Laser Patterning of poly-Si Passivated Contacts for Crystalline Silicon Solar Cells
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2024-02
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Melbourne Authors
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Electrical and Electronic Engineering
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Masters Research thesis
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© 2024 Yijun Wang
Abstract
In crystalline silicon photovoltaics (PV), a pulsed laser can be used as a substitute for a high-temperature furnace step to crystallize a film or activate dopants. In contrast to furnace-based annealing, it can be used to achieve highly localized crystallization/doping activation with controlled dopant profiles. This is useful in advanced architectures that require finely patterned areas, such as the interdigitated back contact (IBC) cell.
In this Thesis, we first utilize a pulsed ultraviolet (UV) laser for phosphorus dopant activation within a ~30 nm thick low-pressure chemical vapor deposited (LPCVD) polycrystalline silicon (poly-Si) passivated contact layer. This method effectively activates doping only within certain regions with controlled dopant profiles, offering results that rival traditional high-temperature furnace annealing techniques. This laser activation step allowed implied open-circuit voltage iVoc values up to 726mV. However, this came at the expense of specific contact resistivities pc, which were significantly higher than that achievable with furnace-based annealing. We attribute this higher pc to a lower dopant concentration across the poly-Si(n+)/SiOx/c-Si interface. Regardless, the optimum fluence produced an iVoc of 712 mV and a pc of 89 milliohms-cm2 which are still compatible with high-efficiency cell designs, underscoring the feasibility and effectiveness of this approach.
Next, we explored the UV laser-based dopant activation for plasma-enhanced chemical vapor deposition (PECVD) poly-Si passivated contact layers with different thicknesses. Despite a similar ability to control dopant concentration, both 50 nm and 10 nm PECVD poly-Si contacts failed to achieve high iVoc, reaching ~685 mV and ~695 mV, respectively. We suggest that this difference in behavior between PECVD and LPCVD films is attributable to the different SiOx layers used in these two contact stacks and suggest that further investigation is required to solve this issue.
Finally, we introduced a novel approach to locally crystallizing an amorphous titanium dioxide (TiO2) layer using a UV laser. The etch resistance of rutile phase TiO2 is much higher than that of amorphous TiO2, allowing the creation of an etch barrier against agents such as dilute hydrofluoric acid (HF) and potassium hydroxide (KOH). We find an appropriate laser fluence to crystallize the TiO2 without ablating the film and show its stability in short dilute HF dips. The resistance of the laser-crystallized TiO2 remains low in KOH etching and will need to be improved before it can serve as a viable etch barrier.
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Dopant activation, pulsed-laser doping, localized doping, poly-Si, LPCVD, PECVD, passivating contacts, TiO2 crystallization, Laser-patterning.