利用瞬态荧光技术研究单重态激子裂变的电荷转移模型

有机材料中单重态激子的裂变过程,由于其在有机光伏器件中的潜在应用而成为一个科学研究的热点.传统的观点采用电荷转移模型来解释激子裂变过程,即认为2个参与裂变的分子之间通过两次的电荷转移来实现分子状态的改变.而在电荷转移的物理图像中,又包括双空穴转移方式和双电子转移方式两种可能性.为了检验电荷转移模型的合理性,将能够发生激子裂变过程的红荧烯分子分别混合于其他4种有机分子中,这4种有机分子被当作间隔分子,用来分离混合膜中掺杂的红荧烯分子.对红荧烯分子与间隔分子,二者间HOMO能级的能量差构成空穴转移的隧穿势垒,而二者间LUMO能级的能量差构成电子转移的隧穿势垒.对4个样品发光衰减曲线的测量与分析表明,激子裂变的速率与电子隧穿势垒的高度具有明显的关联,这从实验角度首次印证了双电子转移模型而否定了双空穴转移模型.

A Charge-Transfer Model for Studying Singlet Exciton Fission with the Transient Fluorescence Technique

Singlet exciton fission in organic materials has become a hotspot of scientific research due to its potential applications in organic photovoltaic devices. Traditionally, exciton fission can be explained with the "charge-transfer" model, in which twice charge-transfer processes between two involving molecules lead to the state conversion. Furthermore, in the physical image of charge-transfer, there exist two possible modes: the double electron-transfer mode and the double hole-transfer mode. In order to test these models, a highly efficient fission material, i.e. rubrene, was mixed into 4 different organic materials, which acted as spacers to separate those doped rubrene molecules. For the rubrene molecules and the spacer molecules, their energy difference between HOMO levels functioned as a tunneling barrier for intermolecular hole-transfer, whereas their energy difference between LUMO levels functioned as a tunneling barrier for intermolecular electron-transfer. Measurement and analysis of the transient fluorescence decay curves of the 4 blending films showed that the rates of singlet exciton fission were highly significantly correlated with the heights of electron-tunneling barriers, thus giving a first experimental conformation for supporting the electron-transfer model and rejecting the hole-transfer model. The results presented in this work are thought to be of significant value for clarifying the physical mechanism of intermolecular singlet exciton fission, for it provides a direct experimental evidence for improving the theoretical computations based on electron-transfer images.