Final Report Summary - QUEST (Quantitative electron and spin transport theory for organic crystals based devices)
Small molecules when assembled together may form crystalline structures, similar to atoms in crystals. These so-called molecular crystals retain some of the properties of their inorganic counterparts (e.g. they can be metals, semiconductors or insulators), but at the same time they have the flexibility in design and composition that only the molecular word can offer (there are only ~90 elements in the periodic table to form inorganic crystals, but millions of different molecules can be used for the organic ones). In addition organic crystals can be made with cheap chemical synthesis, which usually requires low temperature and ambient pressure. This is in contrast with inorganic semiconductors, requiring laborious high temperature processing. Given these advantages it is then not surprising that much effort has been devoted to design organic crystals for a range of applications including, flexible and wearable electronics, sensing, displays, solar energy harvesting, just to name a few.
The main problem with organic crystals arises from their fundamental nature. Molecules interact only weakly with each other so that: 1) the morphology of the crystals is difficult to control and to predict, 2) electron conduction is not efficient, 3) small changes in the structure and composition may affect profoundly their properties, 4) they ultimately require to be interfaced with some inorganic materials (e.g. the metallic interconnects in an electronic device). For all these reasons the design of organic devices has proved so far to be very challenging. The QUEST project aims at constructing an integrated theoretical/computational approach to estimate the properties of organic crystals ahead of experiments. This approach is then applied to the design of novel organic devices.
During its five-year duration QUEST has constructed a complete multi-scale approach to the problem and was able to predict the behaviour of inorganic crystals without the need of experimental information. Our approach consists in interfacing a range of numerical techniques, each one of them operating only within a specific range of length, time and accuracy. When put together these methods allow one to model from atoms to devices. At the lowest level we have used fully quantum-mechanical approaches (e.g. density functional theory) to compute the basic electronic properties of the molecules assembled in a crystal. These methods are highly accurate but can deal only with a few hundred of atoms (a few molecules). At the intermediate level we have constructed a protocol to extract the relevant information obtained by the lower level in order to formulate effective models. These can now deal with several hundreds molecules. Finally such models are solved with either a stochastic approach (Monte Carlo method) or by solving approximate equations of motion. The net result is a complete scheme to go from atoms to devices. The scheme does not necessitate experimental information, and in fact can be used to explain and model the experiments.
The main problem with organic crystals arises from their fundamental nature. Molecules interact only weakly with each other so that: 1) the morphology of the crystals is difficult to control and to predict, 2) electron conduction is not efficient, 3) small changes in the structure and composition may affect profoundly their properties, 4) they ultimately require to be interfaced with some inorganic materials (e.g. the metallic interconnects in an electronic device). For all these reasons the design of organic devices has proved so far to be very challenging. The QUEST project aims at constructing an integrated theoretical/computational approach to estimate the properties of organic crystals ahead of experiments. This approach is then applied to the design of novel organic devices.
During its five-year duration QUEST has constructed a complete multi-scale approach to the problem and was able to predict the behaviour of inorganic crystals without the need of experimental information. Our approach consists in interfacing a range of numerical techniques, each one of them operating only within a specific range of length, time and accuracy. When put together these methods allow one to model from atoms to devices. At the lowest level we have used fully quantum-mechanical approaches (e.g. density functional theory) to compute the basic electronic properties of the molecules assembled in a crystal. These methods are highly accurate but can deal only with a few hundred of atoms (a few molecules). At the intermediate level we have constructed a protocol to extract the relevant information obtained by the lower level in order to formulate effective models. These can now deal with several hundreds molecules. Finally such models are solved with either a stochastic approach (Monte Carlo method) or by solving approximate equations of motion. The net result is a complete scheme to go from atoms to devices. The scheme does not necessitate experimental information, and in fact can be used to explain and model the experiments.