What is HJT Photovoltaic Module Technology
What is HJT Technology?
- HJT solar cells utilize a double-sided structure that efficiently captures both direct and scattered light from both surfaces. The process begins with Plasma-Enhanced Chemical Vapor Deposition (PECVD), where an ultra-thin layer of intrinsic silicon is applied for passivation. After texturing and surface cleaning, P-type silicon doping is introduced on the front side of the monocrystalline silicon wafer, while N-type silicon doping is applied to the reverse side using a similar method.
- Following this, transparent conductive oxide (TCO) and metal layers are deposited on both surfaces using PVD magnetron sputtering technology.
- The final step involves state-of-the-art metallization techniques to create precise metal grids on each side, optimizing the cell’s electrical performance and energy generation capabilities.
Structure of HJT Solar Cells
The HJT cell, short for Heterojunction with Intrinsic Thin Layer (also referred to as HIT), features a symmetrical double-sided structure centered around an N-type crystalline silicon core. On the front side, an intrinsic amorphous silicon thin film is deposited first, followed by a P-type amorphous silicon thin film to establish the P-N junction. The back side is similarly layered with an intrinsic amorphous silicon thin film and an N-type amorphous silicon thin film, forming the back surface field.
Since amorphous silicon has low conductivity, Transparent Conductive Oxides (TCO) are applied on both sides of the cell to facilitate efficient charge conduction. Finally, double-sided electrodes are created using precise screen printing technology, completing the process.
Materials and Components of HJT Solar Cells
Heterojunction solar cells rely on three essential materials: crystalline silicon (c-Si), amorphous silicon (a-Si), and Indium Tin Oxide (ITO), each playing a critical role in their structure and performance.
Crystalline Silicon (c-Si)
Crystalline silicon is the cornerstone of the photovoltaic industry, widely used in the form of wafers for solar cell manufacturing. In HJT solar cells, only monocrystalline silicon is utilized due to its superior purity and efficiency, making it ideal for high-performance applications.Amorphous Silicon (a-Si)
Amorphous silicon emerged in the 1970s as a suitable material for thin-film photovoltaic technology. Although it naturally contains density defects, these are resolved through hydrogenation, resulting in hydrogenated amorphous silicon (a-Si:H). This modification improves its bandgap and doping capability, making it an indispensable component in HJT cell production.Indium Tin Oxide (ITO)
Indium Tin Oxide is the preferred material for the Transparent Conductive Oxide (TCO) layer in HJT solar cells. Renowned for its reflectivity and electrical conductivity, ITO enhances the performance of optoelectronic devices, serving as a crucial contact layer. Its precise deposition is vital for maximizing the efficiency of HJT solar cells.
How Do HJT Solar Cells Work?
Working Principle of Heterojunction Solar Cells
Heterojunction solar cells operate based on the photovoltaic effect, similar to other solar technologies. Their unique distinction lies in the use of a triple-layer absorber material that combines thin-film and traditional photovoltaic designs. When a load is connected to the module’s terminals, photons are converted into electrical energy, creating a current that flows through the load.
Photon Absorption and Electron-Hole Pair Generation
Photons striking the P-N junction excite electrons, moving them into the conduction band and forming electron-hole pairs (e-h). These electrons are collected by terminals connected to the P-doped layer, generating a current that flows through the load. After completing the circuit, the electrons return to the rear contact of the cell and recombine with holes, closing the e-h cycle. This continuous cycle enables electricity generation.
Reducing Surface Recombination
Surface recombination, a phenomenon where electrons pair with holes on the surface of standard c-Si photovoltaic cells, limits their efficiency by preventing electrons from contributing to current flow. Heterojunction cells overcome this issue by incorporating a passivating thin film made of hydrogenated amorphous silicon (a-Si:H) with a wider bandgap. This buffer layer separates highly recombining contacts from wafer layers, allowing charge flow to generate high voltage while minimizing recombination losses.
Three-Layer Photon Absorption
Heterojunction cells utilize all three semiconductor layers to convert photons into electrical energy:
- Outer a-Si:H Layer: Absorbs the initial photons and converts them into energy.
- c-Si Layer: Handles the majority of photon conversion due to its superior energy efficiency.
- Rear a-Si:H Layer: Converts any remaining photons, completing the process.
This three-step photon absorption process allows single-sided heterojunction solar cells to achieve efficiencies as high as 26.7%.
Advantages of Heterojunction（HJT） Technology
- High Efficiency: Equipped with advanced heterojunction (HJT) solar cells and half-cell technology, achieving module efficiencies exceeding 22.87%.
- Large-Sized Cells: Utilizes 210mm HJT solar cells, offering a larger surface area for optimal sunlight absorption and increased energy output in a compact design.
- Low Degradation: Features a non-polarizing TCO film that eliminates LID, LeTID, and PID effects, ensuring power degradation remains below 11.1% over 30 years for long-term stable performance.
- Simplified Manufacturing: Streamlined production process with only four main steps—texturing, amorphous silicon deposition, TCO deposition, and screen printing—compared to the more complex PERC (10 steps) and TOPCon (12-13 steps) processes.
- Thin-Film Technology: Combines crystalline silicon with amorphous silicon thin-film technologies, delivering superior light absorption and excellent passivation.
- Stable High-Temperature Performance: Maintains a low power temperature coefficient of -0.24%/°C, ensuring minimal power loss and consistent energy output in high-temperature environments.
Shingled
Additional Power Gain: HJT cells, with symmetric front and back structures and optimized grid design, achieve a backside utilization rate exceeding 95%, delivering over 30% additional power gain compared to PERC and TOPCon technologies.
Superior Low-Light Performance: By incorporating an intrinsic thin-film (i-a-Si:H) between crystalline and doped silicon layers, HJT cells effectively passivate surface defects, resulting in higher open-circuit voltage, broader light absorption, and faster startup in low-light conditions.
Low-Temperature Process: The silicon-based thin-film used to form the pn-junction allows soldering temperatures below 250°C, reducing thermal stress and preventing high-temperature damage to the cells.
No Cell Cutting: HJT half-cell manufacturing avoids cell cutting, minimizing micro-crack risks and maintaining structural integrity.
High Flexibility: The advanced structure of HJT cells enhances flexibility, reducing the likelihood of microcracks during transportation and installation, and improving the reliability of solar power systems.
Comparison ofHJT, TOPCon, and PERC Technology
Heterojunction (HJT) solar panels deliver high bifacial output and exceptional performance with low temperature coefficients, maximizing power generation efficiency while reducing electricity costs. These panels are particularly well-suited for European regions with elevated summer temperatures and find ideal applications in agricultural photovoltaics, solar carports, and photovoltaic fences.
| HJT | TOPCON | PERC | |
|---|---|---|---|
| Bifaciality | 95% | 85% | 70% |
| Power generation efficiency | 22.87% | 22.28% | 21.2% |
| Initial performance degradation in the first year | 1% | 1.5% | 2% |
| Average annual performance degradation from the second year | 0.35% | 0.4% | 0.45% |
| Temperature coefficient | -0.243%/°C | -0.32%/℃ | -0.35%/℃ |
Future Forecast for HJT Solar Cells
Given the numerous advantages of heterojunction (HJT) technology, its adoption by more companies is expected to increase in the near future. With a manufacturing process that requires four fewer steps than PERC, HJT offers significant cost-saving potential. While PERC has long been a dominant choice in the industry, its complex production process and lack of high-temperature performance advantages make it less competitive compared to HJT.
According to the ITRPV 2019 report, HJT cells are projected to capture 12% of the market share by 2026 and 15% by 2029.
HJT Solar Panels from Maysun Solar
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