Evaluation of Heterocycle-Modified Pentathiophene-Based Molecular Donor Materials for Solar Cells

Two novel solution-processable acceptor–donor–acceptor (A–D–A)-structured organic small molecules with diketopyrrolopyrrole (<b>DPP</b>) as terminal acceptor units and pentathiophene (<b>PTA</b>) or pyrrole-modified pentathiophene (<b>NPTA</b>) as the central donor unit, namely, <b>DPP</b>_<b>2</b><b>(PTA)</b> and <b>DPP</b>_<b>2</b><b>(NPTA)</b>, were designed and synthesized. We examined the effects of changing the central bridging heteroatoms of the five-ring-fused thienoacene core identity from sulfur [<b>DPP</b>_<b>2</b><b>(PTA)</b>] to nitrogen [<b>DPP</b>_<b>2</b><b>(NPTA)</b>] in the small-molecule donor material. Replacement of the bridging atom with a different electronic structure has a visible effect on both the optical and electrical properties: <b>DPP</b>_<b>2</b><b>(NPTA)</b>, which contains much more electron-rich pyrrole in the central thienoacene unit, possesses red-shifted absorption and a higher HOMO level relative to <b>DPP</b>_<b>2</b><b>(PTA)</b> with the less electron-rich thiophene in the same position. More importantly, substitution of the bridging atoms results in a change of the substituting alkyl chains due to the nature of the heteroatoms, which significantly tailored the crystallization behavior and the ability to form an interpenetrating network in thin-film blends with an electron acceptor. Compared to <b>DPP</b>_<b>2</b><b>(PTA)</b> with no alkyl chain substituting on the central sulfur atom of the <b>PTA</b> unit, <b>DPP</b>_<b>2</b><b>(NPTA)</b> exhibits improved crystallinity and better miscibility with PC₇₁BM probably because of a dodecyl chain on the central nitrogen atom of the <b>NPTA</b> unit. These features endow the <b>DPP</b>_<b>2</b><b>(NPTA)</b>/PC₇₁BM blend film higher hole mobility and better donor/acceptor interpenetrating network morphology. Optimized photovoltaic device fabrication based on <b>DPP</b>_<b>2</b><b>(NPTA)</b>/PC₇₁BM (1.5:1, w/w) has resulted in an average power conversion efficiency (PCE) as high as 3.69% (the maximum PCE was 3.83%). This study demonstrates that subtle changes and tailoring of the molecular structure, such as simply changing the bridging heteroatom in the thienoacene unit in D/A-type small molecules, can strongly affect the physical properties that govern their photovoltaic performances