This paper mainly studies TOPCon (Tunnel Oxide Passivated Contact) battery technology that can be applied to large-scale production. This technology can improve the passivation of the battery surface and promote the transfer of most carriers, thereby improving the open circuit voltage and fill factor of the battery. At present, Yingli introduces TOPCon technology on the basis of Panda batteries. The battery efficiency can reach 21.6%, the open circuit voltage reaches 676mV, and the fill factor reaches 80%. There is still much room for improvement in open circuit voltage and fill factor. Panda-TOPCon double-sided battery technology It will greatly reduce the cost per watt of N-type battery and enhance its competitiveness.

Keywords: Panda-TOPCon battery; tunneling layer; Poly-Si layer

1 Research background and content

With the improvement of the quality of silicon wafers, surface recombination of crystalline silicon cells has become the main factor restricting their efficiency, and surface passivation technology is particularly important. TOPCon has become a research hotspot as a new type of passivation technology. This technology is to grow an ultra-thin tunnelable oxide layer and a highly doped polysilicon layer on the surface of the battery. The passivation and high doping of the oxide layer The field passivation of the heteropolysilicon layer can greatly reduce the rate of minority carrier recombination. At the same time, the highly doped polysilicon layer has good conductivity for the polyson, so the TOPCon battery has a high open circuit voltage and fill factor. In 2015, FraunfhoferISE uses N-type fused silicon wafers, pyramid velvet on the front, boron diffusion, aluminum oxide plus silicon nitride laminated film for passivation and anti-reduction, TOPCon technology on the back, and metallization on both the front and back. /Pd/Ag laminated structure, the cell efficiency reached 24.9% [2-4].

At present, the TOPCon technology British interest rate first applies this technology to the N-type battery production line to prepare high-efficiency and low-cost Panda-TOPCon double-sided batteries with an efficiency greater than 21.5%.

2 Battery structure

The Panda-TOPCon battery is based on the conventional Panda battery with a tunnel oxide layer and a phosphorus-doped polysilicon layer, as shown in Figure 1. The battery adopts Czochralski N-type monocrystalline silicon wafer, the front side is boron diffusion emitter, silicon oxide/silicon nitride passivation antireflection layer and metal electrode; the back side is tunnel oxide layer, doped polysilicon layer, nitride Silicon passivation anti-reflection layer and metal electrodes.

3 Research results and discussion

3.1 SiOx/polySi passivation performance study

Take N-type Cz silicon wafers of 156cm×156cm, and carry out double-sided boron diffusion (B|B) symmetric structure, double-sided SiOx/polySi (P|P) symmetric structure, front boron diffusion + back SiOx/polySi (B|P ) Asymmetrical structure. After that, these three samples were simultaneously subjected to double-sided silicon nitride film deposition and sintering treatment. Using the Sinton tester to measure the ImpliedVoc (mV) of these three samples, the test results are shown in Figure 2.

It can be seen from Figure 2 that after double-sided SiOx/polySi passivation, ImpliedVoc can reach more than 700mV. Even with B|P structure, ImpliedVoc is also above 680mV, which is significantly higher than the ImpliedVoc value of the conventional B|B structure. It can be seen that SiOx/polySi The layer can play a good passivation effect.

3.2 Effect of SiOx/polySi on battery electrical performance

In order to further verify the effect of SiOx/polySi on battery electrical performance, 60 N-type Cz silicon wafers are divided into three groups, which are conventional Panda batteries. G2 and G3 are TOPCon batteries. The main difference between the two is the thickness of polySi. It can be clearly seen from Figure 3 that the TOPCon junction battery has an open voltage of up to 660mV and a maximum of 667mV, which is 15mV higher than the Panda battery, and the current gain also reaches 150mA. Although the FF is reduced by 1.5%, the overall Eff has been significantly improved .

3.3 Research on the influence of back doping performance on FF

In order to study the influence of the doping concentration in polySi and the depth of BSF on the battery electrical performance, 150 N-type Cz silicon wafers were divided into three groups, and the polySi layer was divided into low (L), medium (M), and high (H). For different kinds of doping, Figure 4 shows the ECV diagram after doping. The square resistances corresponding to the three different doses are 52Ω/□, 43Ω/□, and 22Ω/□, respectively, and the distribution of phosphorus ions gradually increases.

Table 1 lists the battery parameters corresponding to three different doping concentrations. It can be seen from Table 1 that as the impurity concentration of the doped layer of the polySi layer increases, the battery fill factor increases slightly, but after the doping depth reaches a certain level, the open voltage and current decrease greatly, which is mainly due to the high The doping concentration will affect the tunneling oxidation effect.

3.4 SiOx/polySi layer optimization

At present, Panda-TOPCon batteries are still in the process of process optimization. By adjusting the process parameters, the SiOx layer and the polySi layer are matched with different thicknesses. Table 2 lists the battery parameters for four different thickness combinations.

It can be seen from Table 2 that different thickness combinations of SiOx layer and polySi layer have little effect on battery efficiency, and it can be seen that this process has a wider process window. On the other hand, group A2 corresponds to a higher fill factor, which also gives the direction of increasing the fill factor. At the same time, the highest conversion efficiency of 21.6% was obtained in this experiment, and the maximum open voltage reached 678mV. Figure 5 shows the I-V diagram of the cell.

4 Conclusion 

This paper studies the N-type Panda-TOPCon battery technology that can be mass-produced, and a large-size TOPCon battery with a conversion efficiency of more than 21.5% is prepared on the Yingli Panda production line. The study found:

1) The SiOx/polySi structure has a significant passivation effect;

2) The doping concentration of polySi has little effect on FF, but the high doping concentration will reduce the cell open voltage;

3) The film structure of SiOx/polySi has a certain influence on the battery fill factor.

4) Effect of different battery back-field preparation methods on efficiency

High temperature phosphorus diffusion (efficiency 20.5%)

Ion implantation (21% efficiency)

TOPCon (efficiency 21.5%) phosphorus-doped polysilicon

TOPCon technology can use LPCVD or PECVD to replace phosphorus ion implantation, saving equipment investment. The use and maintenance costs are extremely low, and the battery conversion efficiency is higher. Compared with selective back field technology (efficiency 21.5%), it can also save laser equipment investment costs. Compared to double-sided PERC batteries, TOPCon batteries are more suitable for double-sided batteries and are compatible with P-type and N-type batteries.