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Study Shows How to Tune Electronic Energies in Organic Semiconductors

Physicists at the Dresden Integrated Center for Applied Physics and Photonic Materials (IAPP) and the Center for Advancing Electronics Dresden (cfaed) at the TU Dresden, along with scientists from Tübingen, Potsdam, and Mainz could show how electrostatic forces can be used to tune electronic energies in organic semiconductor films.

Dr Frank Ortmann, cfaed Independent Research Group Leader. (Image credit: TU Dresden)

A distinct set of experiments supported by simulations could reduce the effect of particular electrostatic forces that the molecular building blocks exert on charge carriers. The research was reported recently in Nature Communications.

In organic-semiconductor-based electronic devices like light-emitting diodes, solar cells, transistors, or photodetectors, charge transport levels and electronic excitations are vital concepts to define their operation principles and performances.

However, the corresponding energetics are more complicated to access and to tune when compared to traditional inorganic semiconductors such as silicon chips, which remains a universal problem. This is applicable to the measurement as well as to the controlled external impact.

One tuning knob leverages the long-range Coulomb interactions, which is improved in organic materials. The current research explores the dependence of the energies of excitonic states and of charge transport levels on mixture composition and molecular orientation in the organic material. Excitons are bound pairs of a hole and an electron that are formed in the semiconductor material upon light absorption.

Researchers turn to blend composition when the components contain different organic semiconducting materials. The study outcomes show that the energetics in organic films can be set by modifying a single molecular parameter, known as the molecular quadrupole moment in the pi-stacking direction of the molecules.

An electric quadrupole can contain two positive and two equally strong negative charges, forming two oppositely equal dipoles. In the most fundamental case, the four charges are arranged alternately at the corners of a square.

The authors further associate device parameters of organic solar cells, for example, the photocurrent or the photovoltage, to this quadrupole moment. The outcomes help to describe recent innovations of device efficiency in organic solar cells, which are based on a new category of organic materials.

Since the observed electrostatic effect is a common property of organic materials—including the purported “small molecules” and polymers—it can help enhance the performance of all kinds of organic devices.


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