The research teams led by Professor Michael Grätzel from EPFL, the University of Oxford, and Yokohama University have independently developed solid-state dye-sensitized solar cells (DSSC) with a conversion efficiency exceeding 15%. In just half a year, the efficiency increased by nearly 4 percentage points, significantly outperforming other organic solar cell technologies (Figure 1).
This DSSC uses a hybrid organic-inorganic perovskite material, CH₃NH₃PbI₃, as the photosensitive dye, while an organic hole-transport material (HTM) replaces the traditional liquid electrolyte (Figure 2). The DSSC developed at EPFL includes layers such as glass, FTO, TiO₂, CH₃NH₃PbI₃, HTM, and gold. Meanwhile, the version created by Oxford University and others also incorporates aluminum oxide (Al₂O₃) alongside TiO₂. As a hybrid solar cell made from both organic and inorganic materials, these designs have achieved performance levels comparable to crystalline silicon for the first time.
**Solid electrolytes boost efficiency dramatically**
The initial concept of this DSSC structure was introduced in 2009 by Professor Yosuke Miyaki’s team at Yokohama University. At that time, many researchers were experimenting with quantum dots as sensitizers for quantum dot-sensitized solar cells. However, Miyaki pointed out that “quantum dots suffer from low efficiency and issues like current leakage.†This led the team to explore CH₃NH₃PbI₃ instead.
CH₃NH₃PbI₃ not only efficiently absorbs light across a broad spectrum—from visible to 800 nm—but also allows direct chemical synthesis on porous materials like TiO₂, making it ideal for coating processes.
In 2009, when Miyaki’s team first tested the material, they used a conventional DSSC electrolyte, achieving only 3.8% efficiency. Then, in 2012, researchers at Oxford replaced the electrolyte with a solid HTM called spirodioxime, boosting the efficiency to over 10% for the first time—reaching 10.9%. With further process optimization, the efficiency jumped to 15.36% within about six months.
**Future potential could reach 21%**
Although based on DSSC technology, some argue that this is not a traditional DSSC due to its material composition, component structure, and operating principle, which resemble those of organic thin-film and inorganic CIGS (CuInGaSe) solar cells (Figure 3).
Despite these similarities, the new solar cell has already surpassed conventional DSSC and organic thin-film solar cells in efficiency. It's even expected to exceed CIGS solar cells in the future. Current CIGS efficiency is around 20.4%, but according to Miyaji, with existing materials and technologies, this new cell could reach 17%, and potentially up to 21% in the future.
Moreover, unlike CIGS, this technology doesn’t rely on heavy metals like indium or gallium, allowing for lower-cost production. Its initial development used a coating process, another significant advantage.
However, two major challenges remain. First, the current hybrid material contains lead, which is toxic, though efforts are underway to replace it with tin or copper. Second, there is variability in performance—some trials show 11% efficiency, while others only reach 5%. But optimizing the manufacturing process could resolve this issue. For example, Grätzel’s team used a two-step coating process to produce CH₃NH₃PbI₃, significantly improving both efficiency and consistency.
(Reporter: Nozawa Tetsuru, Nikkei Electronics)
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