Relaxation Dynamics of Photo-excited States and Generation and Transport Processes of Carriers in Pristine C_<60> Single Crystals.

为了了解C_<60>晶体光激发后的整个弛豫过程，我们研究了与光致发光过程相关的载流子产生机制。在本研究项目的第一阶段，我们已经明确载流子的产生源于Frenkel激子的热解离，局域激子的发光通过捕获Frenkel激子进入局域态而增强。为了证实载流子产生与发光过程之间的反相关关系，在同一晶体中测量了光电流和光致发光。用飞行时间法测量了光电流，用显微光谱法观察了ITO电极和云母薄膜的光致发光。光电流和光致发光强度的温度依赖性服从阿伦尼乌斯公式。在100K以上光电流可观测到的范围内，更多的发光来自局域激子的辐射衰变。光电流的热激活行为与光致发光的猝灭表现出反相关性。这种关系可以解释为载流子生成的增强直接导致发光强度的降低。假设热激活居群从弗伦克尔激子转移，它们可以符合99meV的相同活化能。估计的活化能表明，存在于弗伦克尔激子态120meV以下的局域态对载流子的产生没有贡献。另一方面，得到的光电流时间响应为“色散型”，几乎以t^<-□> (α ~ 1)衰减。TOF方法中的这种响应归因于漂移载流子的异常色散。C_<60>晶体中的载流子输运似乎服从跳跃过程，得到的响应表明跳跃位点在势垒高度和距离上存在连续分布。少

Aiming to understanding the whole relaxation processes following optical excitation in C_<60> crystals, we have investigated the carrier generation mechanism in connection with the photoluminescence process. In the first stage of the present research project, we have made clear that the carrier generation originates from thermal dissociation of Frenkel exciton and luminescence from localized exciton is enhanced by capturing the Frenkel exciton into the localized state. In order to confirm anti-correlation between the carrier generation and the luminescence process, photocurrent and photoluminescence were measured in one and the same crystal. The photocurrent was measured by Time-of-Flight method, and the photoluminescence through ITO electrodes and mica thin films was observed by a microspectroscopy method.Temperature dependence of photocurrent and photoluminescence intensities obeys to Arrhenius formula. In the range higher than 100K where the photocurrent becomes observable, the lumi … More nescence originates from radiative decay of localized exciton. The thermally activated behavior of the photocurrent exhibits anti-correlation to quenching of the photoluminescence. This relation is interpreted by that enhancement of carrier generation directly leads reduction of luminescence intensity. They can fit with the same activation energy of 99meV by assuming thermal activated population transfers from the Frenkel exciton. The estimated activation energy suggests that the localized state, which exists below 120meV from the Frenkel exciton state, doesn't contribute to the carrier generation. On the other hand, obtained time response of photocurrent is of "dispersive-type", and almost decays as t^<-□> (α〜1). Such response in TOF method has been attributed to anomalous dispersion of drift carriers. Carrier transport in C_<60> crystals seems to obey a hopping process, and the obtained response suggests the existence of continuous distribution of hopping sites in barrier height and distance. Less