Graphene-passivated nickel as an efficient hole-injecting electrode for large area organic semiconductor devices

Abstract

Efficient injection of charge from metal electrodes into semiconductors is of paramount importance to obtain high performance optoelectronic devices. The quality of the interface between the electrode and the semiconductor must, therefore, be carefully controlled. The case of organic semiconductors presents specific problems: ambient deposition techniques, such as solution processing, restrict the choice of electrodes to those not prone to oxidation, limiting potential applications. Additionally, damage to the semiconductor in sputter coating or high temperature thermal evaporation poses an obstacle to the use of many device-relevant metals as top electrodes in vertical metal–semiconductor–metal structures, making it preferable to use them as bottom electrodes. Here, we propose a possible solution to these problems by implementing graphene-passivated nickel as an air stable bottom electrode in vertical devices comprising organic semiconductors. We use these passivated layers as hole-injecting bottom electrodes, and we show that efficient charge injection can be achieved into standard organic semiconducting polymers, owing to an oxide free nickel/graphene/polymer interface. Crucially, we fabricate our electrodes with low roughness, which, in turn, allows us to produce large area devices (of the order of millimeter squares) without electrical shorts occurring. Our results make these graphene-passivated ferromagnetic electrodes a promising approach for large area organic optoelectronic and spintronic devices. Organic semiconductors serve as a platform for (opto)electronic devices with tunable characteristics by molecular design, enabling versatile device integration, and processing strategies.1 However, ambient processing techniques such as solution processing can facilitate oxidation of metal contacts, resulting in an uncontrolled electronic interface, which is deleterious to performance in semiconductor devices.2,3 New techniques are, therefore, required to control the interface between organic semiconductors and oxidizing metals while maintaining the possibility of solution processing. Graphene has been shown to act as an atomically thin permeation barrier.4–6 Graphene grown via chemical vapor deposition (CVD) directly on the surface of strongly interacting7 catalytic metals, such as Ni, Co, or Fe, acts as a barrier layer to prevent oxidation.8–11 These oxide-free ferromagnetic interfaces have been shown to hold significant benefits within the field of spintronics,12,13 as they enable oxidative fabrication processes, such as solution processing9,10 or atomic layer deposition,14 to be used to fabricate devices with a wider range of relevant materials. One appealing possibility would be to develop graphene-passivated ferromagnets as electrodes9,10 for organic semiconductor spintronics,15–18 where the quality of the electronic interface between the ferromagnetic electrode and the organic semiconductor is of paramount importance.19,20 Another important advantage of an ambient-stable ferromagnetic layer is that it can be used as a bottom electrode in vertical metal-organic semiconductor–metal structures, allowing one to employ techniques such as sputtering to obtain high quality and thickness-controlled metal layers or multi-layers; note that sputtering cannot be used for top-electrodes as it would destroy21 the organic semiconductor. Previous reports using graphene passivated ferromagnets as electrodes for organic semiconductor devices have studied the spin injection properties10 as well as charge injection in lateral organic semiconductor field effect transistors.9 In this work, we investigate few-layer graphene-passivated nickel (Ni/FLG) as a bottom electrode for injection of holes into organic semiconducting polymers in a vertical device structure, demonstrating efficient injection into two standard semiconducting polymers deposited from solution and in air, directly on top of Ni/FLG. Compared to previous reports on graphene-passivated ferromagnetic electrodes, where lithographic techniques had to be used in order to produce micrometer-sized features,8–10,12,14 here, we were able to produce working devices with several orders of magnitude larger active area (4.5 mm2). Our results are, thus, encouraging for the further development of organic optoelectronic and spintronic devices processed from solution under ambient conditions. Nickel was initially sputtered on thermally oxidized silicon wafers, producing films with a thickness of 150 nm. FLG domains were grown on such sputtered Ni films in a custom low-pressure Chemical Vapor Deposition (CVD) reactor (base pressure ∼1 × 10−6 mbar). All substrates were cleaned by sonicating in acetone followed by isopropyl alcohol and blow-dried with a nitrogen gun before loading. Samples were heated to approximately 450 °C using a resistive heater (temperature measurements by a K-type thermocouple) with a rapid ramp rate of 100 °C/min and annealed at ∼1 mbar of H2 for 10 min. This reduces the native oxide prior to graphene growth. After annealing, the H2 flow was stopped and the chamber was evacuated back to approximately base pressure over a period of 5 min. For graphene growth, C2H2 gas was gradually introduced into the reactor via a mass flow controller by incrementally increasing the flow rate over 5 min to achieve a partial pressure of C2H2 of 2.5 × 10−4 mbar. Subsequently, the samples were held at 450 °C in 2.5 × 10−4 mbar of C2H2 for a further 25 min, before rapid cooling (initially ∼300 °C/min) while maintaining the C2H2 flow. All gases were stopped once room temperature had been reached. Upon graphene growth, a roughening of the Ni sputtered on thermally oxidized Si was observed, with an RMS = 67 nm [Fig. 1(a)]. The roughness was found to increase with increasing growth temperature. The roughening of Ni upon graphene growth is explained by grain growth in the sputtered Ni films, occurring at high temperatures during the CVD process: under these conditions, the internal forces in the film are larger than those between the film and the substrate, and diffusion of the film material is appreciable.