Nano-resolved doping of polymer semiconductors can overcome size limitations to create highly integrated flexible electronics, but remains a fundamental challenge due to isotropic diffusion of dopants.
In view of this, researcher Di Chong'an from the team of academician Zhu Daoben of the Institute of Chemistry of the Chinese Academy of Sciences and associate professor Zhang Fengjiao of the University of Chinese Academy of Sciences reported a general method for realizing nanoscale ion implantation electrochemical doping of polymer semiconductors. This method involves confined counterion electromigration to a glass-like electrolyte consisting of a room-temperature ionic liquid and a high glass transition temperature insulating polymer. By precisely adjusting the glass transition temperature (Tg) and operating temperature (T) of the electrolyte, they created a highly localized electric field distribution and achieved anisotropic ion migration almost perpendicular to the nanotip electrode. Confined doping yields excellent resolution of 56 nm and laterally extended doping lengths as low as 9.3 nm. They revealed the universal exponential dependence of doping resolution on temperature difference (Tg − T), which can be used to describe the doping resolution of virtually infinite polymer semiconductors. In addition, they demonstrated their applications in a range of polymer electronics, including organic transistors with 200% performance enhancement and transverse p-n diodes with a seamless junction width of <100 nm. Combined with further demonstration of the scalability of nanoscale doping, this concept may open up new opportunities for polymer-based nanoelectronics. The research results were published in the latest issue of Nature Nanotechnology under the title "Nanoscale doping of polymeric semiconductors with confined electrochemical ion implantation".
【Design Concept】
The low spatial resolution of conventional polymer semiconductor electrochemical doping is due to the global electromigration of the counterions, which in turn is limited by the large size of the counter electrode (CE) and the severe edge field (Figure 1a). Inspired by the physical limitations of the atomic force microscope tip in which the ion beam is penetrated in the single ion implantation method (Figure 1b), the authors hypothesized that a similar focused counterion beam could be achieved by amplifying the counting electrodes to the nanoscale and simultaneously linearly sharpening the edge field (Figure 1c) to create an invisible nanotemplate. As a result, ions in the electrolyte can migrate to the target semiconductor along concentrated field lines, resulting in highly deterministic NEII doping. Based on this concept, the authors utilize a nanoscale AFM tip as CE and reshape the edge field by manipulating the Tg of the electrolyte, as the edge effect in electrochemical doping is closely related to the ionic dynamics associated with the glass transition.
Figure 1.The concept of NEII doping of polymer semiconductors
[Tg depends on doping resolution]
The authors prepared a series of polymer electrolytes with different Tg values by mixing polymethyl methacrylate (PMMA) and 1-ethyl-3-methylimidazole bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) ionic liquids, and explored doping resolution for PBTTT-based films (Figure 1c). Figure 1d shows electrolyte Tg as a function of the EMIM-TFSI weight ratio (WIL). Figure 1e shows the current map of a doped PBTTT film produced by different electrolytes with different Tg at room temperature. In the end, the authors achieved an excellent resolution of 56 nm (the average resolution of the 25 doping sites was 81 nm), which is orders of magnitude higher than that of conventional electrochemical doping. The excellent resolution demonstrates the unique advantages of the proposed concept in achieving nanoscale doping of polymer semiconductors.
However, the observed doping resolution (Rd) is superficial, not intrinsic, as Rd is determined by the CE-electrolyte contact size (Lc) and the extended doping length (Figure 2a). Therefore, the authors defined LDL as (Rd-Lc)/2 to assess the doping limit (Figure 2b). In the low Tg region, Rd and LDL are equally dependent on the electrolyte Tg. However, when Tg exceeds 86 °C, there is a large deviation in LDL, suggesting that the effect of Lc on Rd can no longer be assumed to be negligible (Figure 2c). The theoretical simulations in Figure 2d further support the apparent doping resolution of less than 10 nanometers if the Lc is reduced to a few nanometers.
