Laser-assisted Localisation of Polysilicon Contact for High-efficiency Silicon Solar Cells

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Chinese explanation / 中文解读

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Original abstract

Laser-doped selective emitter (LDSE), as an efficiency-boosting technology, has become a standard feature on the front side of Passivated Emitter and Rear Cell (PERC). Localised polysilicon-on-oxide contacts (LPC) can be a viable upgrade to LDSE on the front side since, compared to LDSE, LPC has a lower metallised recombination current density (J0;metal) while maintaining a similar level of contact resistivity when contacting with silver. Therefore, LPC has the potential to replace LDSE and enable further increments in the efficiency of PERC. Vibrational spectroscopies have been used to extract vital physical parameters during the formation of polysilicon-on-oxide contacts or poly-Si contacts. Raman spectroscopy reveals that the FWHM parameter is sensitive to the evolution of poly-Si crystallinity throughout the fabrication of poly-Si contacts. The FWHM can also show the difference between the poly-Si layers doped with either Boron or Phosphorous. The FTIR study on the phosphorous-doped poly-Si further suggests that poly-Si layers with an ex-situ diffusion process can lead to increased phosphorous oxygen bonding compared to the other poly-Si layers fabricated with an in-situ doping process. The red shift of the FTIR peaks of the interfacial oxide layers in the poly-Si contact samples implies that the interfacial oxide thickness increases during the poly-Si contact formation. The localisation of poly-Si contact has been successfully demonstrated by employing a combination of laser process and a KOH-based etching solution. Contrary to the achievement of LPC using a shadow mask or photolithography, our laser-based approach to localised poly-Si contact has the main advantage of the ease of processing flow. The experimental values of J0;metal and contact resistivity (ρc) of the laser-based LPC are 250 fA·cm-2 and 1.7 mΩ·cm2, an improvement on LDSE. The J0;metal and ρc values are fed into a PERC model in Quokka3, and the estimated efficiency gains when LPC replaces the LDSE on a state-of-art PERC is 0.1 %abs. The model suggests that upgrading the LDSE to LPC in PERC will lead to an efficiency gain of up to 0.5 %abs. The model also suggests that the efficiency gains from reducing J0;metal of the front contacts (either LDSE or LPC) will diminish when J0;metal is lower than approximately 100 fA·cm-2. A techno-economic analysis is performed to evaluate the feasibility of adopting the proposed approach to achieving LPC. Although the process simplicity of LPC is an advantage, the efficiency gains from using LPC on PERC do not fully balance the added cost introduced by the required additional processing steps. However, based on the existing experiments, one key advantage of LPC is its compatibility with aluminium paste instead of conventional silver paste. The techno-economic model indicates that every 10% substitution of the silver paste for aluminium paste can lead to a 2.8% discount on the final dollar-per-watt cost of a finished PERC+LPC cell.

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