Persistent spin coherence and bipolarons.
Persistent spin coherence and bipolarons.
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持久自旋相干性和双极化子。
作者：
J. Behrends;I. Samuel;A. Schnegg;D. Keeble
致编辑——Boehme 和 Lupton 在他们的评论中讨论了有机自旋电子学当前面临的挑战，并正确强调了直接检测参与自旋物种的光谱方法的重要性。我们之前通过脉冲电检测磁共振（pEDMR）从传统本体异质结有机太阳能电池2中观察到室温相干电子自旋拉比振荡持续超过500 ns，该太阳能电池由20%共轭聚合物（MEH-PPV）和80% PCBM（C60）的混合物组成。我们还观察了高微波磁场振幅下的自旋锁定，结果明确证明，负责的两种自旋物种是 S = 1/2。这一观察结果消除了对磁场敏感的自旋相关输运模型，例如极化子对三重态激子的猝灭或两个三重态激子的相互湮灭。考虑到光谱证据和器件物理，我们得出结论，自旋相关的传输机制涉及通过双占据聚合物链段（即双极子态）进行跳跃。 Boehme 和 Lupton 讨论了区分由双极子介导的跳跃和极化子对重组 (P+P−) 引起的自旋依赖性传输的重要性，并以此质疑我们的结论。确实值得回顾一下支持这些模型的证据。 pEDMR 光谱的重叠使解释变得复杂，因此有必要仔细考虑用于这些实验的设备结构和操作条件。值得注意的是，我们的测量是在混合物上进行的，而 Boehme 和 Lupton 研究的是纯薄膜。我们在混合设备上进行的 pEDMR 测量观察到 MEH-PPV 中极化子的 g 值 (2.0028(3)) 特征光谱，使我们能够得出结论，这些是负责的自旋物种。可以解析对光谱的两个贡献，两者具有相同的 g 值，但线宽分别为 0.6(1) mT 和 1.5(1) mT2。 Boehme、Lupton 及其同事 3,4 观察到的纯 MEH-PPV 器件的 pEDMR 光谱也显示了 g = 2.003 处的光谱，包含两个分量，但线宽约为 1.3 mT 和 3 mT。 Boehme 和 Lupton1 显示了来自类似纯 MEH-PPV 装置的新室温拉比振荡光谱（参考文献 1 的图 1b），并将这些结果与我们的混合装置的结果进行比较（参考文献 1 的图 1a）。从所显示的相似性中得出的唯一明确的结论是，在两种类型的设备中，两个 S = 1/2 实体负责自旋相关的传输。自旋锁定的开始取决于两种贡献物质的光谱重叠程度以及较窄光谱成分的线宽。考虑到来自混合器件和纯器件的两个 pEDMR 谱线宽度的差异，起始微波 B1 值（参考文献 1 的图 1a、b）的明显相似性令人费解。 Boehme 和 Lupton 继续展示了室温下纯 PCBM 薄膜的新 pEDMR 光谱（参考文献 1 中的图 1c）。它由两个成分组成，一个较窄，另一个较宽（约 3.5 mT），以约 2.002 的 g 值为中心，与之前纯 C60 薄膜的结果类似，该薄膜表现出线宽约 0.3 mT 和约 3 mT 的一致成分（参考文献 5）。 PCBM 上自由基阴离子 P− 的电子顺磁共振谱的 g 值为 1.9995，100 K 时线宽约为 0.3 mT（参考文献 6,7）。此外，研究表明，位于异质结界面的 PCBM 阴离子的特征是相似的7。然而，负责从富勒烯薄膜观察到的 pEDMR 光谱的自旋伴侣的身份尚未确定。同样重要的是要注意，在纯 C60 薄膜的研究中没有观察到自旋锁定5，这表明该信号本质上与我们在混合设备中观察到的信号不同。 Boehme 和 Lupton 提出，我们的 pEDMR 谱图包含两个独立的 P+P− 重组过程的叠加，一个仅在 MEH-PPV 中，另一个仅在 PCBM 中。我们不同意，没有光谱证据表明共混物的 PCBM 成分中涉及自旋实体。我们观察到的光谱显示出不同的 g 值，并且明显比参考文献中图 1c 中所示的窄。 1. 来自纯富勒烯薄膜的 pEDMR 信号中不存在自旋锁定，这提供了反对 PCBM 组件独有的重组过程的贡献的证据。 PCBM 中自旋实体的缺乏可能令人惊讶，因为明显的自旋相关过程是发生在 MEH-PPV P+ 和 PCBM P− 之间异质结界面处的 P+P− 重组过程。我们一致认为，MEH-PPV 中的 P+P− 复合对自旋相关贡献负责，在纯 MEH-PPV 器件中，其贡献约为传输电流 104 中的 1。 P− 的 g 值与正极化子的 g 值相似8 的证据支持了这一点。然而，在混合装置中情况有所不同；尽管 pEDMR 光谱再次显示极化子 g 值的两个贡献，但线宽与纯器件中的线宽不同，重要的是，器件物理特性因包含 PCBM 而发生显着改变。 PCBM 与高密度异质结界面的存在，在该界面处，P− 转移到 PCBM 时能带偏移呈现约 1 eV 的能量增益，这意味着共轭聚合物组分中存在负极化子的概率可以忽略不计。此外，我们对混合器件的 pEDMR 测量是在低偏压 (U = 1 V) 下进行的，抑制电子注入 MEH-PPV。相比之下，纯 MEH-PPV 装置的 pEDMR 光谱使用中等到高的外加偏压值 (U ~4–15 V)。在这些条件下，P+ 和 P− 的电注入是可能的，因此 P+P− 重组是非常可能的。因此，我们维持我们的解释，即数据表明，解释 MEH-PPV:PCBM 混合器件中 pEDMR 的最可能机制是由弱耦合 P+P+ 前体态的瞬态自旋依赖性双极子形成介导的 P+ 渗流传输。博姆和拉普顿声称双极子模型描述了一种能量上不利的“新粒子”。共轭聚合物中的双极子是“新的”这一说法是错误的：它们在 30 多年前就被提出了9，并得到了实验证据的支持10。有机半导体中磁场敏感输运的关键重要性要求我们充分探索和测试自旋相关输运模型。这需要仔细检查光谱证据和设备物理，并应产生一个有凝聚力和预测性的框架。我们论文的关键结果，即室温下聚合物：富勒烯共混物中的长寿命自旋相干性是无可争议的，并且与纯材料上的工作一起表明了在室温下微秒时间尺度上相干自旋操纵的可能性2,4,5。持续自旋相干性和双极化子
To the Editor — In their Commentary Boehme and Lupton1 discuss the current challenges for organic spintronics, and correctly highlight the importance of spectroscopic methods that directly detect the participating spin species. We have previously observed room-temperature coherent electron spin Rabi oscillations persisting beyond 500 ns by pulsed electrically detected magnetic resonance (pEDMR) from a conventional bulk heterojunction organic solar cell2, comprising a blend of 20% conjugated polymer (MEH-PPV) and 80% PCBM (C60). We also observed spinlocking with high-microwave magnetic field amplitudes and the results proved unambiguously that the