In the next step of TOPCon, double-sided TBC will become mainstream!

From 2024 onwards, TOPCon technology will become the next trend. Photovoltaic capacity will exceed 450GW by the end of 2023, and this technology will occupy the top market share in the coming years.

All TOPCon cells have passivated contacts on the back and most have selective emitters on the front, increasing the voltage to over 720mV and making the cells more efficient than 24.5%. The next step in battery development is to achieve p+ polarity passivated contacts.

There are two options for this: first, transition to selective polysilicon on the front side; second, transition directly to the TOPCon back contact structure (TBC). We will show that the second option is advantageous for several reasons.

Generally speaking, front-side non-metallization is the best solution for maximum efficiency, while back-side metallization and interconnection are very simple. Therefore, we expect TBC to capture the market after 2028. Some powerful components are already on display at SNEC and Intersolar 2023.

Starting from that year: n-PERT to PERC

n-type crystalline silicon (c-Si) technology is on the rise.

In 2011, low-cost double-sided n-type technologies such as n-PERT became an alternative to the dominant Al-BSF technology at that time. However, bifacial PERC quickly outcompeted the competition and became a mainstream technology in 2018, with solar cell efficiencies well above 22%. In 2020, PERC became the "King of the Energy Market". As the silicon wafer size increased from M2 to M6, and now to M10, the levelized cost of electricity (LCOE) of the bifacial photovoltaic system dropped to approximately 1USct/kWh.

In 2023, PERC cell efficiency will reach the limit of 23.5%, and module efficiency will be lower than 22%. This is the opportunity for n-type technologies such as TOPCon, HJT and IBC to become the next trend, which is exactly what is happening now.

The latest rumors from China claim that 1TW TOPCon battery production equipment has been built. The transition from PERC to n-type technology (especially TOPCon) will be as fast as the transition from Al-BSF to PERC.

However, with the adoption of TOPCon technology by the first large Tier 1 manufacturers such as JinkoSolar, JA Solar and LONGi, the decisive phase of mitigation measures has begun. Nowadays, Aixu and Tongwei are also taking the n-type route, and other manufacturers in China and India will follow suit.

The process is similar to moving to PERC. It is foreseeable that TBC will follow the same route. Shows how we predict TBC will penetrate into the photovoltaic market. Currently, before 2025, we have innovators like Sunpower, Trina Solar, State Power Investment Corporation, Aixu and Valoe Cell. Later, early adopters such as Jolywood, Futura New Energy, Insola and CARBON will follow.

However, large first-tier manufacturers will once again take over and TBC will dominate the PV market from 2028 onwards. Increasing the cell voltage from TOPCon's 725mV to the actual maximum of 740mV is simply a repeat of photovoltaic history.

Finally, the modern era of photovoltaics began with the implementation of feed-in tariffs in Germany around 2000, when photovoltaic cells used a very simple Al-BSF structure with a homogeneous emitter passivated by SiNx and a screen-printed metallized contact emitter, a thin layer Surface resistivity is approximately 50Ohm/sq. It was also at that time that mc-Si dominated the photovoltaic market with a substrate area of 10×10cm2, then 12.5×12.5cm2, and finally 15.6×15.6cm2, which was the standard for a long time. .

In 2011, when we started running n-PV workshops in Konstanz, more advanced and lower-cost n-type concepts such as nPERT from Yingli, PVGS and BOSCH Solar and nPERT from MegaCell, Adani and REC entered the market, which mainly It's because of the interest in bifacial applications.

However, starting in 2018, when p-type Al-BSF technology reached the 21% efficiency limit and Longi launched low-cost p-type Cz-Si wafers, bifacial PERC technology gained momentum and dominated the market, Al-BSF and mc-Si technology are completely squeezed out of the market. The nPERT concepts have also lost market share and they now have to be compared with PERC rather than Al-BSF.

Starting from companies such as Trina Solar, REC, Jolywood, and Sunpreme, TOPCon and HJT are also developing rapidly. In order to reduce production costs (increase yield) and replace those small n-type manufacturers that cannot comply with the trend of large wafers such as M10 (half square 18.2×18.2cm2) and G12 (full square 21.0×21.0cm2), Longi, Canadian Solar, Powerful PERC manufacturers such as JA Solar and JinkoSolar have adopted this technology, so the conversion to n-type has also been delayed.

