| PhD Viva

Name of the Speaker: Ragul S (EE16D031)
Guide: Dr. Debdutta Ray
Online meeting link: https://meet.google.com/uff-qetb-zbr
Date/Time: 3rd November 2022, 4.00pm
Title: Doped Graphene and Reduced Graphene Oxide Thin Films as Transparent Conducting Electrodes for Optoelectronic Applications

Abstract

Though graphene exhibits high charge carrier mobility , practically synthesized large-area graphene films suffer from poor conductivity. This is due to the poly-crystalline nature and the presence of other defects, and low intrinsic carrier density when operated near the charge neutrality point (CNP). The application of graphene as the transparent conducting electrode (TCE) in optoelectronic devices requires that it possess low sheet resistance comparable to that of the existing transparent conducting oxides (TCO) such as fluorine/indium doped tin oxides. This can be addressed by using multi-layer films or by doping graphene films. Here we investigate the surface doping of graphene films by organic small molecule interactions and its dynamics by exploring the inhomogeneity due to varying interactions at the adsorption site. Experimental evidence of defect-specific doping effects of organic dopant on graphene with artificially induced controlled defects was recorded. Density functional theory (DFT) based calculations were performed to validate these observations qualitatively. We show that the degree of doping is governed by the nature of graphene film at the localized dopant-interaction site. The relative change in resistance scales proportionally to the initial resistance (or defect density) of the graphene film. This inhomogeneity in graphene doping is adopted to describe the doping-induced discrepancies in the transfer characteristics of graphene channel transistors when operated around CNP. A two-dimensional Weierstrass transform of an ideal Id-Vg characteristics - based on the assumption that the defect distribution to be perfectly random - is hypothesized to capture the deviations from ideal behavior when operating around CNP. This is verified with the reported experimental data along with DFT calculations.Unconventional doping based on polymer ferroelectrics was explored using advanced electrostatic force probe microscopic (EFM) techniques. Ferroelectric doping was realised with polyvinylidene fluoride-trifluoro ethylene (PVDF-TrFE) nano-thin films (about 40 nm - enabling 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) provides a clear insight into the ferroelectric-field dependent doping and parasitics triboelectric surface charge induced doping of graphene. Electrostatic doping-induced Fermi-level tuning is used to map the nature of electronic states in reduced graphene oxide thin films. This was enabled by implementing a modified KPFM setup with in-situ gating-based time-resolved measurements. A hybrid model based on DFT and the experimental analysis was employed to probe the nature of states around CNP in these materials. 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 compared with standard reference devices made with 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 attributed to the intra-device level self-gating effects of the graphene electrodes by the counter cathode under normal operations. An analytical model was developed to capture this effect and very verified along with numerical TCAD simulations. In the last part of the thesis, an in-situ measurement setup to study the dynamics of small molecular doping of graphene was developed. The choice of Si-graphene heterojunction solar cell for this study enabled the investigation of dynamic doping of these devices due to the availability of exposed graphene surfaces in this device architecture. The measurements of the fabricated heterojunction solar cells with this setup enabled a facile route to optimize the thickness of dopant (4.1 ± 0.2 nm) to achieve peak-efficiency operations.