Crystalline silicon cells are moving from the 2.5 era to the 3.0 era. The working principle of the solar cell is the photogenerated volt effect, the sunlight illuminates the semiconductor P-N junction, and the voltage is generated at both ends of the P-N junction, that is, the photogenerated voltage. Crystalline silicon solar cells account for about 95% of solar cells, which is the type of photovoltaic cell with the highest level of industrialization and reliability.
The first generation (2005~2018) conventional P-type batteries: In 2020, the market share of traditional BSF batteries (aluminum back-field batteries) has dropped to 8.8%, basically facing obsolescence.
The second generation (2016~present) PERC and PERC+ batteries: around 2016, with the outbreak of perc battery industry acceptance, the industry entered the 2.0 era. PERC batteries add back passivation and laser slotting on the basis of the traditional aluminum backfield process, of which the purpose of back passivation is mainly to overcome the optical loss and electrical loss of the back surface.
Further, on the basis of PERC, the diffused PSG layer is used as the phosphorus source, and the advantages of the selective heating of the laser are used to secondary dop (phosphorus) on the positive surface, thereby forming a selectively re-doping N++ layer. The introduction of SE technology has enabled PERC batteries to be further upgraded to PERC+, opening the 2.5 era and continuing to this day (86.4% of the market share of per crystal perc/PERC+ in 2020, and BSF down to 8.8%).
At this stage, the PERC+ battery industrialization is mature, and it is still the most economical battery technology, and the conversion efficiency of the mass production line has reached about 23.0% to 23.2%. On the other hand, it is also gradually approaching the upper limit of mass production conversion efficiency, and the industry has begun to explore the next generation of high-efficiency crystalline silicon solar cells.
The third generation (about to open large-scale industrialization) TOPCon, HJT and other N-type batteries: Based on the continuous pursuit of higher conversion efficiency, N-type batteries will gradually begin to replace P-type batteries, which is exactly the stage we are at. P-type batteries diffuse phosphorus to form N+/P structure, although the diffusion process is simple, but face the problem of low upper limit of conversion efficiency; N-type batteries diffuse boron to form P+/N structure, with high sub-sub-life, no photo-induced attenuation advantages. N-type battery representatives include TOPCon, HJT, etc.
TOPCon: Preparation of tunnel-through oxide layer and highly doped polysilicon thin layer on the back surface of the cell.
HJT: Depositing amorphous silicon films on crystalline silicon, the process is simplified, but the requirements are more stringent, and it is the development direction with the best technical ductility.
IBC is a cross-back contact cell, and the amorphous silicon passivation technology is applied to the IBC, that is, it evolves into an HBC cell;
The passivation contact technology is superimposed on top of the IBC, which evolves into a TBC battery.
Comprehensive evaluation of the current situation of high-efficiency crystalline silicon solar cells:
PERC+: At present, the economic advantage is the most obvious, but because PERC+ is a P-type battery technology, there is limited room to further improve the conversion efficiency. At the same time, the light decay is relatively serious, especially the attenuation problem on the back side.
TOPCon: Compared with the PERC+ process, there has been an increase, mainly boron expansion and polysilicon passivation, but it has better equipment compatibility with existing capacity. TOPCon batteries are still high-temperature process batteries, not suitable for thinning, and silicon wafers as the largest component of cell cost, its future cost reduction path is limited.
HJT: natural double-sided power generation battery, bifacial rate > 95%; low-temperature process battery, suitable for thin flakes, large potential for cost reduction; small temperature coefficient, small attenuation in high temperature environment, relatively high power generation; intrinsic amorphous silicon passivation, open circuit voltage is larger. Of course, HJT's most urgent aspects to be solved are the continuous optimization of costs, from the perspective of industry development in 2021, the localization of equipment is progressing smoothly, and it is expected to drop to 350 million yuan / GW level by the end of 2022; in terms of thin flakes, the thickness of 210 half sheets is expected to be reduced to 120 microns; in 2022, the industry will continue to explore the saving of silver pulp consumption in the metallization link, including silver clad copper, copper plating, etc.; printing technology perspectives include laser transfer, steel plate printing, etc.
IBC: Completely gate-free frontal occlusion, which can obtain higher currents than conventional batteries, but the production process is more complicated.
1.2 Surface passivation is the core path to improve efficiency, and HJT achieves double-sided contactlessness
The principle of solar cell operation is the photogenerated volt effect, after the light absorption is generated electron-hole pair, the electron and the hole drift to the corresponding charge selection boundary, at the interface separately to form a positive and negative charge, the collection of charge makes the two sides of the interface form a potential difference, that is, voltage. When an external circuit is connected, the flow of charge forms a path, which generates an electric current.
The optimization of surface passivation technology is the core path of efficient crystalline silicon cell efficiency improvement. Damage to the lattice on the surface of the wafer occurs during the cutting of the wafer. The destruction of the periodic arrangement of ilfilaments leads to the presence of suspension keys, which form a composite center. Passivation is to deactivate the above defects through technical optimization to achieve the purpose of reducing the surface of the charge carrier.
The upgrading of high-efficiency crystalline silicon cell technology, including cell processes such as TOPCon and HJT, is centered on surface passivation technology. From the perspective of technological evolution path, BSF batteries are upgraded to PERC batteries, that is, back-side contact is upgraded to back-line contact; PERC batteries are upgraded to TOPCon batteries, that is, back-line contact is upgraded to back-side contactless; TOPCon batteries are upgraded to HJT batteries, that is, back-side contactless upgrades to double-sided contactless.
