Ohkita Hideo, Cook Steffan, Astuti Yeni, Duffy Warren, Tierney Steve, Zhang Weimin, Heeney Martin, McCulloch Iain, Nelson Jenny, Bradley Donal D C, Durrant James R
Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AZ, United Kingdom.
J Am Chem Soc. 2008 Mar 12;130(10):3030-42. doi: 10.1021/ja076568q. Epub 2008 Feb 16.
We report herein a comparison of the photophysics of a series of polythiophenes with ionization potentials ranging from 4.8 to 5.6 eV as pristine films and when blended with 5 wt % 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]C61 (PCBM). Three polymers are observed to give amorphous films, attributed to a nonplanar geometry of their backbone while the other five polymers, including poly(3-hexylthiophene), give more crystalline films. Optical excitation of the pristine films of the amorphous polymers is observed by transient absorption spectroscopy to give rise to polymer triplet formation. For the more crystalline pristine polymers, no triplet formation is observed, but rather a short-lived (approximately 100 ns), broad photoinduced absorption feature assigned to polymer polarons. For all polymers, the addition of 5 wt % PCBM resulted in 70-90% quenching of polymer photoluminescence (PL), indicative of efficient quenching of polythiophene excitons. Remarkably, despite this efficient exciton quenching, the yield of dissociated polymer+ and PCBM- polarons, assayed by the appearance of a long-lived, power-law decay phase assigned to bimolecular recombination of these polarons, was observed to vary by over 2 orders of magnitude depending upon the polymer employed. In addition to this power-law decay phase, the blend films exhibited short-lived decays assigned, for the amorphous polymers, to neutral triplet states generated by geminate recombination of bound radical pairs and, for the more crystalline polymers, to the direct observation of the geminate recombination of these bound radical pairs to ground. These observations are discussed in terms of a two-step kinetic model for charge generation in polythiophene/PCBM blend films analogous to that reported to explain the observation of exciplex-like emission in poly(p-phenylenevinylene)-based blend films. Remarkably, we find an excellent correlation between the free energy difference for charge separation (deltaG(CS)rel) and yield of the long-lived charge generation, with efficient charge generation requiring a much larger deltaG(CS)rel than that required to achieve efficient PL quenching. We suggest that this observation is consistent with a model where the excess thermal energy of the initially formed polaron pairs is necessary to overcome their Coulombic binding energy. This observation has important implications for synthetic strategies to optimize organic solar cell performance, as it implies that, at least devices based on polythiophene/PCBM blend films, a large deltaG(CS)rel (or LUMO level offset) is required to achieve efficient charge dissociation.
我们在此报告了一系列电离势在4.8至5.6 eV之间的聚噻吩作为原始薄膜以及与5 wt%的1-(3-甲氧基羰基)丙基-1-苯基-[6,6]C61（PCBM）共混时的光物理性质比较。观察到三种聚合物形成非晶薄膜，这归因于其主链的非平面几何结构，而其他五种聚合物，包括聚(3-己基噻吩)，形成的薄膜结晶度更高。通过瞬态吸收光谱观察到非晶聚合物原始薄膜的光激发会产生聚合物三线态。对于结晶度更高的原始聚合物，未观察到三线态的形成，而是观察到一个短暂存在（约100 ns）、归因于聚合物极化子的宽光致吸收特征。对于所有聚合物，添加5 wt%的PCBM导致聚合物光致发光（PL）猝灭70 - 90%，这表明聚噻吩激子被有效猝灭。值得注意的是，尽管激子被有效猝灭，但通过归因于这些极化子双分子复合的长寿命幂律衰减相的出现来测定的解离聚合物 + 和PCBM - 极化子的产率，根据所使用的聚合物不同，观察到其变化超过2个数量级。除了这种幂律衰减相，共混薄膜还表现出短暂的衰减，对于非晶聚合物，这归因于束缚自由基对的双生复合产生的中性三线态，对于结晶度更高的聚合物，则归因于直接观察到这些束缚自由基对双生复合回到基态。根据聚噻吩/PCBM共混薄膜中电荷产生的两步动力学模型对这些观察结果进行了讨论，该模型类似于为解释基于聚对苯撑乙烯的共混薄膜中激基复合物样发射的观察结果而报道的模型。值得注意的是，我们发现电荷分离的自由能差（deltaG(CS)rel）与长寿命电荷产生的产率之间存在极好的相关性，高效的电荷产生需要比实现高效PL猝灭所需的deltaG(CS)rel大得多。我们认为这一观察结果与一个模型一致，在该模型中，最初形成的极化子对的多余热能对于克服它们的库仑结合能是必要的。这一观察结果对优化有机太阳能电池性能的合成策略具有重要意义，因为这意味着，至少对于基于聚噻吩/PCBM共混薄膜的器件，需要大的deltaG(CS)rel（或LUMO能级偏移）来实现有效的电荷解离。
