Heterojunction (HJT) technology was overlooked for many years, but it has been taking momentum for the last couple of years, showing its true potential. HJT solves some common limiting factors for standard photovoltaic (PV) modules, like reducing the recombination process and improving performance in hot climates.
If you want to learn more about HJT technology, this article is for you. Here you will learn about the design and operability of an HJT cell, its difference when compared against popular technologies, benefits, applications, and more.
What is a heterojunction solar panel?
Heterojunction solar panels are assembled similarly to standard homojunction modules, but the singularity of this technology lies in the solar cell itself. To understand the technology, we provide you with a deep analysis of the materials, structure, manufacturing, and classification of the HJT panels.
Materials required to manufacture a heterojunction solar cell
There are three important materials used for HJT cells:
- Crystalline Silicon (c-Si)
- Amorphous Silicon (a-Si)
- Indium Tin Oxide (ITO)
Crystalline silicon is regularly used to create standard homojunction solar cells, seen in conventional panels. There are two varieties of c-Si, polycrystalline and monocrystalline silicon, but monocrystalline is the only one considered for HJT solar cells since it has a higher purity and therefore more efficient.
Amorphous silicon is used in thin-film PV technology and is the second most important material for manufacturing heterojunction solar cells. While a-Si on itself has density defects, applying a hydrogenating process solves them, creating hydrogenated amorphous silicon (a-Si:H), which is easier to dope and has a wider bandgap, making it better for creating HJT cells.
Indium Tin Oxide is the preferred material for the transparent conductive oxide (TCO) layer of the heterojunction solar cell, but researchers are investigating using indium-free materials that will reduce costs for this layer. The reflectivity and conductivity properties of ITO make it a better contact and external layer for the HJT solar cell.
Structure of the heterojunction solar cell
Standard (homojunction) solar cells are manufactured with c-Si for the n-type and p-type layers of the absorbing layer. HJT technology, instead, combines wafer-based PV technology (standard) with thin-film technology, providing heterojunction solar cells with their best features.
The absorber layer of the heterojunction solar cell encloses a c-Si wafer-based layer (blue layer) placed between two thin intrinsic (i) a-Si:H layers (yellow layer), with doped a-Si:H layers (red & green layers) placed on top of each a-Si:H (i) layer. The number of TCO layers varies depending on the HJT cell being monofacial or bifacial, with the rear layer being a metal layer acting as the conductor for monofacial heterojunction cells.
Manufacturing of a heterojunction solar cell
There are several steps involved in the manufacturing process of the heterojunction solar cell. These are the following:
- Wafer processing
- Wet-chemical processing
- Core Layer deposition
- TCO deposition
The wafer processing involves cutting the c-Si cells with a diamond-based saw. Performing this process with extreme delicacy will result in high-quality c-Si layers, which translates to higher efficiency.
During the wet-chemical processing, organic and metal impurities are removed from the c-Si wafer. There are two methods often used for wet-chemical processing, the RCA method involving the use of concentrated sulfuric acid and hydrogen peroxide, and a cost-effective alternative applying an ozone-based process, obtaining similar results.
After the wet-chemical processing, the deposition process using Plasma-Enhanced Chemical Vaporization Deposition (PECVD) is applied, depositing a-Si layers on both sides of the wafer-based layer.
The second part of the deposition process uses Physical-Vapor Deposition (PVD) through sputtering to apply ITO, forming the TCO layer of the heterojunction solar cell. An alternative process uses Reactive Plasma Deposition (RPD) to apply the TCO layer, but this is a less popular option.
The metallization process diverges from regular manufacturing processes because the hydrogen in a-Si:H limits the temperatures to a maximum of 200-220ºC. A specially curated silver paste at low temperatures is used, through a copper electroplating or screen printing process, to place the electrodes on the cell.
Classification of heterojunction solar cells
Heterojunction solar cells can be classified into two categories depending on the doping: n-type or p-type.
The most popular doping uses n-type c-Si wafers. These are doped with phosphorous, which provides them an extra electron to negatively charge them. These solar cells are immune to boron-oxygen, which decreases the purity and efficiency of the cells.
P-type solar cells are better for space applications since they are more resistant to radiation levels perceived in space. The p-type c-Si wafers are doped with boron, providing the cell with one less electron, which positively charges them.
How do heterojunction solar panels work?
Heterojunction solar panels work similarly to other PV modules, under the photovoltaic effect, with the main difference that this technology uses three layers of absorbing materials combining thin-film and traditional photovoltaic technologies. The process involves connecting the load to the terminals of the module, with the photons being converted into electricity and generating an electric current, flowing through the load
To generate electricity, a photon impacts the P-N junction absorber and excites an electron, causing it to move to the conduction band and creating an electron-hole (e-h) pair.
The excited electron is collected by the terminal connected to the P-doped layer, creating the electricity that flows through the load.
After flowing through the load, the electron flows back to the rear contact of the cell and recombines with a hole, ending that particular e-h pair. This is constantly happening as the modules generate electricity.
A phenomenon called surface recombination occurs in standard c-Si PV modules, which limits their efficiency. During this process, an excited electron pairs with a hole at the surface of the material, causing them to recombine without the electron being collected and flowing as an electric current.
