Nanoscale Morphology and Structure of Organic Donor-Acceptor Heterojunctions

Abstract

The scientific effort dedicated to organic electronics in the last decades has led to impressive advancements in the field. Nevertheless, we are still far from fully exploiting the great potentiality offered by organic semiconductors (OSCs). Among the most crucial aspects determining the efficient operation of organic optoelectronic devices, we can find the control of the morphology of the active layers and their interfaces. The functionality of each of the organic layers composing the device, intimately depends on how the molecules arrange, order and orient in the solid film. As a consequence, characterizing the morphological and structural properties of the OSC films at the nanoscale is essential to understand and optimize the fundamental mechanisms operating in the working device. In this regard, the work presented in this doctoral thesis is directed to two branches of organic electronics that are at present deeply debated in the field: molecular doping and organic photovoltaics (OPV). The combination of high spatial resolution characterization techniques was employed with the aim of correlating the morphological traits of the investigated donor-acceptor films with their electronic properties and, in some cases, with the device performance. The first part of the work focuses on the p-type doping of pentacene (PEN) thin film with C60F48. We performed a systematic characterization of the heterojunction formed when evaporating C60F48 on PEN. The influence of the morphology of PEN films with varying thickness on the dopant growth was revealed by a detailed atomic force microscopy (AFM) study, supported by x-ray photoelectron emission microscopy (XPEEM). The repercussions on the local surface work function were determined by Kelvin probe microscopy (KPFM), that demonstrated strong charge transfer between the two molecules but evidencing significant local heterogeneities depending on the film morphology. Further inspection of the electronic properties of the system was carried out by employing photoemission spectroscopy (PES), that allowed to define the energetics and build the energy diagram level of the formed interface. The doping effect of C60F48 was further proven in a PEN-based organic field effect transistor (OFET), by performing an experiment where we evaluated in-situ the improvement of the OFET characteristics upon doping. The characterization of the system was completed by exploring the structural properties of heterojunctions with different architectures. The investigation, based on grazing incidence x-ray diffraction (GIXD), evidenced substantial structural differences depending on the design of the heterojunction. The second part of the thesis aims to unravel the morphology-performance relationship in OPV devices based on non-fullerene acceptors (NFAs). Specifically, we consider two NFAs that only differ in the design of the side-chains. The effects of the different structural evolution of the two NFAs upon thermal annealing on the device performance were examined by performing in-situ grazing incidence wide-angle x-ray scattering (GIWAXS) analysis and complementing with AFM topographical survey. The different thermotropic behaviour of the two NFAs and the considerable consequences on the photovoltaic efficiency are discussed, highlighting the important role played by polymorphism in this class of systems.