The eight advantages of HJT cells
Here are the advantages of HJT (Heterojunction) solar cells:
High Conversion Efficiency
This is mainly due to the N-type silicon substrate and the dual passivation effect of amorphous silicon on the surface defects of the substrate. The current mass production efficiency is generally above 24%, and the technology route for efficiencies above 25% is already quite clear. This involves replacing the existing doping with doped nanocrystalline silicon, doped microcrystalline silicon, doped microcrystalline silicon oxide, and doped microcrystalline silicon carbide on both the front and rear surfaces. In the future, combining HJT with IBC (Interdigitated Back Contact) and perovskite could increase the conversion efficiency to over 30%.
Short Process Flow
The HJT cell process mainly includes four steps: texturing, amorphous silicon deposition, TCO deposition, and screen printing; significantly fewer steps than PERC (10 steps) and TOPCON (12-13 steps). The amorphous silicon deposition mainly uses PECVD (Plasma-Enhanced Chemical Vapor Deposition), while there are two methods for TCO (Transparent Conductive Oxide) deposition: RPD (Reactive Plasma Deposition) and PVD (Physical Vapor Deposition). RPD has a high patent penetration, whereas PVD technology is more mature, and there are many equipment suppliers available.
Low-Temperature Process
HJT cells form the p-n junction using silicon-based thin films, so the maximum processing temperature is the temperature for forming amorphous silicon films (~200°C), avoiding the high temperatures (~900°C) used in traditional thermal diffusion silicon solar cells. The low-temperature process saves energy, and using a low-temperature process reduces thermal damage and distortion to silicon wafers. It also allows the use of thin silicon wafers as substrates, which helps reduce material costs. Panasonic (formerly Sanyo), for example, achieved high-efficiency HJT cells using silicon wafers thinner than 100 microns.
High Open-Circuit Voltage
Because HJT cells insert an intrinsic thin silicon layer (i-a-SiH) between crystalline silicon and doped thin-film silicon, it effectively passivates the surface defects of the crystalline silicon, resulting in a much higher open-circuit voltage than conventional cells. As a result, HJT cells can achieve high photovoltaic conversion efficiencies. The open-circuit voltage of HJT cells has currently reached 750mV.
Low Temperature Coefficient
The performance of solar cells is typically measured at 25°C, but the actual operating environment for photovoltaic modules is outdoors, where high temperatures are particularly important. Due to the heterojunction structure of amorphous silicon thin films/crystalline silicon, HJT cells have better temperature characteristics. The initial reported temperature coefficient for HJT cells was -0.33%/°C, and after improvements, the open-circuit voltage increased, and the temperature coefficient decreased to -0.25%/°C, about half of the temperature coefficient of crystalline silicon cells at -0.45%/°C. As a result, HJT cells have better output performance under high-temperature conditions compared to conventional cells. The thin-film structure of HJT cells also enhances their low-light performance, which is superior to that of conventional cells.
No LID and PID, Low Degradation
Since the substrate of HJT cells is typically N-type monocrystalline silicon, which is phosphorus-doped, it does not experience the same boron-oxygen recombination or boron-iron recombination issues found in P-type crystalline silicon. Thus, HJT cells are immune to the Light-Induced Degradation (LID) effect. Additionally, the TCO film deposited on the surface of the HJT cell prevents the formation of an insulating layer, avoiding any surface charge buildup, which eliminates the possibility of Potential-Induced Degradation (PID).
High Bifaciality
HJT cells have a symmetric front and back structure, and since the TCO film is transparent, they are inherently bifacial. The bifaciality of HJT cells can reach over 90% (with a maximum of 98%), while the bifaciality of dual-side PERC cells is only about 75%+.
Low Carbon Footprint
According to data from Huasheng New Energy, heterojunction cells can reduce the use of raw material silicon, lower energy consumption, and effectively reduce carbon emissions. As of April 2024, Huasheng New Energy’s carbon footprint has decreased to 397 eq/W, which is 200 eq/W less than that of PERC modules. In the future, through a series of cost-reduction and efficiency-enhancement measures, they aim to further reduce the carbon footprint to below 300 eq/W.