Substituted Di(thio)pyranylidenes and their CT Complexes with C60 for Organic Near-Infrared Detectors

Detection of NIR light has attracted significant interest in the last few decades due to the emerging applications in industry, science, medicine and military. Notable examples are optical wireless communication, emerging biomedical imaging, environmental monitoring and night vision.

An organic photodetector (OPD) has the potential to overcome the limits of inorganic photodetectors concerning material processing and integration and combine filterless detection of NIR light with the advantages of organic electronic devices such as low-cost, lightweight and environmental friendliness.

The quality of the OPDs depends on an efficient light harvesting process with high photocurrent and low dark current, and on a high spectral selectivity in the operation spectrum. However, conventional NIR absorber materials are often missing those prerequisites.

Seeking to overcome this material-based problem, a new and promising approach was proposed by Siegmund et al. from the Dresden Integrated Centre for Applied Physics and Photonic Materials (IAPP). This approach exploits the intermolecular CT states formed at the interface between electron donating and accepting materials in a novel type of optical micro-cavity device architecture.

It was demonstrated that the characteristic features of the CT absorption, such as weak intensity, broadness and sensitivity at long wavelengths, can be used in an optical micro-cavity photodetector to yield a high spectral selectivity up to 36 nm and wavelength tuneable spectral responses up to 1550 nm. C60 was used as a standard acceptor choice for vacuum-deposited OSCs and OPDs. The donor component, however, was chosen according to its ability to readily form a CT complex with C60 leading to a broad and far red-shifted CT absorption - an effect that is usually undesired in OSCs, but explicitly utilized in the new cavity concept.

In his study, Siegmund employs two donor materials with the aforementioned properties, namely zinc phthalocyanine (ZnPc) and 2,2',6,6'-tetraphenyl-4,4'-dipyranylidene (TPDP). It is known for both materials that a new red-shifted absorption appears upon blending with C60. The comparison of the devices made with TPDP and ZnPc shows a CT absorption up to 1550~nm for TPDP and up to 1100~nm for ZnPc. Those results identify TPDP as a promising donor material for the CT-based photodetector. By further functionalizing the molecular structure, it is possible to change the absorption characteristics, in particular in favor for a stronger and more red-shifted CT absorption. Up to now, only few attempts have been made to synthesize other members of the TPDP family. Moreover, there was no emphasis on their prospective application in OPDs.

The thesis focuses on the synthesis and characterization of materials with a similar structure to TPDP (here referred to as tetraaryl-substituted dipyranylidenes), with a strong emphasis on their application in OPDs. This includes investigations via cyclic voltammetry (CV), differential scanning calorimetry (DSC) and density functional theory (DFT) calculations. Furthermore, the target materials were tested in vacuum-deposited OSCs, of which the optoelectrical properties were characterized via external quantum efficiency (EQE) spectra and current-voltage (IV) curves.