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 ¹H 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 ¹H 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 ¹H-¹H 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 π-face of the conjugated backbones of P(DTC-TPD) aligned edge-on to the substrate, whereas in contrast the π-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.

中文翻译：

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每个原子都很重要：阐明了共轭聚合物中结构变化对有机光伏的根本影响

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，我们改变了联噻吩稠合环中心的第14组原子（C，Si，Ge），以阐明最小重复单元结构变化对光学，传输和形态特性的影响，最终控制设备性能。仔细的聚合和聚合物纯化产生了三种“单原子变化”的供体-受体共轭交替共聚物，分子量和分散度相似。DFT计算，吸收光谱和高温溶液¹1 H核磁共振（NMR）结果表明，聚（dithienosilole- ALT -thienopyrrolodione），P（DTS-TPD），和聚（dithienogermole- ALT -thienopyrrolodione），P（DTG-TPD）表现出不同的旋转构象相比时聚（cyclopentadithiophene- ALT -thienopyrrolodione），P（DTC-TPD）。固态¹ H MAS NMR实验表明，固相中P（DTS-TPD）和P（DTG-TPD）中反构象的可能性更大。在溶液和固态NMR中观察到的构象变化又影响聚合物的堆积和分子间的相互作用。二维¹ H - ¹H DQ-SQ NMR相关光谱显示P（DTS-TPD）和P（DTG-TPD）的芳族-芳族相关性，而P（DTC-TPD）则不存在。在使用掠入射广角X射线散射（GIWAXS）进行的薄膜链间包装研究中，我们观察到P（DTC-TPD）的共轭主链的π面与底物边缘对齐，而相反P（DTS-TPD）和P（DTG-TPD）的π面平行于表面对齐。聚合物构象和主链取向的这些差异导致与富勒烯PC ₇₁的共混物的OPV性能发生变化BM，其中包含P（DTC-TPD）：PCBM的设备具有较低的填充系数和较低的功率转换效率。超快速瞬态吸收光谱显示，P（DTC-TPD）：PCBM混合物由于最初分离的电荷的双分子重组而具有更明显的三重态形成。通过结合子带隙外部量子效率测量和DFT计算，我们提供的证据表明，更大的电荷重组损失是P（DTC-TPD）的三重态能级水平降低的结果，从而导致更高的重组速率和降低OPV设备的性能。重要的是，这项研究将最终的光伏性能与活性膜的形态特征联系在一起，形态特征是由处理溶液引起的，这是聚合物重复单元结构中单原子差异最小的结果。