Solid‐State Aspects of Conjugated Semiconductors

Abstract

Conjugated semiconductors can exhibit energy bandgaps of 1 eV up to several eVs depending on the choice of the (1) building blocks, (2) the number of such building blocks, and (3) their precise arrangement with respect to each other. Therefore, they cover the region of narrow bandgap materials which absorb and emit light in the infrared region (a) and show reasonable charge transport properties. However, via the visible region (b), they even extend to the ultraviolet spectral range (c), performing like inorganic wide bandgap materials. This versatility and several other advantages such as mechanical flexibility, very high photoluminescence quantum yields (PL QY), easy production of large areas outweigh the intrinsic instability of conjugated molecules to photo-oxidation. Except for applications where long-term stability is not required [1], a proper encapsulation has to be used to introduce conjugated semiconductors for technical applications; such an encapsulation is for example intrinsic to applications such as capacitors and batteries [2].This chapter is organized as follows: after a brief introduction into the class of conjugated semiconductors we will discuss the solid state properties needed in order to achieve the functionalities required for efficient device operation, based on this class of materials. Very different degrees of short and long range order can be obtained in conjugated semiconductors; we will describe them and discuss their consequences on macroscopic properties of films of these materials. Since the properties of the excited states in conjugated semiconductors are both a highly relevant question of fundamental research and essential for applications we will treat the field of excited states spectroscopy in a separate section. This section will be followed by a treatise of device aspects of light emitting devices made from conjugated molecules. Before concluding we will briefly discuss the photophysics of excitation energy transfer and the properties of highly emissive conjugated systems under high excitation densities.

The mobility and relaxation kinetics of charge carriers in molecular materials studied by means of pulse-radiolysis time-resolved microwave conductivity: dialkoxy-substituted phenylene-vinylene polymers

“…(1) chemical doping or continuous 'band-gap' illumination with measurement of the steadystate current [1][2][3][4][5], (2) laser flash-photolysis with time-resolved detection of transient conductivity changes down to picosecond timescales [4][5][6][7], (3) measurement of the current/voltage characteristics of charge injection devices [8,9], (4) time-of-flight (TOF) studies of charge-carrier drift in an electric field [2,[10][11][12].…”