Record-Breaking 'Highest Efficiency' in Coated Solar Cells... Accelerating Commercialization

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[Asia Economy Reporter Kim Bong-su] Domestic researchers have developed a core technology for low-cost, high-efficiency solar cells that can be applied like ink, setting a new record for maximum efficiency.

The Korea Institute of Energy Research announced on the 27th that the solar research team achieved the highest efficiency for a four-terminal solution-processed perovskite·CISSe tandem solar cell through an ink coating method and opened the possibility for mass production. This is a core technology for low-cost, high-efficiency solar cells manufactured by simply applying and coating a solution in an ambient environment.

According to the Shockley-Queisser theory, the maximum efficiency of conventional single-junction solar cells is about the low 30% range. To overcome this, tandem solar cells stack two solar cells with different energy absorption bands (bandgaps) to increase light utilization. Tandem solar cells typically combine silicon or CIGSe-based (compound semiconductors of copper, indium, gallium, selenium, etc.) solar cells as the bottom cell with perovskite solar cells as the top cell. Since the bottom cell mainly uses long-wavelength light, when applying CIGS-based solar cells as the bottom cell, the gallium (Ga) is removed to lower the bandgap, and CISe or CISSe absorption layers are mainly used.

For perovskite·CISSe tandem solar cells, which have advantages such as high efficiency and multifunctionality including flexibility, a solution process for cost reduction of the CISSe bottom cell involves an ink coating method dissolving copper (Cu), indium (In), and sulfur (S) precursors in DMF (Dimethylformamide) solvent. However, to minimize oxygen and moisture exposure during the process, thin-film coating must be performed in an inert gas atmosphere (a nitrogen-filled glove box), which limits mass production feasibility.

Solution Process-Based CISSe Solar Cells

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Focusing on securing mass production feasibility, the research team, while conducting DMF solvent-based research, developed a method to improve thin-film properties and solar cell efficiency by utilizing oxygen in the air. Surface defects generated during CISSe thin-film formation in the DMF-based process cause charge recombination losses and hinder charge transport, which are the biggest factors reducing efficiency. These defects arise because the diffusion and reaction rates of indium (In), a key element necessary for thin-film formation, are limited by various diffusion barriers. The team reduced surface defects by optimizing the heat treatment temperature and combining it with oxygen supply from the air to facilitate indium diffusion and reaction. As a result, using an air ambient solution coating method with high mass production potential, they achieved a new record efficiency of 14.4% (certified efficiency, solution-processed CISSe solar cell category), surpassing the previous highest efficiency of 13.5% made in a nitrogen atmosphere. Furthermore, by integrating this technology with perovskite solar cell technology capable of air ambient solution coating, they achieved a four-terminal tandem efficiency of 23.03%. This efficiency significantly exceeds the previous highest efficiency of 19.4% for solution-processed perovskite·CISSe tandem solar cells.

Dr. An Se-jin said, “We have developed a core technology that simultaneously captures the two goals of high power generation performance and low-cost processing for tandem solar cells,” adding, “We will focus on efforts to realize integrated solar cells and large-area scaling in the future.”

The research results were published as the outside front cover paper in the April issue of the international journal Energy & Environmental Science (IF 38.532).

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