"Organic Transistors with Ultra-Low Doping"

Doping organic semiconductors has become a key technology for highly efficient organic optoelectronic devices,1 in particular for organic light-emitting diodes (OLEDs).2,3 For organic field-effect transistors (OFETs), however, doping is less frequently used.4-9 The relatively large doping concentrations commonly used (several wt.%) present severe challenges for OFETs. Often, doping at these concentrations leads to a degradation of the switching ratio of OFETs and excessively large off-currents.

A rotational shutter system10 was installed in the Lüssem Lab (Fig. 1), which is capable to control the doping concentration in the 10-100 ppm regime, 2-3 orders below the current limit. With the help of this system, the group is able to study the influence of doping in the saturation regime of the transistors and developed a quantitative theory to describe these effects.11

REU students will extend these experiments to the depletion regime of these transistors. Using metal-oxide-semiconductor (MOS) capacitors, they will determine the effective doping concentration in these films and determine the activation energy of the doping process. They will use this information as input parameters for a newly developed numerical model of our devices to fit the experimental results, which will result in an estimation of the doping efficiency.

REU students will be trained in a variety of techniques relevant to thin-film electronics, including vacuum deposition and structuring, devices characterization, and impedance spectroscopy. The students will also learn to numerically model charge transport in organic semiconductors.

Setup of the rotational shutter used to reach doping concentrations in the ppm regime.

Fig. 1: Setup of the rotational shutter used to reach doping concentrations in the ppm regime.

  1. Lüssem, B.; Riede, M.; Leo, K., "Doping of Organic Semiconductors." Phys. Status Solidi, 2013, 210, 9-43.
  2. Meerheim, R.; Lüssem, B.; Leo, K., "Efficiency and Stability of p-i-n Type Organic Light Emitting Diodes for Display and Lighting Applications." Proc. IEEE, 2009, 97, 1606-1626.
  3. Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lüssem, B.; Leo, K., "White Organic Light-Emitting Diodes with Fluorescent Tube Efficiency." Nature, 2009, 459(7244), 234–238.
  4. Ante, F.; Klblein, D.; Zschieschang, U.; Canzler, T. W.; Werner, A.; Takimiya, K.; Ikeda, M.; Sekitani, T.; Someya, T.; Klauk, H., "Contact Doping and Ultrathin Gate Dielectrics for Nanoscale Organic Thin-Film Transistors." Small, 2011, 7(9), 1186-1191.
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  6. Naab, B. D.; Himmelberger, S.; Diao, Y.; Vandewal, K.; Wei, P.; Lüssem, B.; Salleo, A.; Bao, Z., "High Mobility N-Type Transistors Based on Solution-Sheared Doped 6,13-Bis(triisopropylsilylethynyl)pentacene Thin Films." Adv. Mater., 2013, 25(33), 4663-4667.
  7. Wei, P.; Oh, J. H.; Dong, G.; Bao, Z., "Use of a 1H-benzoimidazole Derivative as an n-type Dopant and to Enable Air-Stable Solution-Processed n-channel Organic Thin-Film Transistors." J. Am. Chem. Soc., 2010, 132(26), 8852-8853.
  8. Lüssem, B.; Keum, C.-M.; Kasemann, D.; Naab, B.; Bao, Z.; Leo, K., "Doped Organic Transistors." Chem. Rev., 2016, 116(22), 13714-13751.
  9. Lüssem, B.; Tietze, M. L.; Kleemann, H.; Zakhidov, A.; Leo, K.; Hoßbach, C.; Bartha, J. W., "Doped Organic Transistors Operating in the Inversion and Depletion Regime." Nat. Commun., 2013, 4, 2775.
  10. Tietze, M. L.; Pahner, P.; Schmidt, K.; Leo, K.; Lüssem, B., "Doped Organic Semiconductors: Trap-Filling, Impurity Saturation, and Reserve Regimes." Adv. Funct. Mater., 2015, 25(18), 2701-2707.
  11. Liu, S.; DeWeerd, N. J.; Reeves, B. J.; San, L. K.; Dahal, D.; Radha Krishnan, R. K.; Strauss, S. H.; Boltalina, O. V.; Lüssem, B., "Doped N-Type Organic Field-Effect Transistors Based on Faux-Hawk Fullerene." Adv. Electron. Mater., 2019, 5(6), 1-9.