Heterojunction, PERC, Perovskite… Which solar cells will win the battle for the future photovoltaic market and the race for the highest efficiency? Of the many available cell types, heterojunction technology, alongside PERC technology, is the favourite among photovoltaic experts thanks to its higher efficiency. Panasonic, the manufacturer of the HIT® high-performance modules, is among the small number of producers already offering heterojunction cells and committing to this technology. Now, for the first time, Panasonic explains how the higher efficiency is achieved and what goals the company has in cell development.
Turning first to the technology behind solar cells. How do heterojunction cells differ from other solar cells, especially well-established crystalline and thin-film cells?
Conventional crystalline or thin-film cells generally consist of a single material such as polycrystalline or monocrystalline wafers or a copper-indium-gallium-selenide compound, for example in the thin-film type CIGS. Each of the technologies has its particular benefits. For example, crystalline cells can convert more direct sunlight into electricity than thin-film cells can.
Thin-film cells, on the other hand, perform better in low light conditions. That means they generate more electricity in periods of diffuse light, for example if it is cloudy or if insolation is affected by smog, but also in the mornings and evenings. They also have the edge in terms of temperatures, as shown in the temperature coefficients. The temperature coefficient of a solar cell or module indicates by how much the power and therefore the efficiency decrease per degree Celsius of temperature increase. The smaller the temperature coefficient, the less the efficiency declines as the temperature increases.
Thin-film cells have a smaller temperature coefficient than crystalline modules and therefore lose less power at higher temperatures.
Panasonic combines crystalline and thin-film technology
Heterojunction cells from Panasonic combine the benefits of both technologies as they contain crystalline as well as thin-film technology. In HIT® solar cells, a thin monocrystalline silicon wafer is surrounded by an ultra-thin amorphous (= thin film) silicon layer.
Panasonic is also one of the very few module manufacturers to use n-type wafers. Conventional monocrystalline wafers are based on the p-type, whereas Panasonic uses the n-type because it is less prone to impurities.
In Panasonic HIT® heterojunction cells, a thin monocrystalline silicon wafer is surrounded by an ultra-thin amorphous silicon layer. Source: Panasonic Solar
The main steps in the production process
The first step is wafer production. The ultra-pure monocrystalline silicon ingot is cut using diamond wire saws into silicon wafers that are thinner than a postcard. Each cut wafer will become a substrate for heterojunction cells. In the second step, the wafers are processed. The silicon wafers are cleaned of impurities and textured. Amorphous silicon layers then form the heterojunctions. Transparent electrode layers and charge-collecting grid electrodes are formed to create heterojunction cells with the best energy production values in the world. The power, appearance and other properties of each individual cell are then checked.
While the crystalline wafer at the core of the cell produces a large amount of solar power, the amorphous layer on the surface reduces electron loss. In this way, Panasonic HIT® cells achieve above-average cell efficiency values of 22%. The cell efficiency achieved in lab conditions is also industry-leading. As far back as 2014 it was 25.6%.
The same is true of the modules. The Panasonic module with the highest efficiency is the HIT® N335. With a power of 335 watt, the module efficiency is 20%.
The graphic shows the higher light yield of HIT cells. This is made possible by coating the monocrystalline wafer with an amorphous silicon layer, allowing the light spectrum of amorphous cells to be used as well. Source: Panasonic Solar
High voltage thanks to excellent surface passivation
Above all, a very high voltage is achieved thanks to excellent surface passivation. “The monocrystalline wafers are chemically polished and continually improved. As a result, electron loss is much lower than in monocrystalline cells with rough surfaces,” explains Shigeki Komatsu, General Manager Solar Europe at Panasonic. Combining two silicon types (monocrystalline and amorphous) in the heterojunction cells also offers advantages over, say, a silicon-metal combination.
The higher no-load voltage of Panasonic HIT® cells also has a positive effect. “A higher no-load voltage means that the inverter is activated earlier,” says Komatsu. As a result, direct current is converted into alternative current sooner. That increases the solar power yield and improves the maximum power of the module (Pmax).
