Exciton Binding Energy in Molecular Triads
Exciton Binding Energy in Molecular Triads
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DOI：
10.1021/acs.jpcc.7b03923
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发表时间：
2017-08-17
影响因子：
3.7
通讯作者：
Cuniberti, Gianaurelio
中科院分区：
文献类型：
--
作者：
Kraner, Stefan;Prampolini, Giacomo;Cuniberti, Gianaurelio
预测现有技术的有机光致发光器件的功率转换效率被限制在约15%。该限制可以通过提高载流子迁移率和降低激子结合能来增加。为了实现这一点，我们提出了一个概念的基础上有机三单元组，包括一个供体，间隔，和受体亚分子。基于含时密度泛函理论（TD-DFT）模拟，我们研究了著名的类胡萝卜素-卟啉-C60三单元体的最低激发态，并获得了25 meV的计算激子结合能，证明了实验观察到的和报道的光生电荷分离。此外，我们引入了一种新的三元组，不仅能够改善和控制分离过程，而且还能够改善载流子输运特性。我们使用分子动力学（MD）模拟，以优化一个集群的25个三合会的几何形状。从这个有机团簇中，我们选择了一个三元组与它的四个邻居，再次计算了这种结构的激子结合能，得到39 meV。我们的结论是，激子结合能是稳定的各种不同的基组和泛函，并没有显着改变，如果一个或一个集群的五个三合会模拟。这种稳定的行为发生，因为波函数的变化不会显着影响激子结合能，只要正负电荷之间的距离保持不变。对于光伏应用和基于介电常数约为4的有机材料，我们建议使用大于2纳米的间隔分子。
The power conversion efficiency of state of the art organic photovoltaics is predicted to be limited at about 15%. This limit can be increased by an improved charge carrier mobility and by a lower exciton binding energy. In order to achieve this, we suggest a concept based on organic triads, comprising a donor, spacer, and acceptor submolecule. On the basis of time dependent density functional theory (TD-DFT) simulations we investigate the lowest excited state of the well-known Carotenoid-Porphyrin-C60 triad and obtain a calculated exciton binding energy of 25 meV, justifying the experimentally observed and reported separation of photo generated charges. Further, we introduce a new triad with the ability to not only improve and control the separation process, but also to improve the charge carrier transport properties. We used molecular dynamics (MD) simulations to optimize the geometry of a cluster of 25 triads. From this organic cluster, we picked one triad with its four neighbors, again calculated the exciton binding energy of this structure and obtained 39 meV. We conclude that the exciton binding energy is stable for a variety of different basis set and functionals, and does not significantly change if one or a cluster of five triads is simulated. This stable behavior occurs, since changes in the wave functions do not significantly influence the exciton binding energy, as long the distance between the positive and negative charge remains the same. For photovoltaic applications and based on organic materials with a dielectric constant of about four, we suggest the use of spacer molecules larger than two nanometers.