Negative differential resistance and carrier transport of electrically bistable devices based on poly(N-vinylcarbazole)-silver sulfide composites
© Li et al.; licensee Springer. 2014
Received: 24 February 2014
Accepted: 7 March 2014
Published: 19 March 2014
An electrically bistable device has been fabricated based on poly(N-vinylcarbazole) (PVK)-silver sulfide (Ag2S) composite films using a simple spin-coating method. Current–voltage (I-V) characteristics of the as-fabricated devices exhibit a typical electrical bistability and negative differential resistance (NDR) effect. The NDR effect can be tuned by varying the positive charging voltage and the charging time. The maximum current ratio between the high-conducting state (ON state) and low-conducting state (OFF state) can reach up to 104. The carrier transport mechanisms in the OFF and ON states are described by using different models on the basis of the experimental result.
Organic electrically bistable devices have aroused extensive interests due to their unique advantages such as simple-fabrication process, large memory density, and lower power consumption [1–3]. A wide variety of materials, including conjugated polymers, small organic molecules and inorganic nanocrystals, have been applied to obtain better device performance [4–6]. Among different candidates for electrically bistable devices, colloidal inorganic nanocrystals have been studied extensively due to their unique chemical and physical properties. To date, some different types of inorganic nanocrystals, such as ZnO, Cu2S, and CdSe/ZnS have been embedded into polymers to fabricate electrically bistable devices, which have exhibited clear electrical bistabilities [7–10]. These nanocrystals mentioned above, however, have their intrinsic defects, such as toxicity and instability, which limit their further applications [11, 12]. In the electrically bistable devices based on inorganic nanocrystals, NDR effects standing for the current decreasing with the increasing bias voltage have often been observed, which have aroused much attention since it is considered to be a key feature for their conduction system [13–15]. As promising optoelectronic candidates, Ag2S nanocrystals have the advantages of less toxic and good stability, which are still rarely seen in the reports of organic electrically bistable devices.
In this letter, an electrically bistable device has been fabricated based on the composites containing spherical Ag2S nanocrystals and PVK using a simple spin-coating method. Current–voltage (I-V) measurements as well as retention and reproducibility tests have demonstrated that the devices show good electrical bistability and stability. The NDR effects have been studied by applying different positive charging voltages and the charging time, which can be attributed to the charge trapping/detrapping process in the Ag2S nanocrystals. Moreover, the carrier transport mechanism has been described based on the I-V results.
The electrically bistable devices were fabricated on glass substrates pre-coated with an indium-tin-oxide (ITO) anode, which were alternately cleaned by deionized water, acetone, and ethanol in an ultrasonic environment. Afterwards, the poly(3,4-ethylenedioxythiophene)/poly-(styrene-sulfonate) (PEDOT/PSS) was spin-coated onto the substrate and was annealed at 150°C for 20 min, which could smooth the ITO surface and improved the device stability by hindering oxygen and indium diffusion through the anode. The PVK and Ag2S nanocrystals were mixed and dissolved in chlorobenzene solution with a mass ratio of 1:1. The solution would further form the active layer by the spin-coating method. Finally, a top Al electrode layer of 200 nm thickness was deposited onto the top surface by thermal evaporation under the vacuum of about 1 × 10−6 torr.
Results and discussion
Moreover, the NDR effects under different charging time (0.01 to 1 s, 10 V) were also studied, and the corresponding I-V characteristics in the NDR region are given in Figure 3b. It can be seen that the absolute current value at Voff increases as the charging time is increased from 0.01 to 0.3 s. This indicates that more charges have been seized by trap centers with longer charging time, which results in larger discharging current in the NDR region. However, the I-V characteristic saturates when the charging time of the applied voltage reaches 0.3 s, indicating the traps in device will be completely occupied after a certain charging time, which may be attributed to an oxidation process related to the oxygen vacancies on the surface of Ag2S nanoparticles .
In contrast, the experimental I-V result in ON state can be well described by an ohmic model, which is depicted in Figure 5c. It can be seen that a distinct linear relationship between logI and logV, with a slope of 1.2 in the positive (10 to 0 V) region. The theoretical fitting illustrates that the current of the device is approximately proportional to the applied voltages, which is close to the Ohmic law (I∝V) . The results reveal that the traps in the Ag2S nanospheres have filled by the carriers during the aforementioned TCLC process. The trapped carriers lead to the rise of the internal electrical field at the Ag2S/PVK interface, which can change the conductivity of the device. All the results of the theoretical fitting are consistent with the charge trapping mechanism.
In summary, organic bistable devices based on Ag2S-PVK composites were fabricated by a simple spin-coating method. Obvious electrical bistability and NDR effects have been observed in the devices due to the existence of the Ag2S nanospheres. The NDR effects can be controlled by varying the charging voltages and charging time. The maximum ON/OFF current ratio can reach up to 104. The carrier transport can be described in terms of the organic electronic models, and the carrier transport mechanism alters from the thermionic emission to the ohmic model during the transition from OFF state to ON state, which is closely associated with the charge trapping/detrapping process in the Ag2S-PVK composites.
This work was partly supported by the National Science Foundation for Distinguished Young Scholars of China (No. 61125505), the National Natural Science Foundation of China (Grant No. 61108063), and the author (A. W) is also grateful to the financial support from Beijing JiaoTong University (2012RC046).
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