TY - JOUR A1 - Koopman, Wouter-Willem Adriaan A1 - Natali, Marco A1 - Donati, Giovanni P. A1 - Muccini, Michele A1 - Toffanin, Stefano T1 - Charge-exciton interaction rate in organic field-effect transistors by means of transient photoluminescence electromodulated spectroscopy JF - ACS photonics N2 - Organic light-emitting transistors (OLETs) offer a huge potential for the design of highly integrated multifunctional optoelectronic systems and of intense nano scale light sources, such as the long-searched-for electrically pumped organic laser. In order to fulfill these promises, the efficiency and brightness of the current state-of-the-art devices have to be increased significantly. The dominating quenching process limiting the external quantum efficiency in OLETs is charge-exciton interaction. A comprehensive understanding of this quenching process is therefore of paramount importance. The present article reports a systematic investigation of charge-exciton interaction in organic transistors employing time resolved photoluminescence electro-modulation (PLEM) spectroscopy on the picosecond time scale. The results show that the injected charges reduce the exciton radiative recombination in two ways: (i) charges may prevent the generation of excitons and (ii) charges activate a further nonradiative channel for the exciton decay. Moreover, the transient PLEM measurements clearly reveal that not only trapped charges, as already reported in literature, but rather the entire injected charge density contributes to the quenching of the exciton population. KW - photoluminescence quenching KW - charge density KW - exciton dynamics KW - organic KW - field-effect transistor KW - light emission KW - optical spectroscopy Y1 - 2017 U6 - https://doi.org/10.1021/acsphotonics.6b00573 SN - 2330-4022 VL - 4 IS - 2 SP - 282 EP - 291 PB - American Chemical Society CY - Washington, DC ER - TY - JOUR A1 - Koopman, Wouter-Willem Adriaan A1 - Natali, Marco A1 - Bettini, Cristian A1 - Melucci, Manuela A1 - Muccini, Michele A1 - Toffanin, Stefano T1 - Contact Resistance in Ambipolar Organic Field-Effect Transistors Measured by Confocal Photoluminescence Electro-Modulation Microscopy JF - ACS applied materials & interfaces N2 - Although it is theoretically expected that all organic semiconductors support ambipolar charge transport, most organic transistors either transport holes or electrons effectively. Single-layer ambipolar organic field-effect transistors enable the investigation of different mechanisms in hole and electron transport in a single device since the device architecture provides a controllable planar pn-junction within the transistor channel. However, a direct comparison of the injection barriers and of the channel conductivities between electrons and holes within the same device cannot be measured by standard electrical characterization. This article introduces a novel approach for determining threshold gate voltages for the onset of the ambipolar regime from the position of the pn-junction observed by photoluminescence electromodulation (PLEM) microscopy. Indeed, the threshold gate voltage in the ambipolar bias regime considers a vanishing channel length, thus correlating the contact resistance. PLEM microscopy is a valuable tool to directly compare the contact and channel resistances for both carrier types in the same device. The reported results demonstrate that designing the metal/organic semiconductor interfaces by aligning the bulk metal Fermi levels to the highest occupied molecular orbital or lowest unoccupied molecular orbital levels of the organic semiconductors is a too simplistic approach for optimizing the charge injection process in organic field-effect devices. KW - electro-modulation microscopy KW - organic field-effect transistors KW - threshold voltages KW - contact resistance KW - photoluminescence Y1 - 2018 U6 - https://doi.org/10.1021/acsami.8b05518 SN - 1944-8244 VL - 10 IS - 41 SP - 35411 EP - 35419 PB - American Chemical Society CY - Washington ER -