However, as mentioned above, since PERC will reach its efficiency limit from 2021 (open circuit voltage Voc is about 695mV), Jinko, the first large-scale Tier 1 manufacturer, announced major plans to move to TOPCon, which only started to raise concerns A real interest in TOPCon.

In 2023, TOPCon's production capacity will increase to approximately 450GW, HJT to 50GW, and IBC/TBC to 40GW. The key difference between TOPCon and PERC is the passivated backside deposition technology, which is currently undergoing a race to be the most cost-competitive. In the field of polysilicon deposition, the transition from LPCVD to PECVD and PVD has also been very rapid.

This is another reason why TOPCon will be unrivaled in the coming years and keep HJT (another "emerging" n-type technology) at lower capacity levels, HJT will now be compared to TOPCon instead of PERC. A logical step in the future is to "upgrade" TOPCon to TOPCon back contact (TBC), since to achieve a voltage of about 740mV a carrier-selective contact (passivated contact) on the p+ polarity is necessary.

Since the absorptivity of the polysilicon layer is too high to allow full area application on the front, it is easier to use on the back. Another very important reason for the success of TBC is that it is easier to adopt screen-printed contacts on the back, such as copper or aluminum contacts on the back, which will be covered in Section 4.

We believe that double-sided TBC with copper or aluminum screen printing will become mainstream from 2028 onwards. At the same time, the photovoltaic industry is developing stack technology with n-type crystalline silicon cells as the bottom cells. As far as we know, 4-terminal stacks with perovskite on the front and low-cost copper or aluminum screen-printed bifacial TBC cells are the most suitable candidates to enter the market after 2030. However, the stability, uniformity and many other properties of perovskites still need to be significantly improved.

IBC technology in the photovoltaic market

Several IBC concepts exist in the photovoltaic market. Each two-sided concept, such as PERC, nPERT, TOPCon and HJT, has its corresponding IBC technology. We will not discuss HJT below, as only the first three coexist in the photovoltaic market currently. In addition, HJT IBCs developed by Meyer Burger and other companies also prefer the TBC direction because adding an amorphous silicon layer to the IBC structure is much more complicated.

LONGi produces so-called POLO IBC batteries with aluminum and silver screen-printed contacts. The open circuit voltage of this technology is limited to approximately 725mV. SPIC and ValoeCell currently produce ZEBRA technology products, which was developed by ISC Konstanz and is an upgraded version of nPERT (BiSoN) technology.

To our knowledge, the ZEBRA technology is the only one that uses POCl3 and BBr3 diffusion residues (PSG and BSG) as passivation layers, eliminating the need for an AlOx passivation layer. In addition, this solar cell using standard diffusion (no carrier-selective contacts) has a maximum open circuit voltage of up to 705mV.

The evolution of TOPCon to TBC, so far, Maxeon and Aixu have used other methods such as electroplating or screen printing to produce this battery. We believe that this technology will be another major breakthrough after TOPCon, but using copper and aluminum screen printing technology. Currently, we are combining ZEBRA with TOPCon (TOUCAN) technology to further develop this technology. Starting from 2024, State Power Investment Corporation and Futura New Energy will be the first to adopt this technology.

Three IBC technologies currently coexisting in the photovoltaic market

1. IBC without passivated contacts, 0PC, provided by SPIC and ValoeCell, power 705MW;

2. IBC with n+ passivated contacts, 1PC, provided by Longi, power 720MW;

3. IBC with both n+ and p+ passivated contacts, the so-called TOPCon back contact, provided by TBC, Maxeon and Aixu, with a power of 735MW.

The first concept is called ZEBRA technology [5, 6, 7] and is a 250MW M6 solar cell produced by the State Power Investment Corporation and an 80MW G1 solar cell produced by ValoeCell. SPIC manufactured the only IBC bifacial module (ANDROMEDA 2.0), as shown in Figure 11.

The second 1PC POLO-IBC concept battery is produced by Longi in a 30GW factory; TBC (2PC) is produced by Maxeon with a capacity of 0.6GW, and Aixu (so-called ABC) has a capacity of 0.5GW with plans to expand to 6.5 GW.

Votura plans to produce 1GW of polyZEBRA cells (TBC) in China, which was announced at the Intersolar 2023 conference in Munich. SPIC has not yet confirmed plans to produce TBC starting in 2025.

Characteristics of all IBC solar cell and module technologies on the market, or ipvStuttgart technologies under development.

