Project Title:Crystal Engineering for Organic Electronics

Project Title:Crystal Engineering for Organic Electronics

Abstract

This project is focused on engineered solid state molecular structures, i.e. crystal engineering, of small molecule organic semiconductors. We propose to advance the understanding of polymorphism from a thermodynamic point of view and to identify crystal structures with the potential of high charge carrier mobilities. The novel approach is not only in combining theory and experiments, but also in exploring the possibility of crystallizing known materials in new structures that can exhibit thus far unreported electrical properties. The project is based on the on the on-going research activities of the Solid State Group at the Dpt. of Industrial Chemistry “Toso Montanari”, focused on the study of structural and dynamical properties and photoinduced processes in organic materials, mainly by means of spectroscopic methods.

Research Project

The taste of chocolate, the physiological absorption of drugs such as paracetamol, the critical temperature of a molecular superconductor and the colour of a pigment are all properties that can be influenced by the crystal structure. Thisis just one of the many reasons why the control of the crystal structure, that is of polymorphism, is an important aspect of the exploitation of a material. Polymorphism, which can be defined as the capability of a molecular species to assemble in different crystal structures, is a well-known phenomenon but its relevance for some classes of compounds has been only recently outlined. This is true, for instance, for the fascinating and promising field of molecular materials for electronics where the possibility of predicting and controlling polymorphism and to extend such a possibility to thin film structures would in fact give a better control of the transport properties.

Besides the advantages on processing, research in organic electronics and spintronics has been fuelled by two unique issues: 1) there are virtually infinite possibilities for synthesizing molecules with optimal energy levels and electronic structures for desired applications; 2) the light elements (carbon, hydrogen, nitrogen, sulfur, etc), which are the constituents of organic molecules, induce very weak spin-orbit couplings so that organic semiconductors may exhibit long spin coherence times (>10 microsec). These unique properties are at the basis of organic logic elements (e.g. field effect transistors) and organic spintronic devices (spin valves). As actual devices are made of several thousands of molecules packed in the solid state, charge carriers or spin need to diffuse mesoscopic distances, the crystalline arrangementis bound to play a crucial role on device performance. This theme is central for the important and timely challenges listed here:

i) There has been a massive research effort in synthesizing organic semiconductor molecules with optimal energy levels to tune optical gap (light emission/absorption) or charge injection barriers from electrodes. However, in Van der Waals solids such as organic compounds properties are determined not only by the molecular structure, but also by the way molecules interact in the crystal lattice. It remains difficult to foresee how a certain compound will crystallize and how the packing will influence the device performances. For devices, it is also crucial to obtain crystalline thin films, with structures whose formation may be assisted or driven by the interface.

ii) There is a lack of control of polymorph formation in electronic devices, which is very important for ensuring performance reproducibility. Besides, these materials can easily undergo morphological changes and recrystallizations, resulting in an undesired crystalline form, which may cause performance degradation.

The keyword that is central to the issues identified above is certainly Crystal Engineering. In particular, Crystal Engineering in the field of organic electronics with a broader scope.

We propose to study the preparation and isolation of crystal polymorphs of semiconductors, the prediction of their thermodynamic stability, the integration of crystalline semiconductors in thin film devices and the performance of different polymorphs on different technologically relevant substrates.

Addressing the questions identified above requires an interdisciplinary approach and a combination of crystal growth techniques and state of the art experiments.

Work Plan

The project has 3 scientific objectives:

1) Crystal growth from the vapour and from solution of materials with the most promising charge carrier mobility and spin coherence time.

2) X-ray and micro-Raman characterization.

3) Experiments on thermodynamic stability, charge carrier transport and spin transport in field effect transistor and spin valves.

The three objectives will be pursued through the following steps

1) We will explore different techniques for crystal growth and deposition. High purity single crystals will be grown in a two zones furnace in a stream of inert gas. Solution growth will be used exploring different solvents and solvent mixtures aiming to obtain crystals with controlled dimensions for simple integration into thin film devices.

2) The relative thermodynamic stability of polymorphs will be assessed using thermogravimetric measurements such as TGA and DSC. Lattice phonon spectra will be measured by low wavenumber Raman spectroscopy. This will help to quickly screen the polymorphs in bulk and thin film phases, where also information about molecular orientation at the interphase will be gathered. The most promising materials will be implemented in field effect transistors for charge carrier mobility measurements. Here we will search for a complete understanding of the charge transport by performing experiments at different temperatures also using light as a probe for charge carrier transport, ie the optically active polaron bands.

Management, maintenance and possible implementation of the experimental set-up available at the department of Industrial Chemistry “Toso Montanari” will be required in the course of the research activity.