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

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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