The role of the electrode configuration on the electrical properties of small-molecule semiconductor thin-films
Graphical abstract
Introduction
Organic electronics has been considered one of the main pathways toward novel functional technologies [1], [2], [3], [4]. The use of organic materials for electronics has several advantages over the conventional silicon-based technology, such as low processing temperatures, compatibility with flexible substrates, and broad chemical tunability [5], [6], [7]. The organic nature of such materials is itself a strong appeal for new developments, for example, to reach biocompatibility in electronic devices [8], [9] and to develop entire environment-friendly electronics [10] – from the manufacturing processes to the device disposal. The combination of organic and inorganic materials has also been considered as a potential strategy to add functionalities to existing devices [11], [12], [13], [14]. In this case, both the organic-inorganic interfaces and the multilayer-compositions are fundamentally relevant to determine the device final electrical properties [15], [16], [17].
Among the variety of existing devices, the organic thin-film transistor (OTFT) is considered one of the principal elements for several applications, such as sensors and biosensors [18], [19], identification tags [20], [21] and flexible displays [22]. Since the first OTFT demonstrations, about 30–35 years ago [23], [24], [25], these devices have experienced continuous and fast improvement of performance [22], [26]. The operation of OTFTs has been reported to depend not only on the properties of the semiconducting channel layers but also on characteristics that arise from the device architecture [27], [28]. It has been demonstrated that OTFT geometrical features, such as the aspect ratio [29], [30], [31], the arrangement of the materials' layers [32], [33], and the position of the electrodes [34], [35], are key to achieve high electrical performance.
In practice, the OTFT performance is evaluated in terms of the organic semiconductor charge carrier mobility (μ), whereas high mobility is desirable for many applications [7]. The higher the mobility, more critical becomes the carrier injection limitation from the electrodes in controlling the device performance [36], [37]. It has been shown that the proper choice of the electrode material can lower the intrinsic injection barriers [4], [38]. Alternatively, self-assembled monolayers are effective in lowering the contact resistance of bottom-contact (BC) OTFTs [35], [39], [40]. For top-contact (TC) devices, the insertion of dopants at the OSC/electrode interface has been validated as a strategy to enhance the charge carrier injection [41], [42]. Recent studies have demonstrated that different OTFT structures can display dissimilar electrical characteristics while using the exact same materials [32], [35]. Staggered OTFTs (viz. BC/top-gate or TC/bottom-gate) exhibit contact resistances that are orders of magnitude lower than those found in their coplanar counterparts [43], [44], [45], [46], [47]. Besides, the charge carrier injection in TC and BC devices has been demonstrated to be affected by the gate voltage as well [48]. Finally, extrinsic effects, such as the migration of metal clusters into the OSC in TC devices during the contact formation [49], [50], and low-mobility regions in the contact areas of BC electrodes [31], [51], [52], may lead to changes in the carrier injection properties. Although many efforts have been realized to reduce the contact resistance of OTFTs, this parameter has exhibited a crucial dependence on non-intrinsic phenomena that deserve a detailed investigation [4].
In this work, we demonstrate that space charges localized at the OSC/substrate interface strongly affect the electrical conduction in an OTFT-like device. By using two-terminal devices designed to minimize interfacial contact effects [7], we have evaluated the charge transport characteristics of Au/OSC/Au planar structures, based on the small-molecule OSC prototype copper phthalocyanine (CuPc) commonly used in OTFTs [7]. Here, CuPc devices were characterized as a function of the electric field (E), the operation temperature (T) and the electrode configuration (BC and TC architectures). The electrode position has a crucial role on the electrical properties of CuPc thin-films devices: while TC configuration showed an ohmic behavior, the BC ones were strongly affected by space charges. By performing finite-element calculations, we have verified that space-charge-limited-current (SCLC) is an extrinsic contribution from the OSC/substrate interface and, therefore, related to the device architecture. Traps responsible for the space-charge conduction are found to have origin at the OSC/substrate interface, a relevant region in OTFTs [53]. The present work contributes to understand the origin of contact-related issues in OTFTs from the perspective of the OSC charge transport mechanisms and their dependence on the device electrode configuration.
Section snippets
Experimental details
To investigate the role of contact position on the electrical properties of OSC thin-films, we have fabricated devices in two different configurations, namely TC and BC architectures, as shown in Fig. 1a–b. Both architectures were assembled onto flat glass slides (roughness ∼1 nm) with Au electrodes (50 nm thick) deposited by e-beam evaporation in high vacuum (5 × 10−6 Torr) at a rate of 0.5 Å/s. For the BC devices, a Cr adhesion layer was deposited prior to the Au electrode under similar
Results and discussion
Fig. 2 shows the current vs. electric field (I-E) characteristics for TC and BC devices (50 nm CuPc film) at 317 K. The current values were normalized by the electrode width to allow the direct comparison of the two distinct architectures.
CuPc films have shown to be stable, presenting reproducible and hysteresis-free I-E characteristics. In addition, no rectification has been observed for both architectures, as shown in the Supporting Information. From Fig. 2, we observe that both TC and BC
Conclusion
In summary, this study shows that the electrode configuration has fundamental role on the as-measured electrical properties of thin organic semiconductor films in a planar-contact configuration. The current vs. electric field characteristics for different temperatures and thicknesses provide clear evidence that space-charge limited conduction arises from traps at the OSC/substrate interface. Finite-element calculations corroborate the experimental findings. The space-charge regime is probed
Acknowledgments
The authors acknowledge Angelo L. Gobbi and Maria Helena O. Piazzetta from LNNano/CNPEM (Brazil) for the substrates containing BC electrodes, and the Brazilian funding agencies CNPq (Project 483550/2013-2), FAPESP (2014/25979-2), and CAPES for their support.
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