The technical characteristics of high-efficiency crystalline silicon solar cells currently industrialized (or expected to be industrialized in the future) are as follows:
PERC (P type): emitter and backside passivation battery, on the basis of conventional BSF batteries, add a backpass layer (alumina) to reduce the back surface composite, through laser slotting to form a local back electrode.
TOPCon (N type): Tunneling through the oxide layer passivation contact cell, depositing a very thin layer of silicon oxide on the back of the N-type silicon wafer, and then depositing a layer of re-doped polysilicon film to achieve tunneling passivation on the back side to increase the open circuit voltage.
HJT (N type): Amorphous silicon is used on the basis of the N type silicon wafer substrate to form a heterojunction and act as a passivation layer, the heterojunction open circuit voltage is relatively higher, and the outermost layer is prepared with a transparent conductive oxide layer (TCO).
TBC (Type N): The advantage of IBC (referring to cross-back contact battery) is that the front is not gated and the current is improved. The IBC is combined with THE TOPCon to form a TBC battery by superimposing passivation contact technology.
HBC (Type N): The advantage of IBC (referring to cross-contact batteries) is that the front is not gated and the current is improved. The IBC is combined with the HJT to form an HBC cell using an amorphous silicon passivation layer.
1.3 Downstream customer acceptance of high-efficiency crystalline silicon cells is ultimately determined by LCOE
Downstream customer acceptance of high-efficiency crystalline silicon cells depends on the core metric of electricity cost (LCOE). KWh cost = (full life cycle cost) ÷ (full life cycle power generation). The cost of photovoltaic power generation projects includes initial investment costs, operation and maintenance costs, financial costs, and tax costs. For the end customer, the pursuit of LCOE means a comprehensive evaluation of the stability, reliability and power generation efficiency of the power station in the whole life cycle.
There are many influencing factors of LCOE, the core of which is around the system cost (the main component of the initial investment cost) and the amount of power generation. End consumers tend to opt for a technology path with higher power generation efficiency and lower BOM costs over the entire life cycle. Taking HJT components as an example, the initial investment at this stage is relatively high, but we must also pay attention to its high bifaciality of 90% to 95%, low attenuation, good low light effect, no LID/PID effect and other characteristics, from the perspective of the whole life cycle dimension, the above advantages will dilute its LCOE.
The price cost of the N-type battery pack is higher than that of the P-type, and the acceptance of overseas markets is higher than that of Domestic. We emphasize that for end customers, the final comparison is LCOE, and the overseas market is relatively more accepting of N-type battery modules. As component product prices, which are an important component of initial investment costs, we use the latest data from PV infoLink for comparative analysis:
Component product price difference: The current topCon component product price is 0.13 yuan/W~0.15 yuan/W compared with PERC; the price of HJT component products is more than 0.35 yuan/W higher than PERC. For N-type battery modules, it is critical to reduce their initial investment costs as quickly as possible by reducing costs and increasing efficiency, so as to maximize their LCOE benefits.
The penetration rate of bifacial components continues to increase, and the proportion is expected to reach 50% in 2022: it is expected that more bifacial projects will choose N-type modules in the future, mainly due to their higher bifaciality and lower temperature coefficient advantages.
In December 2021, a new energy industry debuted N-type battery module quotations. Monocrystalline N-type 182 double-sided battery (mainstream efficiency >24.5%), quoted at RMB 1.21/W, USD quoted at US$0.169/W; monocrystalline N-type 182 bifacial double-glass module (mainstream power >550W), quoted at RMB 1.99 RMB/W, US dollars quoted at US$0.278/W.
1.4 PERC+ is expected to approach the upper limit of mass production efficiency in 2022
The conversion efficiency of crystalline silicon cells achieves a qualitative leap. In 1954, Bell Labs G. Pearson and D. Charpin successfully developed the first practical monocrystalline silicon solar cell with a conversion efficiency of 6%. In 1985, the efficiency of silicon solar cells at the University of New South Wales in Australia exceeded 20%, in 1999 it announced that the conversion efficiency of monocrystalline silicon solar cells reached 24.7%, and in 2009 it reached 25% after the solar spectral correction and maintained this record for 15 years, which is a milestone event in the research of monocrystalline silicon solar cells. In 2014, Japan's Panasonic and the United States' SunPower successively broke through the conversion efficiency to more than 25%. The continent's first solar cell with practical value was born in 1959. In 2007, the mainland surpassed Japan to become the world's largest producer of solar cells. In 2017, perC replaced BSF as the mainstream technology trend of solar crystalline silicon cells, and ushered in a major expansion of the industry that continues to this day. Over the past 10 years, the conversion efficiency of large-scale mass production of crystalline silicon cells has increased from 18% to more than 23%.
The conversion efficiency of the P-type battery mass production line is approaching the bottleneck, and the era of N-type battery mass production is gradually approaching. According to the latest research conclusions, the theoretical efficiency of HJT and TOPCon batteries is 28.5% and 28.7%, respectively, which has obvious efficiency advantages compared with PERC+. The paper published by LONGi shows that the theoretical efficiency of HJT batteries can reach 28.5%, which is an improvement from the previous 27.5% research results of the German ISFH agency. The theoretical efficiency of the TOPCon battery is 28.7%. From the perspective of theoretical efficiency, HJT and TOPCon are not much different.