Analysis of a transparent organic photoconductive sensor

The electro-optical performance of transparent photoconductive sensors based on stacks of organic layers is investigated. The photoconductive sensors are composed of interdigitated electrodes covered with a stack of two transparent organic compounds: a hole transport layer 1,3,5-tris[(3- methylphenyl)phenylamino]benzene (m-MTDAB) and an exciton generation layer 3,4,9,10-perylenetetracarboxylic bis-benzimidazole (PTCBI). The photocurrent through the device is measured as a function of the voltage across the electrodes for different illumination levels. Based on the measurements we can explain the working principle of the photoconductive sensor and compare the performance of four different stacks. In order to study the optical sensitivity in more detail, a photoconductive device with two parallel electrodes is manufactured and illuminated by a line-shaped laser beam that covers only a fraction of the gap between the electrodes. The current through the photoconductive sensor is measured as a function of the position of the local illumination for a set of voltages. The experimental results confirm that there is a high-field space charge region near the cathode.