1. Poly+ annealing mechanism

① Depositing a tunneling oxide layer (SiO₂) and a doped amorphous silicon layer on the back to form a selective carrier transport channel (enabling electron tunneling and hole blocking).

② Annealing process: high temperature crystallization of amorphous silicon layer, activation of P phosphorus doping and reduction of contact resistance.

 

  1. Factors affecting Poly+ annealing efficiency

① Contact passivation: The thickness of the tunneling oxide layer needs to be controlled at 1.5nm (too thin passivation is poor, too thick tunneling failure → contact resistance ↑);

② Doping and film thickness: poly-Si doping concentration/film thickness directly affects the back contact resistance (insufficient film thickness → metal penetration-induced recombination ↑ → Voc↓; insufficient doping concentration → contact resistance ↑ → FF↓);

③ Parasitic absorption: A too-thick poly-Si film leads to increased light absorption loss (reducing Isc), so the film thickness should be minimized while ensuring passivation and contact performance (typically, 50nm is the optimal balance).

④ Crystallinity: grain boundary recombination↑ (Voc↓);

⑤ Phosphorus diffusion depth: excessively deep diffusion → increased back-surface recombination current.

 

  1. Improvement direction

① Adjust the thickness of the poly doped layer (comparison of lightly doped/heavily doped area doping amount ±10–20%);

② Optimize annealing parameters (temperature by ±5–15%, soaking time by ±10%, and verify contact performance);

③ Use local thin poly technology (such as poly finger to reduce parasitic absorption).

 

  1. Consider from the perspective of contact, passivation, and parasitic absorption of the film layer:

① For contact passivation, there is an optimal range for the thickness of the tunneling oxide layer (generally considered to be optimal at 1.5nm). If the film thickness is too thin, the passivation effect is not good; if the film thickness is too thick, the tunneling effect is weakened or even fails (manifested as increased contact resistance and increased square resistance after annealing).

② The doping concentration and film thickness of poly-Si need to be controlled, and the two need to be focused on. The size of the back contact resistance and the recombination of the metal-semiconductor contact area are determined by the combination of the two. The thickness of the poly-Si film is too low, and the back metallization may penetrate the poly-Si thickness, the contact area is compounded, and the Voc is reduced; the doping concentration is not enough, the contact resistance is too high, Rs increases, and FF decreases.

③ The parasitic absorption problem of the film layer, the parasitic absorption rate of the poly-Si film layer is high. Film thickness optimization principle: Under the premise of ensuring passivation and contact, reduce the poly-Si film thickness as much as possible. It is generally believed that 50nm thick doped poly-Si can ensure the passivation effect, mainly considering the back metal sintering depth.