Development goal: Increase the cell efficiency of off-the-shelf modules to more than 24 per cent
By continually improving the existing cell properties, Panasonic wants to boost efficiency even further. As mentioned above, cell efficiency under lab conditions was 25.6% even as far back as 2014. In off-the-shelf HIT® modules, Panasonic wants to increase efficiency to over 24%. To achieve this, the main focus is on the following properties:
Reduced electron-hole recombination
The intrinsic amorphous layers surrounding the monocrystalline wafer hold the key to efficiency improvements. Optimising these layers and electron-hole recombination minimises losses in HIT® cells. In electron-hole recombination, differently charged particles, which were separated in the solar cell by sunlight and are now moving freely, are reoriented so that power is produced.
Optimisation of grid structure and amorphous layers
To achieve greater efficiency, it is vital to transport the electrical carriers efficiently from the monocrystalline wafer. Panasonic is optimising the amorphous layers (p-type and n-type) for maximum conductivity and reduced recombination. Panasonic is also improving the arrangement of the electrical grid structure in order to minimise electrical resistance and to increase the optically active surface area. The optically active surface area is increased by progressively making the busbar thinner. On the downside, this increases electrical resistance. So the perfect balance between the optically active area and low resistance must be achieved.
Better photoelectric effect on the rear of the cell
The symmetrical arrangement of the amorphous layers (sandwich structure) means that Panasonic heterojunction cells can produce solar power on the back as well as the front. They can therefore be described as bifacial. Panasonic has optimised the rear of the heterojunction cells to improve the light yield. This helps to boost module efficiency and power generation.
No light-induced degradation (LID)
The cell structure has a positive effect on light-induced degradation (LID). LID is a typical phenomenon in monocrystalline modules based on p-type wafers the first time they are exposed to sunlight. The LID effect is minimised because Panasonic uses n-type wafers in its heterojunction cells.
Extremely thin = highly flexible
Another benefit of Panasonic heterojunction cells is their high flexibility. HIT® cells can be made extremely thin and flexible.
High efficiency is possible even with a thickness of just 100 micrometres. Their flexibility means they are suited to applications such as car roofs. Panasonic HIT® cells are being used on the Prius Plug-in-Hybrid (PHV) which was presented in 2017 by Toyota.
The Panasonic solar cells support high power (about 180 W) on the limited surface area of a car roof. Their high flexibility also means they can withstand snow loads better and are less susceptible to microcracks during transport.
No half-cell modules
Everyone seems to be talking about half-cell modules at the moment. Panasonic has no plans to offer such modules. “We do have experience of half cells from our ‘honeycomb modules’”, is how Shigeki Komatsu explains the decision. The name comes from the honeycomb design of the cells. Because of the shortage of silicon at the time, cells were produced from ingots with cut corners. “In our experience it is much more complex to produce smaller cells than normal cell sizes. With smaller cells, many more cells need to be soldered, and the risk of cell failure increases with the number of cells. If everything works perfectly, half cells actually produce more power, but this does not make up for the higher fault rate and increased production cost.”
Benefits at module level
At module level, too, Panasonic HIT® modules have various advantages. For example, the sophisticated frame design improves stability and reduces soiling. The frame is not bolted but simply plugged in. The modules also benefit from Panasonic's drainage system at the corners of the module frame, reliably removing water from the module glass. This improves the self-cleaning properties of the modules while improving long-term performance thanks to reduced soiling and dust collection.
In addition, the new generation of HIT® modules has a 40-mm thick module frame. The stronger frame means the module can withstand wind and snow loads up to 5,400 pascal (Pa). The module also has official approval for the attachment of installation brackets on the short sides. On 1 June 2018, Panasonic started offering a linear performance warranty for several models of the HIT® module series. Back in 2017, the company extended the product warranty for its high-efficiency HIT® photovoltaic modules in Europe to 25 years.