A high-performance material for perovskite solar cells

#Pioneering Technology #Open Innovation #Environment #Printer

The thin, bendable next-generation perovskite solar cell

As a renewable energy source, solar cells are increasingly being adopted all over the world including Japan. We now see solar panels installed on the roofs of houses and buildings and even abandoned farmland. Most of these are silicon solar cells, which consist of a silicon structure formed over a glass substrate. As a result, they can only be installed in places that can handle the weight of their glass substrate.

In recent years, perovskite solar cells have garnered attention as a plausible next-generation replacement for silicon solar cells. Perovskite is named after Lev Perovski, the Russian mineralogist who discovered it over 200 years ago. The perovskite solar cell is thin, lightweight, and bendable, which solves the issues that come with silicon solar cells. But that is not its only advantage: it can also generate electricity from LED lights, fluorescent lights, and other indoor light sources that are much weaker than direct sunlight. It therefore holds tremendous potential for installation on a greater variety of places compared to silicon solar cells, such as the walls and windows of houses and buildings, vehicle roofs, the walls of sound-proof barriers along highways, and even clothing.

The thin, lightweight, and bendable perovskite solar cell

Canon’s unique, high-performance material: Killing two birds with one stone

However, there were still two issues that stood in the way of the perovskite solar cell’s practical application: its low durability, and the challenge of ensuring stable mass production.

Perovskite solar cells are made up of multiple layers of materials. As shown by the illustration below, these layers are:
- Electrodes made of gold (Au);
- Hole transport layer (HTL), which generates electron flow
- Perovskite layer, which generates electricity; and
- Electron transport layer (ETL), which transports electrons
- Transparent electrode

The structure of a conventional perovskite solar cell

The perovskite layer, which is responsible for photoelectric conversion (electricity generation), is easily affected by moisture, heat, oxygen, and light, which makes it less durable than silicon solar cells and gives it a short lifespan. While using protective materials can prevent the effects of moisture, heat, and oxygen, prolonged light exposure causes the halogen ions that form the perovskite crystals to move, breaking the crystalline structure and making it unable to convert light to electricity. In other words, perovskite solar cells have two conflicting properties: they do not generate electricity unless exposed to light, but they also cease generating electricity with prolonged exposure to light. The challenge of maintaining a certain level of photoelectric conversion efficiency while improving durability was a huge technical barrier.

The second issue is the inability to achieve stable mass manufacturing due to the perovskite layer’s unevenness, which increases the likelihood of incomplete coverage by the HTL layer. When these exposed parts of the perovskite layer come into contact with the electrodes, they can cause a short circuit*. A short circuit in just one area is enough to significantly reduce the power generation efficiency of the entire solar cell, and the greater the surface area, the higher the chances of short circuits. This makes it difficult to ensure the stable mass production of perovskite solar cells with a large surface area.

*When the electrical circuit breaks down and causes the electrodes to stop functioning

Photoelectric conversion efficiency and longevity are important aspects of solar cell performance, and the two weaknesses of perovskite solar cells mentioned above were significant barriers to their wider adoption—at least until Canon developed a new high-performance material that resolved them.
This new material can do two things: prevent the halogen ion movement that causes the perovskite layer to deteriorate, and move the charge generated in the perovskite layer.
A real-world demonstration* showed that when this newly developed high-performance material is applied between the perovskite and HTL layers to form a new layer, it can maintain a high rate of photo-electric conversion efficiency while increasing durability. The new layer covers the bumps in the perovskite layer, creating a smoother surface that enables the stable mass production of the perovskite solar cell. As it also improves durability, the burden of maintenance and repair is also reduced.

*Demonstrated through joint research with Toin University of Yokohama, the creator of perovskite solar cells.

Cross-section illustration of a perovskite solar cell that employs a layer of the newly developed high-performance materials

Applying a technology cultivated in multifunction devices and printers

Canon produces in-house the core components that determine product competitiveness, including the materials they are made of. The company developed the high-performance material applying technology that it had accumulated through the development of multifunction devices and laser printers.
The layer structure and the function of each layer of photosensitive drums, which are core components of multifunction devices and laser printers, are very similar to those of perovskite solar cells, so Canon’s engineers hit upon the idea of applying the technology developed for them. Photosensitive drums have a layer that generates electric charges just like the perovskite layer in a solar cell, and this layer is also sandwiched between the HTL layer on top and ETL layer below. By harnessing the materials technology used in photosensitive drum to modify existing materials, the engineers developed a new material that improved perovskite solar cell durability without reducing its photoelectric conversion efficiency.

Focusing on the technical compatibility of current perovskite solar cells with photosensitive drums

Perovskite solar cells accelerate the transition to a carbon-free society

With their photoelectric conversion efficiency improved to the same level as silicon solar cells, perovskite solar cells have become the focus of increasing attention for the past few years. The new, high-performanced material that Canon developed have removed the two significant barriers—durability and stable large surface area mass production—that stood in the way of its widespread adoption, paving the way for its proliferation.

The possibility of mass producing highly durable perovskite solar cells with large surface areas has the potential to significantly change the state of energy around the world. Firstly, because the cells are lightweight, bendable, and comparatively thinner, they can be installed on walls and other places where existing silicon solar cells cannot. They are also likely to eventually replace silicon solar cells, which are approaching the end of their lifespan.

Canon will continue to contribute its technology to ensure that the new solar cell becomes an effective means toward achieving a carbon-free society.