Every Atom Counts: Elucidating the Fundamental Impact of Structural Change in Conjugated Polymers for Organic Photovoltaics
Every Atom Counts: Elucidating the Fundamental Impact of Structural Change in Conjugated Polymers for Organic Photovoltaics
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DOI：
10.1021/acs.chemmater.8b00590
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发表时间：
2018-05-08
影响因子：
8.6
通讯作者：
Reynolds, John R.
中科院分区：
文献类型：
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作者：
Lo, Chi Kin;Gautam, Bhoj R.;Reynolds, John R.
由于许多基于共轭聚合物的有机光伏（OPV）材料提供了可观的太阳能转换效率（高达13%），因此深入了解主要重复单元结构如何影响器件性能非常重要。在这项工作中，我们已经改变了第14族原子（C，Si，Ge）的中心的联噻吩稠环，以阐明最小的重复单元结构的变化对光学，传输和形态学性质的影响，最终控制设备的性能。仔细的聚合和聚合物纯化产生了三个“一个原子的变化”的给体-受体共轭交替共聚物具有相似的分子量和分散性。DFT计算、吸收光谱和高温溶液H-1核磁共振（NMR）结果表明，聚（二噻吩甲硅烯-alt-噻吩并吡咯二酮），P（DTS-TPD）和聚（二噻吩甲硅烯-alt-噻吩并吡咯二酮），P（DTG-TPD）与聚（环戊二烯噻吩-alt-噻吩并吡咯二酮），P（DTC-TPD）相比，表现出不同的转动构象。固体H-1 MAS NMR实验表明，P（DTS-TPD）和P（DTG-TPD）中的反构象在固相中占优势的可能性更大。在溶液和固态NMR中看到的构象变化反过来影响聚合物堆积和分子间相互作用。二维H-1-H-1DQ-SQ NMR相关谱显示P（DTS-TPD）和P（DTG-TPD）具有芳香-芳香相关性，而P（DTC-TPD）不具有芳香-芳香相关性。在使用掠入射广角X射线散射（GIWAXS）进行的薄膜链间堆积研究中，我们观察到P（DTC-TPD）的共轭骨架的pi面与衬底边缘对齐，而相比之下，P（DTS-TPD）的pi面和P（DTG-TPD）平行于表面排列。聚合物构象和主链取向的这些差异导致与富勒烯PC 71 BM的共混物的OPV性能的变化，其中含有P（DTC-TPD）：PCBM的器件具有较低的填充因子和较低的功率转换效率。超快瞬态吸收光谱显示P（DTC-TPD）：PCBM共混物具有从最初分离的电荷的双分子重组形成的更显著的三重态。结合亚带隙外量子效率测量和DFT计算，我们提出的证据表明，更大的电荷复合损失是一个较低的三重态能级P（DTC-TPD）的结果，导致更高的复合率和较低的OPV器件性能。重要的是，这项研究将最终的光伏性能与活性膜中的形态特征联系起来，这些形态特征是由加工溶液引起的，并且是聚合物重复单元结构中最小的一个原子差异的结果。
As many conjugated polymer-based organic photovoltaic (OPV) materials provide substantial solar power conversion efficiencies (as high as 13%), it is important to develop a deeper understanding of how the primary repeat unit structures impact device performance. In this work, we have varied the group 14 atom (C, Si, Ge) at the center of a bithiophene fused ring to elucidate the impact of a minimal repeat unit structure change on the optical, transport, and morphological properties, which ultimately control device performance. Careful polymerization and polymer purification produced three "one-atom change" donor-acceptor conjugated alternating copolymers with similar molecular weights and dispersities. DFT calculation, absorption spectroscopy, and high-temperature solution H-1 nuclear magnetic resonance (NMR) results indicate that poly(dithienosilole-alt-thienopyrrolodione), P(DTS-TPD), and poly(dithienogermole-alt-thienopyrrolodione), P(DTG-TPD) exhibit different rotational conformations when compared to poly(cyclopentadithiophene-alt-thienopyrrolodione), P(DTC-TPD). Solid-state H-1 MAS NMR experiments reveal that the greater probability of the anticonformation in P(DTS-TPD) and P(DTG-TPD) prevail in the solid phase. The conformational variation seen in solution and solid-state NMR in turn affects the polymer stacking and intermolecular interaction. Two-dimension H-1-H-1 DQ-SQ NMR correlation spectra shows aromatic-aromatic correlations for P(DTS-TPD) and P(DTG-TPD), which on the other hand is absent for P(DTC-TPD). In a thin-film interchain packing study using grazing incidence wide-angle X-ray scattering (GIWAXS), we observe the pi-face of the conjugated backbones of P(DTC-TPD) aligned edge-on to the substrate, whereas in contrast the pi-faces of P(DTS-TPD) and P(DTG-TPD) align parallel to the surface. These differences in polymer conformations and backbone orientations lead to variations in the OPV performance of blends with the fullerene PC71BM, with the device containing P(DTC-TPD):PCBM having a lower fill factor and a lower power conversion efficiency. Ultrafast transient absorption spectroscopy shows the P(DTC-TPD):PCBM blend to have a more pronounced triplet formation from bimolecular recombination of initially separated charges. With a combination of sub-bandgap external quantum efficiency measurements and DFT calculations, we present evidence that the greater charge recombination loss is the result of a lower lying triplet energy level for P(DTC-TPD), leading to a higher rate of recombination and lower OPV device performance. Importantly, this study ties ultimate photovoltaic performance to morphological features in the active films that are induced from the processing solution and are a result of minimal one-atom differences in polymer repeat unit structure.