Doped Graphene and Reduced Graphene Oxide Thin Films as Transparent Conducting Electrodes for Optoelectronic Applications

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Name of the Guide:

Speaker : S Ragul (EE16D031) Date : January 24, 2022 (Monday) Time : 3:00 pm Venue : Google Meet ( Guides : Dr. Debdutta Ray and Dr. Soumya Dutta ABSTRACT : In the first part of the talk, studies on polymer ferroelectric doping of graphene and reduced graphene oxide (rGO) thin films investigated using advanced electrostatic force probe microscopic (EFM) techniques will be presented. The solution spin casting at elevated temperature of polyvinylidene fluoride-trifluoro ethylene (PVDF-TrFE) into Nano-thin films enabled the low voltage poling desired for this study. The ferroelectric characterization of PVDF-TrFE using dynamic contact EFM and the induced doping effect in graphene using Kelvin probe force microscope (KPFM) will be discussed. These studies can be used to design ferroelectric-based FETs that can achieve lower sub-threshold slopes. Further, electrostatic doping induced Fermi-level tuning is used to map the nature of the density of states around the charge neutrality point in rGO thin films on SiO2/Si substrates. This was enabled by implementing a modified KPFM setup with in-situ gating-based time-resolved measurements. The results are in agreement with the DFT and tight-binding model-based calculations. Following this, the reflection of these electrostatic effects as parasitic in the electrical performance at the device level was explored. The experimental devices were fabricated with graphene/poly (3-hexyl thiophene) : phenyl [6,6] C61 butyric acid methyl ester bulk heterojunction blend/aluminum diode structure and a standard reference device made of indium tin oxide/poly (3,4-ethylene dioxythiophene) polystyrene sulfonate in place of graphene. On comparison, the graphene devices showed larger reverse bias currents. This was mapped to the intra-device self-gating effects of the graphene electrodes by the counter cathode under normal operations. Details of the analytical equations adopted to capture this effect, along with the numerical TCAD results, will be presented. The developed model can also be adapted to other devices like light-emitting diodes, perovskite-based devices, thin-film a-Si devices and FETs with appropriate modifications. In the last part of the talk, the development of an in situ measurement setup to study the dynamics of small molecular doping of graphene will be presented. The choice of Si-graphene heterojunction solar cell enabled the dynamic doping studies at the device level due to the availability of exposed graphene surfaces in their device architecture. The response of the fabricated heterojunction solar cells measured with this setup will be discussed.