Excited state quantum modulations in organic semiconductors

Abstract

Organic semiconductors are formed by coupling organic molecules with π-conjugated structure through van der Waals interactions. Different from traditional inorganic semiconductors, such as silicon, the core element of organic semiconductors is carbon atom. Generally, in a π-conjugated organic molecule, carbon atoms are polymerized by sp or sp2 hybridization, forming small molecules or polymers due to the molecular size. Taking the polymer cis -polyacetylene as an example, each carbon atom has three 2sp2 hybrid orbitals in a plane and one 2p z orbital orthogonal to this plane. The 2sp2 orbitals give rise to three σ-bonds, two of which are formed with neighboring carbons and one with a hydrogen. The electron in the 2p z orbital of one carbon atom will be paired with the electron in the neighboring carbon’s 2p z orbital, such that this electron can be delocalized along the chain, denoted as π-electron. For such an organic molecule, according to the Peierls instability, it will be dimerized and form a single-double bond alternation lattice structure, behaving in a semiconductor character. Through doping, the conductivity of organic molecules can be greatly improved and become good conductors. This is the reason why the functional properties of organic molecules first attracted great attention in the 1970s, by which, Heeger, MacDiarmid and Shirakawa won the 2000 Nobel Prize in Chemistry. When organic molecules form solids, the packing of molecules will ultimately determine their functional properties and applications, such as organic polymer films and organic small molecule crystals. Over the past few decades, due to the flexibility and ease of processing, organic semiconductors have become very promising materials in functional devices, such as field-effect transistors, light-emitting diodes, solar cells, as well as organic spin valves. In general, the functional processes of these organic devices are closely related to the quantum dynamics of the formed excited states, including soliton, polaron, bipolaron, exciton, biexciton, charge transfer state, and trion, etc. These excited states are spatially localized due to the self-trapping effect of organic molecules, which arises from their strong electron-lattice interactions. Especially, these excited states behave in abundant and unusual charge-spin relations. For these reasons, the functional processes in organic devices are usually more unique and complex compared with conventional inorganic devices. To date, although many kinds of organic functional devices have been successfully developed, there are still many unsolved questions in their functional mechanisms. The excited state research is the key to solve these questions, and thus the core of organic semiconductor physics and device research. In view of this, through the combination of theoretical and experimental methods, we studied the quantum effects and modulation mechanisms of different excited states in organic semiconductors and their hetero structures. The main contents included are as follows: Quantum dynamics and field modulations for the photoexcitation, interfacial charge separation, and resonance energy transfer in organic photovoltaic systems; spin injection and transport in organic semiconductors, organic magnetoresistance effect, excited ferromagnetism, spin current and organic chiral spin effects. On the one hand, these studies greatly enrich the understanding for the physical phenomena and mechanisms in organic semiconductors; on the other hand, they should greatly promote the designs and applications of organic functional devices.