In a significant breakthrough, the research team led by Professor Michael Gratzel from EPFL, along with collaborators from the University of Oxford and Yokohama University, has developed a solid-state dye-sensitized solar cell (DSSC) that achieves a conversion efficiency exceeding 15%. This represents a remarkable increase of about 4 percentage points within just six months, far outperforming other organic solar cell technologies (Figure 1).
This new DSSC utilizes a hybrid organic-inorganic perovskite material, CH₃NH₃PbI₃, as the photosensitive layer, replacing the traditional liquid electrolyte with an organic hole-transporting material (HTM). The structure of the DSSC developed at EPFL includes layers such as glass, FTO, TiO₂, CH₃NH₃PbI₃, HTM, and gold. Meanwhile, the version created by the Oxford team incorporates aluminum oxide (Al₂O₃) alongside TiO₂. These innovations mark the first time that a hybrid organic-inorganic solar cell has achieved performance comparable to conventional crystalline silicon cells.
One key factor behind this advancement is the use of a solid electrolyte, which significantly boosts the efficiency of the device. The concept was initially introduced in 2009 by Professor Yosuke Miyaki’s team at Yokohama University. At that time, researchers were exploring quantum dot-based solar cells, but these faced challenges like low efficiency and current leakage. As a result, the team turned their focus to CH₃NH₃PbI₃, a material known for its broad light absorption range, extending up to 800 nm, and its ability to be directly synthesized on porous materials like TiO₂, making it ideal for coating processes.
Despite initial efficiency levels around 3.8% using traditional electrolytes, the introduction of a solid HTM called spirodioxime in 2012 helped boost the efficiency past 10%, reaching 10.9%. Further process improvements led to a rapid rise in efficiency to 15.36% within six months.
Looking ahead, researchers believe that this technology could push conversion efficiencies even higher—potentially reaching 21% in the future. While it shares similarities with both organic thin-film and CIGS (CuInGaSe) solar cells, it has already surpassed both in efficiency. Moreover, it avoids the use of rare and toxic elements like indium and gallium, offering a more cost-effective and environmentally friendly alternative.
However, there are still challenges to overcome. One major concern is the presence of lead (Pb) in the hybrid material, which poses health risks. Researchers are actively exploring substitutes such as tin (Sn) and copper (Cu). Another issue is the variability in performance between different trials, with some achieving around 11% efficiency while others only reach 5%. However, optimizing the manufacturing process, such as using a two-step coating method for CH₃NH₃PbI₃, can help reduce these inconsistencies.
Overall, this innovative approach marks a promising step forward in the field of photovoltaics, combining the advantages of both organic and inorganic materials to create a high-efficiency, low-cost solar cell. (Reporter: Nozawa Tetsuru, Nikkei Electronics)
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