Transient and local illumination of an organic photoconductive sensor

In this paper we investigate the performance of a transparent photoconductive sensor based on a double layer of organic materials (m-MTDAB / PTCBI) which are deposited on two interdigitated transparent ITO electrodes. The performance of the sensor is demonstrated with electro-optical measurements: the I(V) curves consist of two linear sections meeting at a knee voltage V _t. Linear regression performed on the I(V) curves below V_t show that the conductance is a power law of the luminance incident on the device. We present a model to describe the behaviour of the sensor below V _t. We present measurements of I(t) for a transient illumination of the sensor. Plotting the inverse of the current as a function of time we find that the transient is consistent with the model for voltages below V _t. For voltages above V_t we find that the sensor behaves like a resistor in series with a space charge (SC) region. We present a local illumination experiment that confirms the existence of a SC region between the electrodes of the photoconductive sensor for V<V_t. The space charge region is located near the cathode of the sensor.