Frank Steinbacher

Imaging with organic and hybrid photodetectors

Organic semiconductors provide exiting new opportunities for the realization of flat panel image sensors as they can be processed from the solution phase on large areas at low cost. In particular the high charge separation efficiency obtained in a bulk heterojunction (BHJ) enables the realization of organic photodiodes (OPDs). The spectral sensitivity of OPDs can be tailored to cover wavelengths ranging from the visible to the near infrared region. These sensitivities match perfectly to a variety of X-ray scintillators enabling a further improvement in the sensitivity range. In combination with an amorphous silicon (a-Si) thin film transistor (TFT) backplane technology, visible, near infrared (NIR) and X-ray image sensors have been realized. Thin film OPDs have been used in combination with a cesium iodide (CsI) scintillator in a traditional stacked geometry, proofing state-of-the art performance. Even more, it is possible to blend X-ray absorbing particles directly into the organic semiconductor thereby enabling quasi-direct X-ray converters with the promise to achieve a modulation transfer function (MTF) that is as high as in direct converting materials such as amorphous Selenium.

Cheap p- and n-doping for highly efficient organic devices

Electrically doped, organic transport layers are important for todays high efficiency organic (opto-)electronic devices. Doped organic layers have a strongly increased free charge carrier density compared to their undoped counterparts and also improve the charge carrier injection from adjacent electrodes into the organics. For practical applications, especially in optoelectronics, these layers have to have low absorption in the wavelength range of interest. The two nearly colorless p - and n -doping materials, rhenium heptoxide and cesium carbonate, are investigated focusing on their conductivity enhancement, injection improvement, and voltage drop over doped transport layers in organic light emitting diodes. They show very good doping properties already at moderate doping concentrations and prove that they can be used in variable thicknesses without a significant voltage increase. This makes them cheap, low absorbing alternatives to todays, well-established doping systems.