Photoinduced Recovery of Organic Transistor Memories with

Mar 13, 2017 - The devices also showed good current switching responses from ON to OFF states for a number of cycles and maintained bistable current s...
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Photoinduced Recovery of Organic Transistor Memories with Photoactive Floating-Gate Interlayers Yong Jin Jeong,† Dong-Jin Yun,‡ Se Hyun Kim,*,§ Jaeyoung Jang,*,∥ and Chan Eon Park*,† †

Polymer Research Institute, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 37673, Republic of Korea ‡ Analytical Science Laboratory, Samsung Advanced Institute of Technology (SAIT), Suwon 16678, Republic of Korea § School of Chemical Engineering, Yeungnam University, Gyeongsan, North Gyeongsang 712-749, Republic of Korea ∥ Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea S Supporting Information *

ABSTRACT: Optical memories based on photoresponsive organic fieldeffect transistors (OFETs) are of great interest due to their unique applications, such as multibit storage memories and flexible imaging circuits. Most studies of OFET-type memories have focused on the photoresponsive active channels, but more useful functions can be additionally given to the devices by using floating gates that can absorb light. In this case, effects of photoirradiation on photoactive floating-gate layers need to be fully understood. Herein, we studied the photoinduced erasing effects of floating-gate interlayers on the electrical responses of OFET-type memories and considered the possible mechanisms. Polymer/C60 composites were inserted between pentacene and SiO2 to form photoresponsive floating-gate interlayers in transistor memory. When exposed to light, C60 generated excitons, and these photoexcited carriers contributed to the elimination of trapped charge carriers, which resulted in the recovery of OFET performance. Such memory devices exhibited bistable current states controlled with voltagedriven programming and light-driven erasure. Furthermore, these devices maintained their charge-storing properties over 10 000 s. This proof-of-concept study is expected to open up new avenues in information technology for the development of organic memories that exhibit photoinduced recovery over a wide range of wavelengths of light when combined with appropriate photoactive floating-gate materials. KEYWORDS: photoinduced recovery, organic field-effect transistors, optical memories, floating gates, fullerene as multibit storage memory cells and flexible imaging circuits.9,18−20 When the OFET-type optical memories are exposed to light, a number of excitons are generated in the organic semiconductor layer. These excitons are devided into holes and electrons in the presence of gate electric field, and then these charge carriers can be trapped in the underlying charge-storage layers. As a result, such devices can exhibit bistable current states with light-assisted programing and erasing processes.9,20,23,24 To better utilize these effects, a photoactive interlayer film can be incorporated into an OFET-type optical memory between the semiconductor and dielectric layers. Such an interlayer can influence the charge transport in semiconducting channel by altering the dipole moment and/or capacitance of dielectric films.25−28 Polymer−nanoparticle composite films have also been employed as interlayers of

1. INTRODUCTION Organic field-effect transistors (OFETs) have attracted significant attention as a key switching device in nextgeneration flexible electronics.1−4 With rapid progress in OFETs in recent years, the integration of additional functions into the OFETs have been widely proposed.5−8 For example, nonvolatile memories based on OFETs have been developed due to the advantages of nondestructive read-out properties and good compatibility with soft integrated circuits on flexible substrates.9−17 Beyond the OFET-type memories modulated only by electrical stress, recently numerous studies have attempted to utilize light as the impetus for electrical responses and have thus expanded their applications to optical memories.18−22 OFET-type optical memories have many advantages because they do not require high voltage biases for programming and erasing. In particular, with the aid of light bias, small magnitudes of electrical stresses can be enough to produce large memory windows (i.e., threshold difference after programing/erasing processes). Thus, OFET-type optical memories have highly specialized potential applications such © 2017 American Chemical Society

Received: February 17, 2017 Accepted: March 13, 2017 Published: March 13, 2017 11759

