HJT Solar Panel: Features, Pros & How It Compares to Other Techs

Crystalline silicon (c-Si) technology-based solar panels have long been dominating the market. But did you know there is one advanced panel technology that innovatively combines different forms of silicon materials and has achieved remarkable performance compared to traditional c-Si panels? That is the HJT solar panel!

This step-by-step tutorial will walk you through the features and benefits of the HJT solar panel. You will also comprehend how it compares to competing panel technologies.

Basics: What Is the HJT Solar Panel?

Heterojunction (HJT) solar panels were invented in the 1980s by the Japanese company Sanyo Electric (a subsidiary of Panasonic), with the first commercial products released in 1997. At the heart of this technology is to improve the efficiency of traditional solar cells by combining crystalline silicon (c-Si) with amorphous silicon (a-Si) thin-film layer to create a hybrid cell.

HJT Cell Materials

In HJT cells, the c-Si material used is typically monocrystalline silicon, which boasts exceptional light absorption efficiency. This material property enables it to convert a significant proportion of incident sunlight into electricity.

On the other hand, the a-Si material is used to complement a couple of weaknesses of the c-Si material and introduce several prominent strengths. 

To kick things off, a-Si has a higher bandgap of approximately 1.7eV, compared to the narrower 1.1eV bandgap of c-Si material. This higher bandgap is advantageous in HJT cells as the a-Si material can absorb higher-energy photons more efficiently which enhances the overall absorption capability of the HJT cell.

Secondly, in contrast to the indirect bandgap of c-Si, the bandgap property of a-Si enables it to absorb light more efficiently even in thinner layers, contributing to the potential for more flexible HJT panel designs.

Also, a-Si material is effective at passivating the surface defects of the c-Si wafer. Its low-temperature processing and ease of deposition make the integration with c-Si material more straightforward and cost-effective, making it an excellent material choice for reducing surface recombination.

By combining its distinct properties, a-Si material demonstrates improved performance in low-light conditions as well as low temperature coefficient.

Other materials used in an HJT cell include coating, transparent conductive oxide (TCO) layers, electrodes, encapsulation and metal frames.

Typical Structure of a HJT Cell

As illustrated in the figure below, each layer in a heterojunction solar cell plays a unique and crucial role in achieving high efficiency.

• Monocrystalline Silicon Substrate: Serves as the primary absorber layer where most of the light absorption and electron-hole (e-h) pair generation occur. In HJT cells, n-type silicon is frequently utilized as it provides enhanced carrier mobility and reduced LID.
• Intrinsic a-Si Layers (i-a-Si:H): As shown in the figure, these undoped layers are placed on both sides of the c-Si substrate to provide surface passivation.
• Doped Amorphous Silicon Layers: These layers are placed on top of/below the i-a-Si:H layers and doped to create the p-type and n-type regions. The p-doped a-Si layer acts as the emitter, forming a heterojunction that aids in charge separation, while the n-doped a-Si layer serves as a BSF, improving passivation and thus reducing recombination.
• TCO Layers: Usually composed of ITO, ZnO or FTO, TCO layers are applied on the front (and sometimes the back surface) of the cell to act as conductive layers that also allow light to pass through, enhancing light absorption and reducing electrical losses.
• Anti-Reflective Coating (ARC): It is applied to the surface of an HJT cell to reduce the reflection of sunlight and increase light absorption.
• Encapsulation & Protective Layers: Encapsulation is typically done using ethylene-vinyl acetate (EVA) or polyvinyl butyral (PVB), with a front layer of tempered glass and a backsheet. Together, they protect the cell from environmental factors such as moisture, dust and mechanical damage.
• Metal Electrodes
• Panel Frame

How a HJT Solar Panel Works

HJT panels optimize light absorption, charge separation and electricity generation by combining crystalline and amorphous silicon layers.

The general working principle can be described as follows:

Sunlight penetrates primarily through TCO and a-Si layers to reach the c-Si wafer, which generates electron-hole pairs. Intrinsic a-Si layers reduce recombination losses, while p-type and n-type doped layers create electric fields that efficiently separate and direct these charge carriers toward the metal contacts.

The TCO layers conduct the charge carriers with minimal resistance, allowing light to pass through. As electrons move to the front contact and holes to the rear contact, an electric current is established, flowing through an external circuit to produce usable electricity. 

The ‘Bifaciality’ of HJT Solar Modules

Although single-sided configurations are available, many efficient HJT solar modules on the market are designed with dual-sided light absorption, following the structure explained above.

By equipping the panel with dual-sided TCO and tempered glass, bifacial HJT modules can produce up to 20-30% more energy compared to monofacial modules, depending on the installation environment and albedo of the ground surface.

