Role of Interfacial, Static and Dynamic Disorder on High Performance Organic Field-effect Transistors

Open Access
Author:
Adhikari, Jwala Mani
Graduate Program:
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 24, 2016
Committee Members:
  • Enrique Daniel Gomez, Dissertation Advisor
  • Enrique Daniel Gomez, Committee Chair
  • Michael Anthony Hickner, Committee Member
  • Michael John Janik, Committee Member
  • Qing Wang, Committee Member
Keywords:
  • organic semiconductors
  • organic single crystals
  • thin film disorder
  • polymer gate dielectric
  • rubrene
  • C8BTBT
Abstract:
The internet and electronic devices have become integral to our everyday life. As the world is beginning to march towards the Internet of Things, surging interest has been focused on low-cost functional electronics. However, the industry of electronics is inundated with traditional inorganic semiconductors. Silicon and germanium based inorganic semiconductors are rigid and fragile, requiring expensive processing. On the other hand, organic semiconductors based electronic devices, namely field effect transistors (FETs) and light-emitting diodes (LEDs), offer a feasible alternative to their inorganic counterparts. Not only are organic semiconductors soft and flexible, but they also possess advantages such as solution processability and chemical modifications. Despite the lucrative advantages, the performance of organic semiconductor-based devices is significantly lower. For instance, the highest hole mobility in one of the best performing organic FETs is two orders of magnitude lower than in inorganic transistors. Moreover, the behavior of organic semiconductors is very complex and is readily susceptible to their surroundings. Due to such complex nature of organic semiconductors, an unambiguous road map to high-performing organic FETs (OFETs) is still missing. The focus of this dissertation is on elucidating and tuning different factors that govern the performance of OFETs. An OFET is a complex structure comprised of three metal electrodes, an active layer (semiconducting layer) and a gate insulator (dielectric layer). One of the key parameters that determine the performance of OFETs is charge carrier mobility. The charge mobility of an OFET is affected by various factors—mainly the chemical structure of an active layer material, the morphology of an active layer and the interactions at the semiconductor-dielectric interface. To understand the role of chemical structure on the performance of organic semiconductor, octyl side-chains are added to [1]benzothieno[3,2-b]benzothiophene (BTBT) core. BTBT is a liquid crystalline material with poor solubility and weak semiconducting properties. The addition of alkyl side-chains is expected to increase the solubility of the core. However, the average FET hole mobility in the single crystalline BTBT with two octyl side-chains (C8BTBT) (μ = 6.0±1.0 cm2/V-1s-1) is three orders of magnitude higher than in the single crystalline BTBT core itself. The use of a single crystalline layer rules out any morphological effect on the performances. Moreover, the intrinsic charge transport parameters of single crystals predicted using Density Functional Theory (DFT) calculations remain essentially the same. Since both morphology and crystal structure seem to have virtually no effect on the measured charge mobilities of BTBT derivatives, what else could have affected the charge mobilities? It is hypothesized that inter-molecular dynamics may be an unaccounted factor. To probe the collective inter-molecular motions—also known as phonon—inelastic neutron scattering (INS) is performed. The results from INS show that lattice vibrations are significant in the BTBT core. In contrast, the vibration modes are completely diminished in C8BTBT, suggesting that the addition of octyl side-chains suppresses lattice vibrations in the BTBT core. The alkyl side-chain assisted reduced electron-phonon coupling may have led to the enhanced hole mobility in C8BTBT. In addition, the morphology of C8BTBT thin films is also optimized to improve an average hole mobility in FETs. It has been found that by melting and quenching spun-cast C8BTBT films, the average hole mobility of FETs is improved by a factor of five. Grazing incidence wide angle X-ray scattering (GIWAS) results show that the melting and quenching enhanced crystal texturing and led to stronger orientation order in the C8BTBT films. The melting and quenching process is believed to be vital in obviating the processing history and in controlling the thermodynamic driving force for crystallization. The effect of a gate insulator on the performance of FETs is also investigated using a novel, photopatternable, high-k fluoropolymer, poly(vinylidene fluoride-bromotrifluoroethylene) P(VDF-BTFE), where the BTFE moieties enable cross-linking through thermal- or photo-curing of dielectric materials with relative permittivities between 8 and 11. Organic single crystal field effect transistors based on rubrene active layers and crosslinked P(VDF-BTFE) gate dielectrics have shown hole mobilities as high as 12 cm2V-1s-1, three times higher than the average hole mobilities for devices comprising PVDF-based fluoropolymers or SiO2 as the dielectric layer. FETs comprising cross-linked P(VDF-BTFE) dielectric layers show the smallest interfacial trap density among all other fluorinated PVDF-based polymers, leading us to believe that cross-linking P(VDF-BTFE) films reduces energetic disorder at the dielectric-semiconductor interface. Fourier transform infrared spectroscopy results suggest that crosslinking enhances the population of trans conformations with respect to the neat polymer, demonstrating that cross-linking minimizes interfacial charge traps and hence enhances hole mobility by tuning chain conformations.