DOI: 10.1021/acsami.7b02365 ACS Appl. Mater. Interfaces 2017, 9, 11759−11769

Research Article

ACS Applied Materials & Interfaces

prepare thin PS or PVN charge blocking layers on the polymer/C60 interlayers, solutions of PS or PVN in chloroform (a nonsolvent for C60) were dropped onto prespun composite samples. A 50 nm thick pentacene film was deposited onto each of the polymer/C60-coated substrates at a rate of 0.1−0.2 Å s−1 by using an organic molecular beam deposition system. The 100 nm thick gold source and drain (S/ D) electrodes were deposited onto the substrates by performing thermal evaporation through a shadow mask. The channel lengths (L) and widths (W) of the OFETs were 150 and 1000 μm, respectively. All fabrication steps were performed in ambient air (RH: 40 ± 10%). The areal capacitance (Ci) was measured using an Agilent 4284 Precision LCR meter by fabricating Au/dielectrics/highly n-doped Si substrates capacitors. The film thicknesses were measured with an ellipsometer (FQTH-100, J.A. Woollam Co., Inc.). The electrical measurements were performed by using a Keithley 4200 SCS in a N2rich glovebox. The intensity of exposed white light was below 500 μW cm−2. Light with wavelengths of 350 or 580 nm was obtained by using a Hg (Xe) arc lamp light source with a graiting monochromator. The morphologies of the polymer/C60 composites and the pentacene layers were investigated with atomic force microscopy (AFM, Bruker Nanoscope). The energy levels of the polymer/C60 composites and the energy level alignment of the pentacene layers/composites were characterized by performing UV photoemission spectroscopy (UPS) measurements and recording ultraviolet−visible (UV−vis) absorption spectra (Cary, Varian Co.). The crystalline pentacene films were analyzed based on the two-dimensional grazing incidence wide-angle X-ray diffraction (2D-GIXD) patterns and the θ−2θ mode out-ofplane X-ray diffraction (XRD) patterns collected by using the synchrotron X-ray beam sources at the 6D and 5A beamlines at the PAL, respectively.

OFET-type memories, which incorporate organic nanoparticles as charge-storage materials.29−31 Organic charge-storage nanoparticles, which in this study were employed as floating gates, have more potential in charge-storage applications than metallic nanoparticles because of the ease of control of their morphologies and electronic structures as well as the feasibility of simple film fabrication with solution-processing techniques.17,29,31 Recently, it has been reported that photoerasable memory characteristics could be achieved by light illumination on OFET-type memories that incorporate vacuum-deposited or solution-processed floating gates of organic charge-storage small molecules.32 This interesting photoreset effect opens up a new strategy for information delivery in the field of OFET-type optical memories with regard to the convenient elimination of stored charges.32 However, the origin of the photoerasable memory characteristics of this system and the role of the organic floating gates have not yet been fully investigated. Therefore, an in-depth study on the mechanism of the photoerasing process is strongly required to better utilize the photoreset effects in OFET-type optical memories.32 Such a study might make it possible to design innovative organic memories that can be reset at any time by illumination of desirable wavelength light if we employ tailor-made organic nanoparticles as floating gate materials. Here, we introduce photoactive polymer/nanoparticle composite interlayers to fabricate OFET-type photoerasable memories and investigated the mechanism of their recovery under light illumination. Fullerene (C60) is employed as organic nanoparticles in composite interlayers, which has been widely used due to its excellent charge-storage properties in flexible memories.17,31,33 In addition, since C60 molecules are typical ntype semiconductors, they can be excited by photoirradiation, generate Frenkel excitons and serve as photoactive floating gates in OFET-type optical memories.33−36 We investigated electrical characteristics of the OFET-type memories with polymer/C60 composite interlayers and their electrostatic charge behavior at the interface between the semiconductor and the composite interlayer. On the basis of this, we concluded that C60 molecules in composite interlayers effectively generated excitons and mainly contributed to the elimination of trapped charges. Because of the charge-storage and exciton-generation properties of the composite interlayers, the memory devices exhibited bistable current states after voltage-driven programing and light-driven erasing processes. The devices also showed good current switching responses from ON to OFF states for a number of cycles and maintained bistable current states for a significant retention time.

