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Defects and disorder

Unlike inorganic semiconductors, organic semiconductors are not held together by electronic bonds, but rather by comparatively weak van-der-Waals interactions between the molecular building blocks. This has significant effects on the resulting band structure, optical properties and the transport of charge carriers or excitons. Two well-known phenomena are particularly noteworthy here, namely the spin-allowed splitting of a singlet exciton into two triplet excitons or the formation of so-called excimers through the structural relaxation of excited molecules. These processes can compete with each other and may be desirable or undesirable depending on the application. Fundamental investigations of the relationship between the packing motif, the optical properties and the dynamics of optical excitations are therefore crucial for the development of theoretical models with high predictive power.

Illustration of individual metastable defects in form of fluctuations

In very small-sized objects like perovskite nanocrystals, the impact of individual, metastable defects is evident in the form of PL fluctuations (“blinking”). Further analysis of the intensity fluctuations allows for characterization of individual non-radiative centers beyond ensemble-averaging.