TOPCon is a tunnel oxide passivated contact solar cell technology based on the selective carrier principle. Its structure is an N-type silicon substrate battery. A layer of ultra-thin silicon oxide is prepared on the back of the battery, and a thin layer of doped silicon is deposited. The two together form a passivation contact structure, which effectively reduces surface recombination and metal contact recombination. It provides more room for further improvement of N-PERT cell conversion efficiency. This issue of Millennial Solar will introduce this high-tech photovoltaic cell design.

Working principle

TOPCon technology is compatible with the N-PERT bifacial cell production line, and large-scale production of N-type monocrystalline bifacial TOPCon cells can be achieved through simple modifications to the N-PERT bifacial cell production line. The tunnel oxide layer can be obtained by the thermal oxygen method. After reducing the oxygen partial pressure, the oxidation rate is slow, and a silicon oxide film layer with controllable thickness and uniformity can be obtained. The poly layer can be achieved by thermal decomposition of silane at high temperatures.

An ultra-thin tunnel oxide layer and a highly doped polysilicon thin layer are prepared on the back side of the cell. The two together form a passivation contact structure. This structure provides good surface passivation for the back side of the silicon wafer. The thin oxide layer can allow multi-carrier electrons to tunnel into the polysilicon layer while blocking the recombination of minority carrier holes, and then electrons are transported laterally in the polysilicon layer and collected by the metal, thereby greatly reducing the metal contact recombination current and increasing the open circuit voltage and short-circuit current of the cell. 

Production principle

LPCVD equipment can complete the preparation of tunnel oxide layer and poly layer in one stop. The two-in-one process of thermal oxygenation and deposition of poly layer can greatly increase production capacity and reduce equipment costs, which has great advantages over other equipment. After the hot oxygen process is completed, the poly layer is deposited under low pressure. In addition to saving time, more importantly, it can protect the ultra-thin silicon oxide layer. On the one hand, the oxide layer will not be further damaged during the unloading process, the tunneling effect is lost; on the other hand, the oxide layer will not be polluted in the air.

There are two ways to dope phosphorus in the Poly layer, namely in-situ doping and non-situ doping. In-situ doping is direct doping during the deposition of poly. Non-in-situ doping is done after the poly layer is deposited. Two methods can be used, one is ion implantation plus annealing, and the other is phosphorus diffusion. The ion implantation process has better single-sidedness, but the equipment is expensive, the production capacity is low, and the cost of large-scale mass production equipment is high. The one-sidedness of the phosphorus diffusion process is slightly worse, but this problem can be solved through process design, and phosphorus diffusion equipment is already very mature in the photovoltaic market, the price is low, and the process is very stable. All things considered, in the mass production of TOPCon cells, LPCVD is the most cost-effective choice to complete the preparation of the tunnel oxide layer and the Poly layer, followed by phosphorus diffusion.

POLY In-line 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

We will share the TOPCon solar cell technology here today. The commercial application of this technology is relatively mature. If you have any questions, you can contact us to discuss it.