3. RESULTS AND DISCUSSION 3.1. Polymer/C60 Composite Interlayers and “Photoinduced Recovery” of OFETs. We selected PS and PVN as the polymer matrixes for the polymer/C60 composites and prepared the composite films on substrates. Since C 60 molecules undergo π−π interactions with the phenyl and naphthalene units of PS and PVN, they can form stable composite films. First, we measured UV−vis absorption spectra of the composite films to obtain the information about the absorption of C60 molecules, as shown in Figure 1. We prepared PS/C60 and PVN/C60 composite films with PS to C60 ratios in the range 10:0 to 8:2 and PVN to C60 ratios in the range 10:0 to 7:3, respectively. In Figures 1a,c, it can be seen that the light absorption at wavelengths in the range 300−400 nm of the PS/C60 and PVN/C60 composites increases as the C60 to polymer ratio increases. This trend probably arises from the increase in the number of C60 molecules in the composites since this peak is the characteristic absorption of C60 due to its delocalized π-electron system. Additional absorption in the range 500−700 nm was observed, as shown in Figures 1b,d, when pentacene was deposited on the PS/C60 and PVN/C60 composites as a semiconductor layer. These peaks are in good agreement with reported pentacene UV−vis absorption spectra.37,38 The AFM images shown in Figure 2 and Figure S1 of the Supporting Information revealed that both single polymer and polymer/C60 composite films had very smooth and featureless surfaces with root-mean-square (rms) roughnesses below 0.3 nm (Figures 2a−d). Composite films with various blending ratios also had the same surface morphology, which indicated that PS/C60 and PVN/C60 formed well-mixed composite films for various C60 proportions. Figures 2e−h illustrate the morphologies of pentacene films grown on the single polymer or polymer/C60 composite layer. All samples had similar pentacene morphologies with micrometer-scale large grains. Moreover, the out-of-plane XRD and 2D-GIXD

2. EXPERIMENTAL SECTION Polystyrene (PS, Mw = 192 000), polyvinylnaphthalene (PVN, Mw = 175 000), anhydrous dichlorobenzene, anhydrous chloroform, and pentacene were purchased from Aldrich Co. Poly(4-chlorostyrene) (PS-Cl, Mw = 250 000) and poly(vinyl cinnamate) (PVCN) were purchased from Polyscience Co. All chemicals were used without further purification. Highly n-doped Si substrates with 100 nm thick thermally grown SiO2 layers (Si/SiO2) were cleaned by using a boiled acetone solution, rinsing multiple times with acetone. After treating each substrate with UV-ozone (UVO) treatment for 20 min, polymer/ C60 composites dissolved in dichlorobenzene were spin-coated onto Si/SiO2 substrates and baked at 90 °C for 30 min to remove any residual solvent. The spin-coating speeds were adjusted to obtain film thicknesses if approximately 30 nm. PS, PVN, PS-Cl, and PVCN were used as the polymer matrixes of the polymer/C60 composites. To 11760

DOI: 10.1021/acsami.7b02365 ACS Appl. Mater. Interfaces 2017, 9, 11759−11769

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not severely altered by blending C 60 with dielectric polymers.40,41 Bottom-gate top-contact pentacene OFETs were fabricated on SiO2/Si substrates, all with a polymer/C60 composite interlayer between the pentacene and SiO2 layers, as shown in Figure 3a. The OFETs with PS/C60 and PVN/C60 interlayers both exhibited typical p-type transfer characteristics (Figures 3b,c) with saturation field-effect mobilities (μ) up to 0.22 cm2/ (V s) (the drain voltage (VD) was −40 V). The electrical characteristics of OFETs are summarized in Table 1 and Table S1. Transfer curves of both OFETs shifted toward the negative direction after the application of gate bias (VG) of −50 V for 10 ms, which may result from trapping of holes into polymer/C60 interlayers under negative (−) VG. Holes might be trapped in the deep-level states between HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) levels of C60 molecules or those of polymers (Supporting Notes S2). Positive space charges stored in the polymer/C60 interlayer impeded the formation of hole channels despite the application of (−) VG. Interestingly, both transfer curves were returned to their initial states by exposure of the OFETs to white light. Such photoinduced recovery arose consistently in the OFETs regardless of the interlayer thickness (Figure S3 and Supporting Notes S3) and the type of polymer (see the results for OFETs with a styrenic polymer/C60 interlayer namely PS-Cl/C60 and a PVCN/C60 interlayer in Figure S4). From these observations, it seems likely that the interface between the pentacene layer and the composite interlayer plays an important role in the photoinduced recovery. To verify our speculation, we inserted thin PS or PVN films between the pentacene and PS/C60 (PVN/C60) interlayers as the blocking layer (see the schematic illustration shown in Figure 3d) and measured transfer characteristics. Inserting thin PS or PVN films produced very little effect on the underlying polymer/C60 composite films (Figure S5 and

Figure 1. UV−vis absorption spectra of (a) PS/C60 composite films, (b) pentacene thin films deposited on PS/C60 (8:2), (c) PVN/C60 composite films, and (d) pentacene thin films deposited on PVN/C60 (7:3) composite films.

