Advances in Long-life Perovskite Solar Cells

Perovskite solar cells are transforming the photovoltaic industry. These solar cells have shown a significant increase in their power conversion efficiency, and there is a possibility of achieving even higher efficiencies. Due to this potential, perovskite technology has gained much scientific research interest.

 

Image used courtesy of Adobe Stock

 

However, there are challenges to its practical applications due to its instability under thermal stress and humidity. Perovskites react with water molecules, and it has been proven difficult to make these solar cells durable and efficient simultaneously. To achieve these specifications, the interface between the charge carrier transport layers and the perovskite layer must be impermeable to water molecules.

Researchers at the National Institute for Materials Science (NIMS) have developed a 1 cm2 perovskite solar cell capable of operating continuously for more than 1000 hours with a power conversion efficiency of more than 20%. They added water-repellent atoms to the interface between the electron transport layer and the perovskite layer to achieve high durability while maintaining efficiency. Moreover, these solar cells can be fabricated on a plastic surface at 100 degrees Celsius, making them flexible and lightweight.

 

Issues with Perovskite Photovoltaics

Perovskite materials can be the major manufacturing component for solar panels due to simpler manufacturing and high-power conversion efficiency. These photovoltaics are usually made of hybrid organic-inorganic halide perovskites. Among these compounds, Methylammonium lead triiodide (MAPbI3) is the most popular organic halide perovskite for photovoltaic applications. It is inexpensive and simpler to manufacture, making it the obvious choice for the mass production of perovskite solar cells.

Solar manufacturers can alter the properties of halide perovskites by changing the chemical constituents to achieve the target electrical and optical properties. However, they have poor long-term stability as the active layer is unstable.

Most halides can't withstand thermal stress, and exposure to humidity causes irreversible damage to the active material. In these cells, oxygen acts as an electron scavenger in perovskite, forming highly reactive superoxide. Therefore, the extended exposure of the film to oxygen and light will decrease its longevity.

 

Achieving Perovskite Solar Cells

Most perovskite solar cells absorb sunlight through the perovskite-based active materials and generate electrons and holes. These two charge carriers travel separately in their respective transport layers. To improve efficiency and durability, these layers and the interfaces between them must be impermeable to water molecules.

Previous studies have suggested that surface passivation can improve the device's performance and stability by mitigating defects and enhancing moisture tolerability. In this study, published in Advanced Energy Materials, the researchers added a hydrazine derivative with water-repellent fluorine atoms (5F-PHZ) at the interface between the electron transport layer and the perovskite layer.

The researchers found that the interface prevented water molecules that have penetrated the electron transport layer from making contact with the perovskite layer, successfully improving the cell's durability. Moreover, it reduced the number of crystalline defects on the surface of the perovskite layer, boosting the power conversion efficiency from 18.10 percent to 22.29 percent.

 

Schematic of the perovskite solar cell. Image used courtesy of NIMS

 

The researchers also added a phosphoric acid derivative (MeO-2PACz) to the interface between the hole transport layer and the perovskite layer, minimizing defects in the transport layer and improving the power generation efficiency. They developed a 1 cm2 perovskite photovoltaic that can operate continuously for 1000 hours with a power conversion efficiency of around 20%. In addition, it can withstand thermal stress, allowing it to be fabricated on plastic at 100 degrees Celsius.

The researchers plan to develop more efficient and durable perovskite solar cells by creating a database of molecules that can improve the interfacial properties of the perovskite crystalline surface.