The Future of Indian Solar Market: TOPCon Technology

Ankit Singhania, Director & Co-Founder, Navitas Solar

The Tunnel Oxide Passivated Contact (TOPCon) structure got more consideration for development of high-performance solar cells by the introduction of a tunnel oxide layer between the substrate and poly-Si, which is best for attaining interface passivation. When the Si solar cell’s efficiency approaches 23%, most of the recombination loss is due to metal contact. Therefore, contact passivation has been an enthusiastic investigation matter in the photovoltaic industry for several decades.

Out of the various resources and compositions, the TOPCon technology remains the industry’s greatest interest. In TOPCon technology, the metal does not come into direct contact with the wafer. Alternatively, a thin tunneling oxide is applied, followed by a layer highly doped with n or p type silicon that contacts the metal at the ends. The tunneling oxide does not affect the operation of the device, as the tunneling oxide blocks one type of carrier. Therefore, these structures are commonly referred to as the passivating contacts.

To improve the quality of passivation of the tunnel oxide layer, the physical properties and thermal stability of the thin layer must be considered. TOPCon silicon solar cell has a boron diffused front emitter, a tunnel-SiOx/poly-Si/ SiNx:H structure at the rear side, and screen-printed electrodes on both sides. This structure can result in a median efficiency of 23.91%, and the highest efficiency of 24.58%, independently. The conversion efficiency of interdigitated back-contact solar cells has reached up to 26% by enhancing the opto electrical properties for both-sides-contacted of the cells.

In comparison with the traditional silicon type solar cell, the TOPCon solar cell attains the subsequent purposes:

• The extreme fine layer of oxides satisfy the dangling bond which appear at the surface of the single crystal resulting in increased conversion efficiency.
• Depending on the conductivity type of the substrate, the extreme fine oxide layer allows to speed up the electron or hole transfer.
• With the consequence of the extreme potential in conductivity of the heavily doped silicon, the resistance of the junction can be decreased and the current output could be improved.

The development of appropriate pastes from metal is also evolving by the time of the photovoltaic manufacturing. According to the LCOE (levelized cost of electricity), n-type TOPCon cells are favorable contestants for mainstream after PERC efficiency is extended up to 23%.

There is a high concentration of electrons in the junction, thus the outcome is high loss of recombination of electrons in the solar cell, when the metal is in directly in-contact with the wafer of silicon. Previously, there were two approaches to reduce the carrier recombination of the metal precisely in contact with the silicon wafer.

• By minimizing the interface capacity and by means of acquired smoothly doping. That is used in Passivated Emitter and Rear Cell (PERC) technology, a traditional PN junction assembly with native doping only in the emitter contact area.
• By employing a slight film of the passivated silicon level, to distinct the silicon wafer from the metal. This process is Hetero-junction with Intrinsic Thin-layer (HJT) solar cell, which are about 26.7% efficient.

Fraunhofer Institute of Germany projected the N-type solar cell by complete distinct charge carrier selective back contact is a dual-faced connection solar cell, with a highest efficiency record about 26.7%. As shown in figure below of average cell conversion efficiency, it is expected that N-type TOPCon will achieve more than 28% after the year 2028.

Projected total wafer capacity by 2023’s end is estimated to reach around 1172 GW. The majority of this newly added capacity is attributed to N-type TOPCon cell technology. N-type wafer capacity is expected to reach 676 GW accounting for 57.7% of the total capacity. Other technologies like HJT has been also evolved with the time and will capture a significant percentage of the market in future but N- type TOPCon technology will dominate the Indian solar market by the next year.

According to the studies, N-type TOPCon solar cell bifaciality and conversion efficiency is inspected by optimizing the finger width of rear side contact, which is shown in the figure below. As the width of the rear side finger increases, the efficiency of the rear side cell decreases but the efficiency of the front side of the cell increases slightly. The optimum value of rear side finger width comes out to be 100 μm that gives the rear side efficiency 19.85% and front side efficiency 23.35% with the optimum efficacy on both side is 27.37% as shown in the figure below.

We, at Navitas Solar, a leading module manufacturing company believe that in the upcoming years, N –Type TOPCon technology will dominate the Indian solar Market. We at Navitas Solar will be manufacturing N-type TOPCon modules up to 20 bus bars with power generation capacity of 720 watts per panel. N- Type TOPCon Bifacial panels will be the choice of consumers in the upcoming future.