Excitons need space to separate: Free carrier production in organic solar cells

Aug 16, 2022 02:56 PM ET
  • Solar cells based upon organic molecules offer potential advantages over conventional devices for transforming light into electricity. These organic solar cells could be inexpensive, durable, and very easy to make. However, organic cells do not yet have the performance that matches conventional devices.
Excitons need space to separate: Free carrier production in organic solar cells
Image: Obadiah Reid and Al Hicks, National Renewable Energy Laboratory

Scientists' efforts to improve performance have been limited by their restricted understanding of how electrons delighted by light (or "photoexcited") come to be "free carriers."

In concept, free carriers flow across a material as well as emerge as an electric current. Prior clinical researches recommend that photoexcitation leads to a tightly bound pair consisting of an electron as well as a hole. These research studies did not define just how to overcome the strong binding forces to form free carriers. This new research study reveals that more sites on neighboring molecules can approve electrons, explaining exactly how free carriers form directly.

Published in Materials Horizons, this study developed a new model called Distribution Range Electron Transfer (DRET). Previous models for the generation of free carriers in organic solar cells have usually invoked new physical phenomena to explain experimental results. Particularly, they have stated that free carriers can form with performance that comes close to 100% in a product where opposite charges are traditionally challenging to separate and also make use of.

In this new research, scientists suggest a simpler option utilizing well-established concepts. They rely upon a straightforward model of processes that transfer electrons in molecules, called Marcus theory, a model for which Rudy Marcus was awarded a Nobel Prize in 1992. The new DRET model can unlock new pathways to reliable organic solar cells.

Developed by researchers from the National Renewable Energy Laboratory (NREL), the new model explains free carrier generation in organic solar cells making use of recognized concepts consisting of Marcus theory for electron transfer, incorporated with a consideration of the degeneration connected with the charge-transfer interface as well as the possibility of long-range transfer events. The model shows that existing design rules for solution-phase electron transfer processes can be applied to organic solar systems.

These rules consist of, first, that the driving force is determined using the widely known Gibbs energy term. Second, that the role of reorganization energy is identified. Third, aspects that control the range dependancy of the digital coupling are recognized.

The model fits experimental data gathered using time-resolved microwave conductivity experiments to map out the normal, optimal, and upside down routines absolutely free carrier generation performance. Qualitative agreement with lengthy observed behaviors in organic solar devices gives a unified system for comprehending the items of photoinduced electron transfer in both remedy and also solid-phase systems. Combination of this new model that properly captures the distinct molecular nature of the components making up the hierarchical framework of organic solar cells can make it possible for scientists to develop materials for extra efficient charge generation processes.

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