Improving Efficiency and Stability of Next-Generation Solar Cells

Korea Institute of Energy Research, Core Technology Development

Domestic researchers have developed a core technology that can significantly improve the efficiency of next-generation solar cells.

The Korea Institute of Energy Research announced on the 14th that it has newly developed a hole transport material that can further enhance the efficiency and stability of next-generation solar cells.

The research team confirmed that simply substituting the central sulfur atom (S) of a low-cost phenothiazine-based self-assembled monolayer (SAM) with oxygen (O) or selenium (Se) showed higher efficiency and stability compared to commercial hole transport materials. Hole transport materials not only facilitate smooth hole transport within perovskite solar cells but also suppress the recombination of generated charges, thereby improving device performance. A self-assembled monolayer refers to a regularly well-aligned monolayer spontaneously formed on the surface of a given substrate.

The Korea Institute of Energy Research is shining light from the next-generation perovskite solar cells it developed onto the lighting of a model house. Photo by Korea Institute of Energy Research.

Until now, the low efficiency of inverted-structure perovskite solar cells compared to conventional structures has been primarily attributed to the lack of suitable hole transport materials. Through the development of hole transport materials based on self-assembled monolayers, a foundation has been established to achieve high efficiency. Perovskite solar cells are the most notable solar cells due to their high light absorption and capability for low-temperature solution processing. Inverted-structure perovskite solar cells, which swap the positions of the electron transport layer and hole transport layer within the cell, allow low-temperature processing of all materials and exhibit less hysteresis in current-voltage curve measurements compared to conventional perovskite solar cells, making them easier to integrate with silicon solar cells in tandem structures and actively researched.

Since inverted-structure perovskite solar cells have been developed based on organic solar cells, the hole transport layer typically uses the organic semiconductor material PEDOT:PSS, which is most commonly used in organic solar cells. PEDOT:PSS can effectively transport holes between the transparent conductive substrate and the light-absorbing layer due to its high electrical conductivity; however, its strong acidic nature corrodes the transparent conductive substrate and light-absorbing layer, shortening the device's lifespan.

The research team developed a new hole transport material that improves efficiency and stability through a very simple process of substituting only the key element within the phenothiazine material that forms the self-assembled monolayer. They designed new molecules by substituting the sulfur atom, the central atom of the phenothiazine head group, with oxygen or selenium, which belong to the same group in the periodic table and have similar chemical and physical properties, and applied them to the hole transport layers of inverted-structure perovskite and organic solar cells.

Perovskite solar cells with three new self-assembled monolayer hole transport materials showed high efficiencies of 22.73% (selenium), 21.63% (sulfur), and 21.02% (oxygen). In the case of organic solar cells, a high solar cell efficiency of 17.91% (selenium applied / 111% compared to commercial PEDOT:PSS efficiency) was achieved. This was confirmed to be because the formation of the self-assembled monolayer lowers the work function of the substrate, significantly reducing energy loss used for hole transfer from the light-absorbing layer to the substrate.

In particular, selenium, which showed the highest efficiency and stability, has a high polarization characteristic that reduces interfacial defects through strong interaction with the light-absorbing layer and prevents non-radiative recombination losses at the interface, greatly enhancing stability. In the case of perovskite solar cells, 98% of the initial performance was maintained after 500 hours of continuous efficiency measurement, and in organic solar cells, stability was significantly improved by more than twice compared to commercial PEDOT:PSS.

The research team stated, “We have developed an original self-assembled monolayer-based hole transport material that can simultaneously improve the efficiency and stability of inverted-structure perovskite solar cells and organic solar cells. We expect this to provide an opportunity to take a step forward in the commercialization of next-generation solar cells and high-efficiency multi-junction solar cells using perovskite solar cells as the top cell.”

The results of this study were published in the international materials journal ‘ADVANCED FUNCTIONAL MATERIALS (IF 19.924).’

This content was produced with the assistance of AI translation services.