and showed no restrictions or differences in the outcome of the solar cell quality or stability. Based on these results it is proposed to implement LECO as an approach to decrease the LeTID-sensitivity by combining LECO with modified firing processes. We claim that a combination of reduced peak firing temperatures with the LECO process leads to a decrease in LeTID sensitivity without any drawbacks in the cells’ efficiencies.
... The Laser Enhanced Contact Optimization (LECO) process is a fast, local and laser-based process for the optimization of metal semiconductor contacts and directly addresses the mentioned issue. LECO generally reduces the contact resistance between silicon and the metallization without harming the passivation [3]. Using LECO was previously shown to improve the cell efficiency up to +0.14 %abs for p-type PERC Ultra Low Doped Homogenous Emitter Cz Silicon Solar Cells featuring a special LECO-paste developed by Heraeus Photovoltaics [4]. ...
... (Fig. 4, right). As already discussed in [3] this gain arises, because of the specific LECO paste and process advantages of enabling improved metal semiconductor contacts without harming the passivation. Previously presented results from 2019 [4] showed a lack in fillfactor. ...
Screen-printed metal-semiconductor contacts as the mainstream-metallization of industrially produced silicon solar cells are currently a major topic in research, as they still show high potential for cell efficiency improvement. The Laser Enhanced Contact Optimization (LECO) process developed by the CE Cell Engineering GmbH enables the generation of a new contact structure that is capable of generating low ohmic metal-semiconductor contacts while preserving the contact passivation. In this study, we investigate the impact of the LECO process on monocrystalline p-type PERC solar cells that feature a front side metallization paste that was especially developed for the LECO process. As highlight from this investigation, a gain in short circuit current can be reported, additionally to a previously reported gain in open circuit voltage enabled by LECO.
... Within the project, we investigated three different possibilities to deal with this effect. Because the SC effect leads to partially underfired contacts, the LECO [12] process can be used to heal these areas. As it can be seen, in the EL image and in the corresponding value representing the series resistance in Figure 7a, the same contact quality can be achieved, when applying the LECO process on cells with high series resistances. ...
Being a key technology in silicon solar cell production, the firing process conducted in infrared (IR) lamp powered conveyor belt furnaces is well established in the market. The purpose of the project FEUERDRACHE is to advance the classic firing process by raising throughput, energy efficiency and process control via increasing the process variability. To achieve the latter, an industry-oriented furnace is expanded by innovative features, which are thoroughly investigated: 1) faster belt velocity; 2) novel IR lamps; 3) thermography as an inline-capable alternative to thermocouple measurements 4) high-power laser as an alternative to a heated chamber.
This article presents a successful laser-powered co-firing process for highly efficient Si solar cells as a more compact and energy-efficient alternative to the conventional firing process in an infrared (IR) lamp-powered heat chamber. The best cell group reaches with laser firing only 0.1%
$_{abs}$
lower cell efficiency compared to the best group with conventional firing, demonstrating the industrial potential of this laser firing technology. Adding the laser enhanced contact optimization (LECO) process after firing improves the cell efficiency for laser firing to the level of conventional firing, demonstrating the potential of the combination of the laser firing and the LECO process.
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