No / Low Degradation TOPCon at System Level

• TOPCon modules have even recorded power gain during the LID and LeTID testing protocols, according to Jolywood
• TOPCon modules feature a low temperature coefficient of puts 0.32%/^°C
• TOPCon also scores high in bifaciality of 80% compared to PERC
• While TOPCon was mainly promoted to residential segment by the early adaptors, Jolywood is mainly promoting the products for utility scale applications

TOPCon not only results in higher efficiency at the cell level, and higher module power as a consequence, but the technology also performs well outdoors in real-world operating conditions. Its key benefit over the incumbent PERC technology is that the current industrial practice is to implement the technology on n-type based wafers. This fact is entitlement enough for it to be free from light induced degradation (LID). Jolywood's R&D director Jia Chen presented the company's internal as well as third-party testing data in his presentation The Benefits & Applications of Jolywood N-type TOPCon Technology at TaiyangNews' Very High Power Modules Conference 2021.

The in-house testing data showed that the TOPCon modules, when subjected to testing conditions of LID, actually showed a slight power increase. The results of testing TOPCon modules at Fraunhofer ISE are very close to Jolywood's internal testing. Whereas the PERC modules suffered a power loss of up to 5% in the LID testing sequence, Jolywood's products showed a slight power gain of 0.5%. As for LeTID, another important degradation mechanism, Liu Zhifeng, VP technology at Jolywood, presented another set of test results in his talk Advantages of TOPCon for Commercial Cell/module Production at TaiyangNews High Efficiency Solar Conference 2020. According to the quoted test results from TUV, PERC modules typically suffer a degradation of 2 to 3% due to LeTID, n-PERT technology was unaffected by the phenomenon, whereas for TOPCon modules the power increased slightly by 1%. Jolywood also deeply investigated this degradation phenomenon in its own lab and found that LeTID test resulted in a slight increase in power in most of the test cases. Zhifeng explained three phenomena in detail for the lower degradation (if not for power gain).

• The solubility of hydrogen in n-type wafer is significantly lower than in p-type. That means less hydrogen would in-diffuse into the n-type silicon bulk than p-type silicon during the firing. The lesser the hydrogen, lower the formation of H-P (N-Si) bond formation, thus lower the degradation.
• The binding energy of a P-H pair in n-type silicon (0.41 eV) is much lower than binding energy between a B-H pair in p-type silicon (0.5 eV). Thus, in annealing conditions, the process is quicker, so is the recovery.
• The diffusivity of hydrogen in n-type silicon is often measured to be orders of magnitude higher than the diffusivity of hydrogen p-type silicon. Thus, hydrogen effusion is more rapid for n-type, which may lead to accelerated recovery.

Betting on this low degradation, both Jolywood and JinkoSolar are guaranteeing a first-year degradation limit of 1% and the spec for annual degradation is 0.4%, while the same specs for PERC products are 1.5% to 2% and 0.45% to 0.55%, respectively.

Low temperature coefficient is yet another advantage of the TOPCon cell architecture compared to p-PERC. While Jolywood puts 0.32%/^°C as the spec for the temperature coefficient of power for its TOPCon modules, the PERC modules from leading suppliers are rated with 0.34 or 0.35%/^°C. According to Jolywood's Jia, every 0.05%/^°C decrease in temperature coefficient has the potential to reduce the LCOE by 0.9% to 1.3%. Counting on these benefits, Jolywood is primarily promoting its products to arid regions such as the Middle East, where the temperature as well as the ground albedo is high – the latter being beneficial for bifacial (see What's Top In TOPCon Solar Cell Technology).

Bifaciality is an important aspect of the TOPCon structure. On one hand, the structure suffers from undesired absorption of light in the doped polysilicon layers, leading to lower bifaciality compared to its peer n-type technologies such as n-PERT or HJT. One way to improve the bifaciality is to reduce the thickness of the polysilicon layer. However, Chen emphasizes that film thickness is just one of several factors influencing bifaciality including shading due to metallization, rear surface morphology (textured or polished), rear passivation quality and finally the wafer quality. Of course, reducing the layer thickness not only improves the bifaciality, but also the throughput of the deposition tools, ultimately helping to reduce costs. While Chen would not disclose the exact film specifications, "about 100 nm" is the ballpark figure he says. On the other hand, the bifaciality of commercial TOPCon modules is certainly above the mark; JinkoSolar boasts 85% bifaciality with its TOPCon modules. Even Jolywood's 80% is higher than the 70% spec for most of the PERC modules out there. According to Jia, each 10% increment in bifaciality reduces LCOE by 0.7% to 1.1%.

When it comes to its applications, the promise of high efficiency that comes with TOPCon led its early adopters such as LG to promote their products mainly for rooftop applications. Given the space constraints and high energy demand per available area, this application commands a price premium over C&I and utility counterparts. In contrast, while Jolywood also has products for residential, its core focus is utility. According to Jia, the TOPCon products of this company, in addition to being part of several installations under China's "Top Runner" scheme focused on technology, were also used in several utility projects in different parts of the world. He further emphasized that the company's products are best suited to hot regions such as the Middle East, to which the company has shipped modules worth about 900 MW so far, while its total n-type module shipments totaled 5 GW by end of 2021. Even JinkoSolar's latest TOPCon products were designed keeping the demands of large-scale power plants in view, i.e., larger modules and high power (see Putting Contacts On TOPCon Cells).

This brief article is taken from our recent TaiyangNews report on TOPCon Solar Technology, which is available for free download here.

Our TaiyangNews Conference on High Efficiency Solar Technologies, on Day 3, focussed on TOPCon Cells. To learn more about the conference and view the presentations from various technology leaders click here