Organic semiconductors are used in many fields of photonics. Displays fabricated using organic light emitting diodes (OLED) can be found in modern smartphones and tablets, whereas organic solar cells (OPV - organic photovoltaics) are emerging with demonstrated efficiencies above 10%. Organic photodetectors (OPD) are another very interesting domain, with ultrathin active layers (order of tens of nanometers) providing performance comparable to bulk inorganic devices. Thanks to a multitude of possible compounds, parameters such as response spectrum, cut-off wavelength etc. can be easily tuned. Because of very high absorption coefficient and low refractive index, issues such as crosstalk or reflection can be minimized. Another exciting feature is the low processing temperature and thus feasibility of using a flexible foil as substrate, leading to rollable or curved photodetector arrays.The performance of organic based photodetectors has grown considerably in the recent years in terms of high speed, high frequency response and detectivity so as to match the required custom specifications for practical applications such as imaging. However, further attention needs to go towards the understanding of the effects of scaling during miniaturization of these devices, while looking for the preservation and/or optimization of their opto-electrical properties. Therefore, in this thesis, emphasis is given on the photodetectors based on organic conjugated polymer materials for imaging applications. These organic photodetector devices normally suffer from a perimeter-to-area dark current density scaling, hence, special attention is on the understanding of this scaling effect as well as optimization of the fabrication process in order to get low leakage (dark) current, and therefore, large dynamic range of the OPDs.
Photodetectors with different geometries and sizes from 0.08 cm2 down to 1.95E ?5 cm2 were fabricated, using two types of edge cover layer. Isolated islands of two different organic films were patterned with photolithography in order to study the dark current mechanisms. Electrical, optical and morphological properties were characterized for both patterned and non-patterned devices. The patterning process used to fabricate the isolated devices does not affect the optoelectrical and morphological characteristics of the photodetectors, and therefore their performance. Moreover, it does not induce or accelerate degradation of the organic films.
The dark current density further decreases after full patterning is performed, while preserving all device characteristics. It is proposed to replace the semitransparent edge cover layer by the opaque, if thicker films can be achieved, as it might be possible to decrease the contribution of the perimeter leakage. Once the patterning process is performed, it is possible to implement these isolated devices as photodetectors arrays for imaging applications with an Ion/Ioff ratio of 6 orders of magnitude.
It is proposed to perform a theoretical study of the dark current mechanism at the interface between the polymer and the metallic contact edges to model and simulate the perimeter recombination at reverse bias conditions. Moreover, further measurements such as electroluminescent and spectroscopy might aid to characterize the carrier transport in the OPDs. The project was aimed within the R&D department of large area electronics. Moreover, all the fabrication and characterization were performed in the facilities of IMEC
