Investigation of hole-injection in α-NPD using capacitance and impedance spectroscopy techniques with F4TCNQ as hole-injection layer: Initial studies
Introduction
Organic semiconductors (OS) have gained considerable attention due to their potential applications in organic light-emitting diodes (OLEDs) [1], [2], [3], [4], organic photovoltaics (OPVs) [5], [6], [7], [8] and organic thin-film transistors (OTFTs) [9], [10]. For all these applications, understanding the OS charge transport is vital and this has led to the development of variety of techniques in electrical characterization of OS [11], [12], [13], [14]. In these methods, the Gaussian disorder model is widely used to depict the density of states (DOS) in the organic layers of the device [15], [16], [17]. The analysis of the dependence of capacitance on applied voltage (C–V) is considered to be the most reliable method to explain the charge transport and interface properties in organic semiconductors [18], [19], [20], [21]. Recent studies by Tripathi and Mohapatra have demonstrated the role of interfacial density of states in the charge transport across different organic structures using C–V characteristics [22]. Similar to C–V, capacitance–frequency (C–f) technique is also widely used to study the interfacial defects and traps in the organic devices [18], [19], [23], [24]. The electrical properties of organic materials and their interfaces are also very well studied using impedance spectroscopy (IS) [25], [26], [27], [28], [29], [30]. In addition to the study of charge injection and transport properties in organic semiconductors, IS measurements have also been performed to probe the degradation in OLEDs [23].
The study of charge transport properties in the widely used hole transport material N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (α-NPD) is very much relevant even today since it offers a basic step in the understanding of complicated device structures [22]. An ohmic contact is necessary to efficiently inject charge carriers in the device [31]. The well-known hole-injection material 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) plays a crucial role in forming a nearly ohmic contact between ITO anode and α-NPD by lowering the energy barrier between them [32], [33]. An improvement in hole injection is also accomplished through p-type doping of organic semiconductors [34]. F4TCNQ acts as a p-type dopant which is conventionally co-evaporated (bulk doped) with α-NPD [35], [36]. On the contrary, sequential doping of F4TCNQ in between ITO and α-NPD layers is simpler and cost-effective; and is on par with co-evaporation technique. The increase in hole-transport layer conductivity due to the diffusion of sequentially deposited F4TCNQ into it has been studied using secondary ion mass spectroscopy [32].
In this work, we report the capacitance and impedance based studies on devices consisting of α-NPD sequentially doped with F4TCNQ. In order to study the transport properties of the holes thus injected, the charge recombination is neglected in the device. This is achieved through hole-only devices wherein the electron injecting electrode has a large barrier with the lowest unoccupied molecular orbital (LUMO) of α-NPD.
Section snippets
Experimental
All the materials were obtained from commercial sources and were used directly without any further purification. In this investigation, we have fabricated devices with three different configurations (D0, D1 and D4). Patterned indium tin oxide (ITO, XinYan Technology Ltd., Hong Kong) was used as the bottom anode, F4TCNQ (Sigma Aldrich) was used as a hole injection material, α-NPD (Sigma Aldrich) was used as the hole transport material and aluminum (Al, Alfa Aesar) was used as cathode. Prior to
Results and discussion
It is clearly discernible from the current density versus voltage (J–V) characteristics [Fig. 2(a)] that insertion of F4TCNQ and the increase in its layer thickness in the device lead to an enormous increase in current density through the device. Also, the turn-on voltage of the device decreases as the F4TCNQ layer thickness increases. At reverse bias voltages, the current density is almost negligible and increases exponentially at the turn-on voltage. This clearly indicates an improvement in
Conclusion
In summary, we have explored the charge transport at the organic interface formed between a hole-injection layer (F4TCNQ) and a hole transport layer (α-NPD) using capacitance (C–V and C–f) and impedance based electrical techniques. The significance of sequential doping is highlighted and the device configuration can be extended to potential organic devices especially OLEDs. The thickness of the F4TCNQ layer plays a crucial role in device properties and is proportional to the charge carrier
Acknowledgment
The authors acknowledge Department of Information Technology (DIT), India for financially supporting the work through a research project grant and National Institute of Technology Karnataka, India for supporting the work through research fellowship and for providing infrastructure.
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