results revealed that the pentacene molecules all formed typical thin-film phase crystals with similar crystallinities on these composite layers (Figure S2 and Supporting Notes S1).39 These results demonstrated that the morphology and crystallinity of the overlying pentacene layer, which are critical factors in the performance of pentacene-based OFETs, were

Figure 2. AFM topography images of (a) PS, (b) PVN, (c) PS/C60 (8:2), and (d) PVN/C60 (7:3) thin films and pentacene thin films deposited on (e) PS, on (f) PVN, on (g) PS/C60 (8:2), and on (h) PVN/C60 (7:3). The inset graphs of (e), (f), (g), and (h) display the corresponding AFM cross-sectional height profiles of the films. 11761

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Figure 3. Schematic illustrations of (a) an OFET with a polymer/C60 interlayer and (d) an OFET with a charge blocking layer and a polymer/C60 interlayer used in this study. Transfer characteristics and their changes under voltage or photo bias of the OFETs (VD = −40 V) with (b) PS/C60 (8:2) and (c) PVN/C60 (7:3) interlayers. Transfer characteristics and their changes under voltage or photo bias of the OFETs with (e) a PS charge blocking layer deposited on a PS/C60 (8:2) and (f) a PVN charge blocking layer deposited on a PVN/C60 (7:3) interlayer.

Table 1. Electrical Characteristics of the OFETs Used in This Study Vth,ave [V] OFETs PS PS/C60 (8:2) PVN PVN/C60 (7:3)

Ci [nF/cm2] 22.2 21.9 21.8 21.4

μave [cm2/(V s)]

Ion/off

± ± ± ±

10 −106 105−106 105−106 105−106

0.28 0.27 0.26 0.26

0.04 0.05 0.03 0.03

5

Initial Vth,ave [V] −6.84 −8.73 −11.58 −10.31

± ± ± ±

1.2 1.2 2.1 2.5

−50 V 10 ms

light 30 s

−12.14 −23.68 −16.88 −31.81

−12.10 −8.87 −16.60 −10.65

C60 molecules could also have a chance to be trapped in the interlayer. Negative space charges stored in the polymer or polymer/C60 interlayers lead to the accumulation of extra holes in the active channel even under a positive (+) VG and therefore to shift in the transfer curves toward the positive direction. As shown in Figure 4a, the transfer curves of OFETs with a polymer-only (PS) interlayer showed substantial shifts in the positive direction after the application of a series of PBSs under white-light illumination. Figure 4b plots the corresponding threshold voltage shifts (ΔVth) of PS-only samples; these results suggest that ΔVth increases as the applied VG increases, which is consistent with previous reports.20,28 Such ΔVth dependence upon the magnitude of VG can be explained in terms of the correlation between the applied VG and the density of charges trapped in the PS interlayers.32 In other words, large ΔVth of OFETs with a PS interlayer can be a clear signal of large amount of electron trapping in PS interlayers. In contrast, transfer curves of OFETs with PS/C60 interlayers exhibited significantly reduced ΔVth compared to those of PS-only samples (Figure 4c). The variations in ΔVth with PBS time for both devices are summarized in Figure 4d (the ΔVth values were obtained from the curves in Figure S7, and see the

Supporting Notes S4). As shown in Figures 3e,f, the negatively shifted transfer curves after applying VG (−50 V) did not return to their initial states in the presence of the blocking layers. The recovery of the initial states of the transfer curves also did not occur after exposure to white light for a much longer time (5 min) (Figure S6). On the basis of these results, we concluded that the “photoinduced recovery” might be due to some interactions between the pentacene and the C60 molecules incorporated in the composite interlayers. To check if the “photoinduced recovery” can be observed in the case of electrical instabilities due to electron trapping, we performed a positive bias stress (PBS) test under white-light illumination. The number of trap sites for the minority charge carrier (i.e., electrons) is expected to be small in p-type OFETs, so we exposed the pentacene OFETs to white light to induce full electron trapping during the PBS test.20,42,43 Exposure of semiconductors to white light results in the generation of excitons that are separated into electrons and holes. The electrons produced in the pentacene layer are then transferred to the polymer or polymer/C60 interlayers and trapped there under the applied vertical electric field with positive (+) VG. The electrons produced in the polymer/C60 interlayer due to 11762