Bifacial HJT panels have higher efficiency rates compared to traditional c-Si-based bifacial panels. Some studies show that by combining the merits of both c-Si and a-Si materials, they can achieve an efficiency of over 25% and also outperform other advanced bifacial techs, such as bifacial TOPCon and PERC.

With a high bifaciality, HJT panels are a superior choice for maximizing energy yield.

HJT Panels vs. Other Panel Technologies

The distinctiveness of HJT solar panels stems from the pioneering composition and architecture of their semiconductor materials. Although there are few comparable commercial products, examining traditional bifacial panels and PERC panels can provide a deeper understanding of the HJT technology.

HJT Panels vs. c-Si Based Bifacial Panels

Traditional c-Si-based bifacial panels have become a well-established technology after several decades of development. They can be made of either monocrystalline or polycrystalline cells and allow them to capture sunlight from both the front and back.

Influenced by the idea behind traditional bifacial panels, HJT panels are capable of dual-sided absorption and they further enhance their light absorption capability by leveraging the integration of a-Si materials.

Both panel technologies provide higher efficiencies and energy production. Furthermore, HJT panels generally have better performance than traditional bifacial panels under low-light conditions and high temperatures.

HJT Panels vs. PERC Panels

PERC panels employ a straightforward and cost-effective passivation technique to reduce surface recombination, a common phenomenon in solar panels, thereby boosting efficiency. Unlike PERC, HJT panels feature more intricate and costly passivation layers, which drive up the initial cost of the products.

While PERC modules can also be engineered to be bifacial panels, their bifaciality is inferior to that of HJT panels. Moreover, with their lower degradation rates, better performance in low-light conditions and low temperature coefficient in mind, the comparably more expensive HJT panels are optimized for long-term cost savings.

In other words, PERC panels may be more ideal for many standard applications seeking higher-efficiency solutions with affordable prices.

Recent Developments in Heterojunction Solar Panels

Heterojunction solar panels have seen significant advancements in recent years. 

Leading HJT solar panel manufacturers have tried to implement back contact and contact-free laser transfer printing to minimize the area of grid lines, thereby increasing the active area of the cells and improving overall efficiency.

Some have also developed flexible HJT solar cells with efficiencies surpassing 25%, depending on the thickness of the cells. These cells are thinner and lighter, which can open up new applications in portable electronics, BIPV, solar vehicles and other areas, emerging as a new competitor to conventional thin-film panels.

In addition, researchers from the Chinese Academy of Sciences have developed a model to predict the lifetimes of HJT modules, considering factors like temperature, humidity and UV irradiation. This model helps in understanding and mitigating degradation, building more efficient and robust HJT panel products.

What Are the Pros and Cons of HJT Panels?

Here are some rounded-up pros and cons of heterojunction panels to assist with your decision.

Pros of HJT Panels

• High Efficiency: HJT panels are among the most efficient solar technologies available, with average products achieving conversion efficiencies above 22% and top-tier or bifacial products exceeding 25%.
• High Bifaciality: Bifacial HJT products exhibit a high bifaciality factor of over 90%, making them ideal for projects that can take full advantage of reflected sunlight.
• Reduced Degradation: Harnessing the benefits of premium n-type monocrystalline silicon wafers and their distinctive structure, HJT panels are less susceptible to light-induced degradation (LID), enhancing their long-term performance.
• Temperature Resilience: These panels have a low temperature coefficient, meaning they maintain high efficiency even in high-temperature environments.
• Good Performance in Low Light: HJT solar panels perform well under low-light conditions, such as during cloudy days or early mornings and late evenings, leading to higher energy yield throughout the day.
• Versatility: These panels are generally thinner and lighter, making them applicable in various applications, including residential, C&I installations, as well as BIPV projects. 

The Cons

• Higher Initial Cost: Manufacturing HJT panels requires specialized equipment and expertise, which in turn results in higher initial cost. This might be a barrier for some consumers with a moderate budget.
• Limited Availability: As a relatively newer technology, HJT panels are not as widely available as more established panel types. This could limit options for some consumers who have a plan to scale up.

Wrapping Up

HJT solar panels improve efficiency and overall performance by transforming their semiconductor materials and associated architecture, being a landmark achievement in the industry’s history.

On the flip side, this innovation represents a key pathway to innovate solar panels, namely, by combining the benefits of different semiconductor materials into one. Beyond the heterojunction, the emerging crystalline silicon-perovskite tandem cell technology also reflects this concept.

While currently with a higher initial cost and limited availability, as the technology matures and production scales up, some of these drawbacks may be mitigated, making them an increasingly attractive option for solar projects.