two spin species responsible were S = 1/2. This observation eliminated models for the spin-dependent transport that provide sensitivity to magnetic fields, such as quenching of triplet excitons by polarons or mutual annihilation of two triplet excitons. Considering both the spectroscopic evidence and the device physics we concluded that the spin-dependent transport mechanism involved hopping via doubly occupied polymer segments, that is, bipolaron states. Boehme and Lupton discuss the importance of distinguishing between spin-dependent transport resulting from bipolaron-mediated hopping and polaron-pair recombination (P+P−) and in so doing question our conclusions. It is indeed worthwhile to review the evidence supporting these models. The overlap of pEDMR spectra complicates the interpretation, so careful consideration of the device structures and operating conditions used for these experiments is necessary. It is important to note that our measurements were made on blends, whereas Boehme and Lupton studied neat films. Our pEDMR measurements on blend devices observed a spectrum at the g-value (2.0028(3)) characteristic of polarons in MEH-PPV, enabling us to conclude that these were the responsible spin species. Two contributions to the spectrum could be resolved, both with the same g-value but with linewidths of 0.6(1) mT and 1.5(1) mT, respectively2. The pEDMR spectrum from a pure MEH-PPV device observed by Boehme, Lupton and co-workers3,4, also shows a spectrum at g = 2.003 comprising two components, but with linewidths of approximately 1.3 mT and 3 mT. Boehme and Lupton1 show a new room-temperature Rabi oscillation spectra from a similar pure MEH-PPV device (Fig. 1b of ref. 1) and compare these results with those from our blend device (Fig. 1a of ref. 1). The only unambiguous conclusion that follows from the similarity shown is that in both types of device two S = 1/2 entities are responsible for the spin-dependent transport. The onset of spin locking depends on the degree of spectral overlap for the two contributing species, and the linewidth of the narrower spectral component. The apparent similarity of the onset microwave B1 value (Fig. 1a,b of ref. 1) is puzzling, given the differences in linewidths for the two pEDMR spectra from the blend and pure devices. Boehme and Lupton proceed to show a new pEDMR spectrum for a pure PCBM film at room temperature (Fig. 1c in ref. 1). It comprises two components, one narrow the other broad (~3.5 mT), centred at a g-value of ~2.002, and is similar to a previous result from a pure C60 film that exhibited coincident components with linewidths of ~0.3 mT and ~3 mT (ref. 5). The electron paramagnetic resonance spectrum of the radical anion, P−, on PCBM has a g-value of 1.9995 and a linewidth of ~0.3 mT at 100 K (refs 6,7). Furthermore, it has been shown that the signature of the PCBM anion localized at the heterojunction interface is similar7. However, the identity of the spin partners responsible for the observed pEDMR spectra from fullerene thin films has yet to be established. It is also important to note that no spin locking was observed in the study of pure C60 films5, indicating this signal is different in nature from that we observed in the blend device. Boehme and Lupton