TOPCon Technology - Here's Why it is Buzz Technology in Solar Industry

The TOPCon solar cells are generally made from N-type cells by adding some additional layers of SiO2 and phosphorus-doped poly-silicon. Their role is to improve the passivation at the contacts leading to higher open circuit voltage of the solar cells and hence higher cell efficiency.

July 25, 2023. By News Bureau

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TOPCon has been a buzzword in the solar industry since 2016, but it officially entered mass production only in 2019. We all know that various cell technologies are competing in the market, and manufacturers are constantly looking for new alternative technologies to develop better and more efficient panels because the limits of PERC cells have been reached. TOPCon technology (Tunnel Oxide Passivated Contact) is a cutting-edge cell technology that is causing a stir in the industry.

What is TOPCon Technology?
Solar cells are made from silicon wafers. In order to generate voltage from sunlight (hence the name photo-voltaic), the silicon wafer is doped with chemicals. When the silicon wafer is doped with boron it creates P-type silicon, whereas doping with phosphorus creates N-type silicon. When the N-type and P-type silicon come in contact, a junction is formed (the P-N junction) which generates an electric field in the material and helps in developing the voltage across the solar cell. An N-type solar cell basically means the silicon wafer was initially N-type (i.e. phosphorus doped) on which some boron was diffused on the top side to create the P-N junction. Since the bulk silicon is free from Boron, these N-type cells do not degrade in sunlight like the P-type cells (a phenomenon called LID, due to the formation of Boron-Oxygen complexes in P-type solar cells).

The TOPCon solar cells are generally made from N-type cells by adding some additional layers to the cell. The additional layers (of SiO₂ and Phosphorus-doped poly-silicon) are added at the rear side of the solar cell before adding the metallic contacts. Their role is to improve the passivation at the contacts (basically facilitating the transfer of the light-generated electrons from the silicon cell to the metallic contact with minimum losses). This leads to higher open circuit voltage of the solar cells and hence higher cell efficiency.

TOPCon Solar Cell Operation

Fig. 1: (a) 3D view of the TOPCon solar cell [1], and (b) Cross-sectional view of the TOPCon solar cell [2]

Sunlight is incident on the solar cell from the top side (and also from the bottom side in the case of bifacial cells) and gets absorbed in silicon material.

Absorption of sunlight generates electron-hole pairs. Actually, the electrons in the valence band absorb the light energy and jump to a higher energy state (conduction band) in which it can flow freely from atom to atom (not bound to any one atom). Holes are basically the vacancies left behind in the Valence Band when the electron jumps to the conduction band. These holes can be considered as positive charge carriers (but they move in the valence band in the opposite direction to electrons).

At the junction of the P-type and N-type regions, there is an electric field that causes the photo-generated electrons to flow to the N-type region and the corresponding Holes to the P-type region. This separates the electron from the hole, which would otherwise recombine to give backlight.

These photo-generated electrons that cross the P-N junction region, now move to the bottom side of the N-type layer towards the rear contact.
Electron-hole pairs are also getting generated in the bulk (N-type layer). If these holes are present near the rear contacts, then they can cause recombination (as Metal - Silicon Contact region acts as a powerful recombination centre), leading to the loss of the photo-generated electrons.

Here the concept of Tunnel Oxide Passivated Contact (TOPCON) comes into play. The 1-2 nm thin layer of SiO₂ tunnel oxide acts like a selective gate allowing only electrons to pass through it to the rear side (and stopping any holes). Further, the n+ Poly silicon layer creates an electric field that attracts the electrons (and repels the holes). This ensures that a high number of electrons reaches the bottom side silver contacts where they are collected by the interconnect ribbons and sent to the external load circuit.

Pros of TOPCon Technology

Manufacturing Process
It can be manufactured in the same machines as P-type Multi-Busbar modules (like the Mono PERC production lines), which means that the module manufacturers have to bear no additional cost to manufacture TOPCon modules.

Higher Efficiency
Compared to P-type Mono PERC cells, TOPCon cells are more efficient at converting solar energy into electricity, increasing both cell and module efficiency. The maximal efficiency of TOPCon cells is 28%, while that of PERC cells is only about 24 percent.

Lower Degradation
In comparison to PERC panels, TOPCon modules exhibit a reduced rate of power degradation both in the first year and over the course of the panel's 30-year operational life.

Lower Temperature Coefficient
The percentage of power production lost by a solar panel with each degree of temperature increase is known as the temperature coefficient. In hot conditions, it affects the power generation of PV modules.

Since TOPCon cells are less impacted by temperature increases, TOPCon modules' effectiveness will be higher in hot climates compared to PERC Modules.

Bifaciality Factor
TOPCon cells have a higher Bifaciality factor than PERC modules. It is an important factor as bifacial solar modules are getting more and more popular in the market.

Excellent Low Light Performance
Since TOPCon modules are more efficient in low light, the daytime power generation period is extended and the annual energy output of the PV system is increased.

Rayzon adopted TOPCon technology because it allows improving the overall performance of the solar panel system which increases its electricity production by absorbing more photons from sun rays in the highest latitudes area of the earth.

In conclusion, Rayzon Solar is one of the leading manufacturers in the solar sector adopting cutting-edge TOPCon technology for commitment to sustainability. We urge you to choose solar panels to generate electricity and move from black energy to green energy. Let's make the future more vibrant and sustainable.

- Hardik Kothiya, Director, Rayzon Solar