Simulation tools for high frequency flexible electronic devices
Final Report Abstract
A theoretical investigation on the nature of charge transport in organic molecules, as well as device characterization in OFETs, on the entire dimension scale has been presented. On the atomistic scale, ab initio methods of DFT and NEGF were used to study the electronic structure and charge transport through SAM-forming molecules for the active region of a FET (i.e. SAMFET). The molecules of interest were three different types of organo-phosphonates. The difference arising from the addition and size of the side chain substituted aromatic groups. A total of five molecular configurations were considered, with the latter two involving an overlap betweenthe side chains. This was done with the aim to represent a thin monolayer of SAM-molecules given specific boundary conditions. Analyses in gas phase produced a trend of decreasing band gaps. Presence of the aromatic groups, their size, and overlap were concluded to be the main factors towards the observed trend. Molecules were then attached to Si/Al2O3 substrates and capped by gold contacts to construct the desired device configurations. Charge transport simulations were carried out and transmission probabilities extracted. From these probabilities, conductive band gaps were identified observing a similar trend of molecular band gap reduction, to that in gas phase. Molecules with side chain aromatic groups exhibited higher evaluated currents. Furthermore, molecular orbital projections corresponding to high transmission probabilities indicated that both inter- and intra-chain flow of charges can occur in these molecules, allowing them to be suitable candidates for active regions in FETs. Macro-scale investigations involved using the DD model for the characterization of OFETs.Important sub-models responsible for accurate description of events taking place within organicsemiconductors were highlighted and considered in the simulation procedure. The effect of fixed charges, interface and bulk traps were investigated. Based on these investigations several scenarioswere outlined for the fitting process of experimental transfer and output OFET characteristics. Itwas concluded that one scenario was insufficient to describe the transistor characteristics over theentire voltage range within device operation. This range was divided into two separate regimes,were each regime was described by a different set of parameterizations. An argument for thisregime separation was presented. Once fitting was achieved, this allowed for the extraction ofimportant device performance measures including the threshold voltages and charge mobilities. Extracted values were found to be close to, if not similar, to what have been reported fromexperimental studies. At the top of the dimension scale, circuit level modelling was used to identify low conductanceregions in organic thin films. The equivalent experimental procedure is SPCM. This involvedshining a laser light onto a specific spot along the thin film. Difference between measured photocurrents(post illumination), dark currents (pre-illumination), and hence local spot properties were depicted in the form of colormaps highlighting the strength of that difference. To investigate this process from a theoretical perspective, a theory of network resistors was applied. Manipulation of block resistances provided the necessary effect of light illumination, and in a similar fashion colormaps were drawn out. The model was initially validated and then used as a comparison with experimental findings. Results were successfully reproduced. Trap densities corresponding to the identified low conductance regions (i.e. defective) were also evaluated. Something that SPCM cannot provide.
Publications
-
"Modeling and simulation of trap densities in organic thin films," Proc. IEEE International Conference on Nanotechnology (2017), Pittsburgh, Pennsylvania, USA
M. Darwish, A. Gagliardi
-
"Program FFlexCom – High frequency flexible bendable electronics for wireless communication systems," Proc. IEEE International Conference on Microwaves, Antennas, Communications and Electronic Systems (COMCAS) (2017), Tel-Aviv, Israel
T. Meister, F. Ellinger, ....., M. Darwish, A. Gagliardi, .....
-
"A resistor network simulation model for laser-scanning photo-current microscopy to quantify low conductance regions in organic thin films," Org. Electron. 62, 474-480 (2018)
M. Darwish, H. Boysan, C. Liewald, B. Nickel, A. Gagliardi