propose that our pEDMR spectrum from the blend comprises a superposition of two independent P+P− recombination processes, one exclusively in MEH-PPV, the other exclusively in PCBM. We disagree, there is no spectroscopic evidence for the involvement of spin entities within the PCBM component of the blend. The spectrum we observed showed a different g-value and is noticeably narrower than that shown in Fig. 1c in ref. 1. The absence of spin locking in pEDMR signals from the pure fullerene films provides evidence against a contribution from a recombination process exclusive to the PCBM component. The lack of involvement of spin entities from the PCBM was perhaps surprising, as an obvious spin-dependent process would be a P+P− recombination process occurring at the heterojunction interface between MEH-PPV P+ and PCBM P−. We agree that P+P− recombination in MEH-PPV is responsible for the spindependent contribution, comprising on the order of 1 in 104 of the transport current4, in the pure MEH-PPV devices. This is supported by evidence that the g-value for P− is similar to that for the positive polaron8. However, the situation is different in the blend devices; although the pEDMR spectra again show two contributions at the polaron g-value the linewidths are different from those in the pure devices, and importantly the device physics are markedly altered by the inclusion of PCBM. The presence of a high density of heterojunction interfaces with PCBM, at which the band offsets present a ~1 eV energy gain for P− transfer to PCBM, means that the probability of negative polarons being present in the conjugated polymer component is negligible. Further, our pEDMR measurements on the blend devices were performed with low bias (U = 1 V) inhibiting electron injection to MEH-PPV. By contrast, the pEDMR spectra from the pure MEH-PPV devices used medium to high applied bias values (U ~4–15 V). Under these conditions the electrical injection of P+ and P− is possible so P+P− recombination is highly plausible. In consequence, we maintain our interpretation that the data suggest that the most probable mechanism explaining the pEDMR in the MEH-PPV:PCBM blend device is the percolation transport of P+ mediated by transient spin-dependent bipolaron formation from weakly coupled P+P+ precursor states. Boehme and Lupton claim the bipolaron model describes an energetically unfavourable ‘new particle’. The suggestion that bipolarons in conjugated polymers are ‘new’ is wrong: they were proposed more than 30 years ago9 and are supported by experimental evidence10. The key importance of magnetic fieldsensitive transport in organic semiconductors requires that we fully explore and test the models for spin-dependent transport. This requires careful examination of spectroscopic evidence and the device physics, and should result in a cohesive and predictive framework. The key result of our paper, namely longlived spin coherence in a polymer:fullerene blend at room temperature is unchallenged and, together with work on the neat materials, suggests the possibility of coherent spin manipulation on the microsecond timescale at room temperature2,4,5. The Persistent spin coherence and bipolarons
影响因子：
8.6
作者：
Behrends, J.;Schnegg, A.;Keeble, D. J.
通讯作者：
Keeble, D. J.