New Technique Improves Stability, Efficiency of Solar Cell Modules

A group of scientists from the Okinawa Institute of Science and Technology Graduate University (OIST) has used a new fabrication method to create perovskite solar modules that have enhanced efficiency and stability with fewer defects.

Perovskite solar cell devices require multiple layers to function. The active perovskite layer absorbs sunlight and generates charge carriers. The transport layers transport the charge carriers to the electrodes, releasing a current. The active perovskite layer is formed from many crystal grains. The boundaries between these grains, and other defects in the perovskite film, such as pinholes, lower the efficiency and lifespan of the solar devices. Image Credit: Okinawa Institute of Science and Technology Graduate University.

The researchers’ findings were recently published in the Advanced Energy Materials journal on January 25th, 2021.

The efficiencies of perovskites have increased from 3.8% to 25.5% in slightly more than 10 years, and as such, they are regarded as one of the most viable materials for state-of-the-art solar technology. Besides this, perovskite solar cells can be produced economically and have the ability to be flexible, which further boosts their versatility.

However, there are two barriers that continue to limit the commercialization of perovskite solar cells—the difficulties with upscaling and their lack of stability over the long term.

Perovskite material is fragile and prone to decomposition, which means the solar cells struggle to maintain high efficiency over a long time. And although small-sized perovskite solar cells have a high efficiency and perform almost as well as their silicon counterparts, once scaled up to larger solar modules, the efficiency drops.

Dr Guoqing Tong, Study First Author and Postdoctoral Scholar in the Energy Materials and Surface Sciences Unit, Okinawa Institute of Science and Technology Graduate University

The Energy Materials and Surface Sciences Unit is headed by Professor Yabing Qi.

The perovskite layer within a functional solar device is located in the center, closely packed between two electrodes and two transport layers. When the active layer of perovskite absorbs solar light, it creates charge carriers which subsequently generate a current by flowing to the electrodes through the transport layers.

But the flow of these charge carriers from the perovskite layer to the transport layers can be disrupted by pinholes within the perovskite layer and also by defects at the boundaries between individual perovskite grains, thus decreasing the efficiency. And at these defects sites, oxygen and humidity would also begin to degrade the perovskite layer, reducing the device lifespan.

Scaling up is challenging because as the modules increase in size, it’s harder to produce a uniform layer of perovskite, and these defects become more pronounced. We wanted to find a way of fabricating large modules that addressed these problems.

Dr Guoqing Tong, Study First Author and Postdoctoral Scholar in the Energy Materials and Surface Sciences Unit, Okinawa Institute of Science and Technology Graduate University

At present, the majority of the solar cells produced so far have a thin layer of perovskite—measuring just 500 nm in thickness. Theoretically, a thin layer of perovskite enhances efficiency because there is less distance for the charge carriers to travel and thus reach the transport layers above and below.

However, during the fabrication of larger modules, the team observed that a thin film usually developed more pinholes and defects.

Hence, the investigators opted to produce solar modules that measured 5 x 5 cm2 and 10 x 10 cm2 and contained perovskite films with twice the thickness.

But fabricating thicker films of perovskite comes with its own set of complexities. Perovskites are essentially a group of materials that are often produced by collectively reacting several compounds as a solution and then enabling them to crystallize.

But the researchers found it difficult to dissolve lead iodine that had a sufficient concentration required for the thicker films; lead iodine is one of the precursor materials used for forming perovskite. The team also discovered that the crystallization step was uncontrollable and fast, and, hence, the thick films contained several tiny grains, with additional grain boundaries.

Hence, to boost the solubility of lead iodine, the investigators added ammonium chloride. This method also enabled lead iodine to be more uniformly dissolved in the organic solvent, leading to a more consistent perovskite film that had fewer defects and relatively larger grains.

The researchers subsequently removed ammonia from the perovskite solution, and this decreased the impurity level inside the perovskite film.

On the whole, the solar modules measuring 5 x 5 cm2 in size had 14.55% efficiency, which is more than the 13.06% efficiency in modules fabricated without ammonium chloride. The new solar modules were also able to operate for 1600 hours—more than two months—at an efficiency of more than 80%.

The larger solar modules measuring 10 x 10 cm2 had 10.25% efficiency and continued to remain at high-efficiency levels for more than 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 really exciting.

Dr Guoqing Tong, Study First Author and Postdoctoral Scholar in the Energy Materials and Surface Sciences Unit, Okinawa Institute of Science and Technology Graduate University

The study was funded by the Proof-of-Concept Program at the OIST Technology Development and Innovation Center. The new results are an encouraging step forward in the quest to create commercial-sized solar modules with stability and efficiency to match their silicon counterparts.

In the following stage of their study, the researchers have planned to further improve their method by creating the perovskite solar modules using vapor-based techniques instead of a using solution, and they are currently striving to scale up to 15 x 15 cm2 solar modules.

Going from lab-sized solar cells to 5 x 5 cm2 solar modules was hard. Jumping up to solar modules that were 10 x 10 cm2 was even harder. And going to 15 x 15 cm2 solar modules will be harder still. But the team is looking forward to the challenge,” Dr Tong concluded.

Scientists from the OIST Energy Materials and Surface Sciences Unit show off the perovskite solar modules in action, powering a fan and toy car. Video Credit: Okinawa Institute of Science and Technology Graduate University.

Journal Reference:

Tong, G., et al. (2021) Scalable Fabrication of >90 cm2 Perovskite Solar Modules with >1000 h Operational Stability Based on the Intermediate Phase Strategy. Advanced Energy Materials. doi.org/10.1002/aenm.202003712.

Source: https://www.oist.jp/

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