A two-dimensional model for an in-plane organic photo-conductive bilayer sensor

Space-charge limited surface currents between two semi-infinite planar electrodes embedded in a uniform dielectric medium

“…This work is the continuation of previous work we did on the development of a transparent organic photoconductive sensor [18,28], made with typically 80 µm wide ITO electrodes, separated by a 20 µm gap and covered by a 40 nm thick hole transporting material (m-MTDAB) and a 20 nm thick electron transporting material (PTCBI) [18]. Excitons are generated in the PTCBI-layer, diffuse towards the interface and dissociate into an electron in the PTCBI-layer and a hole in the m-MTDAB-layer.…”

“…We showed that this is due to a space-charge occurring near the cathode [19]. Using a numerical model [28] we could reproduce the observed behavior using reasonable values for the material parameters and assuming that most of the conduction occurs in the PTCBI-layer, whereas most of the holes remain trapped in the m-MTDAB-layer Contrary to the assumptions made in section 4, and as explained in [28], the fabricated photoconductor has effectively an electron injecting cathode since the active layers covering the cathode form a reverse biased heterojunction (above the anode the heterojunction is forward biased and forms an effective sink for electrons). This explains why there occurs a linear regime without space-charge limitation for small voltages.…”

“…To check the analytical solutions given in the appendix for the electric field and for the charge density we compare (A.3)(A.4) and (A.7) with the results of a numerical model, which uses (10) for calculating the electric field but takes into account drift and diffusion [28] (see figure 2). Boundary conditions for the numerical model were chosen to obtain conditions of single carrier injection: we used a Poole-Frenkel field emission formula for both carriers and adapted the barriers so that only holes are injected.…”

“…Data pertaining to the linear regime has been published in [28] and is reproduced in figure 7, augmented with data for the SCL-regime. Data is shown as a function of the irradiance (either a 639.6 nm laser irradiance or a display backlight luminance converted to an equivalent laser irradiance).…”

“…For the linear regime the triangles show the current I(V ), and the squares show the slope of the current dI/dt just after switching off the irradiance, but with the bias voltage still applied. This quantity enables us to judge the dependence of the generation rate on the irradiance [28]. In the linear regime the generation rate of electron/hole pairs is in equilibrium with the recombination rate and depending on the recombination mechanism this determines the values of the electron and hole densities.…”