Charge transport in three orthogonal directions in a single crystal of the conjugated polymer poly(3-hexylthiophene)
Polymer Materials
Final Report Abstract
The initial experiments performed from July 2013 to ca. July 2014 unexpectedly but clearly showed that P3HT crystals generated at the University of Freiburg and measured at the University of Oldenburg exhibited unstable, often opposite, and not reproducible characteristics. We believe that the main causes for such characteristics were: (a) change of structure of the P3HT single crystals (especially at the level of the fold surface, i.e., at the crystal/electrode interface) during storage of the samples at room temperature, causing (b) a large contact resistance at the P3HT/gold interface. We concluded that the formation of the stabile P3HT crystal/electrode interfaces, which exhibit an Ohmic resistance, requires significantly more time than what has been planned for sample preparation within the framework of the present DFG project. We thus decided that for the successful achievement of the goal of the DFG project, we needed a) to understand the origin of the high resistance at the P3HT/gold interface and b) to find an alternative organic crystal, which is stable enough to allow for such a fundamental study. To this end, we used a single crystal of a small molecule, squaraine (SQ), where the study of charge transfer in three orthogonal directions was possible and storage of crystals at room condition did not lead to changes of their properties. From this point of view, we thus concluded that SQ single crystals can be used as an alternative organic crystalline system for our investigations. Applying four gold electrodes on the squaraine crystal allowed on the same crystal to measure twelve current-voltage (I-V) characteristics (four-, three-, and two-probe configurations for two current directions). Furthermore, we have developed in detail a method allowing to determine the eight contact resistances (four for the injecting and four for the extracting electrodes, respectively) and the corresponding current dependences of gold-squaraine-gold structures, in particular for the case, where contact resistances were significantly larger than that of the organic material. From fifteen samples investigated in total, only one sample showed contact resistances smaller than the resistance of squaraine for which the four-probe configuration could be used. We decomposed each of the measured total I-V characteristics into three components, representing charge transfer through the injecting electrode, the squaraine crystal, and the extracting electrode. For the gold-squaraine-gold structure with contact resistances larger than that of squaraine, the subsequent novel findings can be summarized: (a) Different values of height and width for injecting and extracting holes across the same gold-squaraine interface can be attributed to the action of permanent dipoles present at this interface. (b) The injection process occurs in two steps for low voltages. Holes are initially injected into interfacial trap states, from where they eventually hop to the transport states of the HOMO band of the SQ crystal. For large voltages, the injection is dominated by direct tunneling of holes from metal Fermi level to the HOMO band. (c) By extraction of holes from the SQ crystal to the gold electrode, an increase of the extracting resistance with increasing current could be observed at low voltages, which is identified as a consequence of the filling of interfacial trap states with holes from the HOMO band. For large voltages, holes are extracted directly from the HOMO level to the gold Fermi level. (d) If the contact resistance of the extracting interface is larger than that of the organic crystal and that of the injecting interface, a part of the injected holes remains within the organic crystal due to the blocking action of the extracting interface, but not due to the availability of traps in the crystal. In fact, the concentration of traps is negligible. Consequently, the measured I-V characteristic of the organic single crystal shows the dependence I ~ Vγ with γ > 2.0. In many papers, such dependence is attributed to space-charge-limited current mechanism caused by traps within organic semiconductors. We clearly show that such an interpretation can only be valid, when the contact resistances of metal-organic-metal structures are smaller than that of the organic material. For a gold-squaraine-gold structure with contact resistances smaller than that of squaraine, reproducible measurements of the voltage V23, dropping between electrodes 2 and 3 of a four-probe measurement, was realized. The understanding of the measured I14-V23 curve (I14 is current between electrodes 1 and 4) was achieved with a space-charge-limited current model. The resulting value of the hole mobility amounted to an unexpectedly high value of μ = 1.7x105 cm2/ Vs. Unfortunately, the I-V characteristics, which allowed us to determine this high value of μ , could only be measured twice and only for one sample. Based on this surprising value of μ, we have attempted to find answers to the following questions: (e) How can we explain such a high hole mobility of μ =1.7x105 cm2/Vs? (f) Can such I-V characteristic only be described with the space-charge-limited-current model? Unfortunately, we did not find answers to the above formulated questions within the available timeframe of the present project. The understanding of this result requires additional investigations.
Publications
- Variation of the contact resistance with electric current for gold electrodes on a squaraine single crystal, Phys. Stat. Solidi A 212, 2738 (2015)
L.V. Govor, G. Reiter, and J. Parisi
(See online at https://doi.org/10.1002/pssa.201532241) - Influence of interfacial trap states on injecting and extracting of charges across a metal-organic interface, J. Physics D: Applied Physics 49, 135306 (2016)
L.V. Govor, G. Reiter, and J. Parisi
(See online at https://doi.org/10.1088/0022-3727/49/13/135306) - Tunneling of charge carriers across a gold-squaraine interface, Phys. Lett. A 380, 1493 (2016)
L.V. Govor, G. Reiter, and J. Parisi
(See online at https://doi.org/10.1016/j.physleta.2016.02.025) - When hole extraction determines charge transfer across metalorganic-metal structure, Europhys. Letters 113, 57002 (2016)
L.V. Govor, G. Reiter, and J. Parisi
(See online at https://doi.org/10.1209/0295-5075/113/57002)