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C60 interlayer recovered its initial state after exposure to a white light for 30 s, while the PS-only samples did not recover (Figure 4a). These results suggest that photoinduced recovery characteristics can be observed from the OFETs when they contain a polymer/C60 interlayer, regardless of the type of the trapped charge carrier (i.e., holes or electrons). 3.2. Possible Mechanisms for the Removal of Charges Trapped in Polymer/C60 Interlayers. We propose two possible mechanisms explaining the “photoinduced recovery” illustrated in Figure 5. Figure 5a shows the first possibility: Frenkel excitons are generated initially in pentacene under visible light and move to the pentacene/composite interface; subsequently, charge-exciton annihilation occurs at the pentacene/C60 heterojunction, which leads to the neutralization of trapped charge carriers. This mechanism has been widely investigated in previous studies.32,44 The other scenario is shown in Figure 5b. When exposed to light, C60 molecules are excited and generate Frenkel excitons. These excitons can effectively neutralize the trapped charge carriers via chargeexciton annihilation. High energy carriers that are separated from the excitons diffuse into the pentacene/composite interface and are eventually removed from C60 molecules (Figure S8 and Supporting Notes S6 for further explanation). This mechanism is similar to the operation principle of nanocomposite photodetectors consisting of ZnO nanoparticles embedded in a P3HT matrix.45 Both mechanisms can explain why “photoinduced recovery” has been disappeared after inserting a thin PS (or PVN) film between the pentacene and the PS/C60 (or PVN/C60) interlayer: the polymer-only layer hinders the transport of exciton-derived carriers between the pentacene and C60 molecules. In other words, the polymeronly layer acted as “charge blocking” layers, as illustrated in Figure S9. Both mechanisms could be responsible for the “photoinduced recovery” because both pentacene and C60 molecules can be excited by white light and generate Frenkel excitons. However, we think that it is worthwhile to determine which mechanism is dominant in order to make use of photoinduced recovery in future memory devices.

Figure 4. Shifts in the transfer characteristics (ID1/2 vs VG curves) of OFETs with (a) PS and (c) PS/C60 (8:2) interlayers for various VG bias conditions under light illumination for 10 min (VD = −10 V). (b) Corresponding threshold voltage shift (ΔVth) of the devices in (a) and (c) as a function of the bias VG values. (d) ΔVth values of OFETs with PS and PS/C60 (8:2) interlayers as functions of the bias stress time. During the bias stress tests, a VG of 60 V was applied under light illumination.

Supporting Notes S5 for further explanation). The smaller ΔVth of OFETs with PS/C60 interlayers compared to those of PSonly samples might result from the lower levels of electron trapping in the PS/C60 interlayers. Moreover, it should be noted that photoinduced recovery characteristics have also been observed in the case of electron trapping. The curve consisting of green circles in Figure 4c shows that the OFET with a PS/

Figure 5. Schematic diagram showing the possible mechanisms at the pentacene/composite interface for the “photoinduced recovery” characteristics. 11763

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Figure 6. (a) Schematic diagrams showing light absorptions of C60 and pentacene molecules. Shifts of transfer characteristics (ID1/2 vs VG curves) for OFETs with PVN/C60 (7:3) interlayers when a VG of 70 V is applied for 30 s (red circles), and they are exposed to light with a wavelength of (b) 350 nm for 30 s (blue circles: 40 μW cm−2) and (c) 580 nm for 30 s (blue circles for low intensity: 40 μW cm−2; green circles for high intensity: 300 μW cm−2).

according to the literature.48−51 These considerations provide the theoretical background of the second mechanism in Figure 5b. The different PBS results for OFETs with PS and with PS/ C60 layers in Figures 4a−c could be explained by the second trap removing mechanism. Adding floating-gate materials provides the location of trapped electrons, and the use of C60 as floating-gate materials enable to store many electrons because C60 is n-type semiconductor. In this regard, more electrons should have been trapped in the PS/C60 layers. Nevertheless, less Vth shift and nonincreasing ΔVth over VG of 40 V happened in transfer curves of PS/C60 devices (Figure 4c). It means few electrons were trapped in PS/C60 composite layers under a given bias condition (light and (+) VG). We thought the photoexcited excitons in C60 molecules is the key element to explain these results. When the electrons separated from Frenkel excitons in pentacene moved to composite interfaces under vertical (+) VG, the excitons in C60 molecules triggered the charge-exciton annihilation with entering electrons from pentacene. Therefore, the photoexcited excitons in C60 molecules interrupted the formation of electron trap in composite layers regardless of the magnitude of gate bias voltage. Looked at in these points, the first mechanism in Figure 5a was hard to explain less ΔVth of PS/C60-OFETs in PBS test under white-light illumination. 3.3. Transistor Memory Devices Employing PVN/C60 as Floating-Gate Interlayers. OFETs incorporating polymer/C60 interlayers can be utilized as novel OFET-type optical memories with unique “photoinduced recovery characteristics”. In detail, bistable current states can be prepared by applying voltage and photo biases for writing and erasing processes, respectively, as a result of charge trapping and charge-exciton annihilation in photoactive floating-gate interlayers consisting of polymer/C60 composites, respectively. The feasibility of erasing memories with a photo bias alone without the need for a voltage bias offers a simple and smart strategy for information delivery.32 To demonstrate such an optical memory device, we used PVN as the polymer matrix for the polymer/C60 floatinggate interlayer. PVN has been extensively studied as a polymer electret material for OFET-type memories because when compared to PS it has a relatively low band gap with low LUMO and high HOMO levels due to the long π-conjugation

