Yu Qing-Cai, Fu Wei-Fei, Wan Jun-Hua, Wu Xiao-Feng, Shi Min-Min, Chen Hong-Zheng
Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University , Hangzhou 310012, People's Republic of China.
ACS Appl Mater Interfaces. 2014 Apr 23;6(8):5798-809. doi: 10.1021/am5006223. Epub 2014 Apr 11.
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, DPP2(PTA) and DPP2(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 [DPP2(PTA)] to nitrogen [DPP2(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: DPP2(NPTA), which contains much more electron-rich pyrrole in the central thienoacene unit, possesses red-shifted absorption and a higher HOMO level relative to DPP2(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 DPP2(PTA) with no alkyl chain substituting on the central sulfur atom of the PTA unit, DPP2(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 DPP2(NPTA)/PC71BM blend film higher hole mobility and better donor/acceptor interpenetrating network morphology. Optimized photovoltaic device fabrication based on DPP2(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.
设计并合成了两种新型的可溶液加工的给体-受体-给体（A-D-A）结构有机小分子，它们以二酮吡咯并吡咯（DPP）作为末端受体单元，以五噻吩（PTA）或吡咯修饰的五噻吩（NPTA）作为中心给体单元，即DPP2(PTA)和DPP2(NPTA)。我们研究了在小分子给体材料中，将稠合五元噻吩并苯核心结构的中心桥连杂原子从硫[DPP2(PTA)]变为氮[DPP2(NPTA)]所产生的影响。用具有不同电子结构的桥连原子进行取代，对光学和电学性质均有显著影响：在中心噻吩并苯单元中含有更多富电子吡咯的DPP2(NPTA)，相对于在相同位置含有电子丰富程度较低的噻吩的DPP2(PTA)，其吸收光谱发生红移且最高占据分子轨道（HOMO）能级更高。更重要的是，由于杂原子的性质，桥连原子的取代导致了取代烷基链的变化，这显著地调整了结晶行为以及与电子受体形成薄膜共混物中互穿网络的能力。与在PTA单元的中心硫原子上没有烷基链取代的DPP2(PTA)相比，DPP2(NPTA)表现出更好的结晶性以及与PC71BM更好的混溶性，这可能是由于在NPTA单元的中心氮原子上有一个十二烷基链。这些特性赋予了DPP2(NPTA)/PC71BM共混膜更高的空穴迁移率以及更好的给体/受体互穿网络形态。基于DPP2(NPTA)/PC71BM（1.5:1，w/w）优化制备的光伏器件，其平均功率转换效率（PCE）高达3.69%（最大PCE为3.83%）。这项研究表明，分子结构的细微变化和调整，例如简单地改变D/A型小分子中噻吩并苯单元的桥连杂原子，能够强烈影响决定其光伏性能的物理性质。
