Donor or Acceptor? How Selection of the Rylene Imide End Cap Impacts the Polarity of π-Conjugated Molecules for Organic Electronics

Three molecular semiconductors are compared and evaluated in organic field-effect transistors and organic solar cells. The molecules are constructed from the dyes diketopyrrolopyrrole (DPP), perylene diimide (PDI), and N-(alkyl)benzothioxanthene-3,4-dicarboximide (BTXI). The compound PDI-DPP-PDI (1) has previously been reported and used as a nonfullerene acceptor. The compounds PDI-DPP-BTXI (2) and BTXI-DPP-BTXI (3) were synthesized using direct (hetero)arylation methods and fully identified using NMR spectroscopy and mass spectrometry. All three compounds were characterized using UV-visible spectroscopy, cyclic voltammetry, and density functional theory calculations. Increasing the BTXI content results in a progressive destabilization of the electronic energy levels. For all compounds, no significant changes in the optical absorption spectra are observed when compared to a combination of the constituent optical absorption spectra. Compound 1 exhibits electron transport characteristics and functions as an electron acceptor in solar cells that produce a power conversion efficiency of 5%. Compound 2 exhibits unbalanced (electron transporting dominate) ambipolar charge transport characteristics and performs better as a nonfullerene acceptor in solar cells. Compound 3 exhibits balanced ambipolar charge transport characteristics and performs best as a donor in solar cell devices. The ability to tune the optical and charge-carrier transport characteristics of these panchromatic dyes through direct (hetero)arylation synthesis offers a distinctive way to create organic semiconductors that span a range of device performance metrics.