To reduce surface recombination, HJT cells separate the highly recombinative-active (ohmic) contacts from the wafer-based layer using a passivating semiconductor film with a wider bandgap layer made out of a-Si:H. This buffer layer makes the charge trickle slow enough to create a high voltage, but fast enough to avoid recombination before electrons are collected, increasing the efficiency for the HJT cell.
During the light-absorbing process, all of the three semiconductor layers will be absorbing photons and converting them into electricity.
The first photons arriving will be absorbed by the exterior a-Si:H layer, converting them into electricity. The majority of the photons, however, are converted by the c-Si layer, which has the highest solar conversion efficiency among the materials in the cell. The remaining photons are finally converted by the a-Si:H layer at the rear side of the module. This three-step process is the reason why monofacial HJT solar cells have achieved solar efficiencies of up to 26.7%.
Heterojunction vs. Traditional crystalline silicon panels
Heterojunction technology is based on traditional c-Si panels, improving the recombination process and other major flaws. In this section we compare how both technologies differ, helping us understand how a few modifications in the structure of the cell impact the overall performance of the module.
|Heterojunction (HJT)||Crystalline Silicon (c-Si)|
|Monocrystalline Silicon (mono c-Si)||Polycrystalline Silicon (poly c-Si)|
|Materials (Absorber Layer)||
|Structure||Mono c-Si wafer-based layer encased in a-Si:H passivating layers||Mono c-Si p-n junction||Poly c-Si p-n junction|
|Lifespan||30 years||25-30 years|
|Highest Recorded Efficiency||26.7%||25.4%||24.4%|
Heterojunction solar panel improves deficiencies found in standard c-Si modules, reducing surface recombination. This technology holds a higher recorded efficiency and improves the lifespan of the modules. As a result of the improvements, HJT panels have a lower temperature coefficient, resulting in better performance under different extreme temperatures.
HJT technology was first developed in the early 1990s, but it became popular these last decades, which explains the 5% market share and higher production costs, but this is only a temporary setback that is expected to be surpassed in the near future.
Heterojunction vs. Bifacial panels
The structure of bifacial panels is similar to the heterojunction solar panel. Both include passivating coats that reduce resurface combinations, increasing their efficiency.
HJT technology holds a high recorded efficiency of 26.7%, but bifacial surpasses this with an efficiency of over 30%. The curious side of it is that the bifacial PV module used to achieve that efficiency combines HJT technology with bifacial, and other technologies.
HJT cells can be designed for monofacial or bifacial usage, which reduces the reasons to compare them against each other since they can be combined to create superior bifacial HJT solar panels. The major difference is that bifacial can use other base technologies differing from HJT technology.
Summing up: What benefits do heterojunction panels offer?
Heterojunction solar panels can be quite beneficial since they have an improved technology with great potential in the solar industry. These are some major benefits of the technology.
With a 26.07% conversion efficiency for monofacial modules and more than 30% for bifacial, heterojunction places itself as one of the most efficient solar technologies in the industry. This makes it convenient for applications with limited space, areas requiring large generation capacities, and others.
Good temperature coefficient
Heterojunction solar cell technology is less affected by changes in temperature. This makes it great for applications in locations with high temperatures, which can negatively affect the performance of standard c-Si modules.
HJT cell has a high bifaciality factor of 92%, making HJT deliver a great performance when designed as a bifacial module. This technology is becoming more popular for utility-scale applications, which seek to take advantage of the albedo resource.
Easy manufacturing process
Heterojunction solar cells have additional steps in the manufacturing process, but this does not highly increase the cost. This technology only involves 5-7 steps during manufacturing, and the price for the necessary equipment is constantly being reduced, showing a great promise for the future of HJT.
Typical applications of heterojunction solar technology
Heterojunction solar panels are extremely versatile, opening the way for the solar industry to further increase applications for solar power. These are some of the most common applications for this technology.
Limited space applications (Solar shingles & BIPV)
HJT high conversion efficiency makes it great for limited space applications. Two popular applications are the manufacturing of solar shingles and Building Integrated Photovoltaic (BIPV) products. Tesla roofs are among the most popular shingles using this technology, which greatly increases the solar efficiency for a home with photovoltaics.
Power source for wearable devices
Reducing the size of wafer-based layers can open the way for integrating HJT technology with wearable devices. This will provide the devices with a power source to extend their autonomy during the day.
Regular monofacial heterojunction solar panels can be used in utility-scale applications, being especially beneficial with bifacial heterojunction solar panels. This will result in solar farms with an average efficiency of over 30%, which does not only take advantage of direct sunlight but also of the albedo resource.
Looking into the future of heterojunction technology
The heterojunction is a promising technology with high recorded efficiencies. The technology makes way for the solar industry to increase the efficiency of the day-to-day PV module and decrease the Levelized Cost of Energy (LCOE) regarding solar power.
The solar industry produced 5GW in heterojunction solar panels in 2019, making HJT technology hold around 5% of the retail market, with the largest manufacturers being Tesla in the US and Panasonic in Malaya and Japan, but this is expected to grow in the future. With an expected price of $0.19/W for 2029-2030, HJT technology could hold 15% of the retail market.
One major limiting factor for HJT technology is the current manufacturing process and cost of materials. With the developing technologies and research with other materials, this could stop being a promise in the future, and it could also increase efficiency and reduce costs for heterojunction solar panels.