Figure 2. Charge accumulation in the X-ray photo-response of organic detectors.

(a) Schematic of the process of modulation of the conductivity induced by X-rays exposure of TIPS-pentacene thin films: (left) in dark the conductivity is due to the intrinsic carriers; (right) under X-ray irradiation: (1) additional electrons and holes are generated; after generation holes drift along the electric field until they reach the collecting electrode while (2) electrons remain trapped in deep trap states within the organic material. (3) To guarantee charge neutrality, holes are continuously emitted from the injecting electrode. As a consequence, for each electron-hole pair created, more than one hole contributes to the photocurrent leading to a photoconductive gain effect. (4) Recombination process takes place, counterbalancing the charge photogeneration in the steady-state. (b) Plots of the 4-probe (black squares) and 2-probe (red circles) measurements upon three switching on and off the X-ray beam (the grey regions in the plot indicate when the device is under irradiation). The X-ray source is a Mo-target tube with 35 kV of accelerating voltage. (c) Schematic of the electrode configuration used for the 4-probe (left) and the 2-probe (right) measurements reported in b.