Spectromicroscopic Insights into the Morphology and Interfaces of Operational Organic Electronic Devices

Language
en
Document Type
Doctoral Thesis
Issue Date
2017-07-06
Issue Year
2017
Authors
Du, Xiaoyan
Editor
Abstract

Organic electronics, e.g., organic field-effect transistors (OFETs), organic solar cells (OSCs) and organic light-emitting diodes (OLEDs), have attracted strong interest in both academia and industry during the last decades due to their unique capabilities offered by organic semiconductors. The micro-/nano-structures in active layers and the interface engineering in organic electronics are extremely important for desired device functionalities. In this thesis, the structure-function relationships and interface engineering in both OFETs and OSCs are investigated exploring standard and emerging characterization techniques with specific emphasis on soft X-ray-based methods. Firstly, OFETs based on small molecules are investigated as prototype systems to understand the micro-/nano-structure formation and its relation to charge transport properties. Specifically, the in-situ/in-operando electronic structure detection using soft X-rays is explored. Secondly, the morphology of more complex bulk heterojunction organic solar cells based on both binary and ternary components are investigated to further understand the charge generation, recombination and transport. In both cases, advanced chemically sensitive characterization methods based on soft X-rays and electrons are explored. Finally, the interface losses in all-solution processed multi-junction organic solar cells based on representative interfacial layers and active layers are discussed. A simple, yet effective and universal approach is developed to overcome the interface losses. In the first part of the discussion (Chapter 5), studies on several small molecule systems conventionally used in OFETs are presented. Firstly, co-existence of polymorphs of pentacene in sub-micron scale was investigated using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and scanning transmission X-ray microscopy (STXM). Secondly, the thin film morphology and charge transport properties of one of the important pentacene derivatives, 6,13-dihydro-6,13-diazapentacene (DHDAP), are investigated. The performance of OFETs based on DHDAP is found to be extremely sensitive to the nature and temperature of the substrates during film preparation. We employ several complementary microscopic and spectroscopic probes to investigate the structure of thermally evaporated thin and thick films evaporated at different device-related conditions. The electronic structure of DHDAP is explored by photoelectron spectroscopy (XPS) and NEXAFS spectroscopy. Atomic force microscopy (AFM) and angle-resolved NEXAFS spectroscopy reveal the growth mode and the orientation of the molecules close to the substrates. STXM and resonant soft X-ray scattering (RSoXS) reveal the crystalline domain size distribution of the thicker films. Finally, the inter-grain boundaries density, the molecular packing and the passivation of interface traps are analyzed and correlated to the transport properties. PDI-FCN2-based n-type OFETs with Si3N4 as dielectric layer are demonstrated. The electronic structure and the temperature-dependent morphology are investigated by STXM. Furthermore, a capacitor-like device is employed for in-situ NEXAFS measurements to investigate the field-effect on the electronic structures of PDI-FCN2. The applied voltage induces changes in the NEXAFS absorption intensity, but does not result in any detectable spectral shifts. For in-operando XPS studies within the active channel of pentacene-based thin-film OFETs, significant charging of the active layer and spectral shifts according to the applied gate-voltage are observed. Thus, probing the variation of the chemical states or binding energies of the molecules within the active channel during device operation remains challenging. The next section of this thesis (Chapter 6) deals with the complex interplay of the morphology and functionality in ternary organic solar cells. A systematic study has been conducted by means of energy-filtered transmission electron microscopy (EFTEM) and RSoXS on the morphology evolution of prototype ternary systems upon adding sensitizers, namely poly(3-hexylthiophene) (P3HT) and indene-C60-bisadduct (ICBA) blends with Si-PCPDTBT and C-PCPDTBT. The model sensitizers have very similar chemical structures but significantly different influences on the nanomorphology of the ternary blends and the solar cell device performance. For deeper understanding, a combined density functional theory (DFT) and artificial neuronal network (ANN) computational approach is utilized to calculate the solubility parameters and Flory-Huggins intermolecular parameters to evaluate the influence of miscibility on the final morphology. Experiments reveal that the domain spacing and purity of ICBA-rich domains are retained in Si-PCPDTBT-based systems, but is strongly reduced in C-PCPDTBT-based ternary systems. The P3HT fiber structure is conserved at low sensitizer contents but significantly reduced at high contents. The theoretical calculations reveal very similar miscibility/compatibility between the two sensitizers and ICBA as well as P3HT. Thus, it is concluded that mainly the crystallization of Si-PCPDTBT drives the nanostructure evolution in the ternary systems, while this driving force is absent in C-PCPDTBT-based ternary blends. Moreover, the photovoltaic performance of ternary systems based on a new wide band-gap polymer is investigated and rationalized based on the above mentioned guidelines of morphology aspects. In the last part (Chapter 7), a systematic study of interface losses in both single-junction and multi-junction solar cells based on representative polymer donors and hole transporting layers (HTLs) using electron spectroscopy are reported. It is found that a facile mixed HTL (m-HTL) containing poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and molybdenum oxide (MoOx) nanoparticles successfully overcomes the interfacial losses in both single- and multi-junction solar cells based on various active layers by reducing interface protonation, promoting better energy-level alignment, and forming a dense and smooth layer. Solution-processed single-junction solar cells are demonstrated to reach the same performance as with evaporated MoOx (˃ 7%). Multi-junction solar cells with polymers containing nitrogen atoms as the first layer and mixed PEDOT:PSS and MoOx nanoparticles as HTL reach a fill factor (FF) of over 60%, and a power conversion efficiency (PCE) of over 8%, while the corresponding stack with pristine PEDOT:PSS or MoOx nanoparticles showed FF smaller than 50% and PCE less than 5%.

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