Significance of molecular and crystal structure on organic semiconductor doping

Abstract

Organic semiconductor devices are gaining importance by opening a wide variety of applications nowadays. A key strategy for developing better performing organic electronic devices is via molecular doping, where a dopant molecule is introduced to increase charge carriers. Despite the advances in the field, very few studies have focused on understanding the structural-property relationships in molecular doping. It is well known that the structure of OSC molecules affect the crystallinity of the films but its implications on the doping mechanism and efficiency of doping have not been fully explored. In this thesis, we explore the dopant/organic semiconductor interfacial properties in relation to the molecular structure of the employed molecules. BTBT derivatives with different side groups in combination with two dopants, which have similar ionization energy but with different molecular shapes have been chosen for the investigation. By using a combination of synchrotron-based X-ray diffraction techniques, AFM and spectroscopic methods, the interfacial properties and evolution of electronic levels during doping have been investigated. First, the structure of the interface formed between C60F48 and C8-BTBThas been investigated. The impact of C60F48 on C8-BTBT OFETs was demonstrated to have a double beneficial role in improving the mobility and stability of C8-BTBT OFETs. Further investigation of the role of organic semiconductor side groups in the interfacial properties has been conducted by comparing the C60F48/DPh-BTBT and C60F48/C8-BTBT interfaces. Different interfacial morphologies are observed in both cases, which strongly affect the nanoscale work function distribution of the interface. To investigate the influence of dopant molecular structure, heterostructure films of F6TCNNQ on C8-BTBT were investigated. Unlike C60F48, F6TCNNQ is planar. It is observed that F6TCNNQ-C8-BTBT charge transfer complexes were forming at the interface during F6TCNNQ deposition, which generates free mobile carriers in the film hence enhancing the mobility and causing a shift in threshold voltage of C8-BTBT OFETs. Finally, characterization of charge transfer complex formation at different temperatures, inverted heterostructures and C8-BTBT-F6TCNNQ co-evaporated films have been performed.

Date of Award	26 Jul 2022
Original language	English
Supervisor	Carmen Ocal García (Director), Eva Maria Pellicer Vila (Tutor) & Barrena Villas, Esther (Director)