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 ≅ 10⁵ to ≅ 10⁸ Ω) was larger than the resistance of the squaraine crystal (≤10⁵ Ω); (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.