With a maximum efficiency of 24.6%, ZEBRA solar cells are the most efficient non-charge carrier selective contact solar cells. The efficiency of ipvStuttgart's laser processing technology is 23.3%, and Jolywood's IBC efficiency has also reached 23%-24%. Compared with 2PC technology, the advantage of 0PC IBC is that its process is very close to PERC.

However, the voltage of this technology is limited to 705mV, which is why 1PC and 2PC IBC are increasingly important. Longi's battery is between 24-25% and has a voltage limit of 725mV. The advantage of LONGi's concept is that it uses p-type Cz-Si silicon wafer and aluminum mesh metallization process, which is closer to PERC.

However, compared with 0PC technology, even if it is closer to PERC technology, its processing is not easy. In addition, since the aluminum structure cannot be easily connected by welding or gluing, the interconnection is also relatively complex. Therefore, the future belongs to 2PC TBC technology currently produced only by Maxeon and Aixu. At voltages approaching 740mV, the efficiency of this technology exceeds 25%. However, currently it is still believed that metallization is done by electroplating (or at least without screen printing). To our knowledge, this approach will not be successful in the future for a variety of reasons. Therefore, ISC Konstanz is first working on advanced copper and aluminum screen-printed metallization for ZEBRA, and will then move to TBC (polyZEBRA) in 2024.

The corresponding component technologies (if any) in the photovoltaic market include SPIC's ANDROMEDA 2.0 at 22.3%, LONGi's POLO IBC at 23.0%, Maxeon at 22.8%, and Aixu's BlackHole technology at 23.5%. However, Longi's p-type POLO IBC was not exhibited at the Intersolar 2023 exhibition. Due to many challenges in the materials and solar cell process itself, we expect its production to be shut down. Therefore, TBC will be the only back contact technology in the future.

The most efficient modules in the photovoltaic market in the third quarter of 2023 are all based on n-type technology (except LONGi's technology), and TBC technology ranks among the top four.

As far as we know, Recom’s BlackTiger components are Aixu’s OEM products. We firmly believe that TBC technology will occupy the top 10-15 positions in the next 4 years, and it is expected that from 2028, TBC technology will replace TOPCon and occupy the highest market share. Figure 7 illustrates why in a very simple way.

The argument for this is also very simple: with TBC technology, the front side of the solar cell is perfect, eliminating the need for selective polysilicon structures and metallization on the front side. The process on the back is much simpler. Allows the use of copper screen printing, or even aluminum screen printing, which is not possible on the front side, without losing power due to shadowing effects. In addition, it is also very easy to connect the cells together using standard lamination technology, especially in the lamination process using three-dimensional interconnect patterns or conductive adhesives or conductive backplane technology. Currently, we prefer to use negative gap technology standard strings to achieve the highest bifacial performance.

This inherent advantage of providing greater freedom for backside metallization makes TBC the most suitable technology for the 1TW annual photovoltaic market, expected to start in 2027. For this purpose, an alternative metallization material is required that should be easily screen-printed for the backside. We'll cover how to achieve this in the following chapters.

STANDARD ZEBRA IBC TECHNOLOGY

As mentioned previously, standard ZEBRA solar cells are manufactured by SPIC in Xining, China, and ValoeCell in Vilnius, Lithuania [15,16]. As far as we know, this is a standard diffused solar cell technology with a maximum open circuit voltage of 705mV. This is due to our proprietary advanced screen printing technology, where only 1.5% of the emitter surface is in contact with the silver contacts.

Compared with the PERC process flow, since ZEBRA IBC technology is based on PERC and TOPCon, its process flow is also very streamlined.

The HJT sequence appears simple, but the complexity of the process is only one of its characteristics. Capacity and output are two other very important parameters that make classic technologies such as PERC, TOPCon and ZEBRA IBC unrivaled in terms of operating costs.

When the average open circuit voltage is 700mV, the average efficiency of the M6 ZEBRA IBC solar cell with 9 gate lines is 24.2%, and the maximum efficiency is 24.6% when the open circuit voltage reaches 704mV. The battery has a silver content of 200mg/W and has great potential for improvement, which will be discussed below. ZEBRA IBC technology is welded in a similar way to standard solar cells and is therefore also suitable as a low-cost product at the module level.

