A HIGH-VOLTAGE INTERRUPTED-FIELD TIME-OF-FLIGHT TRANSIENT PHOTOCONDUCTIVITY APPARATUS

The present paper describes a floating bridge technique and a high-voltage apparatus for carrying out interrupted-field time-of-flight (IFTOF) transient photoconductivity measurements with applied voltages up to 1 kV. Switching transient voltages appearing as a result of the removal and reapplication of the applied bias voltage, which otherwise would obliterate the normal TOF photocurrent signal, have been reduced to an undetectable level by using a floating Schering bridge network with two nulling capacitors. The bias voltage is applied from a single triggerable floating high-voltage switch which utilizes recent commercially available n-channel TMOS transistors and can deliver up to 1 kV in a switching time of approximately 150 ns. By switching the gain of an amplifier during the interruption time, even very small recovered photocurrent signals can be measured with respect to the pre-interrupted signal. The technique has been successfully applied to measure both hole and electron trapping lifetimes in a-Se:0.5%As electroradiographic photoreceptors, and to determine the range of the charge carriers. The distinct advantage of the present apparatus, as well as its shortcomings, over the complementary bias and single nulling capacitor method, which was described previously, are also addressed.