According to the results in Figure 1, C60 molecules in PVN effectively absorb light with a wavelength of 350 nm but do not absorb at 580 nm, whereas pentacene on the PVN/C60 does absorb at 580 nm (this situation is illustrated in Figure 6a). Therefore, we exposed positively prebiased PVN/C60 OFETs to light with a wavelength of either 350 or 580 nm to examine the dependence of the transfer characteristics on the light wavelength. Figures 6b and 6c plot the square-root drain current versus the gate voltage (ID1/2 vs VG) for OFETs with PVN/C60 interlayers under exposure to light with wavelengths of 350 and 580 nm, respectively; the corresponding log ID vs VG curves are shown in Figure S10. The positively shifted transfer curve (red circles) recovered its initial state after exposure of 350 nm light (40 μW cm−2) for 30 s, whereas the 580 nm light did not lead to 100% recovery with slight shifts in the negative direction. Increasing the intensity of the 580 nm light (by a factor of approximately 300 μW cm−2) resulted in a slightly higher shift in the negative direction but the curve was still far from its initial state (Figure 6c). The same trend was found in the recovery of negatively prebiased OFETs (i.e., the hole-trapping case) after exposure to 350 and 580 nm light (Figure S11). We further investigated PBS tests under light with a wavelength of 350 or 580 nm, which were discussed in Figure S12 and Supporting Notes S7. These results indicate that “photoinduced recovery effect” occurs dominantly due to the exposure of light with a wavelength around 350 nm. Considering the light absorption window of C60 and pentacene molecules, it seems likely then that the recovery effect results from the photoresponse of the C60 molecules in the PVN matrix. Though photoexcitation may also contribute to trapped charge removal, we suppose the removal by excitons is a major cause of photoinduced recovery effect (Supporting Notes S8). When excitons are generated in C60 molecules under 350 nm light, the trapped electrons (or holes, depending on the polarity of the applied gate bias) can recombine with photogenerated holes (or electrons) within hundreds of picoseconds. Many trap-assisted recombination (including monomolecular and bimolecular processes), dominated by free carriers recombining with trapped one, have been successfully explained with the Shockley−Reed−Hall (SRH) model.46,47 This model suggests that such recombination occurs in a first-order process on the picosecond time scale 11764

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ACS Applied Materials & Interfaces length of naphthalene.52 We adjusted the PVN to C60 ratio in the composite interlayer from 10:0 to 7:3 to determine the optimal conditions with the goal of maximizing the memory window (excessive addition of C60 more than the ratio of 7:3 ratio made the OFETs turned on in highly positive VG, which is far from 0 V). At first, a negative gate bias stress (−50 V for 10 ms) was applied on the OFET memories, and their photoinduced recovery behavior were monitored. The shifts in the transfer curves of the devices and their corresponding photoinduced recoveries are shown in Figure S13. Figure 7a

the basis of the onset of valence regions in the UPS spectra, we calculated the HOMO energy level of each sample and created the energy diagrams in Figure 7b. As the C60 proportion increased, HOMO levels of PVN/C60 composites approached to that of pentacene, which results in lowering of hole injection barrier. More numbers of holes in pentacene would be easily transferred to the PVN/C60 interlayer because of this lower injection barrier, increasing the probability that holes will be trapped in PVN/C60 during the application of the (−) VG bias. Moreover, F−N (Fowler−Nordheim) tunneling might be able to occur relatively easier through the lower energy barrier between the pentacene and PVN/C60 interlayers with a smaller (−) VG bias (see the schematic description in Figure S16). As shown in Figure 8a, the OFETs incorporating PVN/C60 (7:3) composites as floating-gate interlayers can exhibit

