Ab initio electronic structure of a small band gap polymer: Poly-aminosquaraine

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Abstract
Poly‐aminosquaraine is the prototype of a class of organic polymers which recently has been shown to provide a route towards small band gap materials. We predict that poly‐aminosquaraine has a small band gap of ∼0.5 eV. Our prediction is based upon a detailed analysis of first‐principles calculations of the geometrical and the electronic structure, using the Car–Parrinello technique of simultaneous optimization. We analyze the bands around the Fermi level in terms of a simple tight‐binding model based upon the highest occupied and lowest occupied (HOMO/LUMO) states of the individual squaraine molecules. The small band gap of the polymer is shown to be the result of the small splitting between the occupied and the unoccupied states of the squaraine molecule combined with a favorable hybridization in the polymer. It should be possible to analyze the electronic structure of a wide class of squaraine based polymers in the same way.