Figure 2.LDL determination of doped PBTTT films
【Nanoscale Doping Mechanism】
After identifying the important role of electrolyte Tg in achieving nanoscale doping, the authors evaluated the ion-migration-related kinetics of the electrolyte. As the electrolyte Tg increased from -31 °C to 103.2 °C, the ionic conductivity decreased exponentially, by seven orders of magnitude, from 7×10-5 to 3×10-12S:cm-1 (Figure 3a). At the same time, the doping resolution shows a relationship proportional to the σ ions in the log-logarithmic plot (Figure 3b), suggesting that the relatively low ionic conductivity in the high Tg electrolyte is responsible for the increased doping resolution. When the temperature is raised from 0 °C to 100 °C, the ionic conductivity increases by orders of magnitude and crosses occur at 51 °C (Figure 3c). These two different ion transport models show that the decoupling and coupling of ion kinetics occur with the piecewise relaxation of the insulator matrix in the rubber and glass states of the electrolyte, respectively. The authors further extracted ion activation energies with different Tg values at different operating temperatures (Fig. 3d). As the temperature decreases, the ionic conductivity decreases further, which is attributed to the thermally activated ion transport mechanism. The inhibited ionic kinetics in the glass state confirms the hypothesis of this paper that the segmented relaxation of the insulating matrix is responsible for the increase in activation energy. Finite element simulations (Fig. 3e) demonstrate that NEII doping should be universally achieved by controlling the rubber-glass transition of the electrolyte in electrochemical doping.
Figure 3. Tg-dependent doping mechanism
【Glass transition-mediated nanoscale doping】
Ion migration in the electrolyte is not only related to the electrolyte Tg, but also depends on the operating temperature (T), making the doping resolution potentially sensitive to Tg − T. The experimental results (Fig. 4a,b) not only verify the implementation of nano-resolved doping, but also enable direct mapping of Tg and T-dependent doping resolutions. The key advantage is to achieve a doping resolution of < 100 nm and LDL < 10 nm compared to other restricted doping methods, such as chemical doping and traditional electrochemical doping. Both values are the lowest reported for polymer films to date (Fig. 4c,d). NEII doping has the potential to accelerate the emergence of polymer nanoelectronics.
Figure 4. Tg- and T-related doping resolutions
【Doped Devices】
Together, these experimental results make on-demand NEII doping very valuable for polymeric devices. For example, when a doped array was introduced into an organic thin-film transistor, a 200% increase in field-effect mobility was observed with negligible on/off ratio losses (Fig. 5a, b). However, the applicability of the emerging NEII doping method is bottlenecked by the complexity and slowness of cutting-edge technologies. To address this issue, the authors demonstrate the scaling capability of impression-based NEII doping with an imprint matrix tip. By applying voltage and pressure to the tip array, the authors were able to achieve a doped array of 2×2 cm2 in less than 10 sec (Figure 5D). This unique approach yields a unified resolution of 520±20 nm with a reproducible LDLavg of 82.5 nm.
Figure 5. NEII doped devices
【Summary】
In this paper, we report a general approach to nanoscale doping in polymers using the NEII doping concept. By adjusting the Tg and T of the electrolyte, the authors were able to create a linearly reshaped electric field distribution, which favors nanoscale-confined counter-ion electromigration in the electrolyte. This results in a doping resolution of 56 nm and an LDL of 9.3 nm for PBTTT films, which is close to the length of polaron delocalized extended doping. The research experience shows that the doping resolution is closely related to the Tg − T value of the electrolyte, which provides a practical means to control nanoscale doping. More importantly, this method is similar to ion implantation in silicon electronics and should be universally applicable to a wide range of materials for state-of-the-art polymer electronics. It should be noted that current levels of NEII doping are still insufficient for many highly conductive applications. However, this limitation can largely be addressed by precisely tuning the physicochemical properties of the doped ions. The findings in this paper, along with further achievements in doping level manipulation, provide hope that the NEII doping concept can be used as a powerful tool to explore nanoscale polymer optoelectronics.
Source: Frontiers of Polymer Science