Figure 7. (a) ΔVth values of the OFETs containing PVN/C60 interlayers with various C60 proportions after the application of a VG bias stress of −50 V for 10 ms and light exposure for 30 s. (b) Schematic energy diagram of the PVN, PVN/C60, C60, and pentacene, including the HOMO levels. The energy differences between the Fermi and HOMO levels were calculated from the UPS data.

plots the ΔVth values of the OFET memories as a result of the gate bias stressing and light exposure with respect to the ratio of C60 in composite interlayers. Vth shifted toward a negative value after the application of a VG of −50 V for 10 ms because the holes in pentacene were trapped in the PVN/C60 interlayers under the vertical electric field due to the (−) VG. It should be noted that the ΔVth of PVN/C60 OFET memories increased with increases in the C60 to PVN ratio, presumably due to the more amount of trapped holes in PVN/C60 interlayers. After white-light exposure, all the negatively shifted transfer curves of the OFETs with PVN/C60 returned to their initial states regardless of the C60 ratio, whereas the PVN-only devices did not show the “photoinduced recovery” behavior. Negative bias stress (NBS) results of the OFET memories also confirmed these results (Figure S14 and Supporting Notes S9). To investigate why increasing the C60 proportion leads to increased hole trapping, we analyzed the electronic energy levels at the pentacene/composite interface because the probability of hole trapping should be closely related to charge injection from pentacene to the PVN/C60 composite. Figure S15 shows UPS results for PVN/C60 composites films and pentacene on composite films for various PVN/C60 ratios. On

Figure 8. (a) Shifts in the transfer curves for OFETs with PVN/C60 (7:3) interlayers under various VG biases or light exposure (VD = −40 V). (b) Summary of the ID current states (ON and OFF) with respect to reading VG after the application of various bias conditions.

bidirectional shifts in their transfer curves toward the negative and positive directions the application of (−) and (+) gate biases, respectively, and the shifted transfer curves can be recovered through light exposure. Moreover, we have demonstrated that these voltage-induced bidirectional shifts and photoinduced recoveries can occur reversibly in a single device. The devices were stable for light-only biases. Figure S17 revealed that the transfer curves kept their states even after light exposure for 60 min. Without any voltage bias, light stress did not change the current state in the devices because there is no driving forces for exciton separation and therefore the excitons were not trapped anywhere. These features suggest the possibility of smart optical memories that utilize both holes and electrons as carriers for data storage and light as the means of data erasure. Figure 8b summarizes the ID states of the OFETs with respect to the bias condition and reading voltage. After applying a VG of −50 V for 10 ms, ID had a value below 11765

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ACS Applied Materials & Interfaces

Figure 9. Reversible current switching responses during the WRER tests of the optical transistor memories with PVN/C60 (7:3) interlayers for various applied bias conditions: (a) reading ID at VG = −20 V and VD = −10 V after programing with a VG = −50 V for 10 ms and erasing with a light exposure for 10 s; (b) reading ID at VG = 0 V and VD = −10 V after programing with a VG = 70 V for 10 s and erasing with a light exposure for 10 s.

10−10 A at a VG of −20 V because of hole trapping and storing in the PVN/C60 interlayer (see the red curve in Figure 8a; hole storage). This ID state can be regarded as the OFF current state. Light exposure effectively recovered the transfer curve to its initial position and the ID had a value over 10−6 A at VG of −20 V, which can be regarded as ON current state. In the case of electron trapping and storage, after the application of a VG of 70 V for 30 s, an ID current above 10−6 A was observed at a VG of 0 V, which corresponds to the ON current state (see the blue curve in Figure 8a; electron storage). After photoinduced recovery, the ID current had a value below 10−10 A at a VG of 0 V, and this current state was regarded as OFF current state. Electron trapping requires a higher absolute value of VG (70 V) and more time (30 s) to shift ΔVth above 20 V when compared to the results for hole trapping (−50 V and 10 ms to shift ΔVth above 20 V). This difference probably arises because electron trapping is much slower than hole trapping since the hole mobility is higher than the electron mobility in pentacene.31,53 On the basis of the above-mentioned characteristics, we performed writing (programing)−reading−erasing−reading (WRER) cycling tests on optical transistor memories based on PVN/C60 interlayers. Figures 9a and 9b show the reversible ID switching responses during the dynamic WRER tests for the cases of hole and electron storages, respectively. To shorten the total measurement time, we reduced the light exposure time down to 10 s for the erasure step in the WRER tests. Light exposures of 10 s were found to be sufficient to obtain distinct ON current states (ID above 10−6 A) for the hole-storage case and OFF current states (ID below 10−10 A) for the electronstorage case. In addition, drain voltage was adjusted to −10 V to minimize the effects of applying biases during a whole cycle. As shown in Figure 9a, the optical memories exhibited ON/ OFF ID current switching behavior according to repetitive processes of voltage-driven programing due to hole storage and light-driven erasing over 100 cycles. The optical memories also showed similar WRER switching behavior for more than 100 cycles of repeated voltage-driven programming due to electron

