@article{KoopmanMucciniToffanin2018,
  author    = {Koopman, Wouter-Willem Adriaan and Muccini, Michele and Toffanin, Stefano},
  title     = {High-resolution photoluminescence electro-modulation microscopy by scanning lock-in},
  series = {Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques},
  volume    = {89},
  journal   = {Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques},
  number    = {4},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0034-6748},
  doi       = {10.1063/1.5010281},
  pages     = {7},
  year      = {2018},
  abstract  = {Morphological inhomogeneities and structural defects in organic semiconductors crucially determine the charge accumulation and lateral transport in organic thin-film transistors. Photoluminescence Electro-Modulation (PLEM) microscopy is a laser-scanning microscopy technique that relies on the modulation of the thin-film fluorescence in the presence of charge-carriers to image the spatial distribution of charges within the active organic semiconductor. Here, we present a lock-in scheme based on a scanning beam approach for increasing the PLEM microscopy resolution and contrast. The charge density in the device is modulated by a sinusoidal electrical signal, phase-locked to the scanning beam of the excitation laser. The lock-in detection scheme is achieved by acquiring a series of images with different phases between the beam scan and the electrical modulation. Application of high resolution PLEM to an organic transistor in accumulation mode demonstrates its potential to image local variations in the charge accumulation. A diffraction-limited precision of sub-300 nm and a signal to noise ratio of 21.4 dB could be achieved. Published by AIP Publishing.},
  language  = {en}
}
@article{KoopmanNataliBettinietal.2018,
  author    = {Koopman, Wouter-Willem Adriaan and Natali, Marco and Bettini, Cristian and Melucci, Manuela and Muccini, Michele and Toffanin, Stefano},
  title     = {Contact Resistance in Ambipolar Organic Field-Effect Transistors Measured by Confocal Photoluminescence Electro-Modulation Microscopy},
  series = {ACS applied materials \& interfaces},
  volume    = {10},
  journal   = {ACS applied materials \& interfaces},
  number    = {41},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {1944-8244},
  doi       = {10.1021/acsami.8b05518},
  pages     = {35411 -- 35419},
  year      = {2018},
  abstract  = {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.},
  language  = {en}
}
@article{KoopmanNataliDonatietal.2017,
  author    = {Koopman, Wouter-Willem Adriaan and Natali, Marco and Donati, Giovanni P. and Muccini, Michele and Toffanin, Stefano},
  title     = {Charge-exciton interaction rate in organic field-effect transistors by means of transient photoluminescence electromodulated spectroscopy},
  series = {ACS photonics},
  volume    = {4},
  journal   = {ACS photonics},
  number    = {2},
  publisher = {American Chemical Society},
  address   = {Washington, DC},
  issn      = {2330-4022},
  doi       = {10.1021/acsphotonics.6b00573},
  pages     = {282 -- 291},
  year      = {2017},
  abstract  = {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.},
  language  = {en}
}