Because current price fluctuations in silicon and silver cause holding costs to vary widely, the relative differences between technologies are of greater concern than the absolute values. It is worth noting that the holding costs of PERC and TOPCon are currently at the same level (at the SNEC2023 exhibition, the holding costs of some TOPCon components were as low as 17ct/Wp), while the holding costs of ZEBRA IBC are not much higher. The introduction of higher efficiency and low-cost copper metallization will make double-sided copper ZEBRA unmatched in the future.

ZEBRA IBC technology that goes beyond standards

As mentioned before, the silver consumption of ZEBRA IBC is about 20mg/Wp. Due to the excessive silver consumption, this technology cannot have a significant impact on the photovoltaic market. On the other hand, IBC technology is the most suitable for achieving copper and/or aluminum metallization, as will be discussed below.

Silver-silver contacts have three main tasks during the metallization process. They discharge through the silicon nitride, contact the emitter and base, and prevent copper particles from diffusing into the silicon. Therefore, only 4mg/Wp of silver is needed to achieve the same efficiency as a ZEBRA IBC cell printed entirely with silver. Since these are the research and development results of Konstanz ISC, their efficiency is lower than the mass production efficiency of SPIC. However, it can be clearly seen that the solar cell parameters for copper metallization are the same as the reference values, while those for aluminum metallization are slightly lower.

Extensive testing such as the hot plate method, temperature cycling, and hygrothermal and tensile testing were successful [17], so our copper-ZEBRA technology is expected to be put into series production in the next few years.

The polyZEBRA concept is intended as an upgraded version of standard ZEBRA manufacturing, which has the same back-end [19,20] and will accordingly benefit from development to produce higher efficiencies at the same low holding costs. See the last section for more details on this.

polyZEBRA(TBC) concept

As mentioned before, the ZEBRA concept is very simple and extremely stable during production, but is limited to 705mV. In order to reach the practical short-circuit voltage limit of approximately 740mV, the TBC (polyZEBRA) concept must be adopted, that is, passivated contacts are used at both poles. LONGi is taking an intermediate step, which is to use only one passivation polarity, but the message from Intersolar 2023 is that they will stop producing POLO-IBC products because competitor TOPCon products are simpler and already exceed 720mV .

Futura New Energy plans to put into production 1GW of polyZEBRA batteries in 2024. Since the main patents of Futura New Energy will expire within two years, TBC technology will have greater room for development in the future. Therefore, we believe that the next logical development direction for TOPCon will be double-sided copper metallization TBC. Below, we discuss component costs as well as the cost of generating electricity from a photovoltaic system.

Carrying costs and LCOE

We have shown that TOPCon will replace PERC in 2024 and become the future trend after PERC, and PERC's market share in 2024 will reach 70%. Shortly thereafter, double-sided screen-printed copper and/or aluminum TBC technology will become the next logical evolutionary step in photovoltaic development.

At present, IBC technology has a wide range of applications, from PIPV, BIPV, VIPV to industrialized double-sided mode test systems.

For TBC to become the next hot thing, we need to improve efficiency, reduce costs and make components bi-sided. In short, PERC is to replace Al-BSF, and TOPCon is to replace PERC, all of which are essential. To this end, we are working within the EU IBC4EU [18] project to mature TBC technology for production within the EU.

TOPCon replaces PERC, and the next step is TBC

We have proven that TOPCon will replace PERC in 2024 and become another important product after PERC. The next logical step is to move to TBC.

We expect this to start about four years later, from around 2028, due to the higher efficiency potential of TBC (open circuit voltage of 740mV) and the possibility of replacing silver with low-cost materials such as copper or aluminum. With these two materials, the silver content in these new concept batteries can be reduced to 4mg/Wp. In the next few years, we will promote the industrialization of copper and aluminum ZEBRA and polyZEBRA IBC.

This work will be carried out within the EU project IBC4EU[19] as well as industry projects with many partners such as SPIC, ValoeCell, Votura New Energy, Insola, Carbon, SolarNord and many others.

The upcoming Back Contact Workshop in Hamelin on November 29-30, 2023 will discuss other developments by us and the consortium [20]. On June 23, 2023, the German government issued a letter of intent[21], looking for companies that can commit to building a total of 10GW of photovoltaic capacity in Germany in the first phase.

The technical requirements of this tender are very consistent with double-sided copper screen printing TBC technology. The German government is looking for future technologies that can achieve 24% component efficiency, solder-free connections and reduce the use of critical materials. We are convinced that TBC technology will play a very important role in reviving photovoltaic production in the EU and Germany.