9/20/2023 0 Comments Dark current dark noise![]() ![]() As predicted by the simulations, a systematic/controlled decrease of the trap concentration from ~3.5 × 10 15 to ~1.0 × 10 15 cm −3 indeed results in a one order of magnitude decreased J D. ![]() Using impedance spectroscopy (IS), we detect mid-gap trap site distributions in several OPD devices. By employing drift-diffusion simulations, we show that these higher J D values can be explained when a distribution of trap states, present in the D–A blend, is taken into account. This discrepancy is commonly observed in OPDs and is the main limiting factor for achieving higher detectivities. However, the measured J D is orders of magnitude higher than the ideal, thermally generated dark current, J 0, calculated within the radiative limit 14. In this work, we show that J D indeed scales with E CT and values as low as 10 −7 mA cm −2 are achieved for an E CT of 1.58 eV at −1 V. Therefore, the activation energy of the ideal dark current of organic diodes, based on D–A blends, is determined by E CT. Being usually lower than the gap of the single components, the effective gap of the blend is the characteristic charge-transfer state energy ( E CT). However, in organic diodes formed by a donor–acceptor (D–A) structure, charge-transfer (CT) states are present at the interface 13. ![]() In an ideal diode, in addition to the diffusion current, the dark saturation current ( J 0) comprises a thermally activated component as a result of thermal generation of charges over the gap of the material 12. While the above-mentioned J D suppression approaches lead to an improved OPD performance, a comprehensive understanding of the intrinsic and extrinsic sources of dark current is still missing, which would provide insights for future device optimization using improved materials or architectures. Most frequently used approaches are charge selective layers 4, 5, contact alignment 6, 7, prevention of shunt paths via layer thickness increase 8, and interlayers to smoothen the bottom contact 9, 10, as well as charge transport layer structuring 11. Among the many sources of noise, the shot noise, proportional to the dark current, has been suggested to play a major role 2, especially because OPDs usually operate in reverse bias voltages, where the measured reverse dark current ( J D) strongly deviates from its ideal value.ĭark current suppression in organic diodes has been the subject of several reports in the literature 3. On the other hand, organic photodetectors (OPDs) can be significantly cheaper, but these devices still suffer from a high S n, resulting in rather low detectivities. While their performance is outstanding, devices and imagers are expensive and inflexible. Currently, PDs for the visible and near-infrared spectral region are mainly based on silicon (Si) and indium gallium arsenide (InGaAs) alloys. Besides a high responsivity, a low-noise spectral density ( S n), resulting in a high specific detectivity ( D *), is a key requirement. Light sensing and imaging 1 are important technological fields and create high demand for photodetectors (PDs). By modeling the dark current of several donor–acceptor systems, we reveal the interplay between traps and charge-transfer states as source of dark current and show that traps dominate the generation processes, thus being the main limiting factor of organic photodetectors detectivity. We demonstrate that, in addition to the intrinsic saturation current generated via charge-transfer states, dark current contains a major contribution from trap-assisted generated charges and decreases systematically with decreasing concentration of traps. Here, we show that the shot noise, proportional to the dark current, dominates the noise spectral density, demanding a comprehensive understanding of the dark current. However, the high noise spectral density of these devices limits their specific detectivity to around 10 13 Jones in the visible and several orders of magnitude lower in the near-infrared, severely reducing performance. Recent research on organic photodetectors based on donor–acceptor systems has resulted in narrow-band, flexible and biocompatible devices, of which the best reach external photovoltaic quantum efficiencies approaching 100%. Organic photodetectors have promising applications in low-cost imaging, health monitoring and near-infrared sensing. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |