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

• Published in: ACS applied materials & interfaces 2014-04, Vol.6 (8), p.5798-5809
• Main Authors: Yu, Qing-Cai, Fu, Wei-Fei, Wan, Jun-Hua, Wu, Xiao-Feng, Shi, Min-Min, Chen, Hong-Zheng
• Format: Article
• Language: English
• ISSN: 1944-8244; 1944-8252
• DOI: 10.1021/am5006223

Two novel solution-processable acceptor–donor–acceptor (A–D–A)-structured organic small molecules with diketopyrrolopyrrole (DPP) as terminal acceptor units and pentathiophene (PTA) or pyrrole-modified pentathiophene (NPTA) as the central donor unit, namely, DPP 2 (PTA) and DPP 2 (NPTA), were designed and synthesized. We examined the effects of changing the central bridging heteroatoms of the five-ring-fused thienoacene core identity from sulfur [DPP 2 (PTA)] to nitrogen [DPP 2 (NPTA)] 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: DPP 2 (NPTA), which contains much more electron-rich pyrrole in the central thienoacene unit, possesses red-shifted absorption and a higher HOMO level relative to DPP 2 (PTA) 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 DPP 2 (PTA) with no alkyl chain substituting on the central sulfur atom of the PTA unit, DPP 2 (NPTA) exhibits improved crystallinity and better miscibility with PC71BM probably because of a dodecyl chain on the central nitrogen atom of the NPTA unit. These features endow the DPP 2 (NPTA)/PC71BM blend film higher hole mobility and better donor/acceptor interpenetrating network morphology. Optimized photovoltaic device fabrication based on DPP 2 (NPTA)/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.