What is a heterojunction solar cell?

As a consumer searching for the best solar panels to put on your home, there really isn’t a lot that distinguishes one solar panel from another.

If you’re shopping for a car, there’s probably a hundred features, big and small, that make one vehicle different from another. But when it comes to solar panels, it really comes down to a few things: price, warranty, high temperature performance and - probably most notably - efficiency.

Solar panel efficiency refers to how much light a solar panel converts into electricity. The higher the efficiency, the more electricity you’ll get from the panel for the same amount of light. For a rooftop deployment with limited space, this can be a very important feature.

Efficiency has been climbing steadily over the years as manufacturers keeping finding ways to squeeze more electricity out of the same amount of sunlight. But as the limits of each technology is reached, scientists and engineers need to reach into their bag of tricks to find novel ways to keep efficiency growing.

One of the latest technologies that’s finding its way into the consumer solar panel market is heterojunction solar cells. While Panasonic has had this technology for a few years with their HIT panels, the patents on heterojunction technology expired and 2010, and more manufacturers are starting to deploy it in their products.

Before we get into the list of companies that sell heterojunction solar panels, let’s start with a backgrounder on what it is.

What is heterojunction solar?

Hetereojunction solar cells combine two different technologies into one cell: a crystalline silicon cell sandwiched between two layers of amorphous “thin film” silicon. Used together, these technologies allow more energy to be harvested compared to using either technology alone.

The most common type of solar panels are made with crystalline silicon - either monocrystalline or polycrystalline. The silicon crystals are grown into blocks and then cut into thin sheets, often using a diamond wire saw, to form individual cells.

A less common type of photovoltaic cell is thin film, which is made with a variety of materials, one of which is amorphous silicon. Unlike crystalline silicon, amorphous silicon does not have a regular crystalline structure. Instead, the silicon atoms are randomly ordered. For manufacturing, this means that amorphous silicon can be deposited onto a surface - a simpler and less expensive process than growing and cutting silicon crytals.

By itself, amorphous silicon is less efficient at converting sunlight into electricity. However, it does have the benefit of less costly manufacturing. This lower cost and flexibility in the type of materials that amorphous silicon can be deposited on are a couple important advantages.

With heterojunction solar cells, a conventional crystalline silicon wafer has amorphous silicon deposited on its front and back surfaces. This results in a couple layers of thin film solar that absorb extra photons that would otherwise would not get captured by the middle crystalline silicon wafer.

Diagram of a heterojunction cell

SANYO (now Panasonic) originated heterojunction technology. Their diagram illustrates what this technology looks like: Courtesy Panasonic

How heterojunction solar cells increase efficiency

A solar cell is made of a thin material that captures some fraction of sunlight that hits it. It’s not completely opaque, though. Some sunlight will pass right through the cell, and some will also bounce off the surface.

Heterojunction solar technology takes advantage of this by building a solar panel out of three different layers of photovoltaic material. The middle layer of monocrystalline silicon does most of the work of turning sunlight into electricity.

There’s a top layer of amorphous thin-film silicon that captures some sunlight before it hits the crystalline layer, and it also grabs some sunlight that reflects off the layers below. It’s very thin, so much of the sunlight passes right through. But even so, it generates enough additional electricity to make the added cost worthwhile.

On the backside of the crystalline silicon is another thin-film layer. It captures sunlight that passes through the first two layers. If the panel is a glass-on-glass design with a transparent rear panel, this rear thin-film layer will add a significant amount of electricity due to sunlight that’s reflected off the ground.

By building a panel out of a sandwich of three different photovoltaic layers, a heterojunction solar panel can reach efficiencies of 21% or higher. This is comparable to panels that use different technologies to achieve high performance.

Advantages of heterojunction solar

The main advantages of heterojunction solar cells over conventional crystalline silicon cells are:

• Higher efficiency
• Potentially lower cost compared to other technologies used to improve performance, such as PERC
• Lower temperature coefficient (improved performance in high temperatures)

The efficiency of heterojunction panels currently on the market ranges from 19.9% up to 21.7% with the newest HJT panels from REC Solar. While this isn’t the highest on the market - the current champion are the Maxeon cells offered by SunPower, which reach as high as 22.7% efficiency - it is a significant improvement over conventional monocrystalline cells.

In addition, the other technologies used by manufacturers to reach very high efficiencies can be more costly. For example, SunPower’s Maxeon cells utilize a thick block of copper on the back of each cell. While this approach helps Maxeon cells to be the most efficient cells currently on the market, using that much copper isn’t cheap.

In comparison, amorphous silicon is a relatively cheap technology. While this type of thin-film solar isn’t nearly as efficient as crystalline silicon, it benefits from relatively simple manufacturing. By requiring fewer manufacturing steps than other technologies, heterojunction panels have the potential to be cost-effective than other types.

Finally, HJT panels may have an advantage when it comes to high temperature performance. Solar panels are less effective in high temperatures. This is a well-known phenomenon - in fact, temperature performance is listed on the datasheet of any solar panel. Look for temperature coefficient figures, and PTC, NOCT, or CEC power ratings.

However, one advantage of thin-film solar is that it has a better temperature coefficient than crystalline silicon. This means that high temperatures have less of an impact on thin-film that conventional monocrystalline or polycrystalline silicon.

With two layers of thin-film silicon, heterojunction panels gain an advantage over conventional solar panels when it comes to maintaining high performance as the temperature rises.

Manufacturers that make heterojunction panels

Panasonic/SANYO are the company that originally made heterojunction panels, but since the patent has expired, more manufacturers are using the technology. Here is a list of current companies using HJT:

• Panasonic HIT panels
• REC Alpha panels
• SolarTech Universal

Will we see more companies adopt this technology? I think it’s likely, given its advantages. Many more companies are using PERC technology, which has been around for relatively longer. But PERC requires more complex manufacturing, and does not have the high temperature performance benefit of heterojunction. In the meantime, consumers have some good choices with three established manufacturers offering HJT panels.