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Perovskites are projected to be game-changer in future solar technology. But presently go through from a brief operational lifespan and drops in efficiency when scaled up to a large size.

Scientists have increased the steadiness and effectiveness of solar cell modules. Mixing the precursor materials with ammonium chloride during fabrication.

The perovskite active layer in the improved solar modules is thicker and has large grains, with fewer defects.

Both 5 x 5 cm2 and 10 x 10 cm2 perovskite modules maintained excessive efficiencies for over 1000 hours.

Researchers from the Okinawa Institute of Science and Technology Graduate University (OIST) have created perovskite solar modules. They improved stability and efficiency by the use of a new fabrication method that decreased defects. Their findings were posted on the 25th of January 2021, in Advanced Energy Materials.

Perovskites are one of the most promising materials for the next generation of solar technology. Soaring from efficiencies of 3.8% to 25.5% in slightly over a decade. Perovskite solar cells are low-priced to produce and have the potential to be flexible, growing their versatility. But two barriers still block the way to commercialization: their lack of long-term steadiness and difficulties with upscaling.

“Perovskite material is fragile and prone to decomposition. Which means the solar cells struggle to maintain excessive efficiency over a long time,” stated first author Dr. Guoqing Tong, a postdoctoral scholar in the OIST Energy Materials and Surface Sciences Unit, led via Professor Yabing Qi. “And although small-sized perovskite solar cells have an excessive efficiency and perform nearly as well as their silicon counterparts. Once scaled up to larger solar modules, the efficiency drops.”

In a practical solar device, the perovskite layer lies in the center, sandwiched between two transport layers and two electrodes. As the active perovskite layer absorbs sunlight. It generates cost carriers which then flow to the electrodes via the transport layers and produce a current.

However, pinholes in the perovskite layer and defects at the boundaries between individual perovskite grains. It can disrupt the flow of charge carriers from the perovskite layer to the transport layers, decreasing efficiency. Humidity and oxygen can also begin to degrade the perovskite layer at these defect sites, shortening the lifespan of the device.

Perovskite solar cell devices require more than one layer to function. The energetic perovskite layer absorbs sunlight and generates charge carriers. The transport layers transport the charging conveyor to the electrodes, releasing a current. The energetic perovskite layer is formed from many crystal grains. The boundaries between these grains, and other damage in the perovskite film, such as pinholes, lower the efficiency and lifespan of the solar devices. Credit: OIST

“Scaling up is difficult due to the fact as the modules increase in size, it’s more difficult to produce a uniform layer of perovskite, and these defects become more pronounced,” defined Dr. Tong. “We wanted to track down a way of fabricating large modules that addressed these problems.”

Presently, most solar cells produced have a thin perovskite layer – only 500 nanometers in thickness. In theory, a thin perovskite layer improves efficiency, as the charge carriers have less distance to travel to attain the transport layers above and below. But when fabricating larger modules, the researchers discovered that a thin film often developed more defects and pinholes.

The researchers, therefore, opted to make 5 x 5 cm2 and 10 x 10 cm2 solar modules that contained perovskite films with double thickness.

Scientists from the OIST Energy Materials and Surface Sciences Unit express off the perovskite solar modules in action, powering a fan and toy car. Credit: OIST

However, making thicker perovskite films came with its personal set of challenges. Perovskites are a category of materials that are usually formed by reacting many compounds together as a solution and then permitting them to crystallize.

However, the scientists struggled to dissolve an excessive enough concentration of lead iodine – one of the precursor materials used to form perovskite – that was needed for the thicker films. They additionally found that the crystallization step was quick and uncontrollable, so the thick films contained many small grains, with more grain boundaries.

The researchers, therefore, added ammonium chloride to increase the solubility of lead iodine. This additionally allowed lead iodine to be more evenly dissolved in the natural solvent, resulting in a more uniform perovskite film with many large grains and fewer defects. Ammonia was later eliminated from the perovskite solution, lowering the level of impurities within the perovskite film.

By adding ammonium chloride, the resultant perovskite film had fewer grains of large size, reducing the number of grain boundaries. Credit: OIST

Overall, the solar modules sized 5 x 5 cm2 showed an efficiency of 14.55%, up from 13.06% in modules made without ammonium chloride, and were able to work for 1600 hours over two months at more than 80% of this efficiency.

The larger 10 x 10 cm2 modules had an efficiency of 10.25% and remained at high levels of efficiency for over 1100 hours or nearly 46 days.

“This is the first time that a lifespan measurement has been reported for perovskite solar modules of this size, which is definitely exciting,” stated Dr. Tong.

This work was supported via the OIST Technology Development and Innovation Center’s Proof-of-Concept Program. These outcomes are a promising step forward in the quest to produce commercial-sized solar modules with efficiency and stability to match their silicon counterparts.

In the next stage of their research, the team plans to optimize their method in addition by fabricating the perovskite solar modules using vapor-based methods, rather than by using the solution and is now difficult to scale up to 15 x 15 cm2 modules.

“Going from lab-sized solar cells to 5 x 5 cm2 solar modules was hard. Check out solar modules that were 10 x 10 cm2 was even harder. And going to 15 x 15 cm2 solar modules will be difficult still,” stated Dr. Tong. “But the team is finding ahead to the ultimatum.”

By dmaico

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