Challenges with N-Type Solar Cell Technologies : ToPCoN Vs HJT. The Way Forward...

Conventional cell manufacturers favor TOPCon technology, as it is, like PERC, a high-temperature process, and thus more compatible with the latter. This year, many new PERC lines have been reserved for upgrading to TOPCon technology. Despite announcement of several manufacturers’ engagement in TOPCon, few expansion projects were materialized in the ground, due to difficulties in deposition processes. Only Trina, Suntech, and BYD planned to advance to mass production this year, while Longi and Tongwei claimed to have GW-scale TOPCon capacity arranged respectively, followed by Jollywood planning for a 16GW factory shortly early next year.

Presently, mainstream efficiency of TOPCon cells in mass production is around 23.7-23.8%, with some manufacturers claimed to have achieved 24.0%+. However, as efficiency of PERC markedly rose, TOPCon still fell behind by a certain margin.

MAJOR CHALLENGES OF ToPCoN

• In ToPCoN process, Boron deposition needs?900℃ to 1100℃ due to lower solid solubility in silicon compared to Phosphorous.?Typically, BBr3 is used as the dopant gas for deposition, which also severely damages quartz components. For now, more and more manufacturers turned to BCl3, of which the associated production hardly does harm to quartz components. However, subject to longer bond length of B-Cl, the diffusion Uniformity of BCl3 is slightly inferior to that of BBr3. A clear cut advantages or disadvantages of BBr3 or BCl3 is yet to be a mystery component.
• The requirements of clean room conditions for ToPCoN (Class 10,000) is different from PERC (Class 100,000), so manufacturers have to know this information upfront when they are planning for conversion from PERC to ToPCoN. Else, the achieving the required efficiency remains a question
• Currently, selective emitter (SE) is unable to apply to TOPCon front emitter, for such combination has rather limited rooms for minimizing sheet resistance, and thus greatly affects the improvement of short-circuit current and contact resistance. If that's case the SE (laser tool) will be of no use when we upgrade from PERC to ToPCoN? Nobody is talking about this and an important point to note when we a manufacturer wants to upgrade from PERC to ToPCoN
• Presently, the mainstream ToPCoN process is to grow a 1.5-2nm tunnel oxide layer through thermal oxidation and150-200nm of intrinsic polysilicon layer through LPCVD, and then dope through phosphorus diffusion. However, production efficiency of this process is low. Additionally, given the issue of wraparound polysilicon layer, yield rates of this process is not more than 90-95%, far lower than the 98% of PERC. To break the bottleneck, more and more equipment suppliers started trying new deposition technologies, such as PECVD, PEALD, and PVD. Equipment for new techniques emerged successively, but without mass production data, disputes over TOPCon technology roadmap will always be persisting.
• Both TOPCon and PERC use high temperature sintering silver paste. However, TOPCon uses silver paste on both sides. Taking M6-sized cells for example, a TOPCon cell typically requires 130mg of silver paste, 60mg more than a PERC cell. This metallization costs of TOPCon can never go lower than that of PERC, owing to the application of silver pastes on both sides of a cell. Only by further raising cell efficiency and yield figures can decrease the differences of cost per watt for ToPCoN, as compared to that of PERC.

HETEROJUNCTION TECHNOLOGY (HJT)

With higher potential gain in efficiency and rather simple processes, HJT became an instant hit in the capital market and competing to become a potential market leader in N-type cell technology. However, subject to higher equipment investment and production costs, capacities materialized will be lower, whilst production expansions come online slowly in 2021. Presently, efficiency of HJT cells has officially passed the 24%+ mark. Methods of raising efficiency and the replacement of nc-Si for a-Si, are all highly equipment-dependent. As equipment upgrade, suppliers and manufacturers both claimed to reach a cell efficiency of 25% beyond in mass production by 2022.

MAJOR CHALLENGES OF HJT

• With broader localization of equipment manufacturing, the investment capex for HJT has declined to around RMB 450 million/GW and even RMB 400 million/GW, still higher than the RMB 150-200 million/GW of PERC and RMB 250/GW of TOPCon. The high-intensive capital costs of investments in equipment not only affect the pro-active efforts of upfront investments, but also indicate greater depreciated non-polysilicon costs later. At the moment, low utilization rates of HJT manufacturers result in at least RMB 0.03/W higher depreciated costs than that of PERC.
• Unlike PERC and TOPCon that uses high-temperature silver paste for firing process, HJT applies low-temperature cured silver paste, which is relatively less matured, and thus higher selling prices. HJT requires higher silver paste per unit,?because of its bifacial nature and higher resistivity of low-temperature cured silver paste. For instance, a M6 HJT silver laydown is ~ 200mg, while a M6 ToPCoN required a silver paste laydown of 130mg. Metallization costs of HJT is RMB 0.12/W higher than that of ToPCoN, as calculated with current unit consumption of low-temperature cured silver paste and its exhorbitant prices.?
• To further circumvent the metallization costs associated with HJT, the industry focuses on new technology roadmaps, such as silver coated copper paste and copper electroplating. The development of copper electroplating is slow, given the additional investments, yield rates, and environmental concerns. Some manufacturers completed tests and proceeded to reliability stage for silver coated copper paste, which is expected to enter small rate initial production by next year. It is reportedly that 30% of copper allows cell efficiency to sustain but is less economically competitive, whereas 40% of copper may lead to 0.1-0.3% of losses in cell efficiency and most importantly, reliability concerns owing to exposed copper getting oxidized.
• HJT saw RMB 0.05/W of additional non-silicon costs in the formation of TCO thin films, for its use of indium containing target material. Once HJT cell production explosively increase in the future, indium, a rare metal, is expected to skyrocket prices, despite the gradual replacement of foreign target material with those made in China. Driven by demand from solar cell manufacturers, indium prices in September have reportedly risen by 60% on levels in August. Therefore, cost reduction of HJT relies greatly on minimizing the use of indium, which is presently proposed to be achieved through the substitution with AZO (Aluminium doped Zinc Oxide) and indium recycling. The minimization of indium consumption sees no actual progress for the time being, since most HJT manufacturers are still in the midst of bringing capacities online.
• Laser cleaving process needs to be done prior to all HJT processes, which also induces damages to the structure of HJT cells. Offlate, HJT manufacturers introduced gettering process, which markedly raises the efficiency of HJT cells. Outcomes of gettering process and application of half-cut technique in large wafers will be the trending topics next year.

Final Note

Presently, production capacity and production volume of both TOPCon and HJT are expected to expand continually in the next one to three years. However, having better compatibility with existing PERC production lines, TOPCon enjoys more advantages than HJT in terms of expansion progress. Definitely, in the short term perspective, TOPCon will see both production capacity and volume growing faster than that of HJT. Cost is a crucial factor for the development HJT technology. More and more GW-scale HJT capacity expansions may take place, after silver coated copper paste and copper electroplating technology reach industrial maturity, which is still a million dollar question (considering the fact that copper can become a deep level defect inside silicon). HJT is expected to take few more years to achieve cost reductions (minimizing the use of polysilicon, indium, and silver) and see larger scale productions in next few years to come...