Visualization of Carrier Behavior in Organic Photo-conductor layer of copy machine by Transient Space Charge Measurement

在光导材料的评估中，通常采用飞行时间法来估计载流子的迁移率。在这种方法中，在对材料施加光脉冲后，测量偏置电压下的瞬态电流。瞬态电流显示出一个峰或肩，它的时间滞后于光脉冲，给出了载流子的飞行时间，由此可以解释在一定偏置电压下的迁移率。然而，根据实际载波运动，解释往往非常困难。载流子迁移率的分散，以及注入后载流子的扩散，都会对瞬态电流产生畸变。双注入会使瞬态电流变得非常复杂。因此，通常不可能根据瞬态电流来评估准确的载流子运动。利用空间分辨率测量随时间变化的空间电荷分布可能有助于实现这一目的。脉冲电声法在空间电荷的测量中得到了广泛的应用。我们曾提出用直接测量的瞬态空间电荷来评估载流子迁移率，在该方案中，在偏置电压上叠加一个激励脉冲电压来激励过量的电荷。注入的过量电荷形成一个电荷包，通过空间电荷测量跟踪其运动。用载流子迁移率低至10^<-13> m^2/Vs的聚合物证明了该技术的可行性。本报告中对光导材料的测量概念基本相同，但对载流子激发给出的是光脉冲，而不是电压脉冲。此外，还注意跟踪电荷包的快速运动。观察到，在脉冲照射后，载波包在几毫秒内穿过薄膜。成功地将跃迁与相应的瞬态光电流相关联。复印机的打印速度不是由载流子的平均迁移率决定的，而是由载流子的最慢行为决定的，因为残余的载流子可能会影响下一次打印的质量。传统的飞行时间法只能给出载流子迁移率的平均值，难以用瞬态电流来解释载流子迁移率的色散。瞬态空间电荷表明，表观载流子迁移率的色散是显著的。此外，载流子到达阴极后消失需要一定的时间。这意味着阴极的界面现象在载流子通过材料的输运中是不可忽略的。作者认为，所提出的测量方法将有助于评价影印用光导材料。少

In the assessment of photo-conductive materials, time-of-flight method is often applied to estimate the carrier mobility. In this method, a transient current under a bias voltage is measured after giving a light pulse on the material. The transient current shows a peak or shoulder, its time lag from the light pulse giving the time-of-flight of the carrier from which the mobility under a certain bias voltage can be interpreted. However, the interpretation is often very difficult depending on actual carrier motion. Dispersion in carrier mobility, as well as diffusion of carrier after being injected, will bring a distortion in transient current. Double injection may make the transient current very complicated. Therefore, it is not generally possible to assess the exact carrier motion based on the transient current. Measurement of time dependent space charge profile with spatial resolution may be helpful for this purpose. The pulsed electroacoustic method has been widely applied for the me … More asurement of space charge. We once proposed to evaluate carrier mobility from directly measured transient space charge, In the proposal, an excitation pulse voltage was applied being superimposed on bias voltage to excite excessive charge. The injected excessive charge formed a charge packet, and its motion was traced by space charge measurement. The feasibility of this technique was proved with some polymers of which carrier mobility was as low as 10^<-13> m^2/Vs. The concept of measurement in this report for photo-conductive materials is basically the same, however, a light pulse is given for carrier excitation in stead of a voltage pulse. In addition, a care has been taken in order to trace a quick motion of charge packet. It was observed that the carrier packet traveled through the film within several milliseconds after pulsive illumination. The transition was successfully correlated with corresponding transient photo-current. The printing speed of the photocopy is not determined by the average carrier mobility but determined by the slowest behavior of the carrier, since the residual carrier may affect the quality of the next print. Conventional time-of-flight method gives only the average carrier mobility, and the dispersion of carrier mobility is hardly interpreted from the transient current. The transient space charge showed that the dispersion of apparent carrier mobility is significant. In addition, it took a certain time until the carrier disappeared after it had reached the cathode. This means that the interfacial phenomenon at the cathode is not negligible in carrier transport through the material. The authors believe that the proposed measurement method will help to assess the photo-conductive material for photocopy-use. Less