storage and light-driven erasing (Figure 9b). These WRER test results demonstrated the endurance of the devices regardless of the types of carriers for data storage. We believe that the lightdriven erasure time can be further reduced by applying more intense light with a wavelength near 350 nm (in our study, we used white light with an intensity of 500 μWcm−2 and containing 1% UV). The polymer existence between pentacene and C60 interfaces may contribute to long erasing time. The shallow traps in polymer can impede and delay high energy carriers (separated from excitons) diffusing into the pentacene/ composite interface; however, it probably needs further investigation in this respect. We also investigated the longterm sustainability of our optical transistor memories by monitoring the time dependence of the ON/OFF ID currents states after the application of various bias conditions: 70 V for 30 s (for electron storage), −50 V for 10 ms (for hole storage), and light exposure for 30 s (for both the hole and electron cases). Figure 10 shows the retention characteristics of the memories under the various bias conditions. All the current states were found to be stable for more than 10 000 s. The retention time, which is defined as the time required for an ON ID current state to decrease to half of its initial value, also exceeded 10 000 s. On the other hand, the presence of PVN charge blocking layers on PVN/C60 interlayers did not change the current states in the retention test of their OFET memories after light exposure (Figure S18). These results suggest that C60 molecules in the interlayer successfully store both holes and electrons for significantly long time and therefore can be an attractive alternative to organic floating-gate media for photoresponsive optical transistor memories.

4. CONCLUSIONS In conclusion, we introduced polymer/C60 composites as photoactive floating-gate interlayers to fabricate novel OFETtype optical memories that can exhibit bistable ID current states after voltage-driven programing and light-driven erasing 11766

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ACS Applied Materials & Interfaces ORCID

Se Hyun Kim: 0000-0001-7818-1903 Jaeyoung Jang: 0000-0002-5548-8563 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Center for Advanced Soft Electronics under the Global Frontier Research Program (Grant No. 2013M3A6A5073175). This work was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015R1D1A1A02062369).

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Figure 10. Retention characteristics of optical transistor memories with PVN/C60 (7:3) interlayers measured with reading VG voltages of 0 and −20 V (VD of −10 V) after the application of various bias conditions.

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processes. The presence of the polymer/C60 composite interlayers in pentacene OFETs was found to result in “photoinduced recovery” characteristics in the transfer curves due to the elimination of both trapped electrons and holes. By measuring wavelength-dependent electrical responses to light exposure, we found that the “photoinduced recovery” characteristics occurred predominantly due to trap-assisted recombination with photogenerated free carriers due to C60 molecules rather than with photogenerated free carriers due to pentacene molecules. UPS analysis revealed that increasing the C60 proportion in the polymer/C60 interlayers resulted in increased hole trapping because of the lowered hole injection barrier at the interface between the pentacene layer and the interlayer. These optimized OFET-type optical memories incorporating PVN/C60 (7:3) interlayers exhibited reversible bidirectional shifts and recovery of transfer curves with the application of voltage biases and light exposure, respectively. The memories were found to show good ON/OFF ID current switching behavior with repeated programing/erasing processes over 100 cycles during WRER cycling tests and to maintain bistable ID current states for more than 10 000 s during retention tests of both the hole-storage and electron-storage modes. Collectively, our study opens up the possibility of developing novel optical transistor memories based on organic materials that can be reset with a simple and convenient technique. We believe that the utility of these memories will be further expanded when combined with the engineering of tailor-made floating-gate materials that can absorb light with various wavelength ranges.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02365. Supporting Figure S1−S18 and Supporting Notes S1−S9 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (S.H.K.). *E-mail [email protected] (J.J.). *E-mail [email protected], Fax +82-54-279-8298, Tel +82-54279-2269 (C.E.P.). 11767

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