More Power With Ternary Solar Cells

Schematics of Sequentially Cast Ternary (SeCaT) solar cell for higher conversion efficiencies. (Source: P. & R. Allen)

Schematics of Sequentially Cast Ternary (SeCaT) solar cell for higher conversion efficiencies. (Source: P. & R. Allen)

Researchers from North Carolina State Uni­versity have developed a new strategy for fabri­cating more effi­cient plastic solar cells. The work has implications for developing solar cells with a wider absorption range and increased effi­ciency. As plastic solar cells now rival silicon-based solar cells in power conversion efficiency, researchers want to increase the range of photonic energies that plastic solar cells absorb. Ternary solar cells, in which three materials are mixed together as a light-har­vesting layer, offer a potential solution. However, while ternary solar cells have been manu­factured for years, most of the devices have not been able to meet desired levels of performance mainly due to unfa­vorable mixing.

Masoud Ghasemi, a graduate student in physics at NC State, worked with a team of other NC State physicists led by Harald Ade and chemists from the University of North Carolina at Chapel Hill led by Wei You to identify a way to solve the pro­duction problem. The team proposed a calori­metric tool to study the morphology of a ternary system with two absorption-matched donor polymers and a fulle­rene acceptor. When fabri­cated by the traditional method, which involves mixing all three materials together and then depo­siting them onto a substrate, the system gave poor device per­formance.

“Using thermo­dynamic techniques, we were able to find that this particular mixture was undergoing alloying, in which the donor polymers tend to group up together and push the fullerene away,” Ghasemi says. “This explains why so many conven­tionally produced ternary cells may have low effi­ciency.” The team decided to solve the alloying problem by mixing each polymer sepa­rately with the fullerene, rather than mixing all three materials together at once. They created two distinct mixtures which were layered onto the substrate, creating sequen­tially cast ternary (SeCaT) solar cells, which did not fall prey to alloying.

“The SeCaT solar cells prevent the polymers from mixing due to their layered structure,” Ghasemi says. “This novel design allows fabri­cation of plastic solar cells with wider optical sensi­tivity using cheap and scalable processing steps and with reduced materials selection constraints. Hope­fully this new method can be parti­cularly useful for greenhouse appli­cations toward zero energy farming, as the materials used to demonstrate our method have optical pro­perties compatible to these appli­cations.” (Source: NCSU)

Reference: M. Ghasemi et al.: Panchromatic Sequentially Cast Ternary Polymer Solar Cells, Adv. Mat., online 29 November 2016; DOI: 10.1002/adma.201604603

Link: Dept. of Physics and ORaCEL, North Carolina State University, Raleigh, USA

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