Abstract
The short exciton diffusion length associated with most classical organic semiconductors used in organic photovoltaics (5-20 nm) imposes severe limits on the maximum size of the donor and acceptor domains within the photoactive layer of the cell. Identifying materials that are able to transport excitons over longer distances can help advancing our understanding and lead to solar cells with higher efficiency. Here, we measure the exciton diffusion length in a wide range of nonfullerene acceptor molecules using two different experimental techniques based on photocurrent and ultrafast spectroscopy measurements. The acceptors exhibit balanced ambipolar charge transport and surprisingly long exciton diffusion lengths in the range of 20 to 47 nm. With the aid of quantum-chemical calculations, we are able to rationalize the exciton dynamics and draw basic chemical design rules, particularly on the importance of the end-group substituent on the crystal packing of nonfullerene acceptors.
Highlights
The short exciton diffusion length associated with most classical organic semiconductors used in organic photovoltaics (5-20 nm) imposes severe limits on the maximum size of the donor and acceptor domains within the photoactive layer of the cell
Adjusting the thickness of the organic semiconductor and measuring the OPV performance allows the study of LD within the organic layer, without the morphology-related complexities encountered in organic BHJs35
Exciton transport remains a bottleneck for high-efficiency organic photovoltaics
Summary
The short exciton diffusion length associated with most classical organic semiconductors used in organic photovoltaics (5-20 nm) imposes severe limits on the maximum size of the donor and acceptor domains within the photoactive layer of the cell. Recent work has shown that fused-ring acceptors such as indacenodithiophene end-capped with 1,1-dicyanomethylene-3-indanone (IDIC) exhibit long exciton diffusion length with a diffusion constant of at least 0.02 cm2/s21 This is consistent with the large domain sizes of 20–50 nm often reported for high-efficiency NFA-based BHJ cells[8,23,24,25,26]. It is not yet fully understood why the exciton diffusion length in NFAs, such as IDIC, is significantly higher than in amorphous and other polycrystalline organic semiconductors (typically 5–20 nm)[20]. Key relationships between LD and the chemical structure of the NFA are identified, leading to important design guidelines for future generation NFAs
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