Variation of the contact resistance with electric current for gold electrodes on a squaraine single crystal

We examined the dependence of the contact resistances between gold electrodes and a single crystal of small organic molecules, namely, squaraine (2,4-bis[4-(N, N-diisobutylamino)-2,6-dihydroxyphenyl]), on the electric current. Applying four gold electrodes on the crystal allows to measure twelve current-voltage (I-V) characteristics. The dependence of all eight contact resistances (four for the injecting and four for the extracting electrodes, respectively) on the electric current was determined. We found: (i) the contact resistance of all electrodes (ranging from similar or equal to 10(5) to similar or equal to 10(8) Omega) was larger than the resistance of the squaraine crystal (<= 10(5) Omega); (ii) for the same electrode, the values of the resistance for charge injection and charge extraction were different; (iii) the values of all contact resistances were not constant, but depended on the value of the current. Furthermore, each of the measured total I-V characteristics could be decomposed into three components representing charge transfer through the injecting electrode, the squaraine crystal, and the extracting electrode. We demonstrate that, for the correct interpretation of the I-V characteristics measured on metal/squaraine/metal structures, determination of the contact resistances at the interface between metal electrode and organic crystal is ultimately crucial. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim