Scientists at the Korea Institute of Energy Research (KIER) have developed a CIGS solar cell with ultra-thin glass (UTG), an emerging substrate known for its exceptional flexibility and stability.
“We achieved an impressive record device efficiency of over 17% for cadmium-free flexible solar cells by employing an optimized silver alloying strategy at significantly lower substrate temperatures,” researcher Donghyeop Shin told pv magazine. “This method effectively balances high photovoltaic performance with mechanical flexibility, overcoming the brittleness that typically arises from high-temperature processes on UTG substrates.”
The researchers built the cell with 90 μm-thick UTG provided by South Korea's Unique Technology Integral (UTI). Using DC magnetron sputtering, they also applied molybdenum (Mo) back contacts onto the cleaned UTG and silver (Ag) layers and the Mo contacts.
In a further step, they deposited a 2.0 µm-thick CIGS absorber onto the UTG substrate through a three-stage co-evaporation process with a multisource evaporator, followed by sodium (Na) and rubidium (Rb) alkali post-deposition treatments.
The cell also uses a cadmium-free buffer layer made of zinc oxide (ZnO) and magnesium oxide (MgO), instead of commonly used cadmium sulfide (CdS), as well as a thin Ag precursor layer to enable effective grain growth at the lower temperatures required by the use of UTG and protect the cell from transparent conductive oxide (TCO) potential sputtering damage.
Tested under standard illumination conditions, the device achieved a power conversion efficiency of 17.81%, which the scientists said is in line with that achieved by similar cells using CdS buffer layers.
“Additionally, large-area 60 cm² flexible modules manufactured using these innovative techniques have demonstrated promising efficiencies exceeding 10%, highlighting their potential for commercial scalability,” Shi stated. “The research makes significant strides in enhancing the performance of flexible solar cells by systematically addressing the interaction between silver incorporation and UTG substrates at lower temperatures while implementing cadmium-free buffer layers and optimizing intrinsic zinc oxide layers.”
The new cell concept was introduced in the study “High-efficiency cadmium-free Cu(In,Ga)Se2 flexible thin-film solar cells on ultra-thin glass as an emerging substrate,” published in the Journal of Alloys and Compounds.
“Overall, this work makes a significant contribution to the development of eco-friendly solar technology, presenting a viable pathway toward efficient, scalable, and sustainable solar solutions well-suited for integration into portable electronics, automotive applications, and curved-surface installations,” Shin concluded. “Furthermore, these flexible solar cells can be utilized as the bottom cell in a tandem device configuration combined with perovskite solar cells.”
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