Stacked for transparency

Abstract

Semi-transparent organic solar cells are suitable for integration into various settings like buildings, automotive vehicles, and greenhouses as electricity-generating elements. For these applications, an important figure of merit is the light utilization efficiency — defined as the product of transparency to visible light and the power conversion efficiency. There is a trade-off between these two parameters: materials with strong absorption of visible light (particularly electron-donor polymers) afford higher power conversion efficiency. This trade-off makes improving light utilization efficiency challenging. Now, Haiyang Chen, Juan Zhu, Yaowen Li, and colleagues at Soochow University and the Chinese Academy of Sciences achieve a light utilization efficiency exceeding 6% by enhancing charge transport efficiency and reducing visible absorption through engineering of the electron-donor polymer’s stacking behaviour.

The researchers introduce an additive that forms hydrogen bonds with the electron-donor polymer, suppressing its aggregation and favouring a head-to-tail slipped (or J-type) stacking configuration. This stacking mode is characterized by a longer intermolecular conjugation length, which both reduces the absorption of visible light (improving transparency) and facilitates charge transport (enhancing power conversion efficiency). Furthermore, the additive increases the crystallinity of the electron-acceptor molecule, increasing its absorption of near-infrared light and further improving power conversion efficiency. With the inclusion of an optical interference layer, the research team achieve a power conversion efficiency of 13.31% and average visible transmittance of 45.23%, resulting in a light utilization efficiency of 6.02%. To address practical viability, the researchers explore the compatibility of their approach to large-area solar modules and perform preliminary plant growth experiments.