High-quality p-type tunnel oxide passivated contacts (p-type TOPCon) is a feasible technical solution to further improve the efficiency of TOPCon silicon solar cells. Chemical vapor deposition technology can prepare doped polysilicon layers and has become one of the most promising industrial routes for preparing TOPCon structures. POLY 5000 is a Built-in Thin Film Thickness Tester specially designed for photovoltaic processes. It adopts leading micro-nano film optical measurement technology and 100% Poly-si deposition process monitoring. It can perform fast and automatic 5-point synchronous scanning of samples. Using POLY 5000 can optimize the film thickness characteristics of the polysilicon layer and ensure cell yield.

TOPCon cell process

The TOPCon cell process is generally: texturing, diffusion, then preparation of the back tunnel layer and doped polysilicon layer (Poly-Si), then preparation of the front Al2O3 film layer, front and back SiNx film preparation, and finally screen printing of the front and rear electrodes and sintering .

The TOPCon structure consists of front SiNx film, Al₂O₃ film, P-type emitter (p+), N-type silicon wafer substrate, SiO2 film, N-type polysilicon film (Poly-Si), and back SiNx film.

The function of each film layer:

●Front SiNx film (about 75nm): Since SiNX is rich in hydrogen atoms, defects on the surface and inside the body can be chemically passivated during heat treatment, thereby reducing the recombination of surface electrons. At the same time, due to the optical properties of SiNX, front and back anti-reflection effects can also be achieved;

●Backside SiNx film: In order to avoid damage to the film layer by the slurry during the subsequent metallization and sintering process, SiNX relies on its chemical stability and is mainly used to protect the backside film layer; at the same time, it achieves anti-reflection effect;

●Al2O3 film (≤5nm), due to its high negative charge density, can provide good field effect passivation for P-type semiconductors such as the back of PERC cells and the front of TOPCon cells, that is, adding a layer of highly stable charge near the surface The dielectric film creates a gradient electric field near the surface, reducing the surface electron concentration and thereby reducing the recombination rate of surface electrons and holes.

●Ultra-thin tunneling layer SiO2 (<2.0 nm) and N-type polysilicon film (100~200nm): the two together form a passivation contact structure as the passivation layer on the back of the cell, and the highly doped polysilicon (Poly-Si) layer and The energy band bending at the interface caused by the difference in work function between N-type silicon substrates allows electrons to have enough energy levels to occupy after tunneling, making it easier to tunnel;

The edge of the valence band occupied by holes is in the forbidden band of Poly-Si and is not easy to tunnel. Therefore, the ultra-thin oxide layer can allow multi-carrier electrons to tunnel and block minority carrier holes from passing through, thereby separating electrons and holes, reducing Composite, depositing a layer of metal as an electrode achieves a passivated contact structure without opening holes.

Methods of preparing polysilicon layers

For doped polysilicon layers, there are generally three preparation methods. Two of them are chemical vapor deposition (CVD) methods: LPCVD method and PECVD method. There is also a sputtering method that belongs to the physical vapor deposition (PVD) method.Among them, LPCVD can complete the preparation of oxide layer and intrinsic polysilicon layer at the same time, and the industrial application technology is very mature.

But during LPCVD deposition, there are two problems.

First, during the preparation process, the tunnel layer and polysilicon layer that appear on the side and front of the cell will inevitably be attached to form a package.

Second, the way to solve this problem is to "remove plating". The process flow is as follows:

●HF acid single-sided cleaning to remove the phosphosilicate glass PSG in the surrounding plating area (i.e. front and side);

●Double-sided cleaning with KOH alkaline solution to remove doped polysilicon in the surrounding plating area (i.e. front and side). The backside PSG layer serves to protect the tunnel oxide layer and doped polysilicon layer;

●HF acid double-sided cleaning to remove SiO2 (i.e. front and side) and back PSG in the surrounding plating area;

POLY Built-in Thin Film Thickness Tester

E-mail: market@millennialsolar.com

Millennial POLY 5000 Built-in Thin Film Thickness Tester is specially designed for photovoltaic process monitoring. It can quickly and automatically scan the sample at 5 points simultaneously to obtain film thickness distribution information at different locations of the sample. The measurement size can be customized according to the size of the customer's sample.

●Effective spectral range 320nm~2400nm

●Fast, automatic 5-point simultaneous scanning

●Repeatability accuracy <0.5nm

●Ultra-wide measurement range 20nm~2000nm

●Online monitoring and detection achieves zero fragmentation rate

●Realize automatic inspection of the entire production line, greatly saving inspection time

In recent years, many companies have made great efforts to improve the performance of TOPCon cells using mature photovoltaic manufacturing processes such as LPCVD or PECVD and screen printing metallization. Millennial Solar proposes professional optical thin film measurement solutions to help companies become more comfortable in improving process technology and accelerate the development of the photovoltaic industry.