The efficiency of traditional manufacturing solar cells has improved a lot with an efficiency of up to 22% commercially available in the market from PERC solar cells, IBC, HJT, and shingled crystalline cells. Out of the wide range of different cell structures, contact passivation has been a great area of interest for several decades, particularly TOPCON cell technology.
Figure 1: (a) P-Type - PERC Technology (b) N-Type TOPCON structure
Source: AE Solar_ISC Konstanz
The idea of TOPCON technology is that the metal does not come into direct contact with the wafer. TOPCON has an N-Type Silicon substrate and a thin tunneling oxide is applied, followed by a layer of highly doped with n or p poly silicon that contacts the metal at the ends. These tunneling oxide blocks one type of carrier and thus they are called passivating contacts. Compared to the Heterojunction technology (HJT) which has both amorphous and crystalline thin film the manufacturing process is completely different compared to TOPCN technology which is just one upgrade from the PERC structure by just adding a passivation oxide layer. By changing the substrate material from P-Type silicon to N-Type silicon material the existing PERC line in the market can be upgraded to TOPCON structure by just adding the tunneling passivation layer. This means there are capital/manufacturing costs compared to other technologies like HJT /IBC (interdigitated back contact) which requires a different type of architecture, which will be lower.
The TOPCON Solar cell structure:
A typical crystalline solar cell is made of N-type and P-type semiconductors as substrate material. If you want to know more about how are solar panels made reference to one of the previous articles here “Solar Panel Manufacturing”. In TOPCON technology the cell is passivated by an Aluminum oxide (Al2O3) fine layer on the surface of the P-type material and contains a very fine layer of about 2 nm Silicon dioxide (SiO2) tunneling layer and a highly polysilicon thin film below the SiO2 layer. The important part of Topcon technology is that the passivation happens due to the tunneling thin oxide film and the highly doped polysilicon layer helps to attain the minimal amount of recombination.
Process of TOPCON solar cell:
In comparison with the traditional crystalline structure the TOPCON structure has
Ultra-thin oxide layer
Poly silicon doped film
- Ultra-thin oxide layerUltra-thin oxide layer is usually around 1.4 – 1.5 µm. The simplest method to produce the fine layer of oxide is wet chemical dipping in the solution of an acidic mixture at more than 90°C temperature. The extremely fine layer of oxides enhances the conversion efficiencies by attaching to the dangling bonds at the surface of the single crystal. This also allows extreme electron/hole quick transfer.
- The doped polysilicon film
The massive doping of the polysilicon layer is created by LPCVD (Low-Pressure Chemical Vapour Deposition) or PECVD (Plasma Enhanced Chemical Vapour Deposition). With a highly doped polysilicon thin film, there will be an increase in conductivity and the resistance of the junction can be decreased and the current output could be improved.
Fabrication of TOPCON process:
Figure 2 represents the fabrication process. The sequence of cell construction is shown below. The texturizing with potassium hydroxide (KOH) on both sides. After that using a boron tribromide (BBr3) solution boron diffusion is created After cleaning with chemicals, a tunneling SiOx is thermally developed and a pure poly silicon layer is accumulated in an LPCVD system. The boron emitter is passivated with a film of the dielectric layer. The back side SiOx/n+-poly-silicon layer is covered with PECVD SiNx: H. The metallization contacts are screen printed on both sides.
Figure 2: Process diagram of production of TOPCon Solar Cell
Source: Reference 2
TOPCON cell efficiency progress:
The TOPCON solar cell efficiency was reported at the laboratory scale in the first and second quarters of 2013 at about 21.8% and 22.9%. In 2014 the efficiency of TOPCON solar cells was improved to 24.3%. In the last quarter of 2015, the efficiency was reported at about 25.1%. On the laboratory scale, the TOPCON solar cell got an efficiency of about 25.8% which is reported in 2017. N-type mono-crystalline-based bifacial TOPCON solar cells had reached 25.09%. This efficiency is set to increase to 28% after 2028 and TOPCON solar production is set to increase up to 14GW by the year 2025. Figure 3 shows the commercial / laboratory efficiency scale by several institutes and companies for TOPCON solar cells.
Figure 3: TOPCON Solar cell efficiency
To compete with the PERC technology currently available in the market maintaining a high conversion efficiency in mass production is a key aspect. Although recent development has led to a gap in the efficiency between PERC and TOPCON cells, a proper standard process flow for TOPCON solar cells is yet to be established.
- Hasnain Yousuf, & Sanchari Chowdhury. (2021, September). a review on Topcon Solar Cell Technology - . a review on Topcon Solar Cell Technology -. from https://www.researchgate.net/publication/355248522_A_Review_on_TOPCon_Solar_Cell_Technology.
- Chen, D., Chen, Y., Wang, Z., Gong, J., Liu, C., Zou, Y., He, Y., Wang, Y., Yuan, L., Lin, W., Xia, R., “24.58% total area efficiency of screen-printed, large area industrial silicon solar cells with the tunnel oxide passivated contacts (i-TOPCon),” Solar Energy Materials and Solar Cells, Vol. 206, pp. 110258:1-8 (2020).
Authors: Vidhyashankar Venkatachalaperumal, Afshin Bakhtiari