A number of studies show that chemical modification of the semiconductor-dielectric interface can be used to control the threshold voltage (V th ) of organic thin film transistors (OTFTs). A promising chemical functionality to achieve that are acidic groups, which -at the semiconductor-dielectric interfacehave been used to realize chemically responsive OTFTs and easy to fabricate inverter structures. Especially for pentacene based OTFTs, the underlying chemical and physical mechanisms behind the acid-induced V th shifts are not yet fully understood. Their clarification is the topic of the present paper. To distinguish between space-charge and dipole-induced effects, we study the impact of the thickness of the gate oxide on the device characteristics achieving maximum V th -shifts around 100 V. To elucidate the role of the acid, we compare the doping of pentacene by acidic interfacial layers with the impact of hydrochloric acid vapour and investigate the consequences of exposing devices to ammonia. Complementary experiments using 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) as active layer hint toward the central (6 and 13) carbon atoms being subject to the electrophilic attack by the acidic protons. They also prove that the observed V th shifts in pentacene devices are indeed a consequence of the interaction between the acidic groups and the active material. The experimental device characterization is supported by driftdiffusion based device modelling, by quantum chemically simulations, as well as by contact angle, atomic force microscopy (AFM) and X-ray reflectivity (XRR). The combination of the obtained results leads us to suggest proton transfer doping at the semiconductor-dielectric interface as the process responsible for the observed shift of V th .

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Blue emitting GaN-based light emitting diodes (LEDs) show a distinct spectral behavior with respect to temperature and injection current density. Operating LEDs with short current pulses of 500 ns provides a steady state situation, which allows investigating the emission behavior of LEDs at a certain device temperature thereby maintaining thermal equilibrium. The LEDs were examined in a temperature range between 4.2 and 400 K and in a current density range between 2 and 50 A cm À2 . Low temperature investigations showed a blue shift of the electroluminescence spectra (EL) with respect to junction temperature, which is assigned to the radiative recombination of localized excitons. In the elevated temperature region a distinct red shift due to energy gap shrinkage was observed. Further we expect an exciton lifetime reduction at 4.2 K. Additionally, the influence of the driving parameters (pulse injection current or direct current (DC)) in the presence of piezoelectric fields is discussed and separated into band filling effect and occurrence of fields, which screen the quantum confined Stark effect. Low temperature investigations indicate that band filling is mainly responsible for the blue shift of the EL spectra with respect to the injection current.

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