TOPCon technology achieves carrier-selective transport through ultrathin SiOₓ and phosphorus-doped poly-Si (n⁺-poly-Si) layers with theoretical efficiencies up to 28.7%. However, there is a contradiction in the thickness of the poly-Si layer: too thick a layer (>100 nm) increases parasitic absorption losses, while too thin a layer (<50 nm) is not resistant to silver paste corrosion and leads to metallization contact failure.

In this paper, n⁺-poly-Si layers with a thickness gradient of 30-100 nm are prepared by measuring the film thickness of poly-Si layers with a Millennial online poly film thickness tester, and the effect of the thickness on the contact resistivity (ρc), the compound current density (J₀,metal), and battery efficiency, to provide a basis for industrialized thin-layer process.

Preparation process

n-type Cz-Si wafers (330.15 cm²) were used to deposit 1.6±0.2 nm SiOₓ and 30~100 nm intrinsic amorphous silicon (a-Si) layers by LPCVD after fluffing in alkaline solution. a-Si layers were crystallized into poly-Si at 850°C and in situ doped with phosphorus (POCl₃/O₂/N₂). ₂ atmosphere). The phosphosilicate glass (PSG) was subsequently removed with 5% HF solution and the doping concentration was determined by electrochemical capacitance-voltage (ECV) profiling. Finally, Ag paste was printed on the H-grid using screen-printing technique and sintered at 730°C to form metal contacts.

Effect of poly-Si layer thickness on passivation performance

 The ECV profiles of different thicknesses of n⁺-poly-Si layers show that the surface concentrations are all higher than 5 × 10²⁰ atoms/cm³, but the total doping increases with the thickness. The square resistance (R□) increases from 45 Ω/□ to 57 Ω/□ when the thickness decreases from 100 nm to 30 nm, indicating that the insufficient total doping of the thin layer leads to the elevated resistance.

The variation of J₀ value with thickness is small (~0.5 fA/cm²), but J₀ is slightly higher than that of the 100 nm sample at 30 nm due to the shallow depth of phosphorus diffusion (~15-20 nm) and the enhancement of Auger complex. Photoluminescence (PL) tests show that the 30 nm layer is still effectively passivated, corroborating the field-effect passivation of the SiOₓ layer.

The metal contact composite mechanism

J₀,metal increases significantly with the thinning of the poly-Si layer: from 70 to 30 nm, J₀,metal rises from 304 fA/cm² to 545 fA/cm².

SEM analysis shows that Ag slurry corrosion leads to inhomogeneous interfaces in the thin poly-Si layer (<70 nm), with localized regions being penetrated and Ag particles precipitating to intensify the composite. The thick layer (100 nm) has the lowest J₀,metal (26 fA/cm²) due to the high doping concentration and corrosion resistance.

In addition, ρc decreases and then increases with thickness, reaching an optimum value (3.9 Ω-cm²) at 70 nm, indicating that the thickness is needed to balance the doping concentration with the Ag corrosion depth.

Optimization of I-V parameters

The 70 nm thick-layer cell achieved a conversion efficiency of 25.47% with Voc of 723.5 mV, Jsc of 42.01 mA/cm², and FF of 83.8%. Although the J₀,metal (304 fA/cm²) is higher than the 100 nm layer, the current gain compensates for the loss of Voc. It is noteworthy that for thicknesses less than 40 nm, Jsc does not improve due to the reduction of parasitic absorption, but rather the photogenerated electron collection efficiency decreases due to Ag corrosion.

Optical vs. quantum efficiency

The internal quantum efficiency of the 70 nm thick layer is significantly higher than that of the 100 nm layer at < 550 nm short wavelengths, attributed to the reduction of blue light absorption by the low total phosphorus doping concentration. At the same time, the reduced near-infrared parasitic absorption loss of the thin layer increases Jsc by 0.14 mA/cm², which needs to be balanced against the increased reflectance due to front surface weave damage.

This paper investigates the effect of n+poly-Si layer thickness on the metallization contact performance of n-TOPCon solar cells. The results show that the n+poly-Si layer thickness has a small effect on the J₀ value, but a significant effect on the J₀,metal value and contact resistivity. The optimal poly-Si layer thickness of 70 nm allows for low contact resistivity while maintaining a low J0,metal value, thus increasing the cell conversion efficiency to 25.65%. This optimized process can be widely used in the PV industry to reduce processing time and cost.

Millennial Online Poly Film Thickness Tester

Email：market@millennialsolar.com

Adopting micro and nano thin film optical measurement technology, it is able to realize ultra-wide measurement range 20nm-2000nm and 0.5nm ultra-high repeatability accuracy, and it can carry out fast and automatic 5-point synchronous scanning on the sample.

Poly Film Thickness Measurement Range 20nm-2000nm

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Millennial online Poly Film Thickness Tester can monitor the poly-Si layer thickness in real time, which provides data support for the study of TOPCon batteries, and reveals the mechanism by which the n⁺poly-Si layer thickness regulates the efficiency improvement of TOPCon batteries.